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110th Anniversary: Vat Photopolymerization-based Additive Manufacturing: Current Trends and Future Directions in Materials Design Gayan A. Appuhamillage, Nicholas Chartrain, Viswanath Meenakshisundaram, Keyton D. Feller, Christopher Bryant Williams, and Timothy E. Long Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02679 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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Industrial & Engineering Chemistry Research

110th Anniversary: Vat Photopolymerization-based Additive Manufacturing: Current Trends and Future Directions in Materials Design Gayan A. Appuhamillage,† Nicholas Chartrain,§ Viswanath Meenakshisundaram,Ͱ Keyton D. Feller,Ͱ Christopher B. Williams,Ͱ and Timothy E. Long*,†

†Department

of Chemistry and Macromolecules Innovation Institute (MII), §Department of

Material Science and Engineering and Macromolecules Innovation Institute (MII), ͰDepartment

of Mechanical Engineering and Macromolecules Innovation Institute (MII),

Virginia Tech, Blacksburg, VA 24061, USA.

KEYWORDS: Vat photopolymerization, stereolithography, additive manufacturing, 3D printing, future materials design

ABSTRACT: This commentary discusses current capabilities of vat photopolymerization, an additive manufacturing (AM) technique also known as VP, with recent advances in the

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literature, current challenges/limitations, and future outlook in novel materials design. Current trends and recent research advances are broadly discussed covering a spectrum of material classes such as performance, medicine, energy, and active materials in parallel to their importance in diverse technologies. Current challenges and limitations of VP are also discussed in terms of material properties, photodegradation, and material toxicity with directions in future material design to overcome these challenges. This commentary paper is intended to be of broad interest to both chemists and engineers actively involved in the AM field, in terms of future material design and processing for further development of VP-based AM technology.

INTRODUCTION

Additive manufacturing (AM), or 3D printing, fabricates three-dimensional objects in a layer-by-layer process. Owing to its unique ability to create complex geometries that cannot be fabricated using traditional manufacturing techniques (i.e. injection molding, forging, and machining), 3D printing is already used as a manufacturing technique in many fields. Industry experts forecast large capital investments in AM over the next decade1 that may grow the market to $1 trillion annually by 2030.2 Innovations in material ACS Paragon Plus Environment

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and machine design will allow for new applications in

a variety of industrial and

commercial disciplines including medicine,3,4 dentistry,5 aerospace,6 automotive,7 military,8 food,9,10 steel and metal,11,12 tissue engineering,13-15 electronics,16 and pharmaceuticals17,18 with a wide spectrum of materials such as ceramics, solids, metals and alloys, powders, pastes, liquids, polymers, as well as living tissues.

Material jetting, material extrusion, vat photopolymerizaion, and powder bed fusion are the primary means for AM of polymers. The desired manufacturing process is selected depending on the type of polymer to be manufactured and its physical state (i.e. thermoset, thermoplastic, liquid, filament, or powder). In material jetting,19 material (photo-polymers, plastics) is deposited/jetted through a nozzle onto the build platform. Layers are then cured under UV light and the model is built layer-by-layer. Material extrusion consists of two basic techniques, fused filament fabrication (FFF)2,20 and directink-write (DIW).21,22

In FFF, thermoplastics are drawn through a heated nozzle and deposited onto a print bed layer-by-layer. In DIW, the polymer (photo curable polymers, hydrogels) is deposited on a print bed through a nozzle based on its shear-thinning ability, and the model will be ACS Paragon Plus Environment

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built layer-by-layer upon UV curing. Powder bed fusion (PBF)23 methods use either a laser source or high energy electron beam to fuse powder (polymers, metals) together. PBF methods includes direct-metal laser sintering (DMLS), electron beam melting (EBM), selective heat sintering (SHS), selective laser melting (SLM), and selective laser sintering (SLS). Other AM techniques include sheet lamination (ultrasonic additive manufacturingUAM for metals and laminated object manufacturing-LOM for papers) and directed energy deposition-DED for polymers, ceramics, and metals.

VAT PHOTOPOLYMERIZATION-STATE OF THE ART

Overview

Among the aforementioned different classes of AM techniques, VP is considered as the current state of the art owing to its unique superior print resolution, greater efficiency, surface finish, versatility, and printing accuracy. VP usually employs epoxy and acrylate based monomers/oligomers, which are photo-curable via cationic and radical polymerization, finally yielding thermosets.


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In laser-based VP systems such as stereolithography (SLA), manufacturing begins with the deposition of a single layer of photopolymer on the build platform using a recoating mechanism. A UV laser rasters the required pattern on the resin surface, therefore crosslinking the liquid photopolymer into a solid part. Subsequent layers are fabricated by recoating a fresh layer of photopolymer and patterning it with the UV laser. Laser power, scanning speed and exposure time, resin composition, and photo-initiator are key parameters that govern the final quality of printed parts. Another popular VP embodiment is mask-projection vat photopolymerization (MPVP), commonly referred to as DLP or projection stereolithography. MPVP has gained popularity due to its ability to project patterns with feature resolution as small as 30 µm.24 In this embodiment, energy is delivered to the resin by projecting a 2D pattern that represents the layer to be manufactured. MPVP systems may be used in a top-down for printing large-area, multiscale parts or in a bottom-up configuration for high-speed, high-resolution manufacturing.25 The optical resolution of DLP based VP systems is not suitable for manufacturing nanometer scale features. Two-photon lithography fits in this design space.26,27 Here, two beams of UV light are projected into the resin. The resin solidifies


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only in the region where the UV beams interfere constructively. This method enables fabrication of nanoscale features at any point in the 3D space inside the photopolymer. Schematic diagrams in Figure 1 illustrates these three VP systems in detail.

Figure 1. Schematic representations of VP-based AM techniques. A) SLA. Adapted from Stansbury, J. W.; Idacavage, M. J. 3D printing with polymers: Challenges among expanding options and opportunities. Dent. Mater. 2016, 32, 54-64.,28 Copyright (2016), with permission from Elsevier. B) MPVP. C) Two-photon lithography. Adapted from Ian

and Stucker et al.27 Since the inception of these three VP strategies, several modifications have been made to accommodate manufacturing and material requirements. For example, a scanning mask projection VP system was developed to fabricate large area parts for manufacturing performance polymer test specimens.29-31 Here, large-scale manufacturing is achieved by simultaneous projection and scanning of the DLP device over the resin surface. ACS Paragon Plus Environment

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Another

example

of

a

modified

VP

embodiment

is

the

volumetric

vat

photopolymerization.32 Tomographic representations of the 3D object are continuously projected on the encapsulated resin. Regions experiencing constructive interference

rapidly cure into a solid, thereby enabling high-speed manufacturing of parts. Figure 2 demonstrates these advanced additions of VP.

Figure 2. A) Schematic representation of a scanning MPVP system. Adapted with permission from Herzberger, J.; Meenakshisundaram, V.; Williams, C. B.; Long, T. E. 3D Printing All-Aromatic Polyimides Using Stereolithographic 3D Printing of Polyamic Acid Salts. ACS Macro Lett. 2018, 7, 493-497.31 Copyright (2018) American Chemical Society. B) Volumetric VP system, illustrating the schematic diagram (top) and the underlying concept (bottom). Tomographic illumination delivers a computed 3D exposure dose to a photo-curable material thereby enabling high-speed part fabrication. From Kelly, B. E.; Bhattacharya, I.; Heidari, H.; Shusteff, M.; Spadaccini, C. M.; Taylor, H. K. Volumetric additive manufacturing via tomographic reconstruction. Science 2019, 363, 1075.32 Reprinted with permission from AAAS. ACS Paragon Plus Environment

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The remaining sections will provide a detailed discussion regarding recent advances of VP-based AM for several classes of materials.

Performance Materials

Polyimides

Hegde et al. recently introduced a groundbreaking effort to 3D print all-aromatic highperformance engineering thermoplastic, similar to Kapton®, using mask-projection SLA (MPSL) technique.30 These 3D printed structures exhibited an average Young’s modulus of 2.2 GPa, Ultimate tensile strength of ~ 80 MPa, storage modulus (E’) greater than 1 GPa until 300 °C, and a Td5% of 600 °C. These remarkable thermo-mechanical properties of this material along with the scalability of 3D structures using scanning MPSL, offer potential impact on a variety of applications in water filtration, gas separation, automotive, and aerospace industries. Jana et al. later introduced a facile synthetic route for this process. In this work, a commercially available poly(amic acid) (PAA) precursor polymer was directly incorporated with a photo-curable cross-linker, 2-(dimethylamino)ethyl ACS Paragon Plus Environment

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methacrylate (DMAEMA), to form PAA-DMAEMA salts using an acid-base reaction. The photo-curable salt was then 3D printed using SLA and finally imidized to form a polyimide similar to Kapton®.31 This facile strategy offers scale-up opportunities for this highperformance polyimide and will be highly attractive for aerospace and automotive applications.

Graphene

Graphene is an incredibly stiff and conductive material with a Young’s modulus of 1 TPa and an electrical conductivity of 8000 Sm-1. Nevertheless, many graphene foams currently lack structural integrity.

Hensleigh et al. demonstrated the fabrication of

graphene in an ordered manner through unit cell structures.33 Parts were fabricated through a resin comprised of crosslinked graphene oxide sheets into a gel. The gel was then dispersed with diacrylates and photoinitiator for printing. The green part was then subjected to pyrolysis to burn out the organic binder and reduce the graphene oxide to a structural graphene aerogel. The graphene aerogel foams had greater strength and lower density than traditional graphene foams and show potential applications in catalysis, tunable thermal, and fluid flow devices. ACS Paragon Plus Environment

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Elastomers

There is a continuing need for VP-based high-strain elastomers (>200% strain) for commercial applications. VP often utilizes oligomers to avoid high viscosity resins, unfortunately, crosslinked oligomers don’t offer the same performance as crosslinkable high molecular weight polymers. Traditional elastomers owe their strain capacity to their high molecular weight and high molecular weight between crosslinks for thermoplastic and thermoset elastomers, respectively. Currently, Carbon 3D’s EPU 40, an elastic polyurethane, is the only commercial VP elastomer to exceed 200% strain. Carbon 3D reports EPU 40’s stress and strain at break as 8 MPa and 250% respectively.34 Patel et

al. greatly exceeds Carbon 3D’s materials with a combination of commercially available urethane diacrylates from Allnex. Combination of Allnex’s Ebcryl 8413 and 113 with diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) resulted in printed tensile samples with strains exceeding 1000%. Soft actuators and silver coated parts displayed potential utility in soft robotics and flexible electrical switches.35

While there are limited VP-based high-strain elastomers in the literature and commercial sources, recent literature offered elastomers with strains >75%. Scott et al. and Sirrine ACS Paragon Plus Environment

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et al. demonstrated an increase in the strain of hydrogenated polybutadiene and polysiloxane oligomers as the chain extender dithiol content increased, respectively. Upon UV irradiation of the dithiols, chain extension occurred increasing the molecular weight between crosslinks to fabricate a part with lower crosslink density and greater flexibility. Hydrogenated polybutadiene with 0.75 equivalents displayed 80% strain and 0.75 MPa stress at failure36 while polysiloxane with 0.75 equivalents displayed 125% strain and 0.3 MPa stress at failure.37

Ceramics

While VP is typically associated with fabrication of polymeric parts, ceramic printing has made large strides since Griffith and Halloran’s introduction of printing alumina in 1996. Fabrication of ceramics with VP begins with a ceramic-photopolymer slurry (≥35 vol% ceramic) printing resin with the addition of photointiator and dispersants for viscosity control. The resin is printed forming a composite green part, which is subjected to a postprocessing pyrolysis step to remove the polymeric binder and allow the ceramic particles to sinter. While other AM techniques, such as SLS use approximately 100 μm particles, VP utilizes ceramic particles with diameters of tens of microns or nano-scale and at high ACS Paragon Plus Environment

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loadings for achieving bulk densities and isotropic shrinkage. While this general process has remained unchanged, novel additions have been made to create thin layers when viscosities become too high. Song et al. utilized a tape casting recoater for even thin layers and improved resolution.38 Current VP of ceramics use a recoating blade or print using a bottom-up system, enabling the z-stage to dictate the layer thickness. Numerous ceramics have been printed using the techniques described for a variety applications such as silica (SiO2) for investment casting molds,39 alumina (Al2O3) for dental crowns,40 zirconia (ZrO2) for custom cutting tooling,41 yttria-stabilized-zirconia (YSZ) as solid electrolytes in fuel cells,42 barium titanate (BT) and lead-zirconate-titanate (PZT) for piezoelectric components.38,43,44 Until recently, ceramic VP was limited to opaque solids, however, Kotz et.al. in 2016 demonstrated the fabrication of fused silica particles to form glass with comparable transparency and surface roughness on the nanometer scale, comparable to traditionally manufactured silica products.45 Wang et al. and Hazan et al. demonstrated that silica glass and silicon carbide, respectively, are achievable through use of silicon-containing moieties and controlled atmosphere during pyrolysis.46,47 Currently, ceramic resins, which are commercially available, are Formlab’s silica- filled


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ceramic resin, Tethon 3D’s porcelain filled resin, Porcelite®, and Prodways offering four separate resins filled with calcium phosphate, hydroxyapatite, zirconia, or alumina. More ceramic resins will become more plentiful as VP becomes more pervasive and used in industrial settings.

Supramolecular Polymers

The enhanced processability of supramolecular polymers due to the reversibility of supramolecular interactions such as hydrogen bonding, ionic aggregation, guest/host interactions, and π- π stacking, makes them ideal candidates for processing via AM, which opens the door for advances in next generation biomedical devices and tissue scaffolds.48 Currently, VP lacks a significant amount of supramolecular polymers mainly due to processing limitations.49,50 However, some interesting research efforts are discussed as follows. Incorporation of depsipeptide, a derivative of L-alanine, into poly(ethylene glycol) (PEG), brought both supramolecular and biodegradable functionality for successful 3D VP while maintaining cell viability.51,52 Morris et al. reported SLA 3D-printed, ear-shaped, tissue scaffolds using hydrogen bonding interactions between poly(ethylene glycol) diacrylate (PEGDA) and chitosan.53 Yue et al. described ACS Paragon Plus Environment

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SLA-based 3D printing of antimicrobial composite resins consisting of semiinterpenetrating networks via hydrogen bonding and ionic interactions coupled through the printing of diurethane dimethacrylate (UDMA), glycerol dimethacrylate (GDMA), and quaternary ammonium (QA) modified methacrylates.54

In another interesting study,

Gonçalves et al. fabricated precise 3D structures via VP, using supramolecular biomaterials like DNA, bovine serum albumin (BSA), and gelatin for future DNA-based micro-optics, biomedical, and 3D water-based applications.55 Figure 3 highlights recent advances of performance materials recently fabricated via VP systems.

Figure 3. Recent advances of performance materials, additively manufactured via VPbased systems. A) 3D printed Kapton. Adapted from Hegde and Long et al.30 B) 3D microACS Paragon Plus Environment

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architected graphene (MAG) assemblies, from left to right octet-truss, gyroid, cubooctahedron, and Kelvin foam. Adapted from Hensleigh and Worsley et al.33 with permission of The Royal Society of Chemistry. C) Highly stretchable epoxy-urethaneacrylate based elastomers, showing their stress-strain behavior with various epoxy acrylate-urethane diacrylate ratios (left) and snapshots of stretching a transparent elastomer specimen by about ten times (right). Adapted from Patel and Magdassi et al.35 D) 3D printed fused silica glass, illustrating formation of fused silica glass from the 3D printed composite through thermal debinding and sintering (top) and demonstration of the high thermal resistance of printed fused silica glass (bottom). Reprinted by permission from Springer Nature: Springer Nature, Nature, Kotz, F.; Arnold, K.; Bauer, W.; Schild, D.; Keller, N.; Sachsenheimer, K.; Nargang, T. M.; Richter, C.; Helmer, D.; Rapp, B. E. Three-dimensional printing of transparent fused silica glass. Nature 2017, 544, 337.45 Copyright (2017). E) 3D printed UDMA/GDMA/QA based supramolecular materials, from left to right 3D printed molar tooth model, clear dental splint, and tensile properties of a 14 mol% UDMA/GDMA/QA_C12 3D printed tensile test bar and a test bar prepared in a polymerization mold by conventional photoillumination. Adapted from Yue and Ren et

al.54

Materials for Medicine

Tissue Engineering

The success of VP for medical devices such as teeth aligners and in-ear hearing aid shells prompted research into the use of VP for tissue engineering.56 The goal of tissue engineering is to combine cells, a supporting structure or scaffold, and stimuli (i.e. growth


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factors, electrical stimulation) to repair or replace tissue that is damaged or diseased. Advances in materials over the past two decades have helped improve the resolution, degradability, cytotoxicity, mechanical properties of tissue scaffolds fabricated with VP. In addition, various chemical factors and additives have been used to improve adhesion and growth of cells seeded on the printed tissue scaffolds. Finally, recent work has demonstrated the possibility of incorporating cells into the resin, avoiding the need to seed cells onto the scaffold after printing. Many researchers have focused on the use of synthetic photopolymers for scaffold fabrication with VP because of their rapid photopolymerization, chemical tunability, and good mechanical properties.56 Early work developed poly(propylene fumarate) and diethyl fumarate (PPF/DEF) resins for the fabrication of biodegradable scaffolds with VP.57 Scaffolds fabricated with PPF/DEF have been used with many cell types owing to good mechanical properties, low cytotoxicity, and excellent printability. A more recent example of PPF/DEF printing was demonstrated by Kim et al., who fabricated tissue scaffolds with pore sizes of 170 μm using the photopolymer.58 Upon post-curing, they seeded pre-osteoblasts on the fabricated scaffolds, followed by culturing in a multi-


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stimulus bioreactor system for cell proliferation. A combination of high resolution VP with a multi-stimulus bioreactor system fabricated tissue-scaffolds for potential utility in bone tissue regeneration. Neiman et al. has employed VP to fabricate photopolymerizable PEG-based hydrogel scaffolds with multiple open channel geometries coupled to commercially available filters such as polycarbonate and polyvinylidene fluoride, designed for post-seeding with primary hepatocytes.59 High cell viabilities and robust hydrogel-filter bonding were achieved by optimizing SLA energy dose, photoinitiator concentration, and pretreatment conditions. Due to the ability to incorporate various biofunctional ligands and the tunability of the mechanical and transport properties of the printed hydrogel scaffolds, this may become a commercially viable technique for evaluating drug effectiveness and toxicity. Elomaa et al. described preparation of photo-crosslinkable poly(ε-caprolactone)-(PCL) based tissue engineering porous scaffolds by SLA 3D printing.60 Branched PCL oligomers were methacrylated and photo-crosslinked during the printing process using Irgacure 369 photoinitiator, inhibitor, and dye. A solvent-free processing approach yielded scaffolds with a highly interconnected pore network without material shrinkage, precisely matching


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computer-aided design (CAD) files. These exhibit high potential for applications in cell seeding and implanting. In another study, Seck et al. introduced biodegradable PEG/Poly(D,L-lactide) (PDLLA) based hydrogel structures fabricated with VP.61 The utilized photopolymerizable aqueous resin was composed of PDLLA-PEG-PDLLA, visible light photoinitiator, dye, and inhibitor in dimethyl sulfoxide/water. Hydrogels with non-porous and gyroid pore network architectures were fabricated with narrow pore size, excellent pore interconnectivity and mechanical properties. Human mesenchymal stem cells successfully adhered and proliferated. Potential applications include tissue engineering, cell transplantation, and drug delivery.

Pharmaceuticals and Drug Delivery

Recent work has shown that the formulation of new materials for VP can enable applications in pharmaceuticals and drug delivery. For example, Goyanes et al. demonstrated the printing of salicylic acid loaded nose-shaped patches using VP for the treatment of acne.62 The drug was dissolved in mixtures of PEGDA and PEG and the ACS Paragon Plus Environment

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system was crosslinked during printing. These VP fabricated patches offered better resolution and higher drug loading capability (1.9% w/w) without drug degradation during printing. In addition, they also demonstrated higher drug diffusion rates (229, 291 µg/cm2 within 3 hours) for the two VP-based formulations than with fused deposition modeling (FDM). Unlike products currently on the market, VP would allow for the fabrication of patient-specific drug loaded devices that conform to the shape of a patient’s facial features. In a similar study, Wang et al. 2016 described drug-loaded tablets printed using VP. Two model drugs, paracetamol and 4-aminosalicylic acid (4-ASA), were dissolved in a PEGDA-based resin to allow for their incorporation into a printed construct. PEG 300 was also included to promote drug release. No degradation of either drug occurred during VP fabrication of tablets. In contrast, degradation of 4-ASA was observed in tablets fabricated using FDM. This again confirmed the suitability of SLA to 3D print thermolabile compounds owing to the minimal localized heating during the printing process. Moreover, the technology offered simple and fast fabrication of drug-loaded oral-dosage forms with drug-release characteristics that are potentially personalized to each patient.63


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More recently, Martinez et al. fabricated ibuprofen-loaded hydrogels via VP. PEGDAbased resins containing up to 30 wt% water and 10 wt% ibuprofen were successfully fabricated into 3D gels.64 The use of a common drug, processed via VP, is an important step towards future advancement of the pharmaceutical field. Figure 4 illustrates a combination of recent research efforts on materials for medicine using VP processes.


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Figure 4. Recent advances of materials for medicine, additively manufactured via VPbased systems. A) SEM images of 3D printed PPF/DEF scaffolds. Reprinted by permission from Springer Nature: Springer Nature, Nature, Kim, J.; Lee, J. W.; Yun, W. Fabrication and tissue engineering application of a 3D PPF/DEF scaffold using Blu-ray based 3D printing system. J. Mech. Sci. Technol. 2017, 31, 2581-2587.58 Copyright (2017). B) Hydrogel structures 3D printed using an MA-PDLLA-PEG-PDLLA-MA based resin, disk shaped porous (top left) and solid (top right) hydrogel scaffolds and hydrogel scaffold with gyroid pore network (bottom). Adapted from Seck, T. M.; Melchels, F. P. W.; Feijen, J.; Grijpma, D. W. Designed biodegradable hydrogel structures prepared by stereolithography using poly(ethylene glycol)/poly(d,l-lactide)-based resins. J. Control.

Release 2010, 148, 34-41.,61 Copyright (2010), with permission from Elsevier. C) Human nose-shaped 3D printed device using PEGDA/PEG (4:6)-salicylic acid. Adapted from Goyanes, A.; Det-Amornrat, U.; Wang, J.; Basit, A. W.; Gaisford, S. 3D scanning and 3D printing as innovative technologies for fabricating personalized topical drug delivery systems. J. Control. Release 2016, 234, 41-48.,62 Copyright (2016), with permission from Elsevier. D) 3D printed tablets, loaded with paracetamol (top raw) and 4-ASA (bottom raw) from left to right 35% PEGDA/65% PEG300, 65% PEGDA/35% PEG300, and 90% PEGDA/10% PEG300. Adapted from Wang, J.; Goyanes, A.; Gaisford, S.; Basit, A. W. Stereolithographic (SLA) 3D printing of oral modified-release dosage forms. Int. J. Pharm. 2016, 503, 207-212.,63 Copyright (2016), with permission from Elsevier.

Materials for Energy

Piezoelectrics Piezoelectrics, a class of smart materials, have also found recent applications using projection microstereolithography (PµSL). These materials have the ability to accumulate ACS Paragon Plus Environment

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electrical charge/voltage upon exposure to mechanical stress. Using VP-based AM, Cui

et al. recently fabricated free-form, perovskite-based piezoelectric nanocomposites. These metamaterials showed remarkable electromechanical properties with high piezoelectric constants and tailorable flexibility.65 This strategy opens doors for next generation intelligent infrastructure including 3D pressure mapping, directionality detection, and simultaneous impact absorption and monitoring. Chen et al. recently introduced a PµSL-based 3D printing of poly(vinylidene fluoride) (PVDF) upon combining with photocurable 1,6-hexanediol diacrylate (HDDA), Irgacure 819 as photoinitiator, Sudan I as UV absorber, and diethyl fumarate (DEF) as the solvent.66 The optimized ink showed strong piezoelectric properties, in fact, 105.12 × 103

V.m/N compared to 140~330 × 10-3 V.m/N for pure PVDF as reported in literature. In

addition, these 3D printed structures approached a resolution of 7.1 µm with a layer depth of 20 µm. These have potential 3DP applications in customized geometries especially in biosensing and detection fields. This work combined piezoelectric materials with mechanical

flexibility,

chemical

stability,

biocompatibility,

and

solution-based

processability (e.g. PVDF), with photo-curable materials enabling 3D printing. Since


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mostly reported SLA inks that utilize piezoceramics (i.e. lead zirconate titanate, barium titanate) are brittle and not suitable for applications such as flexible biomedical devices, the above examples are highly valuable in filling these types of material gaps.

Ionic Liquids Schultz et al. initially reported AM of phosphonium ionic-liquid (PIL) networks using mask projection microstereolithography. The various 3D printed geometries using the copolymer based on poly(ethylene glycol) dimethacrylate (PEGDMA) and the phosphonium

ionic

liquid

monomer,

4-vinylbenzyl

trioctylphosphonium

bis(trifluoromethanesulfonate)imide (TOPTf2N), poly(PEGDMA-co-TOPTf2N), are shown in Figure 5. These 3D printed materials had low UV light intensity requirements and high digital resolution along with a collection of sound properties like high thermal stability, tunable glass transition temperature, optical clarity, and ionic conductivity for emerging electro-active membrane applications.67 In another recent study, Lee et al. reported use of ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) /photo-polymer 2-[[(butylamino)carbonyl]oxy]ethyl


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acrylate (BACOEA) composites to fabricate VP-based sensing materials for piezoresistive tactile sensors. The performance of these piezoresistive sensors was a function of the degree of crosslinking (using different crosslinker concentrations) and polymerization (by changing UV exposure time) in the IL/polymer composites.68 Figure 5 demonstrates some of the recent advances in materials for energy, additively manufactured via VP systems.


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Figure 5. Recent advances of materials for energy, additively manufactured via VP-based systems. A) SEM images of 3D printed piezoelectric microlattices. Reprinted by permission from Springer Nature: Springer Nature, Nature Materials, Cui, H.; Hensleigh, R.; Yao, D.; Maurya, D.; Kumar, P.; Kang, M. G.; Priya, S.; Zheng, X. Three-dimensional printing of piezoelectric materials with designed anisotropy and directional response. Nat.

Mater. 2019, 18, 234-241.65 Copyright (2019). B) (a) CAD model of St. Basil’s Cathedral, (b) Optical microscopy image and (c), (d) SEM images of 3D printed St. Basil’s Cathedra using the piezoelectric material consisting of PVDF/HDDA/Iigacure 819/Sudan I/ DEF. Adapted from Chen, X.; Ware, H. O. T.; Baker, E.; Chu, W.; Hu, J.; Sun, C. The Development of an All-polymer-based Piezoelectric Photocurable Resin for Additive Manufacturing. Procedia CIRP 2017, 65, 157-162.,66 Copyright (2017), with permission from Elsevier. C) 3D printed various geometries using the phosphonium ionic liquid network poly(PEGDMA-co-TPOTf2N). Adapted with permission from Schultz, A. R.; Lambert, P. M.; Chartrain, N. A.; Ruohoniemi, D. M.; Zhang, Z.; Jangu, C.; Zhang, M.; Williams, C. B.; Long, T. E. 3D Printing Phosphonium Ionic Liquid Networks with Mask Projection Microstereolithography. ACS Macro Lett. 2014, 3, 1205-1209.67 Copyright (2014) American Chemical Society. D) 3D printed piezoresistive tactile sensor from ionic liquid/polymer- EMIMBF4/BACOEA composites. Adapted from Lee and Choi et al.68

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4D printing Future AM industry is advancing towards smart materials. These are defined as materials that change their shape or function upon exposure to certain stimuli such as temperature, light, pH of the medium, solvent, and electric fields.69,70 This class of materials introduces 4D printing which fabricates dynamic/active structures replacing three dimensional stationary ones in conventional AM processes. These mainly includes AM of shape-memory polymers. Lantada et al. reported SLA-based 3D printing of shape memory polymers using an epoxy-Accura 60 for 4D applications such as micro-claw and active springs.71 Yu et al. carried out SLA-3D printing of shape-memory composites using a epoxy-acrylate hybrid photopolymer to be applied in 4D actuators and hinges.72 Zarek et al. recently introduced SLA based 3D printing of a shape memory polymer using a photopolymerization of melted solid methacrylated precursor polymer for potential applications in flexible and responsive electrical circuits.73 The above mentioned examples of 3D printed objects morph their


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shape in response to temperature changes for 4D applications. Overall, this technology is progressing in many advanced fields such as actuators, soft robotics, medical devices, sensors, and wearable electronics that utilize active materials. Figure 6 highlights these recent advances in active materials utilizing VP-based AM. Figure 6. Recent advances of active materials, additively manufactured via VP-based systems. 3D printed micro-vascular shape memory polymer devices and probes via photo-polymerization of epoxy resin; A) Micro-claws and B) Active spring with inner vasculatures (top) which undergo the shape memory effect after heating via injected water through their micro-vasculatures (bottom). Adapted from Lantada and Tanarro et

al.71 C) 3D printed Eiffel tower structure with a epoxy acrylate-hybrid photopolymer undergoing shape-recovery process with a dryer. Adapted with permission from Yu, R.; Yang, X.; Zhang, Y.; Zhao, X.; Wu, X.; Zhao, T.; Zhao, Y.; Huang, W. Three-Dimensional Printing of Shape Memory Composites with Epoxy-Acrylate Hybrid Photopolymer. ACS

Appl. Mater. Interfaces 2017, 9, 1820-1829.72 Copyright (2017) American Chemical Society. D) Fabrication of shape memory-based electrical devices using a molten PCLbased macromonomer; coupled with, carbon nanotubes (left-top and bottom) acting as a shape memory connector, which upon applying a voltage closes the electrical circuit and conductive ink (middle and right) acting as a temperature sensor in its off state (top-right) and on state (bottom-right). Adapted from Zarek and Magdassi et al.73 It is also noteworthy to present recent progress in multimaterial VP. Dolinski et al. recently introduced solution mask liquid lithography (SMaLL), a unique variation of VP to additively manufacture multimaterials. 3D objects with chemically and mechanically distinct domains were successfully manufactured by coupling photoswitches with resin mixtures containing orthogonal, multiple photo-crosslinking systems. Ongoing work includes in-depth characterization of multimaterial interfaces and resin curing optimization using dynamic printing processes.74 Mechanochemistry also represents an emerging tool for designing active materials. Recent efforts ACS Paragon Plus Environment

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deal with chemical reactions induced by direct absorption of mechanical energy and hence, complements the conventional activation methods such as heat, irradiation, and electrochemistry.75 However, only extrusion based 3D printed mechanochromic materials are reported to date.76 Since VP based AM requires efficient photo-curing kinetics of monomers, mechanochemical approaches for VP based AM are challenging. Table S1 summarizes the above mentioned recent advances in VP-based AM.

CHALLENGES AND FUTURE OUTLOOK IN MATERIALS DESIGN Despite the aforementioned remarkable capabilities of VP-based AM, the technique maintains some challenges, which need to be addressed in future materials design. These limitations are broadly categorized into three sub-sections such as material properties, photodegradation, and toxicity.

Material Properties The materials catalogue for VP has traditionally been limited to simple acrylate or epoxy based monomer/oligomer backbones.27 Recently, advances in VP have demonstrated fabrication with previously unprocessable polymers such as fully aromatic polyimides,30,31 polyurethanes from Carbon 3D,77 and poly(ether ether ketone) (PEEK).78 However, the development of new polymers, especially elastomers and commercially important


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polymers such as polyolefins, polyamides, remains very limited. The primary reason is attributed to the inherent viscosity requirements in VP processes. Commercial VP technologies have been limited to printing photopolymers with low viscosities (0.25-10 Pa⋅s).49 Performance elastomers employ high molecular weight backbones to impart mechanical strength, and this results in an increase in photopolymer viscosity. High viscosity photopolymer increases the chance of damaging printed features during recoating, therefore limiting the size of the printable features. To address this issue, researchers have investigated the use of (1) novel chemistry and resin synthesis and (2) novel processing techniques. Novel chemistry, such as simultaneous chain extension and crosslinking36,37 and Carbon 3D methods, have lowered photopolymer viscosity during fabrication, but resulted in the formation of high molecular weight between crosslinks for networks after photocuring. Currently these methods are limited to few material families only (i.e. siloxanes, polybutadienes), but these strategies hold promise for other use with other polymer families too. Modifying resin formulation by using dual-cure chemistries or polymer


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colloids capable of interpenetration, has opened VP to a new range of polymer families allowing process flexibility, even for potential nano-scale printing.79,80 Processing techniques, such as the use of high temperature to lower viscosity, have shown promise for printing new photopolymer systems with low printing times, higher conversion, and better mechanical properties in green bodies.81 While thermal stability of the photopolymer hinders processing at very high temperatures, the process opens the door to unlock new photopolymer systems by combining high print resolution with improved mechanical properties or even desired properties for 4D-printing.81 The addition of non-reactive solvents have also played an important role in lowering photopolymer viscosity.30,31 In addition to lowering viscosity, solvents also allow for dissolution of high performance polymer backbones, making it a very powerful technique for materials expansion. This technique produces organo-gels with very low strength. Nevertheless, several post-printing steps are required here to reduce the shrinkage caused by solvent removal. More attention is required to understand how geometry and oregano-gel strength affect printing parameters, shrinkage, and warpage of the final printed parts. Other polymer parameters such as modulus and photo-curing kinetics should also be considered in materials design for VP-based systems, especially for large scale manufacturing. Moreover, current 3D printable materials also suffer from the lack of concurrent flexibility and high tensile strength that are necessary for engineering load-bearing tissues like tendons, ligaments, and muscles.82 In addition, most naturally available biomaterials such as alginate, chitosan have limited resources, low stability, and poor mechanical properties, which affect the structural integrity of the engineered tissues, if used directly. ACS Paragon Plus Environment

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Therefore, future materials design should be directed towards novel synthetic candidates addressing these issues. Future research based on hybrid scaffolds, which combines both natural and synthetic bodies, would be of much importance.

Photodegradation Another area of concern hindering industrial use of photopolymers is the loss of mechanical properties over time. 3D printed parts exposed to UV light potentially display a reduction in mechanical performance due to potential photo-induced chain-scission.83 While most consumer polymers are susceptible to such damage, more work is required in this area to identify suitable modifiers that improve the UV performance of a printed part, especially in cases targeting for durable mechanical properties. Toxicity Toxicity is an area of importance in designing materials, especially for the pharmaceuticals and tissue engineering fields. Low molecular weight polyacrylate oligomers contain potentially hazardous monomers in the object after printing that lead to regulatory challenges and stability issues in pharmaceutics.64 Therefore, residual analysis is required for pharmaceutical applications. Also, use of less toxic initiators with visible light initiation capability such as lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), Eosin Y disodium salt is also a sound approach in addressing toxic issues in these fields.49 Also, laser


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thermal energy could lead to cellular damages and cytotoxicity.84 Hence, strategies such as controlling printing temperature for rapid gelation of a bio-ink,85 and modifying crosslinking densities86 are helpful to overcome existing limitations.

CONCLUSIONS

In conclusion, this commentary demonstrates the remarkable AM capabilities of VPbased systems, current state-of the art, for a myriad of industrial applications including performance, medicine, energy, and active material classes. We also addressed some existing challenges/limitations in VP-based systems and provided future directions in materials design in order to overcome these hurdles and broaden the scientific and commercial applicability of VP to novel classes of advanced polymeric materials. This commentary was intended for both synthetic chemists and material science engineers for the advancement of AM.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ACS Paragon Plus Environment

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ORCID

Gayan A. Appuhamillage: 0000-0003-4165-8741

Timothy E. Long: 0000-0001-9515-5491

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. SUPPORTING INFORMATION Table containing VP-based 3D printed materials and application category, and related references

ACKNOWLEDGMENTS We acknowledge National Science Foundation (NSF) and Civil, Mechanical and Manufacturing Innovation (CMMI, 1762712) for financial support. ACS Paragon Plus Environment

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Figure 1. Schematic representations of VP-based additive manufacturing techniques. A) SLA. Adapted from Stansbury, J. W.; Idacavage, M. J. 3D printing with polymers: Challenges among expanding options and opportunities. Dent. Mater. 2016, 32, 54-64.,28 Copyright (2016), with permission from Elsevier. B) MPVP. C) Two-photon lithography. Adapted from Ian and Stucker et al.27



Figure 2. A) Schematic representation of a scanning MPVP system. Adapted with permission from Herzberger, J.; Meenakshisundaram, V.; Williams, C. B.; Long, T. E. 3D Printing All-Aromatic Polyimides Using Stereolithographic 3D Printing of Polyamic Acid Salts. ACS Macro Lett. 2018, 7, 493-497.31 Copyright (2018) American Chemical Society. B) Volumetric VP system, illustrating the schematic diagram (top) and the underlying concept (bottom). Tomographic illumination delivers a computed 3D exposure dose to a photo-curable material thereby enabling high-speed part fabrication. From Kelly, B. E.; Bhattacharya, I.; Heidari, H.; Shusteff, M.; Spadaccini, C. M.; Taylor, H. K. Volumetric additive manufacturing via tomographic reconstruction. Science 2019, 363, 1075.32 Reprinted with permission from AAAS.


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Figure 3. Recent advances of performance materials, additively manufactured via VP-based systems. A) 3D printed Kapton. Adapted from Hegde and Long et al.30 B) 3D micro-architected graphene (MAG) assemblies, from left to right octet-truss, gyroid, cubo-octahedron, and Kelvin foam. Adapted from Hensleigh and Worsley et al.33 with permission of The Royal Society of Chemistry. C) Highly stretchable epoxy-urethaneacrylate based elastomers, showing their stress-strain behavior with various epoxy acrylate-urethane diacrylate ratios (left) and snapshots of stretching a transparent elastomer specimen by about ten times (right). Adapted from Patel and Magdassi et al.35 D) 3D printed fused silica glass, illustrating formation of fused silica glass from the 3D printed composite through thermal debinding and sintering (top) and demonstration of the high thermal resistance of printed fused silica glass (bottom). Reprinted by permission from Springer Nature: Springer Nature, Nature, Kotz, F.; Arnold, K.; Bauer, W.; Schild, D.; Keller, N.; Sachsenheimer, K.; Nargang, T. M.; Richter, C.; Helmer, D.; Rapp, B. E. Three-dimensional printing of transparent fused silica glass. Nature 2017, 544, 337.45 Copyright (2017). E) 3D printed UDMA/GDMA/QA based supramolecular materials, from left to right 3D printed molar tooth model, clear dental splint, and tensile properties of a 14 mol% UDMA/GDMA/QA_C12 3D printed tensile test bar and a test bar prepared in a polymerization mold by conventional photoillumination. Adapted from Yue and Ren et al.54



Figure 4. Recent advances of materials for medicine, additively manufactured via VP-based systems. A) SEM images of 3D printed PPF/DEF scaffolds. Reprinted by permission from Springer Nature: Springer Nature, Nature, Kim, J.; Lee, J. W.; Yun, W. Fabrication and tissue engineering application of a 3D PPF/DEF scaffold using Blu-ray based 3D printing system. J. Mech. Sci. Technol. 2017, 31, 2581-2587.58 Copyright (2017). B) Hydrogel structures 3D printed using an MA-PDLLA-PEG-PDLLA-MA based resin, disk shaped porous (top left) and solid (top right) hydrogel scaffolds and hydrogel scaffold with gyroid pore network (bottom). Adapted from Seck, T. M.; Melchels, F. P. W.; Feijen, J.; Grijpma, D. W. Designed biodegradable hydrogel structures prepared by stereolithography using poly(ethylene glycol)/poly(d,l-lactide)-based resins. J. Control. Release 2010, 148, 34-41.,61 Copyright (2010), with permission from Elsevier. C) Human noseshaped 3D printed device using PEGDA/PEG (4:6)-salicylic acid. Adapted from Goyanes, A.; Det-Amornrat, U.; Wang, J.; Basit, A. W.; Gaisford, S. 3D scanning and 3D printing as innovative technologies for fabricating personalized topical drug delivery systems. J. Control. Release 2016, 234, 41-48.,62 Copyright (2016), with permission from Elsevier. D) 3D printed tablets, loaded with paracetamol (top raw) and 4-ASA (bottom raw) from left to right 35% PEGDA/65% PEG300, 65% PEGDA/35% PEG300, and 90% PEGDA/10% PEG300. Adapted from Wang, J.; Goyanes, A.; Gaisford, S.; Basit, A. W. Stereolithographic (SLA) 3D printing of oral modified-release dosage forms. Int. J. Pharm. 2016, 503, 207-212.,63 Copyright (2016), with permission from Elsevier.


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Figure 5. Recent advances of materials for energy, additively manufactured via VP-based systems. A) SEM images of 3D printed piezoelectric microlattices. Reprinted by permission from Springer Nature: Springer Nature, Nature Materials, Cui, H.; Hensleigh, R.; Yao, D.; Maurya, D.; Kumar, P.; Kang, M. G.; Priya, S.; Zheng, X. Three-dimensional printing of piezoelectric materials with designed anisotropy and directional response. Nat. Mater. 2019, 18, 234-241.65 Copyright (2019). B) (a) CAD model of St. Basil’s Cathedral, (b) Optical microscopy image and (c), (d) SEM images of 3D printed St. Basil’s Cathedra using the piezoelectric material consisting of PVDF/HDDA/Iigacure 819/Sudan I/ DEF. Adapted from Chen, X.; Ware, H. O. T.; Baker, E.; Chu, W.; Hu, J.; Sun, C. The Development of an All-polymer-based Piezoelectric Photocurable Resin for Additive Manufacturing. Procedia CIRP 2017, 65, 157-162.,66 Copyright (2017), with permission from Elsevier. C) 3D printed various geometries using the phosphonium ionic liquid network poly(PEGDMA-co-TPOTf2N). Adapted with permission from Schultz, A. R.; Lambert, P. M.; Chartrain, N. A.; Ruohoniemi, D. M.; Zhang, Z.; Jangu, C.; Zhang, M.; Williams, C. B.; Long, T. E. 3D Printing Phosphonium Ionic Liquid Networks with Mask Projection Microstereolithography. ACS Macro Lett. 2014, 3, 1205-1209.67 Copyright (2014) American Chemical Society. D) 3D printed piezoresistive tactile sensor from ionic liquid/polymer- EMIMBF4/BACOEA composites. Adapted from Lee and Choi et al.68



Figure 6. Recent advances of active materials, additively manufactured via VP-based systems. 3D printed micro-vascular shape memory polymer devices and probes via photo-polymerization of epoxy resin; A) Micro-claws and B) Active spring with inner vasculatures (top) which undergo the shape memory effect after heating via injected water through their micro-vasculatures (bottom). Adapted from Lantada and Tanarro et al.71 C) 3D printed Eiffel tower structure with a epoxy acrylate-hybrid photopolymer undergoing shaperecovery process with a dryer. Adapted with permission from Yu, R.; Yang, X.; Zhang, Y.; Zhao, X.; Wu, X.; Zhao, T.; Zhao, Y.; Huang, W. Three-Dimensional Printing of Shape Memory Composites with EpoxyAcrylate Hybrid Photopolymer. ACS Appl. Mater. Interfaces 2017, 9, 1820-1829.72 Copyright (2017) American Chemical Society. D) Fabrication of shape memory-based electrical devices using a molten PCLbased macromonomer; coupled with, carbon nanotubes (left-top and bottom) acting as a shape memory connector, which upon applying a voltage closes the electrical circuit and conductive ink (middle and right) acting as a temperature sensor in its off state (top-right) and on state (bottom-right). Adapted from Zarek and Magdassi et al.73


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