3D Printing of Photocurable Cellulose Nanocrystal Composite for

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3D Printing of Photocurable Cellulose Nanocrystal Composite for Fabrication of Complex Architectures via Stereolithography Napolabel Palaganas, Joey Mangadlao, Al Christopher De Leon, Jerome Palaganas, Katrina Pangilinan, Yan Jie Lee, and Rigoberto Advincula ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09223 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017

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ACS Applied Materials & Interfaces

3D Printing of Photocurable Cellulose Nanocrystal Composite for Fabrication of Complex Architectures via Stereolithography Napolabel B. Palaganas1,3, Joey Dacula Mangadlao1,2, Al Christopher C. de Leon1, Jerome O. Palaganas1,3, Katrina D. Pangilinan1, Yan Jie Lee1, Rigoberto C. Advincula1* 1

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Department of Macromolecular Science and Engineering,

Department of Radiology, Case Western Reserve University, Cleveland, Ohio 44106, USA

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School of Graduate Studies, Mapua Institute of Technology, Intramuros, Manila, Metro Manila 1002, Philippines

KEYWORDS: additive manufacturing, 3D printing, stereolithography apparatus, nanocomposite hydrogel, cellulose nanocrystal ABSTRACT. The advantages of 3D printing on cost, speed, accuracy, and flexibility have attracted several new applications in various industries especially in the field of medicine where customized solutions are highly demanded. Although this modern fabrication technique offers several benefits, it also poses critical challenges in materials development suitable for industry

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use. Proliferation of polymers in biomedical application has been severely limited by their inherently weak mechanical properties despite their other excellent attributes. Earlier works on 3D printing of polymers focus mainly on biocompatibility and cellular viability and lack a close attention to produce robust specimens. Prized for superior mechanical strength and inherent stiffness, cellulose nanocrystal (CNC) from abaca plant is incorporated to provide the necessary toughness for 3D printable biopolymer. Hence, this work demonstrates 3D printing of CNCfilled biomaterial with significant improvement in mechanical and surface properties. These findings may potentially pave the way for an alternative option in providing innovative and cost effective patient-specific solutions to various fields in medical industry. To the best of our knowledge, this work presents the first successful demonstration of 3D printing of CNC nanocomposite hydrogel via stereolithography (SL) forming a complex architecture with enhanced material properties potentially suited for tissue engineering. INTRODUCTION The advent of 3D printing technology has revolutionized manufacturing approach with its adoption in a broad range of industries such as aerospace, automotive, medical, electronics, semiconductor, industrial, construction, and art.1–5 With 3D printing, complicated virtual designs can now be produced physically with ease and seamless execution simplifying the conventional processes, and thus dramatically reducing production cycle time.1–3 In addition, 3D printing eliminates the need for expensive molds, dies, tools, and jigs of the conventional manufacturing approach.6 This, in turn, significantly reduces the production cost per unit; thus, paving the way for highly customizable solutions. In fact, the value-added impact of 3D printing is estimated to be $667 million worldwide from which approximately $241 million comes from US alone.7 Beyond cost and customization, value-added benefits like accuracy, flexibility, and speed have


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further attracted several new applications of 3D printing to specific areas of medicine like tissue engineering and reconstructive surgery where patient-specific solution is needed.8–11 Stereolithography (SL) is one of the foremost and most versatile 3D printing techniques in use nowadays.2,4,5,12,13 Photocurable resins serve as the ink of the 3D printer and undergo spatially controlled curing based on digital design data upon exposure to UV light.4,12 Biocompatibility plays a major role for biomedical applications.14,15 Successful 3D printing of biocompatible polymers may lead to the development of different patient-specific tissue engineering constructs. Biocompatible polymers approved by US Food and Drug Administration (FDA) for clinical applications include polycaprolactone (PCL), poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(lactic-co-glycolic acid) (PLGA), and poly(ethylene glycol) (PEG).15–18 However, among these biopolymers, PEG has gained so much attention due to its relatively high biocompatibility and hydrophilicity, making it more suitable for biomedical applications.14,15,19,20 A functionalized PEG in the presence of an appropriate photoinitiator readily constitutes a resin suitable for 3D printing using stereolithography.21,22 However, the use of neat PEG to form a hydrogel via radical polymerization manifests low tensile strength and ductility that severely deter its widespread use.19,23 Earlier works on 3D printing of PEG focus mainly on the chemical formulation

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photopolymerization reaction using SLA.12 The development of reproducible SLA 3D printing process in obtaining PEG hydrogel took higher priority over tuning of properties of 3D-printed hydrogel material.12 Hence, the need to develop a new system to improve the properties of PEG hydrogel overcoming its inherent limitation and to pave the way for more industry use arises. The incorporation of nanofillers has been a reliable approach to improve and tune different properties of polymers materials.4,19,24–26 A recent study reports the introduction of different


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nanofillers to form polymer composites via 3D printing in order to modify different properties of a material.4 The inherent stiffness,23–27 hydrophilicity,19 and biocompatibility23–26 of cellulose nanocrystal (CNC) present a good material choice as a nanofiller to combine with PEG polymer matrix. Additionally, the specific modulus of CNC is known to be at par with Kevlar and remarkably higher than that of fiber glass and steel.4,26–28 Considered as the strongest natural fiber, abaca plant (Musa textilis) shown in Figure S1 as a source of CNC is especially interesting due to its abundance and renewability, outstanding degradation resistance, and superior tensile strength.28 Therefore, abaca is an excellent source of CNC in the fabrication of sustainable and mechanically robust materials. Previous studies reported the development of CNC-based functional biomaterials having potential for tissue engineering applications.29,30 However, only the conventional manufacturing approach was employed to process these CNC-based precursors.23,31 The conventional method lacks the flexibility in geometrical designs, entails longer curing time for bulk material, and requires expensive molds and dies consequently making customization impractical. Earlier proposition that the standard resin used for the conventional method will work directly with 3D printing via stereolithography apparatus (SLA) is not accurate due to the absence of complexities on curing mechanisms in conventional method.2,12,32 The conditions under 3D printing pose some challenges that would need prior resin modification in order to shift from the conventional method.5,12 To facilitate efficient and effective photopolymerization via 3D printing, resin formulation must be carefully designed allowing faster curing time in a continuous layer-bylayer execution with acceptable resolution and printing repeatability.3,5,12,32 Hence, this novel work delineates the development of CNC-nanocomposite hydrogel precursor suitable for 3D printing via stereolithography to form complex architectures exhibiting enhanced properties


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intended for tissue engineering constructs. This study also aims to show that improvement of different properties of 3D-printed hydrogel can be achieved upon the incorporation of CNC to PEGDA matrix. Furthermore, this work also presents a comprehensive study on the physicochemical, thermomechanical, swelling capacity, and surface properties of 3D-printed PEGDA-CNC hydrogel material. EXPERIMENTAL SECTION Materials Poly(ethylene glycol) diacrylate with an average molecular weight of 575 g mol-1 (PEGDA575, Figure

S2a),

disodium(3Z)-6-acetamido-4-oxo-3-[[4-(2-sulfonatooxyethylsulfonyl)phenyl]-

hydrazinylidene]-naphthalene-2-sulfonate (RO16, Figure S2b), and (2,2,6,6-Tetramethyl-1piperidinyloxy) (TEMPO, Figure S2c) were purchased from Sigma-Aldrich and used as received. Lithium Phenyl(2,4,6 trimethylbenzoyl)-phosphinate (LAP, Figure S2d) was purchased from TCI Chemicals and also used as received. Cellulose nanocrystals (Figure S2e) were extracted from abaca plant fibers originated from the Philippines. Isolation of cellulose nanocrystals (CNCs) CNCs were extracted from abaca pulp fibers (Figure S1f) through acid hydrolysis based from earlier studies with modifications.31,33 Abaca pulp fibers (0.86 wt% of the acid solution) were hydrolyzed using 42 wt% sulfuric acid solution at 55 ⁰C under continuous agitation for 5 hours maximum.26,34 Purification was done by series of washing with deionized water and centrifugation (4400 rpm, 5 mins) until pH was neutral. The supernatant was discarded and then the nanocrystals were allowed to completely dry in a vacuum oven.


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TEM Imaging The morphology of CNC was observed under Tecnai TF30 ST TEM with an accelerating voltage of 300 kV and probe current > 0.6 nA per 1 nm spot. A drop of 0.01 % w/v CNC suspension was deposited onto carbon-coated copper grids. Excess liquid was blotted out by a filter paper. The grid was positively stained by 2 wt% aqueous uranyl acetate. Preparation for 3D Printing of PEGDA-CNC Complex Architecture Hydrogel precursor in Scheme 1 was photocured using radical polymerization at ambient temperature with varying CNC (0-1.2 wt%) loading to understand the influence of CNC on mechanical and thermo-mechanical properties of the nanocomposite hydrogel. Mass ratio of PEGDA to deionized water was maintained at 3:1. Water was used to disperse CNC through sonication techniques for 15 min.35 The photoinitiating system, composed of 0.75 wt% LAP, 0.005 wt% RO16, and 0.01 wt% TEMPO, was then added into the resin under continuous agitation for 30 min. The photocurable resin was poured into the resin tank of Form 2 desktop SLA 3D printer manufactured by Formlabs. The SLA 3D printer laser has a wavelength of 405 nm, a power of 250 mW, spot size of 140 µm, and a vector-scanning capability. The build volume of SLA measures 145 x 145 x 175 mm3. All specimens were printed using 3D print resolution of 50 µm. After 3D printing, all specimens were soaked in phosphate-buffered saline (PBS) for 10 minutes to remove impurities and then allowed to dry under ambient condition. 3Dprinted unfilled specimens, 0 wt% CNC, were labeled pure PEGDA to be the control samples all throughout this study.


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Scheme 1. 3D-printing system for CNC-nanocomposite hydrogel. The photocurable liquid resin is basically composed of the polymer matrix (PEGDA), the nanofiller (CNC), and the photoinitiating (PI) system. The CAD design in STL (Standard Tessellation Language) format must be uploaded to SLA for spatially controlled curing. The resin tank normally holds between 100 mL and 200 mL of resin. The chemical inertness of silicone layer coated on the bottom of the tank helps prevent adhesion of the cured material with the tank and allows it to attach on the platform instead. FT-IR Spectroscopy The FT-IR spectra were generated using Attenuated Total Reflectance (ATR) of Cary 600 FT-IR Spectrometer from Agilent Technologies accumulating 128 scans in the absorbance mode under ambient conditions within the frequency bands ranging from 4000 cm-1 down to 380 cm-1 for each sample. Spectral changes in the functional groups and fingerprint bands upon the incorporation of CNC were observed using Origin software.


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Tensile Measurement Uniaxial tensile testing at a speed of 5 mm min-1 was performed at room temperature on asprepared dogbones having cross-sectional area of 13.75 ± 0.70 mm2 (1.78 ± 0.07 mm x 7.73 ± 0.19 mm) and gage length of 54 ± 2 mm using Zwick/Roell Z050 Germany with 100-N load cell. Measurement was taken from five specimens for every CNC loading. Stress was instantaneously graphed by testXpert II as a function of strain and analyzed using Origin software. Subsequently from the tensile curves, various mechanical properties of the nanocomposite hydrogel were derived. Likewise, the moduli of toughness given as the fracture energy (WB) were automatically determined. Thermal Gravimetric Analysis The amount and rate of change in the mass of the nanocomposite hydrogel were measured as a function of temperature at constant heating rate of 5 ⁰C min-1 using TGA 2050 V5.4A from TA Instruments in an atmosphere of nitrogen purged at rates of 70 and 30 mL min-1. The platinum pan was rinsed with acetone and then torched to disintegrate the remaining contaminants prior use. A small sample of the hydrogel ranging from 5 to 9 mg was contained in the pan under controlled environment and loaded into the TGA machine where the temperature was ramped up to 1,000 ⁰C. All measurement data were graphed and evaluated using Origin software. Differential Scanning Calorimetry DSC thermogram measurements were taken using TA Instrument Q100 set with experiments carried out at a temperature range from -80 ⁰C ramping to 300 ⁰C at a rate of 5 ⁰C min-1. Each of the samples of the hydrogel ( 5 mg) was contained in a hermetically sealed aluminum lid and

pan prior loading into the DSC machine and running the experiment. A separate reference sealed


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pan is used in conjunction with the sealed pan containing each of the hydrogel specimens at a time when loading into the DSC system. All measurement data were graphed and analyzed using Origin software. Dynamic Mechanical Analysis Dynamic mechanical data on rectangular specimens measuring 60 x 12 x 3 mm3 were gathered through DMA Q800 System of TA Instruments using dual cantilever from -120 ⁰C to 75 ⁰C at a rate of 3 ⁰C min-1. Storage moduli and tangent delta (tan δ) for all the specimens were measured as a function of temperature under a strain of 10 µm at 1 Hz. Again, measurement data were plotted and analyzed using Origin software. Contact Angle Measurement Contact angle measurement was conducted through CAM200 of KSV Instrument Ltd. There were five 3D-printed specimens for every CNC loading on which contact angles were measured through

1-2 µL of water by using the static sessile drop method.

Swelling Ratio Five samples of the hydrogel were prepared from each CNC loading and then weighed (M1) before soaking into deionized water for 192 hours. Subsequently, all samples were taken from submersion and then weighed (M2) again after blotting out the excess water using filter paper. The swelling ratio, q, was calculated as the change in weight (M2 – M1) over the original weight (M1) that is before swelling. RESULTS AND DISCUSSIONS Isolation of CNC


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The intermolecular attraction among cellulose chains is intrinsically strong due to the myriad of hydroxyl moieties capable of robust hydrogen bonding.24,34,36,37 This behavior is responsible for unwanted agglomeration of CNC.4,24,37,38 Through the use of sulfuric acid to hydrolyze the amorphous domains of cellulose during the isolation of CNC, the anionic sulfates covalently attached on the surface of CNC promote good dispersion of CNC in water (Figure 1a) by electrostatic repulsion.24,26,36,37 TEM (Figure 1b) and AFM (Figure 1c and S3) imaging techniques confirm the successful isolation of nano-sized individual monodispersed crystals and allow measurement of CNC dimensions which are dependent on the source of CNC and its isolation method. Consistent with previous studies, the diameter of CNC measures 3 ± 1 nm while the lengths average to 246 ± 100 nm with an aspect ratio (length/diameter) of 82 ± 12.26,35

Figure 1. Optical images of CNC (a) re-dispersed in water; (b) TEM image showing diameters of 3 ± 1 nm and lengths of 246 ± 100 nm yielding to an aspect ratio of 82 ± 12; and (c) AFM image showing consistent results with TEM. 3D Printing of PEGDA-CNC Complex Structure Sonication techniques are utilized to guarantee homogeneous dispersion and uniform distribution of CNC to maximize reinforcement to PEGDA hydrogel.4,5,39 The hydrophilic nature of CNC promotes higher wettability of its surface consequently facilitating adsorption of PEGDA on


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CNC, and thereby establishing stronger interfacial adhesion between the matrix and the filler that is expected to provide some strengthening effect on the nanocomposite hydrogel.23,35,36,39 The conventional PEGDA-CNC precursor is not directly suitable for 3D printing (Figure S4a and S4b). In order to achieve highly reactive in situ photopolymerization, there is a need to improve the efficiency of the photoinitiating system in absorbing energy from UV irradiation at longer wavelength. Among limited options, LAP is a good choice because of its solubility in water and a good balance between photon absorptivity and biocompatibility.40 In spite of that, 3D-printed specimens manifest persistent delamination (Figure S4b). For this reason, LAP is sensitized by small mass fraction of RO16 for better absorption of photon energy and effective transmission of the absorbed energy to adjacent molecules.32,41 In this case, the energy can penetrate the preceding cured layer which encountered further curing in parallel with the succeeding layer.12 The overlapped curing has made the interlayer bond stronger that eliminates delamination.12 On the other hand, the interlayer curing can also bring about additional volume in the structure that creates adverse effects in geometrical accuracy (Figure S4c).12 Hence, TEMPO is likewise introduced in small fraction to regulate photopolymerization by maintaining a steady concentration of radicals.3,12,42 This stable species controls the thickness of the curing part by either quenching the photo-excited initiator to a certain extent that limits the number of radicals formed or scavenging some radicals via a reaction forming peroxides that are less reactive with acrylate function.2,3,32,42 The fidelity of fabricating a complex 3D-printed structure has been demonstrated by printing a butterfly structure in Figure 2 that involves intricate design, contours, and angles. The butterfly specimens are 3D printed using a commercial resin from Formlabs (Figure 2a) and the internally formulated PEGDA-CNC precursor (Figure 2b). From here, the printing output of the new


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precursor is comparable with that of the commercial resin in a way of forming the details and the support of the butterfly design. Successful 3D printing of this specimen makes a good indicator to proceed with the tensile dogbones and DMA strips for characterization purposes.

Figure 2. Formlabs SLA 3D printing butterfly test specimen using (a) Formlabs resin and the (b) internally formulated PEGDA-CNC precursor. As PEGDA, CNC, and LAP are all known for its biocompatibility, the resin developed here has a very good potential for biomedical and reconstructive surgery applications where patientspecific solutions are highly warranted.15,20,21,24,28,29,40 Characterization of 3D Printed PEGDA-CNC Hydrogel FT-IR Analysis The final properties of 3D-printed PEGDA hydrogel are influenced by the incorporation of CNC via photopolymerization process in a layer-by-layer fashion using SLA. FT-IR spectroscopy is employed to investigate the presence of CNC with varying concentration in polymer matrix PEGDA. The full spectra (Figure S5a) are generated for pure CNC and 3D-printed specimens – pure PEGDA hydrogel (0 wt% CNC) and PEGDA-composite hydrogel with varying CNC loading (from 0.3 wt% up to 1.2 wt% CNC). Both the functional group (Figure 3a) and fingerprint (Figure 3b) frequencies of 3D-printed pure PEGDA hydrogel appear to be consistent


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with the spectrum of PEGDA shown by Imani et al. and Visenti et al.43,44 Focusing on group frequencies (Figure 3a), 3D-printed PEGDA hydrogels exhibit a band at 2870 cm-1 representing sp3-hybridized C–H stretching vibration while bands at 1724 cm-1 and 1637 cm-1 indicate the existence of C=O (carbonyl) stretching and C=C stretching vibrations, respectively.

Figure 3. FT-IR spectra of pure CNC and 3D-printed specimens – pure PEGDA hydrogel (0 wt% CNC) and PEGDA hydrogel with varying CNC loading (0.3, 0.5, 0.9, and 1.2 wt%) (a) Functional group and (b) Fingerprint frequencies. The presence of the broad band at ~3450 cm-1 signifies H-bonded O–H stretching vibration due to the adsorption of water molecules on PEGDA. As for pure CNC, there are two evident group frequencies. As intensified by hydrogen bonding, the absorption band at ~3329 cm-1 is expected due to the rich hydroxyl groups on the surface of CNC.34,36 The presence of absorption band at 2888 cm-1 specifies an aliphatic C–H stretching vibration. The spectra of the nanocomposite hydrogels resemble that of pure PEGDA having increased band intensities of the first two bands (Figure S5b and S5c). This manifestation confirms the presence of CNC molecules in PEGDA in accordance with Beer-Lambert Law showing the direct relationship between the absorbance and


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the concentration of the absorbing species (Equation S1).45–47 Moreover, PEGDA-CNC specimens reveal the absorption band at around 1033 cm-1 (Figure 3b) representing C–O stretching vibrations, a signature frequency of CNC.48 Mechanical Properties Robustness in tissue engineering constructs is crucial in order to withstand substantial loads during application.49,50 Omission to consider this requirement often leads to tissue breakdown and functional failure.50,51 Thus, uniaxial tensile measurement is conducted in this study to optimize the concentration of CNC as well as to establish its influence on the mechanical properties of the conventional PEGDA hydrogel. Results show that 3D-printed PEGDA hydrogel has a tensile strength of 0.6 ± 0.2 MPa (Figure 4a and Table S1) which improves by 100 % upon the addition of 0.3 wt% CNC (1.2 ± 0.3 MPa). Subsequently, the latter value provides the new material a higher stress amount sustained without experiencing rupture. However, further increase in CNC loading did not mark any significant improvement in the tensile strength of the nanocomposite. The tensile strength of the 3D-printed PEGDA hydrogels in this study is found higher than earlier reported PEGDA hydrogels produced using conventional 2D method (65 KPa for pure PEGDA and 375 KPa for PEGDA with 1.4 wt% CNC as presented in Table S1).23 The significant improvement in tensile strength has triggered a corresponding increase in ductility of PEGDA hydrogels with the same CNC concentration. Figure 4b and Table S1 provide the elongation behavior of the 3D-printed specimens at various CNC loadings. The pure PEGDA exhibits a deformation of 2 ± 1 % that is enhanced by 110 % (5 ± 1.5 %) and 40 % (3 ± 1.3 %) at 0.3 wt% and 1.2 wt% CNC, respectively. Furthermore, 3D-printed PEGDA hydrogels without CNC manifest a tensile modulus of 26 ± 1 MPa, which are significantly higher than 7.5 KPa for pure PEGDA hydrogels and 31 KPa for PEGDA hydrogels with 1.4 wt% CNC produced using


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the conventional 2D method (Table S1).23 Figure 4c shows that there is no significant variation in terms of the tensile modulus of PEGDA hydrogels with varying CNC loading. This implies that the inherent stiffness of CNC has no direct impact on the tensile modulus of the new material. Meaning, the resistance of PEGDA-CNC hydrogel in this study against elastic deformation is primarily dictated by PEGDA-PEGDA interaction. The mechanical behavior of the nanocomposite hydrogels is further illustrated by plotting stress (σ) values as a function of strain (ε) values in Figure S6a through which the tensile moduli as measured by the slope of the elastic region manifest very small amount of variation. The fracture energy of the 3D printed PEGDA hydrogels at various CNC loading is shown in Figure 4d and Table S1. The unfilled 3D-printed specimens (0 wt% CNC) exhibit fracture energy (WB) equivalent to 6 ± 3 mJ and over a unit volume yield to 8 ± 5 GJ·m-3. Whereas, the addition of 0.3 wt% and 1.2 wt% CNC improves fracture energy to 25 ± 14 mJ (35 ± 20 GJ·m-3) and 12 ± 7 mJ (16 ± 9 GJ·m-3), respectively. Thus, fracture energy increases by 300 % and 100 %, respectively. The absorption of fracture energy is known to have a direct relationship with material toughness.52 Even though the tensile modulus shows very little variation, the toughness of the new material is significantly improved by the increase in tensile strength that produces a corresponding increase in ductility (Figure S6b). Toughness of 3D-printed PEGDA hydrogels are found far higher than those produced by 2D method showing 0.21 ± 0.01 kJ·m-3 for unfilled PEGDA hydrogel and its best at 1.4 wt% CNC having 4.78 ± 0.22 kJ·m-3 (Table S1).23 Another study shows toughness of ~1500 Nm·m-2 of PEGDA-alginate-nanoclay 3D-printed using extrusion.53 At the optimum level of CNC, 3D-printed specimens in this study via SLA can reach ~3,600 J·m-2 that is tougher than natural cartilage (1102 ± 136 J·m-2).51


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Figure 4. (a) Tensile strength, (b) Strain/elongation, (c) Tensile modulus, and (d) Fracture Energy for 3D-printed PEGDA hydrogels as a function of CNC loading.

The three-fold increase in fracture energy (Figure 4d and Figure S6b) of 3D-printed PEGDA hydrogel upon the incorporation of small amount of CNC cannot be attributed to the inherent stiffness of CNC alone.39,54,55 The reinforcement mechanism is being influenced by PEGDAPEGDA, CNC-CNC, and PEGDA-CNC interactions.23,39,54–57 Upon the application of an external tensile force, deformation is first endured by PEGDA.54,55


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PEGDA hydrogels fabricated by 2D method manifest much inferior mechanical properties (Table S1) primarily due to under-cured resin trapped within the structure where inefficient photopolymerization occurs. The conventional 2D method of fabrication implements bulk curing at a shorter radiation wavelength of the UV source. This process limits the penetration of UV radiation and so does the depth of curing (Equation S2) at which the effect is governed by the inverse square law.12,58,59 As a result of undercuring, the necessary binding force that comes from PEGDA-PEGDA interaction is not enough to sustain and transmit the load. On the other hand, the longer wavelength of 3D printing is expected to correspondingly increase the depth of curing (Equation S2). Another essential benefit from 3D printing is the manner of curing done in a layer-by-layer fashion that guarantees solidification in the innermost section of the model, which will in turn contribute on the mechanical characteristics of the output structure. The covalent crosslinking via PEGDA-PEGDA interaction provides the primary network to effectively accommodate the externally applied load. The strong affinity between PEGDA and CNC leads to better interfacial adhesion and creates the secondary network for efficient transfer of tension from relatively softer PEGDA to stiffer CNC, which greatly contributes also to the tougher characteristic achieved by the hydrogel.23,39,56,57,60 When these interatomic bonds begin to dissociate, atoms move relative to one another and then new linkage forms between neighboring atoms.57 The series formation of new bonds between PEGDA and CNC is evident in Figure S6c by region DE that provides some degree of plasticity to nanocomposite hydrogels and defers complete rupture of the material.35 The strength of the newly formed bonds can be quantified by the additional stress sustained by the nanocomposite hydrogels as shown by segment BC in Figure S6c. Uniformity in dispersion and distribution of CNC is the key to maximize PEGDA-CNC interaction by attaining higher aspect ratio of CNC and consequently


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larger surface area on which PEGDA molecules adsorb.4,5,23,26,35,36,39,61 The larger surface area of the individual CNC opens more access for interaction with PEGDA molecules. Once the load is transferred to CNC, the rigid CNC bears considerable amount of the load.54,55 In effect, the less rigid PEGDA will only have to deal with small portion of the load as the rest will be sustained by CNC upon transmission. The inherent stiffness of CNC comes from 180degree beta acetal linkage of carbon atoms giving them the full access for intramolecular hydrogen bonding necessary for better stability that provides the structure reasonable resistance to degradation.26,34,36 Upon the dissolution of the amorphous regions of microfibril during the isolation of CNC, the crystallinity goes higher (even above 90 %), which also augments to the stiffening performance of CNC.26,35 Due to this very high stiffness, CNC is able to sustain the additional amount of stress (manifested by segment AB in Figure S6c) successfully transmitted from PEGDA. The variation in mechanical performance between the 0.3 wt% CNC versus higher CNC loading (0.5, 0.9, 1.2 wt%) can be explained by the free volume principle for composite.60 As CNC concentration goes higher (the volume fraction of CNC increases and free volume decreases), the distance between neighboring CNCs gets smaller thus accommodating inadequate PEGDA chains with restrained mobility.39 This greatly influences the failure mechanism in polymer nanocomposite.39 Furthermore, the variation in mechanical performance among the hydrogels filled with CNC greater than 0.3 wt% (0.5, 0.9, and 1.2 wt%) can be attributed to CNC orientation within PEGDA network wherein the resulting mechanical strength contribution of CNC depends on the degree of perfection of the alignment of CNC within PEGDA, which considerably varies during 3D printing.60


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Thermal Properties Thermogravimetric (TG) measurement is done to study the thermal kinetics, stability, and decomposition of the 3D-printed nanocomposite hydrogels relative to the conventional PEGDA hydrogel. It is evident in Figure 5a that there is no significant amount of variation in terms of thermal degradation profiles among the 3D-printed specimens. This only implies that different CNC loadings employed in this study do not influence the decomposition performance of PEGDA hydrogel. However, this good thermal stability demonstrated by 3D-printed specimens up to 400 ⁰C exceeds the range (ambient to 300 ⁰C) of those produced by the conventional method.62,63 In support of this claim, the maximum rates of mass loss occurring at around 400 ⁰C are clearly depicted by the second derivative peak in Figure 5b. This great mass loss is attributed to pyrolysis of PEGDA initiated by bond dissociation.46,63

Figure 5. (a) Thermogravimetric mass loss and (b) derivative mass of a sample of pure PEGDA (0 wt% CNC) and PEGDA with varying CNC loadings (0.3, 0.5, 0.9, 1.2 wt%) heated at a constant rate of 5 ⁰C min-1.


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At the onset of degradation process in Figure 5a, drops in mass (mg) ranging from 4.7 % (0.5 wt% CNC) to 17 % (1.2 wt% CNC) are observed in segment 1. Those were brought about by dehydration process via complete evaporation of water molecules adsorbed on PEGDA.63 The mass of the specimens began to stabilize upon reaching 100 ⁰C in segment 2 but it continued to go down more slowly at 268 ⁰C in segment 3. Then, it was followed by a very sharp fall transition in segment 4. All specimens show a significant deterioration at around 385 ⁰C from 75 % mass except for 1.2 wt% CNC-filled hydrogel in which thermal decomposition happens 10 ⁰C earlier from 68 % mass. The masses dropped to 15 % (pure PEGDA and 0.5 wt% CNC), 10 % (0.3 wt% CNC), and 5 % (1.2 wt% CNC) at 440 ⁰C and then followed by a more gradual mass loss in segment 5 until 600 ⁰C. The mass residues visible at the end of the cycle (segment 6) could be attributed to CNC and to lithium salt contained in the photoinitiator (LAP).63 According to Prime et al., aromatic compounds and other cyclic structures tend to form char upon degradation in nitrogen.63 TG curves for pure CNC and LAP are shown in Figure S7. Figure 5b, confirming three degradation events appeared in Figure 5a, displays two minor mass loss steps by the presence of the first and the third derivative peaks at ranges ambient-100 ⁰C and 440-600 ⁰C, respectively. Dehydration process is responsible for the existence of the first peak while dissociation of the multiple bonds of the compounds may account for the third peak.63 Apparently, the second derivative peak occurring between 300 ⁰C and 440 ⁰C indicates the maxima of mass loss due to pyrolysis of PEGDA.63 DSC is performed in this study in order to observe the physical transformation of 3D-printed specimens across a temperature range and set the operating limits of the new material. Figure 6a reveals the occurrence of distinct thermal transitions to imply semicrystalline polymer network.64 First-order (crystallization and melting) transitions of the DSC curves confirm that the specimens


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contain both the crystalline and the amorphous fractions. An increase in temperature prior to crystallization has prompted the molecules to move around and gain sufficient energy to rearrange among themselves into a more organized structure upon reaching the crystallization temperature (TC).

Figure 6. (a) DSC heating curves exhibiting crystallization temperatures of PEGDA with varying CNC loadings (0, 0.3, 0.5, 0.9, 1.2 wt%) heated at a constant rate of 5 ⁰C min-1 and magnified on (b) the melting curves. Pure PEGDA hydrogel crystallizes at around 92 ⁰C but the incorporation of 0.3 wt% CNC has triggered the hydrogel to crystallize at lower temperature, 82 ⁰C.46 The rest of the CNC loadings at 0.5, 1.0 and 1.2 wt% require higher amount of energy as evidenced by the larger area of the peaks to generate crystallization occurring at 80 ⁰C, 95 ⁰C, and 83 ⁰C, respectively.46 Upon reaching the crystalline arrangement, CNC-filled hydrogels dissipate more heat with respect to unfilled PEGDA hydrogel, which is manifested by higher peaks of the crystallization exotherms.64 Further increase in temperature beyond Tc allows for another thermal transition where polymer crystals absorb energy and consequently break the intermolecular bonding thereby increasing the mobility of these molecules.65 Hence, this indicates the occurrence of


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melting at the temperature measured at the trough for substances of very high purity.64 In the case of semicrystalline materials, melting temperatures are measured at the highest temperature point of the melting endotherm.64 Pure PEGDA hydrogel exhibits melting at 270 ⁰C (Figure 6b). The addition of CNC (0.3 and 1.2 wt%) into PEGDA would require higher temperature (275 and 285 ⁰C) in order to melt the polymer.46 However, this is not the case of 0.5 and 0.9 wt% CNC showing melting temperatures slightly below that of the control (270 ⁰C).

Figure 7. (a) Storage modulus and (b) Tan δ measurement under a strain of 10 µm at 1 Hz via DMA on 3D-printed PEGDA hydrogels with varying CNC loading (0, 0.3, 0.5, 0.9, and 1.2 wt%) heated at a constant rate of 3 ⁰C min-1. DMA is employed in this experiment to take advantage of its greater sensitivity to measure Tg in order to define the minimum temperature operating limit of the 3D-printed specimens. In Figure 7a, the storage modulus of 3D-printed neat PEGDA hydrogel measures ~8700 MPa at 120 ⁰C and increases to its peak of ~9000 MPa at -117 ⁰C prior to its first transition. The storage modulus tends to increase with higher CNC loading and this observation agrees with an earlier study using CNC as nanofiller but of a different polymer matrix.66,67 At the same initial temperature, addition of CNC from 0.3 wt% gradually varied up to 1.2 wt% has raised the


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modulus to ~11500 MPa and to ~12900 MPa, respectively. The notable increases in the storage modulus upon the addition of CNC simply prove the stiffening effect imparted by CNC to the systems of network at the glassy region upon absorbing great amount of load transmitted through PEGDA.66 Increasing the temperature has caused the modulus to slowly decline as the molecules gain more flexibility for movement with the increase of free volume upon material expansion.52 The curves for 3D printed nanocomposites manifest three transitions as compared with the twostep transition exhibited by pure PEGDA (0 wt% CNC). Further increase in temperature led the specimens to alpha (or glass) transition where larger number of molecules began to move and acquire momentum to form crystals.52,67 This is the region in which the amorphous domains of the nanocomposite hydrogel undergo transformation from glassy to rubbery behavior. At the peak of tan δ curve in Figure 7b, the glass transition (Tg) of neat PEGDA hydrogel occurs at around -9 ⁰C that shifts to -10 ⁰C and -20 ⁰C with the addition of 0.3 wt% and 0.5 wt% CNC, respectively.52 Further addition of CNC to 0.9 wt% and 1.2 wt% tends to increase again Tg to -19 ⁰C and -14 ⁰C, respectively.52 Here, 0.3 wt% and 1.2 wt% CNC loadings manifest higher Tg and higher magnitude of tan δ peak height with respect to other CNC concentration. This occurrence can be attributed to better interfacial adhesion between PEGDA and CNC, which provides some degree of restriction for molecular chain movement.66 Moreover, this explains the better mechanical performance of the nanocomposites with 0.3 wt% and 1.2 wt% CNC.46,68 Contact Angle Measurement The hydrophilicity of hydrogels has been a known advantage in tissue engineering to create an interface conducive for cell adhesion, growth, and proliferation.20 Thus, contact angle measurement is done to confirm the surface properties of 3D-printed specimens upon


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incorporation of CNC. Figure 8a and Figure S8 show a decreasing trend on the magnitudes of contact angles upon increasing the concentration of CNC. The hydrophilic nature of CNC, together with the hydrophilicity of PEGDA, further increases the surface energy of the nanocomposite allowing higher interfacial interaction with water. The neat PEGDA hydrogel exhibits hydrophilic surface with mean contact angle of 72 ± 14⁰. The incorporation of 0.3 wt% CNC lowered the contact angle to 19 ± 8⁰. However, increasing the CNC loading leads to a slight increase in contact angle, although still substantially lower compared to PEGDA alone. At higher CNC concentration, it may well be that surface roughness and significant number of exposed CNC come into play.5,69 Increasing the CNC loading from 0.3 to 0.5 wt% may have created a rougher surface, resulting in a slightly increased contact angle. However, as the CNC loading was further increased to 1.2 wt%, due to the elevated concentration of the nanomaterial, it is unavoidable to have significant number of CNCs at the outermost surface layer of the 3D printed material. As a result, the contact angle measurements appeared to decrease due to the hydrophilic nature of those exposed CNC. Furthermore, the decrease of the contact angle upon the addition of CNC suggests good compatibility between PEGDA and CNC as manifested by the higher surface energy of the nanocomposite.70 The significant increase in the mechanical strength of the nanocomposite, as discussed in earlier section of this work, has unveiled the dynamic intermolecular attraction between PEGDA and CNC.35


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Figure 8. (a) Contact angle measurements and the (b) Swelling behavior of 3D-printed PEGDA hydrogel with varying CNC loadings (0, 0.3, 0.5, 0.9, 1.2 wt%). Swelling Behavior The ability to maintain three-dimensional structure in aqueous solution marks another important property for a hydrogel intended for tissue engineering. Figure 8b shows the swelling behavior of 3D-printed nanocomposite hydrogels demonstrating mechanical integrity during immersion in water for 8 days. This suggests stable crosslinks between PEGDA chains and strong interaction between PEGDA and CNC.23 Neat PEGDA hydrogel shows 30 % swelling while that containing 0.3 wt% CNC allows water intake of up to 33 %. Likewise, further increase of CNC concentration tends to lower the swelling percentage to 22 % < q < 27 % as the interfacial interaction between PEGDA and CNC gets more pronounced.23 These stronger adhesive forces tend to make it difficult for water molecules to penetrate the network. In addition, the nanocomposite hydrogel could no longer accommodate much water due to limited free volume with higher concentration of CNC.


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Figure 9. 3D printing of human ear construct using PEGDA-CNC hydrogel via SLA potentially suitable for tissue engineering applications. To finally demonstrate the capability of the current system to fabricate complex architecture, a human ear construct has been 3D printed (Figure 9) potentially suitable for reconstructive surgery of microtia and anotia. This result has also validated the efficiency of the newly formulated photoinitiating system intended for 3D printing of PEGDA-CNC. Overall, this study has successfully demonstrated the photocurable CNC nanocomposite formulation, 3D printing of the nanocomposite complex architectural structures, and different material property improvement especially the mechanical property of the 3D-printed nanocomposite hydrogel making 3D printing and CNC nanocomposite an innovative solution for various biomedical applications. CONCLUSION 3D printing of CNC nanocomposite hydrogels with high degree of repeatability, fidelity, and mechanical integrity of complex design using SLA has been successfully demonstrated in this novel work. The photoinitiating system developed in this study facilitates efficient


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polymerization at λ = 405 nm, eliminates recurrent delamination problem, and consistently controls layer-by-layer thickness intended to demonstrate finer details and intricate contours of robust 3D-printed specimens. The incorporation of CNC results in improvement in the physicochemical and thermomechanical improvement in the properties of the 3D-printed nanocomposite hydrogel, which in turn enhances the toughness and the surface wettability of the 3D-printed material. The resulting 3D-printed nanocomposite hydrogel, due to biocompatibility of its resin precursor, yields to promising attributes allowing for potential use in tissue engineering and reconstructive surgery. The results of this study further advance the potential of the nanocomposite hydrogel for industry application and also facilitates further research from the scientific communities. With the different advantages cited, 3D printing via stereolithography as a new fabrication technique has found its entry to serve several critical applications in various fields like in the case of medical industry where patient-specific solutions are inevitably necessary. With new capabilities such as novel material development, new material failure analysis performance, seamless fabrication of highly customized product, and cost-effective product delivery, augmented with advanced material like the nanocomposite hydrogel in this work, 3D printing is fastly becoming a formidable fabrication technique which can bring great value in the medical industry in the near future. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publication website at http:// DOI: 10.1021/acsami.7b09223.


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Images of abaca, chemical structures, AFM image, different 3D printing defects, FTIR spectra analyses, Beer-Lambert equation, comparative table on mechanical properties, stress-strain curves, relationship between curing depth and radiation wavelength, TGA thermograms, and contact angle measurements.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Phone: +1 216-368-4566 ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from Department of Science and Technology – Philippine Council for Industry, Energy, and Emerging Technology Research and Development (DOST-PCIEERD). Sincerest thanks are also offered to Dr. Cyril Jose Bajamundi for reviewing this paper and sharing valuable inputs. REFERENCES (1) (2) (3)

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3D Printing of Photocurable Cellulose Nanocrystal Composite for

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