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Injectable and 3D Bioprinted Polysaccharide Hydrogels: From Cartilage to Osteochondral Tissue Engineering Janani Radhakrishnan, Anuradha Subramanian, Uma Maheswari Krishnan, and Swaminathan Sethuraman Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b01619 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Injectable and 3D Bioprinted Polysaccharide

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Hydrogels: From Cartilage to Osteochondral

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Tissue Engineering

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Janani Radhakrishnan, Anuradha Subramanian, Uma Maheswari Krishnan and

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Swaminathan Sethuraman*

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School of Chemical & Biotechnology, SASTRA University

Centre for Nanotechnology & Advanced Biomaterials

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Thanjavur-613401, India

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ABSTRACT: Biomechanical performance of functional cartilage is executed by the exclusive

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anisotropic composition and spatially varying intricate architecture in articulating ends of

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diarthrodial joint. Osteochondral tissue constituting the articulating ends comprise superfical soft

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cartilage over hard subchondral bone sandwiching interfacial soft-hard tissue.

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absorbent, lubricating property of cartilage and mechanical stability of subchondral bone regions

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are rendered by extended chemical structure of glycosaminoglycans and mineral deposition

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respectively.

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class of hydrogels investigated for restoration of functional cartilage.

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hydrogels have gained momentum as it offers patient compliance, tunable mechanical properties,

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cell deliverability and facile administration at physiological condition with long term

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functionality and hyaline cartilage construction.

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groups in carbohydrate polymers impart tailorability of desired physicochemical properties and

The shock-

Extracellular matrix glycosaminoglycans analogous polysaccharides are major Recently, injectable

Interestingly, facile modifiable functional

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versatile injectable chemistry for the development of highly potent biomimetic in situ forming

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scaffold. The scaffold design strategies have also evolved from single component to bi- or multi-

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layered and graded constructs with osteogenic properties for deep subchondral regeneration. This

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review highlights the significance of polysaccharide structure-based-functions in engineering

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cartilage tissue, injectable chemistries, strategies for combining analogous matrices with

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cells/stem cells and biomolecules and multi-component approaches for osteochondral mimetic

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constructs. Further, the rheology and precise spatio-temporal positioning of cells in hydrogel

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bioink for rapid prototyping of complex three-dimensional anisotropic cartilage have also been

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discussed.

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KEYWORDS: injectable, osteochondral, bioinks, bioprinting, polysaccharides

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Arthritis, an age-associated degeneration in articular cartilage is the leading cause for disability

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affecting daily activities in about 23% of older adults.1 Around 9.6% men and 18% women

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worldwide over 60 years of age have been diagnosed with symptomatic arthritis.2,3 The disease

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causes joint tenderness, varying degrees of inflammation, joint pain, occasional effusion and

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degeneration of the joint. These sequential events in the disease progression limits the movement

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and function, leading to poor quality of life thereby causing significant societal and economic

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burden equivalent to cardiovascular disease.2,4,5

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imbalance between synthesis and degradation of matrix materials leads to osteoarthritis.2,4

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Although the injury in articular cartilage initiates wound healing response, drawbacks such as

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limited proliferation of terminally differentiated chondrocytes, their catabolic response to

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pathological mediators and avascularity restricting immigration of regenerative cells prevents

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complete regeneration and restoration of native ECM structure and composition.6,7 Current

1. Introduction

Destabilization in tissue homeostasis causing

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treatment modalities such as microfracture, mosaicplasty, autologous chondrocyte transplantation,

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and osteochondral allograft transplantation relieves pain and improves joint function.5,8 However,

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the cartilage produced by these approaches lack integration with neighbouring host tissue and

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most often constitute collagen I, which is inferior both chemically and mechanically to hyaline

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articular cartilage that has collagen II.1,8,9 Hence, once distorted, neither reversion nor retardation

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of degeneration occurs in the native joint, thereby leaving the field wideopen for regenerative

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strategies like tissue engineering and cell based therapies to address the existing lacuna.5

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Articular cartilage, a low friction articulation diarthrodial joints at knees, hips, fingers, and lower

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spine region is an unusual biphasic tissue. Solid matrix of this tissue is composed of collagen II

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and proteoglycans as primary extracellular matrix (ECM) components and fluid phase is

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synovium.10,11 Cartilage is avascular, aneural and alymphatic in nature and unlike other tissues,

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homogenous population of sparse chondrocytes (2–5%) residing in porous matrix contribute to

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the ECM maintenance, which in turn nourishes the chondrocytes.10,12 The components of a few

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millimeters thick cartilage provides required mechanical properties enabling biomechanical

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functions such as dissipation of compressive loads, shock absorption and allows frictionless pain-

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free movement.4,10,13,14 Complementary to the cartilage compressive strength, the underlying

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harder subchondral bone contributes due to its large area and provides anchorage for collagen

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fibrils of articular cartilage.15 However, in the case of arthritic joint evidenced by extensive

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cartilage tissue damage, an imbalance occurs between the matrix synthesis and degradation of the

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major matrix glycocomponents due to loss of chondrocytes and adverse inflammatory

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responses.16 The pathogenesis of severe full-thickness defects involves degenerative changes in

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cartilage and subchondral bone with notable remodelling changes in response to the applied

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stress.15 Polysaccharides possess several characteristics that can be harnessed to address the 3 ACS Paragon Plus Environment

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problems encountered at an osteoarthritic joint.

The sol-gel transition phenomenon of

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polysaccharides and tailorability offers designing of spatially varying multi-layered anisotropic

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constructs that mimic the anisotropic composition and micro-macro structural organization

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thereby providing a viable therapeutic strategy to combat osteoarthritis and related joint disorders.

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The use of polysaccharides for a variety of material science applications with special emphasis on

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biomedical applications has been increasing since it resembles the glycan constituent of native

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extracellular matrix (ECM).17 Owing to their beneficial inherent physicochemical properties

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such as biocompatibility, biodegradability, tailorable functional groups and role in cell signaling,

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research on structural and functional polysaccharide biomaterials have gained pace in the recent

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decades. They find diverse utility ranging from extracorporeal devices to intricate implants

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addressing highly specialized medical requirements.18 Hence, many novel synthetic routes and

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newer sources are being explored recently for polysaccharide polymers though most of the

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currently available polysaccharides have been sourced from natural origin.19,20 Polysaccharides

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have been used to design tissue engineered constructs for blood vessels, myocardium, heart

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valves, bone, articular and tracheal cartilage, intervertebral discs, menisci, skin, liver, skeletal

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muscle, neural tissue, urinary bladder and for transplantation of Islets and ovarian follicles.21,22

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In particular, different types of polysaccharides have been explored as cartilage substitutes for the

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functional restoration of load bearing tissue. It has been found extremely useful for articular

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cartilage whose degeneration causes major disabilities in elders.9,23 Three-dimensional Hyaff 11®,

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Hyalograft–C® implants and intra-articular injections of hyaluronic acid as viscosupplement have

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proven effective for the management of patients suffering from osteoarthritis.9,23

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Tissue engineering integrates the principles of engineering and life sciences towards designing

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various scaffolds as biological substitutes either with or without cells and biomolecules to repair,

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regenerate and restore functional tissue at the lesion.24–27 Success of tissue engineering relies

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largely on the extent to which a scaffold mimics the native extracellular matrix (ECM) of the

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respective tissues in terms of structure and composition.28,29

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biomaterials are being explored for the fabrication of matrices, polymers have gained prime

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significance towards the development of ECM analogues.

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biodegradable properties, and ease of functionalization exhibited by polymers favors tailoring of

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the desired composition and intricate architecture as per requirement.30

Although various classes of

Wide range of mechanical and

Natural polymers

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intrinsically possess diverse functions. This category includes proteins that serve as structural

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materials and catalysts while the polysaccharides play vital role in storage, cell recognition and

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intracellular communication.31 Designing an ideal tissue engineered substitute for the restoration

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of articulation and load-bearing function requires constructs with highly mimetic multi-layered

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osteochondral features. The integration of superficial chondral and mineralized subchondral

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bone regions to establish smooth transitional soft-hard tissue interface in the scaffold is

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pivotal.32,33 Recently, 3D printing and patterning of cellular and matrix materials promises to

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replicate the components of anisotropic osteochondral tissue.

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bioprinting enables reconstitution of intrinsic architectural organization and functional

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performance of the tissue. However, the choice of hydrogel material with cytocompatibility, cell

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dispensability and required viscoelastic properties that ensures stability of printed construct is

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indispensible for bioprinting.34 This review compiles the state-of-the-art in polysaccharide based

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injectable scaffolds, multi-component constructs and printable bioinks for functional restoration

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of cartilage that helps in comprehending the relevance of chemistry on physical and biological

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properties of the cartilage tissue mimics. 5 ACS Paragon Plus Environment

This exciting prospect of

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2.

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Polysaccharides are carbohydrate macromolecules built by repeating monosaccharide units linked

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by glycosidic bonds that play vital roles in living systems.21,35 These abundant polysaccharides

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intrinsically possess desirable properties such as biocompatibility, biodegradability and

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functional groups that facilitate facile chemical modifications for tailorability, cytocompatibility

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and organized macro-structural features making them more promising as biomaterials.21,35

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Complementary to the chemical and structural features, biologically the saccharide units play a

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remarkable role in cell signaling thereby mediating cellular processes at molecular level.21

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Polysaccharides majorly explored for tissue engineering include starch, cellulose, chitosan,

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pectins, alginate, agar, dextran, pullulan, gellan, xanthan and glycosaminoglycans.21 Based on

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the chemistry, these repeating units of saccharide possess both beneficial and detrimental

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properties towards tissue engineering applications, which are tabulated in table 1 and 2. These

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polysaccharides have been used alone or in combinations with other natural and synthetic

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polymers for the fabrication of robust matrices with multiple cues to achieve tissue

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morphogenesis. Polysaccharides can be categorized based on the chemical composition (homo-

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and heteropolysaccharides), structure (linear and branched), function in the organism (structural,

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storage and secreted polysaccharides), charge (cationic, anionic and neutral) or source (animal

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origin, plants, algal and microbial).21,35 Figures 1 and 2 depict the structures of linear and

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branched polysaccharides explored for tissue engineering applications. As the physicochemical

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characteristics and functions depends on the structure of polysaccharides, classification based on

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the structure (linear and branched) with the properties has been discussed in the following

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sections (figure 1, 2 & 3).

Polysaccharides as functional biomaterials

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Figure 1. Chemical structures of linear polysaccharide polymers with disaccharide repeating

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units [A] Hyaluronic Acid; [B] Chitin; [C] Chitosan; [D] Heparin; [E] Chondroitin Sulfate; [F]

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Carrageenans; and [F] Pectins. 7 ACS Paragon Plus Environment

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2.1

Linear polysaccharides

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2.1.1

Hyaluronic acid (HA) is a linear polysaccharide consisting of alternate N-acetyl-D

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glucosamine and D-glucuronic acid linked by β(1→3) and β(1→4) glycoside bonds (figure

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1A).36–38 It is a unique linear non-sulfated proteoglycan in the native ECM of connective tissues,

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particularly cartilage and the synovial fluid, with high water adsorption and retention

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capability.23,36,38 The highly viscous and viscoelastic synovial fluid containing HA as the major

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component exhibits protective physicochemical properties as lubricant and shock absorber in

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articular joint cavities.23,36,39 HA is also known to interact with CD44 glycoprotein receptors on

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the chondrocyte surface and enhance the cellular functions through the chondrocyte-specific

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CD44/HA signaling pathway.36,39,40 It stimulates chondrocyte metabolism and exhibits multiple

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chondroprotective roles such as production of collagen II and proteoglycans. This is achieved

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through associations of HA core with keratin sulfate and chondroitin sulfate, scavenging of

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reactive oxygen species, regulation of immune complex interaction and fibroblast

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proliferation.38,39 Though it has many desirable properties such as lubricating and cushioning

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effects for restoring the viscosity and elasticity of the synovial fluid, poor mechanical strength

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and faster degradation of HA restricts its potential in visco-supplementation therapy for treatment

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of early osteoarthritis.4,39,41,42

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hydrophobic poly(caprolactone) units or suitable chemical modifications such as esterification,

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methacrylation and divinyl sulfone/dialdehyde crosslinking at the carboxyl and hydroxyl

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functional groups of HA.38,43,44 Heris et al., have reported a smart injectable hybrid HA / gelatin

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hydrogel particles with Young’s modulus of 22±2.5 kPa by indentation test and shear modulus of

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75±15 Pa at 1 Hz, embedded in HA network as substrate for viable cell adhesion.45

However, these demerits can be overcome by including

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2.1.2 Chitin or poly(β(1→4) N-acetyl-D-glucosamine) is an aminopolysaccharide sourced

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majorly from the crustaceans (shrimps and crabs) and is the second most abundant natural

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polymer (figure 1B).46,47

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viscosity, polyelectrolyte tendency, polyoxy salt formation and metal chelation.48 Additionally,

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the inherent biocompatibility, biodegradability and low immunogenicity of chitin are

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advantageous for biomedical applications such as wound dressing, immobilization of enzymes

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and cells.46,47,49,50 The microfibrillar structure of chitin films and fibers has promoted its use as a

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bioabsorbable suture material.47 Scaffolds of β-chitin has supported chondrocyte morphology

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and synthesis of specific ECM similar to hyaline cartilage.51 A ternary polyethylene oxide /

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chitin / chitosan scaffold showed cartilaginous regenerative potential as the N-acetyl glucosamine

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of chitin favors chondrogenic expression.52 However, its hydrophobicity and insolubility in

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various media limits its applications and hence chitosan, its deacetylated dissolvable form finds

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wide utility as a biomaterial. 48,53

The properties of this highly basic polysaccharide include high

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Table 1. Various linear polysaccharide polymers, their repeating units along with their

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advantages and disadvantages towards tissue engineering applications

17 Polymer

Repeating units

Properties Beneficial

Disadvantages

Hyaluronic acid (HA)

N-acetyl-D- glucosamine and D-glucuronic acid linked by (1→3) and (1→4) glycoside bonds

High viscoelasticity, chondro-protective, high water adsorption and retention

Poor mechanical strength, faster degradation

Chitin

(1→4) linked N-acetyl glucosamine

High viscosity, polyelectrolyte tendency, metal chelation, enzymatic degradation

Hydrophobicity and insolubility in various media

Chitosan

(1→4) linked D-glucosamine and N-acetyl glucosamine

Process-ability, microbicidal, structural

Brittleness

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similarity to glycosaminoglycans Chondroitin Sulfates

D-glucuronic acid and Nacetyl galatosamine sulfated at either 4- or 6- positions

Water and nutrient absorption, chondroprotective, antiinflammatory

Highly hydrophilic, solvability

Heparin

(1→4)-linked uronic acids (mainly D-glucuronic, Liduronic or L-2-sulfated iduronic) and glucosamine (mainly D-N-acetyl glucosamine and D-di-N-6sulfate glucosamine)

Protein binding, anticoagulation, complement activation, anti-angiogenic, anti-cancer and antiinflammatory

Provokes immunogenic responses

Cellulose

D-glucopyranose linked by β(1→4) glycosidic bonds.

Varied mechanical properties to suit soft and hard tissues, machinable, bioadhesives

Requires intense processing to obtain pure form

Pectins

α (1→4)-galacturonic acids with varying degrees of methylation in carboxylic acid groups

Ionic gels

Highly hydrophilic

Alginate

β-D-mannuronic acid and αL-guluronic acid

Ionic gels, poor protein binding affinity, promotes cell spheroids

Poor mechanical properties, difficult to handle

Carrageenans

(1→3)-linked β-D-galactose and (1→4)-linked α-Dgalactose

Thermo and ionic gelling, structural similarity with glycosaminoglycans, soft to firm gels

Inflammatory

Gellan gum

(1→4)-L-rhamnose-α(1→3)D-glucose-β(1→4)-Dglucuronic acid-β(1→4)-Dglucose

Thermo and ionic gelling, structural similarity with glycosaminoglycans, varying mechanical properties based on acetylation

Slow degradation

Pullulan

α(1→6) linked maltotriose

Flexible with elastic and compressible properties

High hydrophilicity

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2.1.3 Chitosan is a linear aminopolysaccharide consisting of β(1→4) linked D-glucosamine

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residues and N-acetyl-glucosamine groups (figure 1C). It is a semi-synthetic polymer derived by 10 ACS Paragon Plus Environment

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the partial deacetylation of the poorly soluble chitin.7,54 The cationic chitosan possess bioactive

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properties such as biocompatibility, biodegradability and microbicidal property that renders

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versatile applications for this polysaccharide.55

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electrostatically with negatively charged ECM molecules like glycosaminoglycans, which in turn

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links with cytokines and growth factors to regulate the cellular fate processes.55 Additionally,

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nontoxicity, processability, ease of chemical modification via hydroxyl and amine functional

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groups, mechanical properties and unique engineered structures of chitosan are advantageous for

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cell and tissue engineering applications.56–58 Moreover, due to its chemical structural similarity

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with diverse glycosaminoglycans (GAGs), the predominant ECM molecules in cartilage and

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meniscus, chitosan mimics the native micro-environment for chondrocytes and meniscus cells,

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and promotes chondrogenic activity and cartilage-specific protein expression.36,54,59 Chitosan-

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polycaprolactone copolymers blended with varying ratios of collagen II to construct layer by

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layer mimetic porous microstructured scaffolds crosslinked with sodium tripolyphosphate for

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articular cartilage repair. The graded architecture and zonal variation in mechanical properties

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exhibited positive impact on the cellular behavior of chondrocytes cultured.60

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2.1.4 Heparin is another linear highly sulfated glycosaminoglycan with alternating units of

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β(1→4) linked uronic acids (mainly D-glucuronic, L-iduronic or L-2-sulfated iduronic) and

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glucosamine residues (mainly D-N-acetyl glucosamine and O- and N-sulfated glucosamine).61–63

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Existence of reactive groups such carboxyl and sulfate groups (figure 1D) provides charge

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specificity that enables chemical modification as well as electrostatic interactions. The highly

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negative charge of heparin interacts electrostatically with proteins including growth factors,

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proteases and chemokines and regulates various cellular signaling.64 Its affinity towards growth

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factors and cell surface receptors could facilitate stabilization of trophic molecules against

The positive charge of chitosan interacts

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degradation and enable extended localization with enhanced binding.64,65 In addition to cellular

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interactions, heparin exhibits anti-coagulation, complement activation, anti-angiogenic, anti-

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cancer and anti-inflammatory effects in biological system.61,64,66 An enzymatically crosslinked

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injectable heparin and dextran based hydrogel displayed higher storage modulus (~48kPa),

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chondro-compatibilty and cartilage matrix secretion.63

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2.1.5 Chondroitin sulfate (CS) is an anionic polyelectrolyte glycosaminoglycan consisting of

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repeating disaccharide units of β-(1→4) D-glucuronic acid and β-(1→3) N-acetyl galactosamine

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with sulfate groups (figure 1E). CS is the major matrix component of cartilage,67–69 associated

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with proteoglycans to form aggrecan and syndecan that function as structural component and

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receptor respectively.70 Chondroitin sulfate not alone mediates the osmotic swelling pressure for

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matrix expansion and collagen network tension, but also possesses high affinity towards the

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growth factors via electrostatic interaction thereby exhibiting both structural and biological role

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in articular cartilage.70

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activity, water and nutrient retention, potent wound healing for functional cartilage restoration are

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the other major beneficial properties of CS.68,69 Since the presence of CS influences compressive

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strength of the scaffold by promoting proteoglycan secretion, many attempts has been made to

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integrate CS with other polymers like poly(ethylene glycol) through its reactive hydroxyl and

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carboxyl functional groups.68,69

Host tissue integration, chondro-protectiveness, anti-inflammatory

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2.1.6 Carrageenan is a linear carbohydrate polymer with (1→3)-linked β-D-galactose and

22

(1→4)-linked α-D-galactose units, which vary in the degree of substitution and are modified into

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the 3, 6-anhydro derivative depending on the source and extraction conditions.

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disaccharides in the backbone of hydrophilic carrageenans have been in special focus, as its 12 ACS Paragon Plus Environment

Sulfated

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structure resembles glycosaminoglycans. Based on the sulfation of disaccharides, three classes of

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carrageenans are available. They are the least sulfated κ-carrageenan (kappa), highly sulfated λ

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and the ι-type with intermittent sulfation.

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Kappaphycus cottonii forms strong rigid gels, while the highly sulfated lambda (λ) form does not

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self-gel. On the other hand, elastic, dry, soft gels are produced by the iota (ι-type) in the presence

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of calcium ions.

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potassium concentration.71 Instead of ionic gelation, carrgeenans even form thermoreversible

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hydrogels since it can undergo upper critical solution temperature mediated sol-gel

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transformation.71

The κ-carrageenan (kappa) extracted from

Firmness of the κ-carrageenan hydrogels has been tailored by changing

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2.1.7 Pectins are natural polysacchardies found in most primary cell walls and non-woody parts

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of terrestrial plants.21 Repeats of largely methoxylated 1,4-linked α-D-galactosyluronic acid

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constitutes pectin (figure 1G).21,72

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combined with many hydrophobic polymers to improve wetablility in polymeric blends.21,72 In

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addition to biodegradability and cytocompatibility, the facile gelling ability of pectin on exposure

16

to divalent or multivalent cations to form physical hydrogels has gained much attention for tissue

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engineering applications as injectables.21,73,74 Further, the diols of pectins can be oxidized to

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aldehyde groups that form Schiff‘s base by reacting with amino groups to form chemical

19

hydrogels.74

The excellent hydrophilic property of pectin could be

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2.1.8 Cellulose constitutes the primary component of plant cell wall structure and is well known

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as renewable and sustainable biopolymer.75,76 It is a linear homopolymer with D-glucopyranose

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repeating units linked by β-(1→4) glycosidic bonds (figure 2A).76 Apart from its hydrophilicity,

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the predominant equatorial conformation of the glucose residues in cellulose imparts stability to 13 ACS Paragon Plus Environment

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the chair structure, minimizes flexibility and confers exceptional mechanical strength due to

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hydrogen bonding and high cohesiveness.75,77

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2.1.9 Alginate is an anionic block polysaccharide with repeating disaccharide units of β-D-

5

mannuronic acid and α-L-guluronic acid residues linked through 1→4 glycosidic linkages (figure

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2B). It is derived from brown sea weed family which includes Laminaria japonica, Laminaria

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hyperborea, Ascophyllum nodosum, Eclonia maixma, Lesonia negrescens, Macrocystis pyrifera,

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and Sargassum.78–81

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mucoadhesive.80,82,83 The carboxylate groups in guluronic acid residues have been used to form

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ionic hydrogels through their interaction with divalent cations such as calcium, zinc and

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strontium.80,83,84

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crosslinks.85 However, poor binding affinity of alginate to serum proteins restricts adhesion of

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certain mammalian cell types on alginate scaffolds. However, alginate scaffolds have been found

14

to aid non-adherent cells such as chondrocytes to maintain their native spherical morphology.84

15

Apart from the most commonly evaluated ionically crosslinked physical hydrogels, chemical

16

hydrogels have also been reported by modifying alginic acid groups to aldehyde via acid

17

hydrolysis. This step is followed by oxidation to form injectable poly(aldehyde guluronate)

18

(PAG) hydrogel stabilized by exposure to adipic acid dihydrazide for crosslinking molecule.86 A

19

calcium crosslinked alginate hydrogel with rapid curing and homogeneous mechanical stability

20

showed near-elastic property with Young’s modulus of

21

periosteum-derived chondrogenesis.87

This biocompatible carbohydrate is reported to be biodegradable and

The block structures of alginate determines the structure of the ionic

0.17±0.01 MPa and enhanced

22 23

2.1.10

Gellan gum is a high molecular weight anionic exopolysaccharide secreted by the

24

bacteria Sphingomonas paucimobilis (Sphingomonas elodea) during aerobic fermentation.88 The 14 ACS Paragon Plus Environment

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linear tetrasaccharide consists of (1→4)-L-rhamnose-α(1→3)-D-glucose-β(1→4)-D-glucuronic

2

acid-β(1→4)-D-glucose as repeating unit, with one carboxylic side group (figure 2C).89 Gellan

3

gum properties such as non-toxicity, easy processability, injectability, pseudoplasticity and

4

structural similarity of glucuronic acid residue with glycosaminoglycans are advantageous.89 The

5

mechanical properties of gellan gum varies from soft, elastic to hard, brittle in the acetylated and

6

deacetylated forms respectively.89 Gellan gum forms non-toxic, ionic gels and thermo-gels close

7

to body temperature and has been used for various drug delivery and tissue engineering

8

applications.88,90–92 Though this polysaccharide tends to form ordered double helix structure on

9

cooling, true gel network formation has been initiated upon aggregation and ionic crosslinking.88

10

Oliveira JT et al., 2010, reported that the valency of the ions determines the strength of ionic

11

hydrogel. Presence of monovalent cations induces gelation in gellan gum via screening of

12

electrostatic repulsion amongst the carboxylate groups whereas divalent cations connects two

13

carboxylate groups of glucuronic acid molecules in addition to the screening effect.89 Thus

14

divalent cations form stonger gels than monovalent cations and hence influence the viscoelastic

15

behavior too. This gellan gum also forms photocrosslinkable hydrogels via methacrylation.

16 17

2.1.11 Pullulan occurs as a part of the cell wall in the yeast-like fungus Aureobasidium

18

pullulans.5,93 It comprises linear maltotriose residues linked by α(1→4) glycosidic bonds with

19

consecutive α(1→6) glycosidic bonds (figure 2D).5,93–95 Its molecular weight ranges between

20

1,000,000-2,000,000 daltons depending on the culture conditions of fungi.93 This non-ionic

21

polysaccharide is biocompatible (non-immunogenic), non-toxic, blood compatible, non-

22

mutagenic and non-carcinogenic, impermeable to oxygen, non-hygroscopic and non-

23

reducing.93,94 Presence of a unique linkage in its structure imparts structural flexibility with

24

elastic and compressible properties.93 The highly water soluble pullulan has been modified via 15 ACS Paragon Plus Environment

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1

chemical functionalization or blending with other organic or inorganic materials that improve the

2

stability.93,94 Carboxymethylated pullulan conjugated with heparin and hydroxyapatite / pullulan

3

/ dextran composite have been developed as tissue regenerative constructs.93,94

4 5

Figure 2. Chemical structures of linear polysaccharide polymers [A] Cellulose; [B] Alginic acid;

6

[C] Gellan gum and [D] Pullulan.

7 8

2.2.

9

2.2.1. Starch is an abundant, renewable, hydrophilic storage polysaccharide in plants comprising

10

of two polymers – linear α-amylose (20-30%) and highly branched amylopectin (70-80%)

11

consisting of α(1→4) glucan and α(1→4)-glucan with α(1→6) linkages at branches (figure

12

3A).21,24,31,96–98

13

contributes to its inferior mechanical properties with brittleness.77

Branched polysaccharides

It has high molecular mobility and weak interaction between chains that

16 ACS Paragon Plus Environment

However, due to its

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biocompatibility, biodegradability and versatile processability, starch has been used in

2

combination with other polymers and as composites in various tissue regenerative strategies

3

especially orthopaedics and in drug delivery.77,97–99 In a stimuli responsive injectable chitosan-

4

starch combined hydrogel, the increase starch content enhanced the elastic modulus evaluated by

5

dynamic mechanical analysis, induced chondrogenesis of encapsulated adipose derived stromal

6

cells and enhanced carilage matrix secretion.98

7 8

2.2.2. Dextran derived from bacteria consists primarily of repeating α(1→6) linked D-

9

glucopyranose residues with less percent of α(1→2), α(1→3), or α(1→4) linked side chains

10

(figure 3B).100,101 Dextran known for its high hydrophilicity shows low protein adsorption,

11

biodegradability, biocompatibility and has been widely used for development of cell

12

microcarriers, drug delivery vehicles and tissue engineering constructs.100–102 It bears three

13

hydroxyl groups per glucose unit, which facilitates the formation of both physically and

14

chemically cross-linked hydrogels.100,102,103 Dextran derivatized with lactic acid oligomers form

15

physical crosslinks while dextran modified with bifunctional isocyanates, glutaraldehyde, or by

16

partial oxidation of hydroxyl groups to aldehydes that were then crosslinked with gelatin forms

17

chemically crosslinked gels.100 In addition, liquid-liquid phase separation mediated scaffolds has

18

been fabricated from dextran for tissue engineering applications.100 Dextran-tyramine conjugated

19

HA enzymatic injectable biomimetic hydrogel demonstrated high moduli of 370 to 18,000 Pa,

20

enhanced bovine chondrocyte viability, proliferation and matrix secretion.104

21 22

2.2.3. Agar sourced from marine red seaweeds is a complex polysaccharide mixture of linear

23

agarose gelling element and branched agaropectin.

24

comprises of 1→3-linked-β-D-galactose (G) and 1→4-linked 3,6-anhydro-α-L-galactose whereas

The linear agarose polymer skeleton

17 ACS Paragon Plus Environment

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1

branched agaropectin comprises agarose with presence of several substituent groups (eg. sulfate

2

esters, methyl esters, pyruvate acid ketals) at 4-linked α-L-galactose residues (figure 3C).84,105

3

The self-gelling property of agar restricts its processing ability for use as a biomaterial, although

4

it possesses exciting properties and has gained recent attention in the form of nanofibers.105

5

Linear agarose forms hydrogels by hydrogen bonding and has been employed for three-

6

dimensional chondrocytes encapsulation.84

7

synthesis of matrix components on exposure to dynamic deformational loading as well as

8

maintained chondrocyte phenotype upto four days.12

9

tendency to resist invasion of blood vessel that results in low oxygen tension and thereby

10

Chondrocyte-seeded agarose hydrogels induced

In addition, agarose possess intrinsic

supports the formation of avascular cartilage tissue.84

11 12

Table 2. Branched polysaccharide polymers, their repeating units along with their advantages

13

and disadvantages towards tissue engineering applications Polymer

Repeating units

Properties Beneficial

Disadvantages

Starch

20-30% of α-amylose (glucose linked by α(1→4) bonds) and 70-80% of amylopectin α(1→4)linked glucose, branched with α(1→6) linked for every 24 to 30 glucose residues

Versatile processability, imparted with thermoplasticity

Inferior mechanical properties with brittleness

Agar

Agarose [(1→3)-β-Dgalactopyranose-(1→4)-3,6anhydro-α-L-galactopyranose units] and agaropectin [(1→3)-βD-galactopyranose-(1→4)-3,6anhydro-α-L-galactopyranose and 4-linked α-L-galactose]

Self-gelling, agarose resists blood vessel invasion supporting avascularity

Difficult to process, quick gelation, limited degradation of agarose

Dextran

α(1→6) linked D-glucopyranose with less percent of α(1→2),

Swellability and rheology

Very high hydrophilicity

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Biomacromolecules

α(1→3), or α(1→4) linked side chains Xanthan

Two D-glucose, two D-mannose and one D-glucuronic acid. Mannose-glucuronic acidmannose trisaccharide is attached to every second glucose by (1→3) linkage

Pseudo-plastic, rheology similar to HA

Difficult to handle high viscosity

1 2 3

2.2.4. Xanthan gum is an extracellular heteropolysaccharide produced by the bacterium

4

Xanthomonas campestris. Its molecular weight ranges from 2 X 106 to 20 X 106 Da.4,106,107

5

Xanthan consists of repeating units of five monosaccharides comprising two D-glucose, two D-

6

mannose and one D-glucuronic acid units (figure 3D).4,108 The glucose residues forms the

7

primary structure similar to the cellulose backbone. The alternating glucose units are substituted

8

with trisaccharide units of glucuronic acid flanked by mannose units.106,107

9

around half of the terminal mannose units are pyruvated while the inner mannose linked to the

10

backbone are acetylated.106,107 In the fivefold helical secondary structure, the backbone remains

11

protected from the outside environment by wrapping of the side chains via hydrogen bonding.4,106

12

This complexity in structure contributes to its high viscosity at lower concentration, stability at

13

various pH, temperature, ion concentrations and pseudo-plasticity similar in rheology to

14

hyaluronic acid.4 The non-toxic, biocompatible, bioadhesive and biodegradable xanthan has been

15

reported for various medical applications such as wound healing, implantation, drug carriers and

16

as hydrogels components.106,108 The administration of intra-articular xanthan injection has been

17

found to be protective on the articular cartilage in papain-induced osteoarthritic rabbit models.4

18

Further, carboxymethyl derivatization at the glucose residues of xanthan has been reported for

19

microcapsule encapsulation of chondrocytes and act as artificial matrix.107

20 19 ACS Paragon Plus Environment

Approximately

Biomacromolecules

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1 2

Figure 3. Chemical structures of branched polysaccharide polymers [A] Starch; [B] Dextran; [C]

3

Agar and [D] Xanthan.

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1

3. Articular cartilage: Structure, function, injuries, healing responses and therapeutics

2

A comprehensive understanding of the structure, function and injuries of articular cartilage with

3

intrinsic responses ensures progress in the design of appropriate therapeutic modalities.109 The

4

hyaline articular cartilage functions to provide gliding low-friction surface, shock absorption and

5

protects the underlying supporting subchondral bone from pressure which in turn complement the

6

mechanical strength.15,110–112 The biomechanical role especially the compressive properties of

7

cartilage is based on the interstitial fluid swelling pressure of proteoglycans, mostly associated in

8

aggregates comprising complexes of aggrecans.

9

attachment of brush-like chondroitin sulfate or keratin sulfate with the protein core (aggrecan)

10

with hyaluronic acid and a link protein for stabilization.11,110–112 Additionally, hydrated collagen

11

II stabilized by collagen IX and XI, non-collagenous proteins, lesser amounts of small

12

proteoglycans such as biglycan, fibromodulin and decorin contributes to the organization and

13

thereby regulates functions of the cartilage matrix.4,10,110,111 The additional functions of matrix

14

include protection for chondrocytes, mediation of nutrient transfer, serve as store for cytokines

15

and growth factors for chondrocyte homeostasis.110

16

sparsely distributed cells that synthesize and maintain their resident matrix by regulating matrix

17

metabolism.

18

collagenous proteins differentially at varying depths to highly specialized tissue that enable their

19

functions. The compositional and morphological variations in the depth of cartilage matrix is

20

divided into four zones – superficial, transitional, middle (radial) or deep zone and calcified

21

cartilage zone.110 The superficial zone consists of thinnest collagen fibres at highest density to

22

form oriented lamina splendens covering the joint.

23

permeability and contributes to the tensile strength.111 The transitional zone contains highest

24

proteoglycan content, with collagen orientation varying from tangential to random in the

Aggrecan aggregates are generated by the

Chondrocytes are highly specialized,

The chondrocytes precisely organize the collagen, proteoglycans and non-

This resists shearing, monitors fluid

21 ACS Paragon Plus Environment

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1

underlying regions. The collagen fibrils orient perpendicularly in the deeper zone, and delineates

2

at the tidemark thereby differentiating the deep zone from calcified zone.111 This arrangement of

3

collagen improves the integration of soft and hard tissues at the cartilage-bone interface. In

4

contrast, calcified zone comprises collagen X for mineralization and structural integrity. The

5

proper functioning of the cartilage depends on the contributions from each zone.111

6

maintenance of required levels and distribution of proteoglycan and collagen fibers is crucially

7

significant for imparting the compressive and tensile strength respectively of articular

8

cartilage.113 The deepest or lattermost region of diarthrodial bone end is subchondral bone that

9

lies between calcified cartilage and trabecular bone.15 Unlike the cartilage region, subchondral

10

bone is highly vascularized which nourishes itself and the overlying cartilage.15 In addition to

11

providing anchorage for collagen fibrils of adjacent cartilage, the subchondral bone plays critical

12

role in absorption and maintenance of joint shape.15 The mechanical strength of bone is rendered

13

by extracellular matrix composed collagen and other organic components reinforced with

14

inorganic calcium phosphate nanocrystallites for superior rigidity.114

The

15 16

Trauma or degeneration that causes imbalance between the matrix biosynthesis and degradation

17

leads to destruction of articular cartilage tissue with subsequent extensive damage at various

18

zones.16 Such defects or injuries are categorized based on the depth into partial-thickness and

19

full-thickness, with the subchondral bone affected in the latter (figure 4). The avascular nature of

20

cartilage has no access to progenitor cells and blood cells, hindering the ability to initiate normal

21

wound healing process in partial thickness defects, thus exhibiting it poor intrinsic

22

reparability.110,111

23

chondrocyte proliferation with subsequent synthesis of ECM in an attempt to heal the lesion. In

24

most cases, the reparative efforts fail as the chondrocyte number does not increase to meet the

However, metabolic and enzymatic activities at the lesion site promote

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Biomacromolecules

1

demands at the injured site that ultimately compromises functioning.111

2

thickness wounds penetrating the calcified and subchondral bone witness the migration of bone

3

marrow cells followed by spontaneous immune response and subsequent healing process to

4

restore the tissue. The subchondral bone plays integral role in the osteoarthritis pathogenesis as it

5

undergoes bone remodeling induced by the applied stress.15 The characteristic remodeling in

6

osteoarthritis is increased sclerotic subchondral bone, osteophytes spikes at the joint margins,

7

subchondral bone cyst formation and remodeling of trabeculae.115

8

osteoarthritis evidences 20 fold higher turn over of subchondral bone with associated higher

9

secretion of bone markers such as alkaline phosphatase, osteocalcin and osteopontin than normal

10

bone.115 Subsequently, these changes enhances anabolic activity of subchondral bone osteoblasts

11

and contributes to osteophytic bone formation.115 Such immature osteoids in subchondral bone

12

are unmineralized that lacks in the properties of native bone. This eventually leads to bone-bone

13

articulation of abnormal subchondral bone and the presence of unmyelinated free nerve endings

14

causes severe pain experienced in patients.15

15

components including contrasting layers of osteochondral tissue comprising both soft superficial

16

cartilage and hard subchondral bone with irregular defects.

On the contrary, full

The progression of

Thus, the distortion affects the entire joint

17 18

Management of clinical osteoarthritis includes pharmacological, non-pharmacological and

19

surgical options.116

20

administration of simple analgesia, opioid analgesia, non-steroidal anti-inflammatory drugs

21

(NSAIDs), calcitonin, topical applications, nutraceuticals such as glucosamine and chondroitin

22

sulfate, intra-articular injections of hyaluronic acid and corticosteroids.116,117 Management of the

23

condition by awareness, exercise and weight reduction approach are categorized as non-

24

pharmacological approaches.116

Pharmacological therapies mostly aim at alleviating the pain by

In patients refractory to both pharmacological and non23 ACS Paragon Plus Environment

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1

pharmacological therapy, surgical total joint replacement option remains definitive for substantial

2

improvement in pain and function, but has finite life expectancy.109,116 Regenerative modalities

3

such as microfracture, mosaicplasty, allograft transplantation and autologous chondrocytes

4

transplantation reduce pain, restore tissue and improve joint function but lead to the formation of

5

inferior fibrocartilage consisting of collagen I.1,5,8,9 The high proportion of fibrocartilage may fail

6

in lateral integration with neighboring host tissue of the defect site.118 To overcome the existing

7

demerits, tissue engineering based interventional regenerative strategies aim at augmenting the

8

cartilage repair with subchondral bone.

9

substituting the damaged matrix and / or cells, biomolecules in combinations to restore the

10

superficial hyaline cartilage composed of collagen II and the mineralized subchondral bone

11

region. Emergence of injectable glycosaminoglycan mimetic polysaccharide scaffold addresses

12

major unresolved clinical complications due to its facile administration, patient compliance,

13

spatio-temporal distribution of cells and biomolecules, structural and composition mimetic design.

14

Furthermore, the fabrication of multi-layered matrices that are analogous to superficial resilient

15

cartilage and mineralized deeper subchondral zones are most relevant recent strategies in tissue

16

engineering.

These strategies employ biomaterial scaffolds for

17 18

4. Sol-gel transition chemistry of polysaccharide materials

19

Hydrogels are three dimensional, hydrophilic, water-insoluble networks of physical or chemically

20

crosslinked homopolymer or copolymer chains that retain water and possess properties similar to

21

native cartilage.37,83,104,119,120

22

transition from sol state at ambient conditions to gel phase on exposure to physiological

23

conditions are collectively termed as injectable or in situ forming hydrogels.83 An ideal in situ

24

forming injectable hydrogel should be formed in compatible aqueous medium without releasing

Such polymeric hydrogels formed on the rationale of phase

24 ACS Paragon Plus Environment

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1

harmful by-products.

Further, gelation rate should ideally ensure homogeneous mixing of

2

clinically meaningful number of cells with the functional efficacy.84 Such in situ gelling systems

3

permit premixing of cells and bioactive agents thereby facilitating facile spatiotemporal

4

incorporation into the hydrogel depot at the target site.37,121 These hydrogels are superior to

5

preformed scaffolds in terms of improved patient compliance and overcome the risk of implant

6

migration. Other advantages include simple cell encapsulation and ease of clinical

7

implementation via minimally invasive route for the treatment of geometrically irregular, larger

8

and deeper lesions.8,37,83,122,123

9

crosslinking, chemical crosslinking, enzymatic crosslinking, pH-induced gelation, temperature-

10

induced gelation, electric field, magnetic field, ionic interaction, hydrophobic interactions,

11

antigen mediation and their combinations.8,83,101,124–129 The crosslinking chemistry requires the

12

presence of specific functional groups inherently present in the polymer structure or introduced

13

through various chemical modifications. In the case of polysaccharides, such cross-links are

14

established at the carboxyl, amino and hydroxyl groups (Table 3).

15

injectability, simultaneous tailoring of physical properties such as mechanical strength to meet

16

the requirements of scaffolds could be dictated through appropriate functionalization. In addition,

17

injectable hydrogels derived from two or more polymers results in the integration of properties

18

from parent polymers.

19

interpenetrating, semi-interpenetrating or double network hydrogels.130,131 Based on the types of

20

bonds formed or associative interactions that leads to gelation, in situ forming hydrogels are

21

categorized as chemical hydrogels or physical hydrogels.124

The gelation strategies for these systems include photo-

Apart from imparting

Sol-gel transformation strategies facilitate the formation of

22

25 ACS Paragon Plus Environment

Biomacromolecules

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1 2

Figure 4. Functional groups involved in crosslinking chemistries and the resultant bonds formed

3

by [A] Michael addition reaction; [B] Schiff’s base formation; [C] Click reactions and [D]

4

Enzymatic coupling mediated injectable hydrogels. 26 ACS Paragon Plus Environment

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Biomacromolecules

1 2

Table 3. Functional groups of polysaccharides, chemical modifications to desired reactive

3

groups and their respective gelation mechanism

4 Polysaccharide Hyaluronic acid

Functional groups Carboxyl (-COOH) Carboxyl (-COOH) Carboxyl (-COOH) Carboxyl (-COOH) Carboxyl (-COOH) Carboxyl (-COOH) Hydroxyl (-OH) Vicinal diols

Chitosan

Amino (-NH2) Amino (-NH2) Amino (-NH2) Amino (-NH2) Amine (-NH2) Amine (-NH2) Amine (-NH2)

Chemical Modifications Thiolation by carbodiimide mediated Dithiothreitol (DTT) coupling Maleimide functionalization Modification with furylamine Tyramine conjugation Carbodiimide activated 1,3diaminopropane crosslinking Coupling with amineterminated dextran– tyramine conjugates Methacrylation Periodate oxidation to form aldehydes

Gelation mechanism Michael addition

Reference Censi et al., 2010; Jin et al., 2010 104,122

Michael addition Diels-Alder click Enzymatic Thixotropic inter-penetrating networks Enzymatic

Photo crosslinking Schiff’s base formation

Jin et al., 2010 132 Yu et al., 2013 133 Lee et al., 2009 134 Barbucci et al., 2010 22 Jin et al., 2010 132

Levett et al., 2014 135

Monocarboxylated pluronics glycolic acid/phloretic acid

Thermal

Millane et al., 2015; Sheu et al., 2013; Su et al., 2013; Tan et al., 2009 123,136–139 Park et al., 2009 54

Enzymatic

Jin et al., 2009 140

Methacrylic acid and lactic acid grafted via carbodiimide Succinyl modified

Thermal

N-Acetyl-L-Cysteine conjugation Grafted with poly (Nisopropylacrylamide) Methacrylated glucol derivative

Hong et al., 2008 141

Schiff’s base formation Schiff’s base formation Thermal Photo crosslinking

27 ACS Paragon Plus Environment

Tan et al., 2009 136 Zhang et al., 2011 130

Chen and Cheng, 2009 36 Park et al., 2013 142

Biomacromolecules

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Amine (-NH2) Amine (-NH2) Hydroxyl (-OH) Chondroitin Sulfates

Carboxyl (-COOH)

Hydroxyl (-OH) Vicinal diols

N-Acetyl-L-Cysteine conjugation Carbamate Carbodiimide bond activated 1,3Ester bond diaminopropa ne crosslinking Acrylation Derivatized with tri(2carboxyethyl)phosphine (TCEP) Methacrylation Periodate oxidation to form aldehydes

Photo crosslinking Schiff’s base formation

Barbucci et al., 2010 22

Jo et al., 2010 70

Levett et al., 2014 135

Dawlee et al., 2005; Millan et al., 2015 137,144

Kim et al., 2011 145

Periodate oxidation to form aldehydes

Michael addition Schiff’s base formation

Carbodiimide activated 1,3diaminopropane crosslinking Oxidation to form aldehydes

Thixotropic inter-penetrating networks Schiff’s base formation

Barbucci et al., 2010 22

Carboxyl (-COOH) Carboxyl (-COOH)

---

Ionic crosslinking Ionic crosslinking

Vicinal diols

Periodate oxidation to form aldehydes

Schiff’s base formation

Sulfate (-SO4),

---

Ionic crosslinking

Cellulose

Carboxy methyl cellulose

Carboxyl (-COOH)

Pectins

Vicinal diols

Carrageenans

Michael addition

Teng et al., 2010 143

Jin et al., 2011 63

Tyramine coupled using carbodiimide activation Thiolation

Alginate

Michael addition Thixotropic inter-penetrating networks

Enzymatic

Carboxyl (-COOH) Carboxyl (-COOH) Vicinal diols

Heparin

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28 ACS Paragon Plus Environment

Balakrishnan et al., 2013 146

Takei et al., 2010 74

Mishra et al., 2012 73

Cho et al., 2009; Popa et al., 2011; Stevens et al., 2004 71,87,147 Balakrishnan et al., 2014; Millan et al., 2015 124,137 Popa et al., 2011 71

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Biomacromolecules

Dextran

Carboxyl (-COOH) Hydroxyl (-OH) Hydroxyl (-OH) Hydroxyl (-OH)

Hydroxyl (-OH) Vicinal diols

Xanthan

Hydroxyl (-OH)

Thiol

Tyramine conjugation

Michael addition Michael addition Enzymatic

Hydroxyethyl methacrylate conjugation Periodate oxidation to form aldehydes

Photocrosslinking Schiff’s base formation

Carboxymethylation

Ionic crosslinking

Vinylsulfone

Hiemstra et al., 2007 148 Hiemstra et al., 2007 148 Jin et al., 2007; Jin et al., 2010; Teixeira et al., 2012 104,149,150 Pescosolido et al., 2011 151 Zhang et al., 2011 130

Mendes et al., 2012 107

1 2

4.1.

3

This category comprises hydrogels that are formed by covalent interactions between the

4

functional groups in the polymer chains. The commonly encountered covalent coupling reactions

5

are discussed in the following sections.

Chemical hydrogels

6 7

4.1.1. Michael addition: Michael addition reaction is based on the addition of nucleophiles

8

(Michael donor) across the carbon-carbon multiple bonds of activated electrophilic olefins or

9

alkynes (Michael acceptor) to form a ‘Michael adduct’ (figure 4A).152 Polysaccharides have been

10

chemically modified to carry both electrophilic (acrylates, vinyl sulfone, maleimide) and

11

nucleophilic groups (thiol, phosphine).70,102,104,122,132,148,153 Michael addition reactions have been

12

widely employed to tailor the properties of polysaccharide hydrogels for tissue engineering

13

applications. The water-solubility of chondroitin sulfate (CS) was reduced by formation of a

14

hydrogel through phosphine-mediated Michael addition involving the crosslinking of CS-acrylate 29 ACS Paragon Plus Environment

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1

and CS-tri(2-carboxyethyl)phosphine (TCEP).

2

incorporated by grafting acrylic acid and TCEP to the adipic acid dihydrazide linked CS 70. The

3

hydroxyl groups of dextran were substituted to varying degrees with thiol (Dex-SH) and vinyl

4

sulfones as nucleophile and electrophile respectively for Michael reaction. The rheology and

5

degradation behavior of the resultant hydrogel was compared with similar hydrogel formed using

6

tetra-acrylated poly(ethylene glycol) as synthetic polymer electrophile.

7

exhibited high elasticity while delayed degradation was exhibited by hydrogel formed from the

8

synthetic polymer.148 Similarly, other polysaccharides such as hyaluronic acid and chitosan have

9

been modified to carry maleimide and thiol groups that exhibits in situ gelling ability via Michael

10

Both the electrophile and nucleophile were

Both the hydrogels

addition reaction.104,132,143

11 12

4.1.2. Click reaction: Click reactions are those that can be used for a wide range of functional

13

groups and result in high yields. The by-products can be easity separated from the reaction

14

mixture and are performed in easilty removable solvents. Click reaction cross-linked hydrogels

15

are advantageous over traditional physical and chemical crosslinking as they have higher yield

16

with rapid reactivity at mild conditions, superior chemo-selectivity and specificity with non-toxic

17

byproducts.133,154,155 The common click chemistries include azide-alkyne (3+2) cycloaddition,

18

thiol-alkene addition catalyzed by Cu(I) and furan-maleimide (4+2) Diels–Alder (DA)

19

cycloaddition (figure 4C).133,154,156 Since, the micro-molar toxicity of Cu(I) catalyst restricts

20

cellular applications, the catalyst-free Diels-Alder cycloaddition catalyst has gained attention in

21

recent years for cartilage tissue engineering applications.133 In addition to facile encapsulation of

22

cells by pre-mixing, the unreacted azide or acetylene groups can be further functionalized

23

thereby aiding modification of the hydrogel properties.155 An interpenetrating hydrogel network

24

of gelatin, HA and chondroitin sulfate (CS) was developed by the crosslinking of HA modified 30 ACS Paragon Plus Environment

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1

with furylamine at the carboxyl group and furan carboxylic acid modified gelatin via Diels-Alder

2

click reaction.133

3

network hydrogels holds promise for their use as potent matrices for cartilage tissue

4

engineering.133 Meng et al., have recently reviewed the click reactions based polysaccharide

5

modifications for various biomedical applications.157

The tunable mechanical properties and rheology of these interpenetrating

6 7

4.1.3. Schiff’s base formation: The reaction between amines and aldehydes to form Schiff base

8

without any chemical crosslinking agents has been the basis for the formation of injectable

9

hydrogels (figure 4B).155 The gelation kinetics and physical properties of these hydrogels can be

10

tailored by varying the ratio of amine and aldehyde groups, while the remaining functional

11

groups can be conjugated with therapeutic moieties.155

12

hyaluronic acid, chitosan, dextran, cellulose and chondroitin sulfates have been derivatized to

13

form the precursors of Schiff base hydrogels.130,139,144,146,155

14

have been oxidized by periodate that cleaves the vicinal diols in monosaccharide residues and

15

forms aldehydes.74,123 These aldehydes forms hydrogels by crosslinking with amine or hydrazide

16

groups via imine or hydrazine bonds respectively.74,123

17

enhances the degradability of the polysaccharide backbone.74

18

introduction of Schiff’s base crosslinking to an existing disulfide crosslinked in situ forming

19

thiolated chitosan and oxidized dextran hydrogel system enhanced the mechanical properties and

20

reduced the gelation duration.130 Recently, oxidized hyaluronic acid, alginate and chondroitin

21

sulfate based Schiff‘s base crosslinked microtissues have been engineered by cell aggregation to

22

rejuvenate and restore chondrogenic potential of aged mesenchymal stem cells and synthesize

23

collagen II. 137

Various polysaccharides such as

Polysaccharides with vicinal diols

Further, such periodate oxidation

24 31 ACS Paragon Plus Environment

Zhang et al., reported that

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1

4.1.4. Photo-crosslinking: Photo-crosslinked hydrogels are formed by in situ polymerization of

2

acrylate and methacrylate groups carrying monomers or polymers mediated by the free radicals

3

generated by photoinitiators on exposure to light of specific wavelengths.83

4

polymerization occurs at physiological pH and temperature, this in situ forming strategy enables

5

the spatio-temporal distribution of functional bioactive factors and viable cells.83,119,158 Factors

6

such as polymer concentration, photoinitiators and light exposure time can be modified to control

7

the spatio-temporal gelation.159

8

biocompatibility and usage of low viscous monomer or macromer precursor solutions for

9

gelation.119 A variety of chemistries could be achieved with unsaturated C=C carrying groups in

10

polymers such as chitosan, alginate, chondroitin sulfate and hyaluronic acid thereby imparting

11

photo-crosslinking potential.83,160

As the

Photo-crosslinking hydrogels are advantageous for their

12 13

4.1.5. Enzyme-mediated cross-linking: Injectable, biodegradable and biocompatible hydrogels

14

formed upon the coupling of tyramine phenol moieties catalyzed by oxidation reaction of

15

horseradish peroxidase (HRP) enzyme and hydrogen peroxide (H2O2) finds applications as

16

artificial ECM and cargo delivery systems.134,140,161

17

conditions, exhibits tailorable mechanical strength and gelation time.140,162

18

conjugates synthesized by the grafting of glycolic acid / phloretic acid and tyramine at the amine

19

groups of chitosan and hydroxyl groups of dextran respectively formed cross-linked hydrogels

20

catalyzed by HRP and H2O2 (figure 4D).140,149,162

21

hydrogels supported the viability, phenotype retention and ECM production of the encapsulated

22

chondrocytes.140,149,162 HRP-mediated co-crosslinking of tyramine conjugated dextran (Dex-TA)

23

and heparin (Hep-TA) developed enzymatic hydrogels to achieved chondrogenesis by

24

glycosaminoglycan and collagen synthesis. The carboxyl groups of heparin was substituted by

These hydrogels formed at optimal Polysaccharide

Both the enzyme-mediated cross-linked

32 ACS Paragon Plus Environment

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1

tyramine and the increase in ratio of Dex-TA/Hep-TA resulted in faster gelation and increased

2

storage modulus.63

3 4

4.2.

5

Physical hydrogels are stabilized through secondary forces of associations such as electrostatic

6

interactions, hydrogen bonding, hydrophobic forces, etc,. The following sections describe the

7

salient features of physical hydrogels stabilized by each type of secondary associative forces.

Physical hydrogels

8 9

4.2.1. Electrostatic interaction: Oppositely charged ionic polyelectrolytes exhibit electrostatic

10

interactions leading to the formation of hydrogels in situ.83 For example, addition of divalent

11

ions such as strontium (Sr2+), calcium (Ca2+) and barium (Ba2+) cross-links anionic alginate

12

quickly via G-blocks (comprising L-guluronate residues) stacking thereby leading to ionotropic

13

hydrogel network formation with an egg-box structure.147 The egg-box model represents a

14

structure with corrugated surface with interstices that host cations for electrostatic association

15

with their counter ions on the polymer chain.147 Similarly, anionic gellan gum forms hydrogel in

16

the presence of cations such as Ca2+ and Mg2+.88 These ionotropic hydrogels are formed by

17

crosslinking low millimolar concentrations of cations thereby exhibiting low toxicity towards the

18

encapsulated cells.88 The degradation and mechanical properties of these hydrogels relies on the

19

length of the polymer chain and charge density. The electrostatic interactions between two

20

oppositely charged macromolecules form cytocompatible polyelectrolyte hydrogels with rapid

21

gelation.129 For instance, cationic polysaccharide chitosan formed resorbable polyelectrolyte

22

hydrogel on interaction with a natural anionic polypeptide polyglutamate that was found to

23

support the adhesion and proliferationof cells.129 Photocrosslinkable chitosan was reported to

24

exhibit electrostatic interactions with the extracellular matrix components chondroitin sulfate and 33 ACS Paragon Plus Environment

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1

collagen II to form an injectable hydrogel formulation. This composite hydrogel was found to

2

support proliferation and matrix protein synthesis by the encapsulated chondrocytes.163

3 4

4.2.2. Temperature or pH induced gelation: The intrinsic behavior of certain polymers to exhibit

5

sol-gel transition at critical solution temperature (CST) and above critical gel concentration

6

(CGC) has been used for developing injectable hydrogels.126,164 Polymers exhibiting sol-gel

7

transition above CST due to the hydrophobic interactions are collectively termed as lower critical

8

solution temperature (LCST) polymers. On the contrary, upper critical solution temperature

9

(UCST) polymers form gel at temperatures below their CST.164 This reversible transition occurs

10

rapidly to the energetically most favored state which is either a sol or gel.165 For instance, the

11

LCST of poly(N-isopropylacrylamide) (PNIPAM) and its copolymers is around 32°C, above

12

which inter and intra-molecular hydrophobic interactions facilitate the release of water from the

13

vicinity of isopropyl group thereby forming hydrogel.126,160,164 This temperature induced gelation

14

has been combined with other existing strategies to improve the structural integrity of the

15

hydrogel network. Censi et al., developed an in situ forming hydrogel based on the simultaneous

16

Michael addition and thermo-responsiveness of a triblock copolymer poly(ethylene glycol)

17

flanked by N-isopropylacrylamide (PNIPAM)/N-(2-hydroxypropyl) methacrylamide dilactate

18

and thiolated hyaluronic acid which was found to have enhanced structural and mechanical

19

stability.122 Thermosensitive chitosan-pluronic hydrogel was developed by grafting a synthetic

20

monocarboxylated

21

poly(ethylene oxide), to the amino group of chitosan thereby imparting thermoresponsive phase

22

transition at room temperature.54

23

solubilizes in aqueous medium at pH lower than 6.2 and as the basicity increases, the cationic

24

amine groups are neutralized due to deprotonation that is evidenced by gel-like precipitation.

pluronic

polymer,

poly(ethyleneoxide)-b-poly(propylene

Changes in pH mediates gelation in chitosan.

34 ACS Paragon Plus Environment

oxide)-b-

Chitosan

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1

This responsiveness to pH can also be achieved on addition of polyols such as β-

2

glycerophosphate (GP) salt.82,83

3 4

4.2.3. Supramolecular assembly: Supramolecular assemblies stabilized by secondary forces of

5

attraction can be employed as injectables due to their ability to undergo sol-gel transition in

6

response to the environmental stimuli.

7

assembly of polymeric chains into cyclodextrins (CD). Cyclodextrins are water-soluble cyclic

8

oligosaccharides with hydrophobic internal cavities consisting of six to eight α-D-glucopyranose

9

units designated as α-CD, β-CD and γ-CD.166 Self-assembly occurs as the guest linear polymer

10

chains penetrates and threads into the hydrophobic cavities of cyclic host molecules to form

11

inclusion complexes with unique supramolecular architectures.166

12

complex formation includes combination of geometric compatibility and secondary forces such

13

as hydrophobic interactions, hydrogen bonding of hydroxyl groups along the rims of neighboring

14

CDs and van der Waals forces.166 The properties of these supramolecular hydrogels can be

15

tailored by modifying the host and guest moieties. A bivalent β-CD/adamantane has been

16

synthesized and coupled with the HA derivatives as guest polymer that assembles as

17

hydrogels.167

Supramolecular hydrogels are formed by molecular

The forces driving the

18 19

4.2.4. Shear thinning hydrogel: Certain proteins, colloidal systems, peptides and polymeric

20

blends possess phenomenal self-assembling behavior that causes gelation on withdrawal of

21

shear.168,169 Based on this property, highly viscous polymer solution or minimally cross-linked

22

gel deforms or flows under shearing force while being injected and forms hydrogel in situ.169

23

The polymeric chains of HA form random coils and molecular entanglements leading to gelation,

24

while on application of shear stress, it deforms and flows due to the alignment of molecules along 35 ACS Paragon Plus Environment

Biomacromolecules

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Page 36 of 74

1

the direction of stress.170 This unique rheology of HA has been blended with thermal gelling

2

methyl cellulose to develop an injectable hydrogel. The gelation was found to be rapid in this

3

system due to the integration of two gelation strategies.170 Barbucci et al., have studied a class of

4

thixotropic injectable inter-penetrating hydrogels by combining polysaccharides.

5

combinations of negatively charged carboxymethyl cellulose with positively charged chitosan,

6

neutral guar gum and negatively charged hyaluronic acid exhibited interesting mechanical and

7

biological properties as interpenetrating hydrogel networks.22 Among the various polysaccharide

8

combinations, the hydrogel formed by combining oppositely charged polysaccharides exhibited

9

better mechanical properties.22 In a recent approach, Ding et al., have integrated associations

10

based on electrostatic interactions and supramolecular assembly using heparin and α-cyclodextrin.

11

The dual physically bonded hydrogel exhibited shear thinning property, cell compatibility and

12

sustained growth factor delivery.171

The

13 14

5.

15

Ideally, biomaterials to be used as scaffolds for cartilage regeneration should possess properties

16

such as biocompatibility, biodegradability, high porosity, compressibility, non-cytotoxicity and

17

non-antigenicity.172

18

cartilage, provide mechanical support, biochemical cues, and promote cell-matrix interactions for

19

initiating tissue reparative process.14,150 A typical tissue engineering triad comprises of cells and

20

biomolecules as biological components apart from scaffolds as structural support.83,173,174 This

21

section discusses the progress of polysaccharide based injectable scaffolds and its combinations

22

with biological components towards the fabrication of ideal constructs that repair and regenerate

23

the complex functional articular hyaline cartilage. The emergence of hydrogels as in situ forming

24

scaffolds and bioprinted constructs for cartilage tissue has been depicted in figure 5.

Injectable strategies for cartilage tissue engineering

The scaffolds should mimic the native ECM architecture of articular

36 ACS Paragon Plus Environment

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Biomacromolecules

1 2 3

Figure 5. Schematic representation of progressive degeneration of articular cartilage. The white

4

patches in superficial cartilage tissue represents partial-thickness defect of articulating surface

5

and red patches denotes the full-thickness defect of articular surface with subchondral bone. In

6

situ forming hydrogel that fills the irregular osteochondral defect serves as depot for cells and

7

growth factors encapsulated. The fabrication of 3D printed irregular construct and administration

8

has also been depicted.

9 10

5.1.

11

Articular cartilage tissue majorly consists of the extracellular matrix with very low cell-to-matrix

12

ratio.14 Regeneration of this structure based functional tissue could be augmented by scaffolds as

13

temporary

14

morphogenesis.109,175 Scaffold biomaterials should preferably be porous to facilitate infiltration

15

of cells. It should be biodegradable to permit neo-tissue formation, permeable to facilitate

16

nutrient and gas exchange and possess appropriate mechanical strength, apart from being

17

chondro-inductive and chondro-conductive.109

18

properties play a vital role in regenerating the injured cartilage tissue.14,135 Various injectable 37

Polysaccharides as structural substitutes

biocompatible

matrices

that

provide

form,

shape

and

direct

tissue

Thus, development of scaffolds with ideal

ACS Paragon Plus Environment

Biomacromolecules

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Page 38 of 74

1

acellular scaffolds based on polysaccharides have been developed and evaluated for their

2

potential to infiltrate and support the cell fate processes of mammalian chondrocytes. Biji et al.,

3

have developed Schiff’s reaction based injectable hydrogels formed by self-crosslinking of the

4

oxidized polysaccharides namely carboxymethyl cellulose or alginate with gelatin in the presence

5

of borate for cartilage tissue engineering.124,146 Such self-crosslinks result in porous hydrogels

6

with biocompatibility, biodegradability, tunable gelation kinetics and host tissue integration.124,146

7

In addition, infiltration, proliferation, phenotype maintenance and GAG deposition of murine

8

chondrocytes was promoted by these scaffolds.124,146 The aldehyde of oxidized hyaluronic acid

9

reacts with the amino group of water-soluble N-succinyl-chitosan to form Schiff‘s base

10

crosslinked hydrogel within 4 minutes.136 The resultant hydrogels exhibit microporous structure,

11

desired swelling, adequate compressive modulus, in vitro degradation, and support viability and

12

spherioid formation of bovine chondrocytes that were introduced either by surface seeding or

13

encapsulation.136 Though the scaffold acts as a structural substitute and recruits cells to the

14

injured site, this approach would not be potent in the case of larger, deeper defects that suffer

15

severe loss of chondrocytes. Hence, combinational strategies involving scaffolds with cells and

16

trophic factors have emerged as promising alternates for accomplishing cartilage tissue

17

regeneration. These alternates have been discussed in the following sections.

18 19

5.2.

20

The lack of neurovascular supply in the articular cartilage restricts the migration of regenerative

21

cells while poor proliferation of differentiated chondrocytes decreases the intrinsic regenerative

22

potential of the tissue.176–178 Some of the current treatment modalities such as mosaicplasty,

23

autologous chondrocytes transplantion (ACT) and microfracture are based on recruiting

24

chondrocytes and mesenchymal stem cells from bone marrow at the injured site to enhance

Cell / Stem cell carriers

38 ACS Paragon Plus Environment

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Biomacromolecules

1

regeneration.

The ACT procedure involves harvesting chondrocytes from non-load bearing

2

unaffected cartilage surfaces and expansion in vitro prior to transplantation at the injured site.176

3

Besides chondrocytes, stem cells are being sourced to overcome the limitation of chondrocyte

4

availability especially in cases of severely degenerated articular cartilage. The microfracture

5

treatment aims at recruiting bone marrow mesenchymal stem cells by infiltration through the

6

fracture into the defect site.179 Embryonic stem cells, bone marrow stromal cells, synovium

7

derived stem cells and human induced pluripotent stem cells (hiPSCs) have been extensively

8

investigated to attain functional cartilage regeneration and restoration.176,180–184 The role and

9

efficiency of different cells in the scenario of cartilage tissue repair and regeneration have been

10

comprehensively reviewed by several groups.185–188 The features of regenerated tissue largely

11

rely on the cell types involved in the synthesis of matrix components. For instance, autologous

12

chondrocytes therapy generates better hyaline-like cartilage compared to microfracture based

13

mesenchymal cell therapy.179 Success of cell-based therapeutics requires efficient localization

14

and retention of transplanted cells at the injured site, thus necessitating the need for integration of

15

biomaterial matrices with cells.189,190 In comparison with the preformed scaffolds, in situ forming

16

hydrogels augment the uniformity in distribution of cells throughout the scaffolds. The following

17

section discusses on the various polysaccharide based injectable strategies evaluated for its

18

potential as carriers of viable differentiated cells and stem cells. Table 4 summarizes some of the

19

scaffolds evaluated in vitro for chondrogenesis.

20 21

Table 4. Polysaccharide polymer based injectable hydrogel scaffolds that have been evaluated in

22

vitro for cartilage regenerative potential Polymer

Gelation Mechanism

Cell/Stem cells

Study Inference Duration

39 ACS Paragon Plus Environment

Reference

Biomacromolecules

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Oxidized Alginate / Gelatin

Schiff’s base formation

Murine chondrocytes

Carboxymethylated Enzymatic crosslinking Pullulan / Chondroitin Sulfate

Porcine chondrocytes

Chitosan / Alginate / Fibrin with Strontium Ranelate nanoparticles

Human Mesenchymal stem cells

21 days

14 days

14 days

Page 40 of 74

Infiltration of chondrocytes, phenotype retention and functionality

Balakrishnan et al., 2014

Proliferation of chondrocytes and cartilage specific protein expression

Chen et al., 2016 191

Chondrogenic differentiation with enhanced ECM synthesis

Deepthi et al., 2016 192

Infiltration of chondrocytes, phenotype retention and functionality

Balakrishnan et al., 2013

124

Oxidized Carboxymethyl cellulose / Gelatin

Schiff’s base formation

Murine chondrocytes

Alginate / Polyvinyl alcohol

Ionic crosslinking

Chondrocytes 28 days (immortalized human costal chondrocyte cell line) C28/I2 cultured

Chondrocytes increased and deposition of glycosaminoglycans

Cho et al., 2009 147

RGD-Alginate / Hyaluronate

Crosslinking due to Specific interactions between cell and polymer

Primary chondrocytes

6 weeks

In vivo subcutaneous injection into the dorsum of SPF/VAF immunedeficient mice

Park and Lee, 2011 193

Oligo (poly(ethylene glycol) fumarate) (OPF) / PEG diacrylate

Ionic crosslinking

Rabbit marrow mesenchymal stem cells (MSCs)

2 weeks

Chondrogenic differentiation

Park et al., 2007 194

Methacrylated glycol chitosan and hyaluronic acid

Photocrosslinking

Chondrocytes

28 days

Proliferation and ECM deposition

Park et al., 2013 142

Chitosan / Hyaluronic acid

Schiff’s base formation

Bovine articular chondrocytes encapsulation and adhesion

24 hours

Promoted cell survival and retention of chondrocytes phenotype

Tan et al., 2009 139

40 ACS Paragon Plus Environment

146

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Biomacromolecules

onto the scaffold Hyaluronic acid / PEG

Michael addition

Primary Bovine chondrocytes

3 weeks

Production of glycosaminoglycans and collagen II

Jin et al., 2010 132

Chitosan / HA / PNIPAM

Thermoreversible

Articular chondrocytes from New Zealand rabbits

5 weeks

Cell proliferation, differentiation and morphology retention, Increased ECM deposition and mechanical properties

Chen and Cheng, 2009 36

Oxidized gellan gum

Temperature Porcine dependent chondrocytes

150 days

Cell proliferation and specific matrix formation

Gong et al., 2009 195

Dextran-Tyramine

Enzymatic crosslinking

Primary Bovine chondrocytes

28 days

Sustain chondrocyte viability, phenotype retention and de novo ECM synthesis in vitro

Jin et al., 2010 104

Chitosan- beta glycerophosphatehydroxyethyl cellulose

Temperature Human/rat mesenchymal stem cells

28 days

Chondrogenic differentiation

NaderiMeshkin et al., 2014 196

1 2

5.2.1. Chondrocytes: Injectable polysaccharide hydrogels extensively characterized for their

3

physicochemical properties, have been evaluated further using chondrocytes sourced from rabbit,

4

bovine and other mammals for their potential to establish cell-matrix interaction, phenotype

5

maintenance, proliferation and synthesis of hyaline cartilage ECM.132,136,197,198

6

developed by various injectable strategies, the cell sources, duration of the in vitro study and the

7

potential exhibited in cartilage regeneration have been consolidated in table 4.

8

chondrocytes were well dispersed in poly(ethylene glycol) (PEG) - vinyl sulfone precursor

9

solution prior to hydrogel formation in association with thiolated HA through Michael 41 ACS Paragon Plus Environment

Hydrogels

Bovine

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Page 42 of 74

1

addition.199 In addition to tunable gelation time, storage modulus and degradation behavior can

2

be tailored by varying the molecular weight of HA and degree of substitution in the functional

3

groups. The HA-PEG hydrogel exhibited good compatibility with over 95% viable chondrocytes

4

and enhanced production of GAGs and hyaline-specific collagen II for three weeks.199 HA based

5

H2O2 and HRP mediated enzymatic hydrogels (HA-g-dextran-tyramine conjugates) have been

6

demonstrated to form biomimetic hydrogel within two minutes in situ. This hydrogel resembled

7

the macromolecular structure of proteoglycans and exhibited chondrocyte compatibility,

8

increased proliferation and matrix deposition for three weeks.197

9

glycosaminoglycan of the cartilage namely hyaluronic acid and chondroitin sulfate were

10

methacrylated and included with photocurable gelatin-methacrylamide based biomimetic

11

hydrogel to investigate the role of compositional cue on the chondrocytes behavior.135 Presence

12

of glycosaminoglycans in the in situ forming gel positively influenced the chondrogenic

13

redifferentiation of expanded chondrocytes, chondrogenesis, matrix distribution and mechanical

14

properties.135

The most abundant

15 16

Autologous chondrocytes seeded alginate was injected subcutaneously in mice and the construct

17

remained localized and promoted cartilage formation.80 Injectable Michael addition mediated

18

heparin-thiol and PEG-diacryl hydrogel has demonstrated in vitro spontaneous re-differentiation

19

of de-differentiated chondrocytes without growth factor or chondrogenic moieties after one week

20

of culture.200 The cell-hydrogel construct showed re-differentiation and cartilage formation in

21

vivo following subcutaneous implantation in nude mice.200 In a typical partial-thickness rabbit

22

knee defect model, the heparin based cell bearing demonstrated re-differentiation, endogenous

23

TGF-β1 retention, excellent regeneration and integration with the host tissue in 4 months when

24

compared with the cell-free hydrogel.200 42 ACS Paragon Plus Environment

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1

Despite its regenerative potential, the limited availability of differentiated chondrocytes,

2

morbidity of source site, intrinsic low expansion potential and trans-differentiation in vitro

3

restricts the clinical utility of well differentiated chondrocytes from autologous sources.201,202 On

4

the other hand, other allogeneic or xenogeneic sources can induce unfavorable immune responses

5

and transmit diseases.111 Hence, interest towards the investigation of undifferentiated stem cells

6

of various origin for their chondrogenic potential thereby enabling repair and regeneration of the

7

articular cartilage tissue are being explored.

8 9

5.2.2. Stem cells

10

The undifferentiated stem / progenitor cells possess high proliferative potential, differentiate to

11

specialized cells, have good accessibility and availability thereby proving to be a promising

12

alternate source to meet the clinical demands of chondrocytes.111,202,203 Stem cells from various

13

sources such as embryonic, fetal or adult tissues are allowed for expansion and differentiation in

14

vitro or by microenvironment of the transplanted area in vivo.202 The chondrogenic stem cells

15

perform the cellular processes to restore the extracellular matrix and thereby re-establish the

16

articular cartilage.

17

turnaround time for allogeneic therapy and eliminates the requirement for harvesting patients’

18

cells. 201

In addition, increasing numbers of stem cell banks ensures reduced

19 20

Embryonic stem cells (ESCs)

21

ESCs derived from the inner cell mass of blastocysts are highly proliferative, undifferentiated

22

cells with pluripotent ability.203,204 The intrinsic potential of differentiation to several somatic

23

cell lineages challenges the efficient direction of differentiation to specific chondrogenic lineages

24

203

. Several approaches using biomaterials, co-culturing with chondrocytes, 3D culture combined 43 ACS Paragon Plus Environment

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with growth factors, biophysical stimuli, hypoxia and mechanical stimulations have been

2

attempted to achieve chondrogenic differentiation of ESCs.203,204 Development of efficient mode

3

for enhanced differentiation is crucial as undifferentiated ESCs enhances the risk of teratoma

4

formation and can turn tumorigenic in vivo.204 Toh et al., demonstrated the direct differentiation

5

of human ESCs (hESCs) in high-dense microenvironment combined with TGFβ-1 and expanded

6

as hESC-derived chondrogenic cells for 16 population doublings in selective growth factor

7

combination (TGFβ-1, FGF2 and PDGF-bb). Further, the chondrogenic cells cultured in the

8

hyaluronic acid based hydrogel (GlycosilTM) were implanted in rat osteochondral defect model

9

and exhibited chondro-inductive potential without teratoma formation.203

Human iPSCs

10

reprogrammed to possess embryonic stem cell-like epigenetic status have been induced to

11

undergo chondrogenic differentiation in vitro using alginate hydrogel constructs prior to

12

implantation at a rat osteochondral defect site.183 The chondro-induced hiPSCs exhibited repair

13

and regeneration of cartilage with enhanced quality.183

14 15

Adult stem cells

16

Multipotent adult mesenchymal stem cells (MSCs) isolated from different sources like bone

17

marrow, adipose tissue (ASCs), umbilical cord matrix, skin, dental pulp and synovial tissue have

18

been evaluated for their cartilage regenerative potential.44,188,201,205

19

isolated from bone marrow of healthy adult donors’ posterior iliac crest were expanded and

20

encapsulated in thermosensitive hydrogel made from water soluble chitosan and poly(N-

21

(isopropyl acrylamide)) copolymer.

22

chondrogenic differentiation of hMSCs in vitro and showed increased expression of

23

chondrogenic markers such as aggrecan and collagen II.206 Similarly, thermoresponsive chitosan-

24

beta glycerophosphate-hydroxyethyl cellulose exhibited chondrogenic property apart from

Human MSCs (hMSCs)

This injectable cell-polymer complex promoted

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promoting the survival and proliferation of encapsulated bone marrow derived human/rat

2

MSCs.196 In order to enhance the engraftment of bone marrow hMSCs at the injured cartilage,

3

cyto-adhesive pullulan exopolysaccharide was employed. This carbohydrate favored maintenance

4

of viability, enhanced proliferation and chondrogenesis of MSCs in a cartilage explant in vitro

5

model.5 Portron et al., demonstrated the role of hypoxia on chondrogenesis in a polysacchraide

6

scaffold by simulating the native avascular cartilage hypoxic environment via preconditioning of

7

adult stem cells with low oxygen tension in vitro. The chondrogenic pre-disposition of adipose

8

derived stromal cells (ASCs) by hypoxia were further assessed for in vivo regenerative potential

9

using rabbit cartilage defect and nude mice subcutis models.202 Polysaccharide based silanized

10

hydroxypropyl methylcellulose (Si-HPMC) pre-mixed with chondrogenic rabbit and human

11

ASCs were reticulated to form hydrogels prior to administration in rabbit and mice

12

respectively.202 The study concluded that chondrogenic differentiation using specific medium in

13

vitro promoted optimal cartilage regeneration irrespective of oxygen preconditioning.202

14

composite injectable hydrogel reinforced using two-dimensional nanomaterial has been reported

15

for hMSCs encapsulation.

16

hydrogel exhibited high cell viability, physiological stability and shear thinning phenomenon

17

ideal for cell delivery in cartilage tissue engineering and bioprinting.207 While the stem cells

18

encapsulated in polysaccharide constructs were induced for chondrogenic differentiation using

19

growth factors in culture media, other approaches of simultaneous loading of specific growth

20

factors with stem cells have also been examined.113,194,206

A

This nanosilicate reinforced methacrylated kappa–carrageenan

21 22

5.3.

23

Growth factors are a class of bioactive polypeptides that mediate cell signaling and influence

24

cellular fate processes thereby regulating homeostasis and involving in reparative processes.208

Bio-molecular signal delivery

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There are several such signaling growth factors involved in the development of articular cartilage

2

that serve as therapeutics for defects in articular cartilage.208 Growth factors designated for

3

cartilage therapeutics should essentially be anti-catabolic and pro-anabolic by decreasing the

4

inflammatory cytokines and stimulating chondrocyte ECM deposition respectively.208 Tropic

5

factors such as transforming growth factor-β (TGF-β) superfamily, bone morphogenetic protein-2

6

(BMP-2), BMP-7, insulin-like growth factor-1 (IGF-1), fibroblast growth factor (FGF), platelet-

7

derived growth factor (PDGF), etc., augments cartilage repair by displaying variety of vital roles

8

in regeneration.113,208–210 These growth factors have been explored alone or in combinations for

9

cartilage regeneration. For instance, co-encapsulation of TGF-β3 and bFGF in an injectable

10

thermo-responsive hydrogel, promoted chondrogenic differentiation of rabbit MSCs by mediating

11

molecular and cellular processes that resemble in vivo chondrogenesis.211

12 13

TGF-β1 is a 25 kDa protein that potentially promotes chondrogenic differentiation, proliferation

14

and cartilage ECM synthesis.87,212,213

15

degradable injectable hydrogel demonstrated proliferation of chondrocytes for over 21 days while

16

minimizing de-differentiation.212 On culturing periosteum in polysaccharide gels such as agarose

17

and alginate with TGF-β1, proliferation and differentiation of chondrogenic precursor cells in the

18

cambium layer promoted chondrogenesis.87 Such autologous periosteal-derived cartilage tissue

19

formation is advantageous as template for directional tissue evolution and act as a source for

20

chondrogenic growth factors with minimal morbidity at donor site.87

21

chondrogenic commitment in stem cells prior to implantation was demonstrated by the

22

administration of autologous adipose stem cells (ASC) in the treatment of rabbit articular

23

cartilage defect.

24

chondrogenic ASC enhanced collagen II and aggrecan expression while down regulating collagen

Co-encapsulation of chondrocytes with TGF-β1 in a

The efficiency of

Injectable gellan gum combined with TGF-β1 and BMP-2 preconditioned

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I, thereby emerging as a promising construct for cartilage repair.92 Thus, stem cells predisposed

2

to chondrogenic differentiation in vitro leads to improved tissue repair compared to

3

undifferentiated stem cells.92,202

4 5

IGF-1 also known as somatomedin C, primarily acts in an anabolic fashion to increase the

6

synthesis of proteoglycan and collagen II.113

7

localized and sustained delivery to mediate vital cellular processes including differentiation and

8

proliferation of progenitor cells and ECM synthesis.113 The self-crosslinked alginate / gelatin

9

scaffold that gelates within two seconds had demonstrated cell attraction, adhesion and enhanced

10

chondrocyte functionality by the incorporation of bioactive platelet derived growth factor

11

(PDGF-BB), dexamethasone, chondroitin sulfate, and its combinations.124

12

dexamethasone and chondroitin sulfate improved glycosaminoglycan deposition within the

13

matrix, and PDGF-BB increased the chondrocyte proliferation, thus promoting neo-cartilage

14

formation for treating osteoarthritis.124

Growth factors loaded in scaffolds achieve

The presence of

15 16

Other signaling sources such as platelet lysate rich in growth factors and anti-inflammatory

17

cytokines could serve as potent chemoattractants and signaling mediators for cellular processes.

18

Autologous blood derived platelet rich plasma has demonstrated enhanced biological activity

19

such as proliferation and chondrogenic differentiation of mesenchymal stromal cells in dextran-

20

tyramine hydrogel. The injectable dexran-tyramine system with desired mechanical strength and

21

biological cues had integrated into the osteoarthritic site and opens up new vistas towards cell-

22

free approach in cartilage regeneration.150

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5.4.

2

Research on pharmaceutical therapeutics for osteoarthritis focus primarily on developing disease-

3

modifying osteoarthritis drugs (DMOADs) and connective tissue structure-modifying agents

4

(CTSMAs) such as doxycycline (Dx).117 Polysaccharide hydrogel scaffolds that deliver drugs

5

such as doxycycline and resveratrol have been reported to integrate the effects of

6

pharmacological and regenerative strategies.117,138 Doxycycline (Dx) indirectly protects collagen

7

II and aggrecan by inhibiting synthesis of inducible nitric oxide synthase, which suppresses

8

chondrocyte secretion of remodeling proteins, matrix metalloproteinases (MMPs). In addition,

9

Dx had been demonstrated to improve subchondral bone structure, minimize joint-space

10

narrowing and also exhibited anti-inflammatory effects by suppressing inflammatory cytokines

11

(interleukin-1 (α/β) and interleukin-6).117 Electrostatic interactions between Dx and HA based

12

injectable hydrogel crosslinked through zinc chelation have been reported. This hydrogel when

13

used to treat osteoarthritis in rabbit model, exhibited additive effects of both the polysaccharide

14

and chondroprotective drug.

15

property that were useful for treating osteoarthritis.117 Resveratrol (Res), an anti-inflammatory

16

drug modified to carry amino group was allowed to react with oxidized HA to form Schiff‘s base

17

crosslinked hydrogel and was used for chondrocyte delivery.138 The HA/Res hydrogel was found

18

to be chondrocyte compatible. The chondrocytes synthesized ECM through upregulation of the

19

cartilage specific genes collagen II, aggrecan and sox-9 while downregulating genes encoding for

20

MMPs, interleukin-1b. Further, the scaffold also reduced LPS-induced inflammation that is

21

associated with chondrocyte damage which augers well for cartilage regeneration.138

Pharmaceutical strategies

This included viscoelasticity, anti-inflammatory and analgesic

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6. Combinational multi-component design strategies

2

Progress in the design of biomimetic scaffolds has promoted the exploration of combination

3

strategies for engineering complex articular cartilage tissue.214 The lacuna existing in different

4

single component approaches can be overcome by integrating strategies with properties

5

complementing each other.214

6

poly(lactide-co-glycolide) (PLGA) with defined shape and size have been piled to form three-

7

dimensional porous scaffold that demonstrated chondrocyte and stem cell compatibility.141

8

However, mechanical strength and uncontrolled movement of microcarriers in vivo, restrict its

9

utility as scaffolding material. To overcome this limitation, microcarriers have been coated with

10

collagen and incorporated into crosslinkable chitosan grafted with methacrylic acid and lactic

11

acid. This composite scaffold that integrated hydrogel with cell microcarriers exhibited improved

12

elastic modulus and chondrocyte viability with phenotype retention for 12 days.141 Recently, a

13

simple combination of heparin and self-assembling peptide RAD16-I enhanced the chondrogenic

14

commitment of adipose-derived stem cells (ADSC) due to the existence of growth factor binding

15

domain in heparin.65 Further, integration of nanofibrous peptide amphiphiles (RAD16-I) and

16

gellable heparin increased the expression of mature cartilage markers like collagen II and

17

proteoglycans.65

18

variation to mimic the anisotropic architecture of native tissue.

Injectable cell microcarriers of polylactide (PLA) and

The multi-component strategies have been further designed with spatial

19 20

6.1.

21

Typically, the degenerative changes in osteoarthritis are associated with both articular cartilage

22

and underlying subchondral bone.215

23

mediated reparative cartilage tissue tends to delaminate that remains a threat in clinical scenario

24

of arthritis. 216 Therefore, designing of constructs that integrates soft cartilaginous phase and hard

Osteochondral mimetic approaches

The spontaneous response or surgical interventions

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osseous phase with soft-hard interface plays crucial role in regenerative process and integrate

2

with neighbouring host tissue.

3 4

6.1.1. Bioceramics and Bioglass

5

Calcium phosphates such as hydroxyapatite (HA) and tricalcium phosphates are bioceramic that

6

potentially enhance biomineralization and are indispensible biomaterials for bone tissue

7

engineering.217 Bioceramic based implants induce the deposition apatite layer on the surface

8

similar to native bone and thereby promotes integration with adjacent host bone.217 Though

9

bioceramics are brittle, the mechanical stiffness, bioactive osteoconductive properties and

10

tailorable biodegradability are inevitable for bone scaffolds.217,218 Bioactive glasses are a class of

11

bioceramics which include 45S5 Bioglass® composed of 45 wt % SiO2, network modifiers of

12

24.5 wt % Na2O and 24.5 wt % CaO, and 6 wt % P2O5 that simulate Ca/P composition of

13

HA.217,219 Bioactive glasses have been reported with rapid bonding to bone compared to

14

bioceramics.217,219

15

mechanical properties of bone layer and interfacial properties has become a promising strategy in

16

the fabrication of multi-layered osteochondral scaffold.

17

osteochondral constructs with bioceramics in calcified layer have been tabulated in table 5.

Incorporation of inorganic bioceramics or bioactive glasses to enhance the

The various multi-component

18 19

Table 5: Bioceramics based subchondral mimetics in various bi- or multi-layered osteochondral

20

strategies for regeneration of functional articuar cartilage Phasicity

Phase

Biphasic

Chondral phase

Biomaterials Bovine decellularized articular cartilage extracellular

Fabrication Modified temperature gradientguided thermalinduced phase separation (TIPS)

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Inference Superior biomechanical properties in vitro. Biphasic scaffold with

Reference Da et al., 2013 220

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Bony phase with compact bone

Multilayered

Gradient

Bilayered

matrix

technique

PLGA / βTricalcium phosphate

Rapid prototyping technique

Cartilaginous Collagen – I layer

chondrogenic and osteogenic induced BMSCs enhanced integration and functional regeneration of osteochondral tissue in vivo.

Intermediate layer

Collagen – I (40%), hydroxyapatite (60%)

Standardized Good early Kon et al., physical processing stability of 2010 221 biomaterial in Standardized pilot clinical study physical processing of 6 months. and neoossification

Subchondral bone layer

Collagen – I (30%), hydroxyapatite (70%)

Standardized physical processing and neoossification

Superficial zone

Chitosan with gelatin

Thermoresponsive hydrogel

Radial zone

Chitosan with hyaluronic acid

Thermoresponsive hydrogel

Calcified zone

Chitosan with β- Thermoresponsive Tricalcium hydrogel phosphate

Cartilage layer

Chitosan

Freeze-dried at 80ºC

Bone layer

Hydroxyapatite in polyurethane sponge

Sintered at 1300ºC

Depth dependent variation in mechanical, rheological and structural properties.

Walker and Madihally, 2014 222

Adhesion, proliferation and differentiation of goat bone marrow stromal cells to osteoblasts and chondrocytes in respective layers.

Oliveira et al., 2006 223

1 2 3

6.1.2. Bi- or Multi-layered osteochondral constructs

4

In view of replicating the contrasting soft cartilage and hard bone tissues, bilayered scaffolds

5

have been designed with β-tricalcium phosphate, hydroxyapatite and bioactive glass constituted 51 ACS Paragon Plus Environment

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in the deep layer.219,220,223 Enrichment of the subchondral bone layer with such bioceramics have

2

improved the mechanical and physical characteristics, as well as enhanced the osteogenic

3

potential of the bilayered construct.219,220,223

4

between the cartilage-like and bone-like layers in the multi-phasic constructs have been designed

5

to mimic soft-hard transitional interface of the osteochondral tissue.221,222 In another approach,

6

the anisotropic architecture of cartilage matrix has been mimicked by varying the depth-based

7

composition of chitosan as continuous phase in the scaffold. The polysaccharide chitosan based

8

temperature sensitive injectable hydrogel consisted of gelatin, hyaluronic acid and β-tricalcium

9

phosphate in the superficial, radial, and calcified zones respectively. The anisotropic hydrogel

10

demonstrated depth dependent variations in mechanical, rheological and structural properties in

11

addition to its gelling nature both in vitro and in in vivo BALB/c mouse subcutaneous model.222

12

Thus, developing integrated constructs could emerge as superior strategy to closely mimic the

13

complex articular cartilage tissue with potential for repair and restoration.

Furthermore, sandwiching an interfacial layer

14 15

7. 3D rapid prototype using polysaccharides bioinks

16

The advent of 3D bioprinting is set to revolutionize the field of regenerative medicine and

17

conventional tissue engineering approaches towards complex tissues such as osteochondral,

18

kidney, brain, blood vessel, etc,.224–226 Bioprinting aims at building intricate heterogeneous 3D

19

architecture of tissue by spatio-temporal positioning of cells and biomolecules with micrometer

20

precision using bioinks that exactly mimic the native tissue.224,225,227 Figure 6A & B shows the

21

various patterns printed using thermoresponsive pluronic hydrogel as bioink. The homogeneity

22

of dispensed print and layer by layer precise deposition of prints have been shown in figure 6C

23

and 6D-E respectively using curcumin (green) and sulforhodamine (red) as fluorophores under

24

confocal laser scanning microscopy (CLSM). Unlike traditional scaffold-based strategies, this 52 ACS Paragon Plus Environment

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bottom-up technology offers high cell seeding density with strong cell-cell communication,

2

which in turn facilitates cell-ECM interactions that aid regeneration of the tissue. Success of 3D

3

construction of tissues largely depends on the bioink properties as it should possess both

4

biological features to pattern cells as well as the physical requirements for printability which are

5

contrasting to each other.151,224,228 The feasibility of printing viable cells (green stained by

6

calcein-AM) using alginate has been shown in figure 7B with absence of dead cells (no red cells

7

– ethidium bromide staining). Figure 7A shows the CLSM micrographs of alginate bioprints

8

loaded with dansylated bovine serum albumin as model protein. The versatile properties of

9

polysaccharides such as injectability, flowability, cell deliverability and cytocompatibility in

10

addition to chondrogenesis have led to their exploration as bioinks for 3D bioprinting of tissues.

11 12

Among the various cartilages in the human system, aesthetic cartilage like ear and articular

13

cartilage have been bioprinted using polysaccharides like alginate, hyaluronic acid, cellulose,

14

gellan gum and dextran that have been investigated for their printability parameters.151,224,225,228–

15

234

16

was used to print 3D cartilage structures such as human ear and sheep meniscus using blueprints

17

of magnetic resonance images and computed tomography images. The shape fidelity of alginate

18

was achieved by nanofibrillated cellulose and it was successfully used to print the human

19

nasoseptal chondrocytes thereby demonstrating the potential of the blend bioink for printing 3D

20

tissues.224 Pescosolido et al., have developed a semi-interpenetrating network (semi-IPN) based

21

on two polysaccharides namely hyaluronic acid and hydroxyethyl-methacrylate-derivatized

22

dextran (dex-HEMA). The bioink dispensed by the print head of bioprinter exhibited shear-

23

thinning rheology and the photopolymerized construct showed appreciable mechanical strength.

A bioink formulated with shear thinning nanofibrillated cellulose and quick gelling alginate

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The HA/dex-HEMA printed scaffolds showed high porosity with well-defined strand spacing and

2

supported the viability of encapsulated chondrocytes.151

3 4 5

Figure 6. Spatio-temporal patterning of thermoresponsive Pluronic hydrogels by 3D bioprinting:

6

[A] Macroscopic views of various patterns; [B] Optical micrographs (4X); Confocal laser

7

scanning micrographs (CLSM) (10X) of [C] curcumin stained bioprint, [D] layer by layer

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printing (Green: curcumin, Red: sulforhodamine) 2D micrograph, and [E] 3D reconstructed

2

image.

3

4 5

Figure 7. Bioprinting of calcium crosslinked alginate hydrogel: confocal laser scanning

6

micrographs (10X) showing [A] different regions of the printed construct loaded with dansylated

7

bovine serum albumin (blue), and [B] merged images of 3D printed hydrogel (grey) loaded with

8

viable cells (green).

9 10

Biofabrication of two compartments consisting of chondrocyte encapsulated hyaluronic acid and

11

osteoblast encapsulated collagen-I have been printed as osteochondral tissue-mimetic

12

structures.225

13

chondrocytes in Dulbecco’s Modified Eagle Medium (DMEM) for chondral section, while

14

collagen-I in DMEM constituted the osteo section. The cell survival and functioning were

15

appreciable for 14 days, thus validating the choice of bioinks.225 An attempt to mimic the various

The polysaccharide bioink was composed of hyaluronic acid, alginate and

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zones of osteochondral tissue by deposition of multiple bioinks to develop clinically relevant

2

sized constructs was reported by Levato et al.

3

expanded in polylactic acid microcarriers and encapsulated in gelatin methacrylamide-gellan gum

4

bioink for printing viable constructs. The osteogenic and chondrogenic differentiation of stromal

5

cells has been established by gellan gum bioink with and without microcarriers respectively.233

6

Though the layer-by-layer assembly of 3D printed constructs mimics the complex structural

7

organization of the native tissue, establishing desirable bio-mechanical properties still remains a

8

challenge in this strategy. Hence, attempts have been made to enhance the mechanical properties

9

of 3D dual cell-laden printed scaffold for osteochondral tissue engineering by sequential

10

dispensing of cell-alginate bioink over a thermoplastic polycaprolactone (PCL) framework.235

11

The fabricated construct retained the viable osteoblasts and chondrocytes in their respective

12

layers.235

Mesenchymal stromal cells (MSCs) were

13 14

8. Translation of cartilage tissue engineering to clinics

15

The ultimate goal of cartilage tissue engineering is to translate a promising therapeutic strategy to

16

the clinics for regeneration and restoration of functional articular cartilage. Polysaccharide based

17

scaffolds have attained a remarkable place among the biomaterials evaluated for clinical

18

regeneration of cartilage tissue in osteoarthritic patients. The FLEXX trial assessed the efficacy

19

and safety of 1% sodium hyaluronate (EUFLEXXA®) for painful knee osteoarthritis therapy at 26

20

weeks. It was observed that the EUFLEXXA® therapy demonstrated significant pain relief in

21

patients with improved joint functioning, treatment satisfaction and health-related quality of life

22

compared to saline-control.236

23

hyaluronate (Hyalgan or Supartz), Hylan G-F 20 (Synvisc) and Orthovisc, a high molecular

24

weight HA are available in the United States.23

A variety of other hyaluronan preparations such as sodium

Though HA intra-articular injections are

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administered to compensate the loss of endogenous HA, the fate or exact role of exogenous HA

2

remains ambiguous. As an effort to decipher the effect of exogenous HA in arthritis, the wear

3

particles released by degradation was monitored in the synovial fluid human knees before

4

EUFLEXXA® injections (four times) using bio-ferrography.

5

magnetically labeled osseous and carilagenous particles in addition to a questionnaire assessment.

6

The study concluded that HA potentially reduced the joint degradation rate.237 A pilot clinical

7

study has evaluated the short-term stability of osteochondral biomimetic multi-layered

8

scaffold.221

9

hydroxyapatite nanoparticles for subchondral bone. The high resolution MRI evaluation of early

10

6 months follow up in thirteen patients (15 defects) confirmed the stability of this acellular

11

gradient scaffold. 221

The study analyzed the

The gradient scaffold has been based on collagen-I and nucleated with

12 13

Autologous chondrocyte implantation (ACI) is the most relied treatment for symptomatic

14

chondral or osteochondral defects.238

15

differentiation, inhomogenous distribution and leakage of cells from the site necessitate

16

integration of scaffolding with cellular therapeutics for patients.238

17

available tissue engineered graft is Hyalograft® C (Fidia Advanced Biopolymers Laboratories,

18

Abano Terme, Italy) consisting of autologous chondrocytes cultured on Hyaff-11®, an esterified

19

form of hyaluronan with controlled degradation and complete resorption in 3 months.239 Two to

20

five year follow-up reports of Hyalograft® C administration for arthritis proved improvement in

21

the repair of cartilage with hyaline-like appearance.38 A monolayer-expanded cartilage cell

22

product, CARTIPATCH (Tissue Bank of France, Lyon, France)

23

dimensional agarose-alginate hydrogel was reported to improve cell phenotype retention.238 In

24

the phase-II clinical study, symptomatic arthritis patients were implanted with the cellular

However, limitations such as chondrocyte de-

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A cell-based clinically

combined with three-

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agarose-alginate hydrogel and investigated for

clinical, radiological, arthroscopic and

2

histological outcomes after a minimum of two years follow-up.238 The arthritic condition was

3

significantly improved in all patients including patients with larger and deeper lesions.238 Such

4

clinical trials at different phases increase the translation of promising tissue engineering

5

approaches from bench-to-bedside of patients for potentially regenerating the injured tissue.

6 7

Though there are numerous preclinical studies in the development of tissue engineered medical

8

products, clinical trials are very limited due to the regulatory issues, differences in

9

patients‘ healing response, large scale fabrication and skilled expertise for handling in production

10

as well as implantation constraints. The introduction of advancements such as tissue engineered

11

products into clinics faces regulatory restrictions as it creates ethical dilemmas for physicians,

12

patients and wider public.240 In contrast to the well-defined defects of preclinical studies, clinical

13

osteoarthritic defect is irregular with varied size and severity based on the patient‘s history.

14

Personalization of osteochondral constructs to integrate with both soft and hard tissue based on

15

the specific requirement of patient remains challenging. Fabrication of large tissue engineered

16

constructs with intricate 3D structures, strictly defined mechanical, chemical and biological

17

characteristics limits the clinical translation.241

18

defined 3D structures comprising of soft and hard zones with interface in between. However,

19

expansion of clinically meaningful number of cells and complex tissue analogous matrices at

20

rapid rate to meet the clinical requirement has not been realized practically.

21

commercialization and industrialization of tissue engineered medical products is nascent, the

22

unmet prime requirements include massive fabrication, sterilization, stringent quality control of

23

3D scaffolds for human trials and affordability of medical expenditure restricts translation of this

24

technology.241–243

Bioprinting technology offeres to fabricate

As

Finally, well trained expertise in handling the complex osteochondral 58 ACS Paragon Plus Environment

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Biomacromolecules

1

constructs at various stages from production to implantation in patients is also bottleneck that

2

holds effective translation of this technology.241

3 4

9.

5

The advent of tissue engineering principles has undoubtedly changed the scenario of

6

osteoarthritis therapeutics to improve the patient’s quality of life. Polysaccharide biomaterials

7

have been extensively studied for fabrication of cartilage engineering scaffolds due to their

8

desirable

9

processability, viscoelasticity and biological activities such as cytocompatibility and

10

chondrogenic potential. Further, the facile modifications of accessible functional groups in

11

polysaccharides have overcome the inferior mechanical properties and faster degradation apart

12

from imparting injectability to these biomaterials. The most challenging aspects of cartilage

13

regeneration include homogeneous distribution of functional chondrocytes, retention of

14

chondrocyte phenotype, restoration of lubricative ECM with collagen-II rich hyaline-like

15

cartilage and long term durability for pain free locomotion. In vitro and in vivo evaluation of

16

injectable polysaccharide based in situ forming hydrogels has demonstrated the ability to deposit

17

collagen-II and aggrecan, which are the key markers of hyaline cartilage. The emergence of

18

injectable and printing technologies for the construction of osteochondral mimetics represents a

19

significant milestone in the repair and regeneration of articular cartilage at complexly injured

20

osteoarthritic sites. In addition, 3D bioprinting strategy could potentially contribute to the next

21

generation personalized therapeutics that addresses each patient‘s requirement as it is an

22

automated, computerized, rapid technology to establish cell-cell communication with complex

23

architecture of native tissues. However, translation to clinics requires meticulous choice of the

Conclusions and Future Perspectives

properties

such

as

tailorable

chemistry,

biodegradability,

59 ACS Paragon Plus Environment

biocompatibility,

Biomacromolecules

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1

appropriate polysaccharide with robust features such as mechanical properties, compatibility,

2

degradability and regenerative potential assessed in suitable pre-clinical trials.

3 4

Corresponding Author

5

*Dr. Swaminathan Sethuraman

6

Director, Centre for Nanotechnology & Advanced Biomaterials

7

Orchid Chemicals & Pharmaceuticals Chair Professor

8

School of Chemical & Biotechnology,

9

SASTRA University,

10

Tamil Nadu, India

11

E-mail: [email protected]

12 13

Author Contributions

14

The manuscript was written through contributions of all authors. All authors have given approval

15

to the final version of the manuscript.

16 17

Acknowledgements

18

The authors wish to acknowledge Nano Mission (SR/NM/PG-16/2007) and the FIST program

19

(SR/FST/ST/LSI-453/2010) of the Department of Science & Technology (DST), Government of

20

India for their financial support. The joint financial support from the Drugs & Pharmaceuticals

21

Research Programme, DST, India and SASTRA University is also acknowledged. First author is

22

thankful to Innovation in Science Pursuit for Inspired Research (INSPIRE), DST, India for Senior

23

Research Fellowship (IF120692).

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Table of Contents

7 8 Crosslinked Gel Osteochondral defect

Z motion X motion

Sol

Y motion

Polymeric Bioink

Printed hydrogel construct

9

3D Bioprinted Gel

74 ACS Paragon Plus Environment