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3D Printing Biocompatible Polyurethane/Poly (lactic acid)/ Graphene Oxide Nanocomposites: Anisotropic Properties Qiyi Chen, Joey Dacula Mangadlao, Jaqueline D. Wallat, Al Christopher C. de Leon, Jonathan K Pokorski, and Rigoberto C Advincula ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11793 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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3D Printing Biocompatible Polyurethane/Poly (lactic acid)/Graphene Oxide Nanocomposites: Anisotropic Properties Qiyi Chen, Joey Dacula Mangadlao, Jaqueline Wallat, Al De Leon, Jonathan K. Pokorski, and Rigoberto C. Advincula* Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106 (USA)

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

KEYWORDS: Fused Deposition Modeling, Thermoplastic polyurethane/Poly (lactic acid) Polymer Blend, Graphene Oxide, Mechanical Enhancement, Thermal Stability, Biocompatibility.


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Abstract: Blending thermoplastic polyurethane (TPU) with poly (lactic acid) (PLA) is a proven method to achieve a much more mechanically-robust material, while the addition of graphene oxide (GO) is increasingly applied in polymer nanocomposites to further tailor their properties. On the other hand, additive manufacturing (AM) has high flexibility of structure design which can significantly expand the application of materials in many fields. This study demonstrates the fused deposition modeling(FDM) 3D printing of TPU/PLA/GO nanocomposites and its potential application as biocompatible materials. Nanocomposites are prepared by solvent-based mixing process and extruded into filaments for FDM printing. The addition of GO largely enhanced the mechanical property and thermal stability of the nanocomposites. Interestingly, we found that the mechanical response is highly dependent on printing orientation. Furthermore, the 3D-printed nanocomposites exhibit good biocompatibility with NIH3T3 cells, indicating promise as biomaterials scaffold for tissue engineering applications.

INTRODUCTION Additive manufacturing (AM), also known as 3D printing, is gaining more and more interests in recent years. Several techniques have been developed for AM, such as stereolithography (SLA), fused deposition modeling (FDM), and selective laser sintering (SLS)1, 2. Among all the techniques, FDM is the most commonly used and also the most cost-effective3,4. It is a filament based printing process in which materials in a filament form are fed into a heating nozzle where the filament is melted, extruded and deposits onto a buildplate to generate a three-dimensional structure in a layer-by-layer fashion. Opposite to traditional manufacturing that is based on the subtraction of materials in a pre-designed mold, AM is processed by adding and joining materials together. Therefore, the use efficiency of materials is higher, and more importantly, the fabrication process does not require any mold or template5, 6. Because of this high degree of design and manufacturing flexibility, AM is becoming increasingly important in various fields. In tissue engineering, AM technique can easily prepare scaffolds with specially designed size, porosity and inter-connected channels which are beneficial for the cell growth7. Pressure sensor can be fabricated into well controlled geometry with AM technique to significantly ease the integration of functional materials for electronic devices8. The mechanical property of one material can be easily tailored by controlling the architectures during 3D printing9. Thermoplastic polyurethane (TPU) is a highly elastic linear polymer composed of soft segments, usually flexible polyester or polyethers, and hard segments, usually diisocyanates with benzyl structure. TPU exhibits good biocompatibility, and also excellent mechanical properties such as good abrasion resistance, high elongation, and moderate tensile and compression strength1. These properties enable TPU to be widely applied in many fields including coatings, foaming, adhesives and tissue engineering. However, TPU does have several drawbacks including poor shape fixity and low mechanical strength10. Poly (lactic acid) (PLA), on the other hand, derived from renewable resources instead of petroleum, has also been commonly used in



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biomedical applications because of its high biocompatibility and biodegradability11. PLA is mechanically strong and rigid, but very brittle with low flexibility and impact resistance. Researchers have studied the blending of TPU and PLA and found that the blending of two polymers can effectively improve the overall mechanical strength, impact resistance, and also shape memory property. By simply adjusting the weight ratios of TPU and PLA, the mechanical properties of the resulting polymers can be easily tailored to satisfy various application requirements11~17. Graphene is a two-dimensional single layer of sp2 hybridized carbon atoms. The special structure of graphene offers superior electrical, thermal, mechanical properties, as well as excellent anti-microbial effect18, 19, 20. Graphene oxide (GO), the oxidized form of graphene with carboxylic acid, epoxide or hydroxyl groups present on the surface, is miscible with wide range of solvents and polymer matrices. It is well established that small amount of GO in a polymer matrix can significantly enhance several properties of the polymer nanocomposites 21, 22, 19, 23, 24. Jing, et al. reported the TPU/GO scaffolds, prepared by thermally induced phase separation, renders improved thermal, mechanical properties, and good cell viability. Huang, et al. reported that the incorporation of GO in PLA offers excellent mechanical strength, anti-UV and anti-bacterial effects. Although, as described above, the TPU/PLA polymer blend has been demonstrated to have excellent and easily tailored properties, the addition of GO in TPU/PLA polymer blend resulting in TPU/PLA/GO nanocomposites, has not been well investigated. while the addition of GO can further enhance several properties of the polymer blend. Besides, as aforementioned advantages of AM technique, it is reasonable to predict that with the aid of AM techniques, applications of TPU/PLA/GO nanocomposites can be further explored, but the study of 3D printed TPU or TPU composites and the investigation of properties of 3D printed objects have never been reported before. In this study, we report for the first time FDM 3D printing of the TPU/PLA/GO polymer blend nanocomposites. In order to have an elastic material, ratio of TPU to PLA is fixed at 7 : 3. Mechanical property, printing orientation effect, thermal property of the FDM printed objects were studied. One of the potential application is, not limited to, tissue engineering scaffold, thus cell viability were also studied to initially demonstrate its biocompatibility.

EXPERIMENTAL SECTION

Materials. Chemical reagents were used without further purification unless otherwise specified. TPU was purchased from Pearlthane. PLA was purchased from Nature Works Ingeo. Dichloromethane (DCM), dimethylformamide (DMF), and alcohol, were purchased from Fisher Chemical. Graphite, concentrated sulfuric acid (H2SO4), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2) were purchased from Sigma Aldrich. Graphene oxide were synthesized based on the procedures developed in our lab.


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Graphene Oxide Synthesis. 3 grams of graphite and 400 ml H2SO4 was added slowly into a 1000 ml flask, followed by stirring at 400 rpm for 10 min. Then 3 grams of KMnO4 were added slowly into the reaction mixture under stirring. 3 portions of 3g KMnO4 were added every 24 hours. On the fourth day, reaction was stopped and each 120 mL of the reaction mixture was mixed with 300 ml ice–water mixture. Then 2 ml H2O2 was slowly added into the solution which turned the color from the dark purple to yellow. To purify the synthesized GO, yellow solution was centrifuged for 10 min at 4400 rpm. After centrifuge, upper liquid was removed and the sediments were collected and washed with deionized water, followed by another centrifugation and washing cycle. For further purification, sediments were then washed with isopropanol repeatedly until the pH was close to neutral. Then, sediments were sieved using No. 80 and No. 100 USA Standard Testing Sieve. Sediments after sieving were then redispersed in isopropanol and dialyzed for 3 days. After centrifugation at 1000 rpm for 30 min and vacuum evaporation, graphene oxide solids were collected.

(Raman spectrum and TEM of prepared GO is shown in Figure S1)

Cell Culture. NIH/3T3 fibroblast cells were maintained in Dulbecco's Modified Eagle’s Medium (DMEM) at 37 °C in a 5% CO2 humidified air environment. Medium was supplemented with 10% (v/v) fetal bovine serum (FBS) and 1% (v/v) penicillinstreptomycin.

Filament preparation and FDM printing. The nanocomposite preparation and FDM printing process is shown in Figure 1a. TPU were dissolved in DMF at 150 mg/ml under stirring at 60 C for 5 hrs. PLA were dissolved in DCM at 100 mg/ml under stirring at room temperature for 10 hrs. As-prepared GO was dispersed in DMF at 5 mg/ml under ultra-sonication for 1 hour. Then three solutions were mixed together under vigorous stirring overnight. In order to have an elastic polymer nanocomposite, the ratio of TPU to PLA is fixed at 7:3. The loading of GO are set to be 0.5 wt%, 2 wt% and 5 wt% and the reference sample is TPU/PLA blend without GO loading. The solution mixtures were then precipitated in alcohol. After vacuum drying at 40 °C for 24 hours, the precipitates were extruded by a mini-extruder to prepare polymer nanocomposite filament which were later directly used in FDM printer. The nozzle size of FDM printer was 0.4 mm. Printing speed was set to be 20 mm/s, layer thickness was 0.1 mm and infill density was 100%. Nozzle temperature and buildplate temperature was set at 210 °C and 60 °C respectively.



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Figure 1. a) TPU/PLA/GO nanocomposites filament preparation and FDM printing process. b) ~ f) photos of FDM printed TPU/PLA/GO nanocomposites. b) 3D printed cuboid specimen and dumbbell specimen for mechanical testing. (0.5 wt% of GO). c) 3D printed Ultimaker robot. (0 wt% of GO). d) 3D printed “RCA Lab”. (black: 2 wt% of GO. yellow: 0 wt% of GO). e) 3D printed microlattice. (5 wt% of GO). f) 3D printed microlattice under bending. (5 wt% of GO).

Live/Dead Staining. 3D printed TPU/PLA/GO nanocomposites were sterilized with 70% EtOH and placed in the wells of a 6 well plate. Confluent NIH/3T3 cells were removed using 0.05 (w/v) trypsin-EDTA and added to each well of the plate at a density of 1.0 x 106 cells in 2 mL of complete growth media and incubated in 5% CO2 in humidified air at 37 °C for 4 days. Media was replaced daily. After approximately 96 hours, unattached cells were removed via aspiration and the nanocomposites washed 3times with 2 mL of DPBS. Clean nanocomposites were carefully transferred to the surface of a clean coverslip in a petri dish and stained using the LIVE/DEAD Viability/Cytotoxicity Kit for mammalian cells (ThermoFisher), as per the manufacturer’s instructions. Images were acquired on a Leica TCS SPE confocal microscope.

Characterization. Scanning electron microscopy (SEM) was performed using Helios Nanolab 650 FESEM. Fourier Transform Infrared Spectroscopy (FT-IR) was measured using Digilab FTS 7000 ATR rapid scan spectrometer in transmittance mode. Thermogravimetric analysis (TGA) was done on TA Instruments, TGA 2050. 3D printed samples were heated from 25 ºC to 800 ºC under nitrogen atmosphere at a heating rate of 10 ºC/min. Differential scanning calorimetry (DSC) was conducted on TA Instrument, 2920 MDSC. 3D printed samples of around 6 mg were sealed in aluminum pan and heated from 25 ºC to 250 ºC under nitrogen atmosphere at a heating rate of 10 ºC/min. Fused deposition modeling 3D printing was completed by using Ultimaker 2+. Compression and tensile testing were recorded by using MTS ReNew upgrade package system. Filament were prepared by using mini-extruder. Cell images were acquired on a Leica TCS SPE confocal microscope.

RESULTS AND DISCUSSION


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To demonstrate the high flexibility of structure design in FDM 3D printing, objects of different shapes have been printed with TPU/PLA at different GO loading. The printed objects are shown in Figure 1b-1f, it can be seen that regular shapes such as dumbbells and cuboids (Fig. 1b), and complex shapes such as robotic Figure (Fig. 1c), letters (Fig. 1d) and micro-lattice (Fig. 1e, 1f) can all be printed with high quality. The flexibility of printed objects is also demonstrated in Figure 1f.

Mechanical Properties To investigate the effect of the addition of GO on the mechanical property of the 3D printed parts as well as the effect of printing orientation, compression testing and tensile testing were performed. For compression testing, all samples were printed into a cuboid shape specimen (Figure S2a), of which the width, length and height are 7 mm, 7 mm, and 15 mm respectively. In order to investigate the effect of printing orientation on mechanical response, each type of samples (including TPU/PLA blend without GO, with 0.5 wt%, 2 wt% and 5 wt% of GO) were prepared in two different printing orientations: standing specimen (Figure 2a) and lying specimen (Figure 2c). For standing specimen, the printing orientation is same as height direction while for lying specimen, the printing orientation is same as width (length) direction. Both types of specimens were performed under compression testing in a way that height direction is parallel with the compression direction (Figure 2b, 2d). Therefore, two kinds of compression testing

Figure 2. a) Schemes of standing specimen 3D printing. b) Illustration of S compression testing. c) Schemes of lying specimen 3D printing. d) Illustration of L compression testing.



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were performed on each sample: “S compression testing”, where standing specimen is used in the testing, and “L compression testing”, where lying specimen is used in the testing. The data is shown in Figure 3 (details shown in table S1). Clearly, compression modulus in both L and S compression constantly increases with the loading of GO. Modulus increases 56% in L compression and 167% in S compression at 5 wt% GO, which suggests that the addition of GO significantly improves the compression strength of TPU/PLA matrix.

Figure 3. a) S Compression testing curves of samples of different GO loading. b) L Compression testing curves of samples of different GO loading. c) Compression modulus of L and S compression testing of samples of different GO loading. d) SEM image of cross-section of S specimen (0.5 wt%). (insert: Image of specimen after S compression testing). e) SEM image of cross-section of L specimen (0.5 wt%). (insert: Image of specimen after L compression testing).


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When comparing results of L compression and S compression, it reveals that for each individual sample, the modulus of S compression is always smaller than that of L compression. Besides, in L compression, stress will decrease and specimen will fail at the final stage while stress constantly increase in S compression (Figure 3a, 3b). This finding exhibits the effect of printing orientation on mechanical response. To understand the cause of printing orientation effect, SEM images of cross-section (cutting face is along the compression direction) of S and L specimens were collected. The cross-section of S specimen includes multiple layers packing together, and it can be seen that significant number of large voids are present between different layers (Figure 3d). The cross-section of L specimen shows the image of one individual layer, and only limited number of small voids are dispersed in it (Figure 3e). As mentioned above, FDM printing is based on a layer-by-layer deposition manner. The adhesion between two layers is physical interaction, thus air will be trapped in the large contact areas between two layers which inevitably generates the voids and loosens the packing of layers. However, within one individual layer, the small contact area of extruded fibers greatly reduced the amount of trapped air and thus provides much stronger packing within each individual layer. The loose packing and large voids between layers in S compression testing lead to the smaller compression modulus when compared to the stronger packing within each individual layer in L compression testing. However, in terms of specimen failure at the final stage, in L compression, each layer will be bended and the inhomogeneous bending results in the expansion of voids between bended layers. Eventually expanded voids lead to the delamination of layers, which causes the failure of specimens. In S compression, instead of expansion, voids will be compressed and therefore delamination is avoided which stops the failure of specimen. Images of specimen after compression is shown in Figure 3d and 3e inserts. The delamination of layers in L compression can be easily seen but only slight bending is found in S compression. The printing orientation effect on the mechanical response also reveals the fact that due to weak adhesion strength between layers, mechanical properties of 3D printed objects are inherently inferior to that of the objects produced by traditional processing. However, 3D printing can easily introduce complex structures (as shown in Figure 1), thus introduce more properties associated with the designed structures.

In tensile testing, each sample were printed into dumbbell shaped specimens (Figure S2b), of which width, length and thickness are 5 mm,22 mm, and 3 mm respectively. The printing orientation is only in lying specimen. The tensile testing curves are shown in Figure 4 (details shown in table S2). The strain at break of pure TPU/PLA is higher than 7, which means that it exhibits good elasticity of the prepared polymer blends. Tensile modulus and yield point has increased by 75.50% and 69.17%, respectively at 0.5 wt% loading of GO which indicates that the addition of GO has significantly improved the tensile strength. However, when GO loading is further increased, both the tensile modulus and yield point decreased. This is possibly because the percolation threshold for tensile strength in the TPU/PLA matrix is below 2 wt%, thus the further addition of GO will reduce tensile strength. The strain at break constantly decreases with the loading of GO which states that the addition of GO decreases the elasticity of the polymer matrix.



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Figure 4. a) Tensile modulus of samples of different GO loading. b) Tensile testing curves of samples of different GO loading.

FTIR Spectra and SEM Images Generally for polymer nanocomposites mechanical property enhancement relies on uniform dispersion of the filler in polymer matrix; and this dispersion is often dictated by interactions between the filler and the matrix. The existence of polar group, such as hydroxyl and epoxy groups, on the surface of graphene oxide can hydrogen bond with urethane groups in the backbone of TPU and carboxylic group in PLA, thus, the strong interaction between GO and the TPU/PLA matrix is expected. In order to determine if hydrogen bonding occurs between GO and PLA/TPU polymer matrix, Fourier Transfer Infrared (FTIR) spectroscopy was used. FTIR provides insight into a materials’ characteristics by monitoring vibrational frequencies at which specific functional groups absorb infrared energy. The FTIR spectrum of TPU/PLA with different loading levels of GO are shown in Figure S3. Peaks at 3332 cm-1, 2935 and 2850 cm-1, 1701 and 1731 cm-1, and 1754 cm-1 are corresponding to the stretch vibration of N-H, asymmetric and symmetric vibration of –CH2-, H-bonded C=O and free C=O in TPU, and –C=O group in PLA, respectively25, 26, 27 In Figure 5a, hydrogen bonding between the matrix and GO was monitored by observing the absorbance peak at 3520 cm-1 which refer to the free stretching of N-H group from the TPU. With increased loading levels of GO, the intensity of the absorbance peak decreases, a result caused by hydrogen bonds between GO (hydrogen bond acceptor) and N-H from TPU, the hydrogen bond donor. This suppression of the free stretching motion of N-H bonds with hydrogen bonding results in reduced peak intensity. In Figure 5b, absorbance peaks at 1731 cm-1 refer to the free stretching of the carbonyl bond (C=O) from PLA and TPU. Decreasing absorbance values at 1731cm-1 are observed with increased loading levels of GO, indicating hydrogen bonding between the matrix and filler material. Results from the FTIR indicate hydrogen bond interactions between the matrix and filler are generated. In order to probe dispersion of the filler in the matrix, scanning electron microscopy was used. SEM images of the cross-section of various loading levels of GO in the matrix are shown in Figure 5c-f (1000x magnification) Figure S4 (5000x magnification) and Figure S5 (10000x magnification). No signs of aggregation of GO is observed in the


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polymer matrix under different magnifications. Since the diameter of GO, as shown in TEM, is around 20 µm, which makes aggregation of GO in the polymer matrix easily observed under SEM, the absence of aggregation is a good indication that GO uniformly disperses in the polymer matrix. It is very apparent that flake-like structures with sharp edges are increasingly appearing and uniformly dispersed in the cross-sectional surface when the loading of GO increases. These sharp-edged flakes largely increase surface roughness, thus the fractured surface becomes rougher as the loading of GO increases. The formation of the flakes is due to the GO embedded in and wrapped by the polymer matrix and the strong interactions between the GO and polymer backbone causes mechanical interlocking28,

29, 30

. Taken together, the FTIR data and SEM images confirm strong

interactions and good dispersion of GO in TPU/PLA matrix.

Figure 5. a - b) FTIR spectrum of the polymer nanocomposites with various loading leveling of GO. a) Absorbance peaks at 3520 cm-1. b) Absorbance peaks at 1731 cm-1. c-e) SEM images of samples of different GO loading. c) 0 wt% GO. d) 0.5 wt% GO. e) 2.0 wt% GO. f) 5.0% wt GO.



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Thermal Stability To determine the influence of GO on thermal property of the 3D printed nanocomposites, Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) have been conducted. Weight loss curves (Figure S6) reveal that after heating to 800 °C, the remaining residue weight continuously increased from 0.69% to 9.72% when loading of GO increased to 5 wt%. Subtracting the weight percentage of GO, more than 4 wt% more polymer blend has remained after 800 °C heating. Derivative weight curves (Figure 6a) show that pure TPU/PLA polymer blend has three degradation stages, 308 °C, 372 °C, and 442 °C, which correspond to the degradation temperature of PLA component, soft segment of TPU and hard segment of TPU, respectively28,

31

. The addition of GO has increased the degradation temperature of PLA at 308 °C toward the

degradation temperature of TPU soft segment at 389 °C and eventually merges together at 5 wt% GO loading. This phenomenon can be explained by interfacial interactions between GO and PLA. As the material is heated, radicals are generated, leading to the degradation of the matrix. Generated radicals, however, will covalently bind with the conjugated structure of GO and thus recede the degradation process32, 33. DSC data of the nanocomposites is shown in Figure 6b (DSC data of pure TPU and PLA shown in Figure S7). In the curve of pure TPU/PLA blend, cold crystallization occurs at 90 °C, followed by a single melting peak at 156 °C. Because TPU is amorphous, the crystallization and melting behavior attributes to the PLA component26. The addition of GO increased the cold crystallization temperature to 117 °C. This is possibly due to the fact that when sufficient loading of GO is present in the polymer matrix, it will confine the motion of PLA chains, restrict the cold crystallization process of PLA and thus increase the crystallization temperature30. Double melting peaks occurs at 156 °C when loading level of GO increased to 2 wt%, and higher temperature peak in this double melting peaks gets stronger. The existence of double melting peak is normally caused by the recrystallization during melting, which generates different crystalline forms30,34. The intensity increase of higher melting peak in the double melting peak indicates better crystalline forms are increasingly generated with the loading of GO. Furthermore, a new melting peak appears at a much higher temperature around 190 °C. The newly appeared melting peak at 190 °C are also likely corresponding to the newly generated crystalline forms with a much higher melting temperature, this is most visible with the 5% GO. These crystalline forms with higher melting temperature suggest that the addition of GO can nucleate and grow more and better crystals in PLA components. Thus the TGA and DSC data both demonstrate that the thermal stability of TPU/PLA blend has been significantly improved by the addition of GO.


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Figure 6. a) TGA derivative weight curves and b) DSC curves of samples at different GO loading.

Cell Culture testing Graphene oxide is known to enhance cellular growth and differentiation of various cell types35, 36, while TPU/PLA polymers are biocompatible polymers previously used for cell scaffold materials25. For these reasons, TPU/PLA GO blends are well suited for cellular culture37. To investigate the effect of graphene oxide on the FDM printed TPU/PLA blend, ultra-thin sheets of TPU/PLA blends containing 0.5, 2, and 5 wt% GO were fabricated by printing a monolayer of material on a glass substrate to support cell adhesion, growth, and proliferation. The ultra-thin nature of the 3D printed polymer is aimed to allow for the passage of light, a critical feature in imaging the cells for the assay. These 3D printed TPU/PLA GO monolayers were evaluated as seeding scaffolds using NIH3T3 mouse embryonic fibroblast cells with a LIVE/DEAD viability/cytotoxicity assay. Briefly, 3D printed monolayers were immobilized on a cover-glass slide, and NIH3T3 cells were deposited onto the monolayers. Follow 96 h incubation, the 3D printed scaffolds were washed and stained with a combination of calcein-AM and ethidium homodimer-1 (EthD-1) to detect live and dead cells, respectively. Live cells are measured by monitoring the presence of calcein, seen in green. Calcein-AM is converted to calcein, a fluorescent molecule, by esterases in healthy cells. Dead cells are measured by monitoring EthD-1 which enters damaged membranes in dead cells and binds to nucleic acid, and is visualized in red. The results of the LIVE/DEAD assay indicate that at all scaffolds support cells growth as only live cells are detected, no dead cells were detected in any scaffold (Representative images seen in Figure 7, complete result images seen in Figure S8). Additionally, the NIH3T3 cells appear to spreading and proliferating on all scaffolds as indicated by their morphology. While all scaffolds support cell growth and proliferation, the TPU/PLA with 0.5% GO shows the highest density of cells across all loadings of GO and even when compared to the TPU/PLA control (see supplemental for more images). It appears that 0.5% GO is optimal for supporting cell-growth. Although further investigation is needed to explain the result, one possible explanation could be attributed to the configuration of GO in the TPU/PLA scaffold. If the loading level of GO is too high, the GO may not ‘lay’ flat



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in the polymer matrix and thus the enhancement in cellular growth is not afforded38. Taken together these results indicate that all scaffolds are non-toxic toward embryonic cells with 0.5% GO providing the greatest enhancement in cellular growth and proliferation as indicated by the LIVE/DEAD assay38, 39, 40.

Figure 7. 96 hours cell culture results of NIH3T3 cells on 3D printed TPU/PLA with different GO loading. a) 0 wt% GO. b) 0.5 wt% GO. c) 2 wt% GO. d) 5 wt% GO. (Green color indicates live cells while red color indicates dead cell)

CONCLUSIONS In conclusion, we have successfully 3D printed elastic TPU/PLA/GO nanocomposites by using solvent mixing process as well as fused deposition modeling technique. Nanocomposites can be easily printed in to complex shapes with high quality. FTIR and SEM images reveals good dispersion of GO in polymer matrix. The addition of GO has significantly enhanced the mechanical properties of polymer matrix, 167% in compression modulus and 75.5% in tensile modulus. The printing orientation leads to different mechanical responses due to the weak adhesion strength between layers during 3D printing. Thermal stability has also been improved, with 90 °C increase in degradation temperature as well as new formation of better crystalline structures. Cell culturing results reveal excellent cell viability of 3D printed scaffolds, indicating limited addition of GO has no obvious


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toxicity to cell growth, and small amount of GO is beneficial for cell proliferation. Based on the above results, the 3D printed TPU/PLA/GO nanocomposite exhibits excellent mechanical properties, thermal stabilities and cell viability, which allows it to be widely applied in many fields, especially as a good potential in tissue engineering scaffolds.

AUTHOR INFORMATION Corresponding Author Rigoberto C. Advincula E-mail: [email protected] Phone: +1 216-368-4566

SUPPORTING INFORMATION Raman spectrum of synthesized graphene oxide. TEM images of synthesized graphene oxide. photos of 3D printed specimen for mechanical testing. Compression testing data details. Tensile testing data details. Full FTIR spectrum of samples of different GO loading. SEM images of cross-section of 3D printed specimens with different GO loading at 5000x and 1000x magnifications. TGA weight loss curves of samples of different GO loading. DSC curves of pure TPU and pure PLA. Cell culture results of NIH 3T3 cells on 3D printed of samples of different GO loading. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT We gratefully acknowledge funding from NSF CMMI NM 1333651 and STC-0423914, and technical support from Dr. David Schiraldi (Department of Macromolecular Science and Engineering, Case Western Reserve University), Dr. Eric Baer (Department of Macromolecular Science and Engineering, Case Western Reserve University), Swagelok Center for Surface Analysis of Materials (Case School of Engineering, Case Western Reserve University), Agilent Technologies, and Park Systems.



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