preparation of thermoplastic polyurethane/graphene oxide composite scaffolds by thermally induced...

Preparation of Thermoplastic Polyurethane/GrapheneOxide Composite Scaffolds by Thermally InducedPhase Separation

Xin Jing,1,2 Hao-Yang Mi,1,2 Max R. Salick,2,3 Xiang-Fang Peng,1 Lih-Sheng Turng2,4

1National Engineering Research Center of Novel Equipment for Polymer Processing, School of Mechanicaland Automotive Engineering, South China University of Technology, Guangzhou, China

2Wisconsin Institutes for Discovery, University of Wisconsin–Madison, Wisconsin

3Department of Engineering Physics, University of Wisconsin–Madison, Wisconsin

4Department of Mechanical Engineering, University of Wisconsin–Madison, Wisconsin

In this study, biomedical thermoplastic polyurethane/graphene oxide (TPU/GO) composite scaffolds weresuccessfully prepared using the thermally inducedphase separation (TIPS) technique. The microstructure,morphology, and thermal and mechanical properties ofthe scaffolds were characterized by Fourier transforminfrared spectroscopy (FTIR), Raman spectroscopy,scanning electron microscopy (SEM), differential scan-ning calorimetry (DSC), thermal gravimetric analysis(TGA), and compression tests. Furthermore, NIH 3T3fibroblast cell viability on the porous scaffolds wasinvestigated via live/dead fluorescent staining andSEM observation. FTIR and Raman results verified thepresence of GO in the composites. SEM imagesshowed that the average pore diameter of the com-posite scaffolds decreased as the amount of GOincreased. Additionally, the surface of the specimensbecame rougher due to the embedded GO. The com-pressive modulus of composite specimens wasincreased by nearly 200% and 300% with the additionof 5% and 10% GO, respectively, as compared withpristine TPU. 3T3 fibroblast culture results showed thatGO had no apparent cytotoxicity. However, high load-ing levels of GO may delay cell proliferation on thespecimens. POLYM. COMPOS., 35:1408–1417, 2014. VC 2013Society of Plastics Engineers

INTRODUCTION

The fabrication of three-dimensional (3D) porous scaf-

folds with an interconnected porous structure holds great

potential in prospective tissue engineering applications.

Numerous approaches have been developed to fabricate 3D

scaffolds such as gas foaming, electrospinning, solvent

casting/particle leaching, and thermally induced phase sep-

aration (TIPS). Each of these methods has certain advan-

tages and disadvantages. The greatest advantage of TIPS

over other approaches is that it is capable of fabricating

suitable scaffolds in one simple process [1]. Polymer scaf-

folds with varied compositions have been prepared using

the TIPS technique in recent years, such as poly(E-capro-

lactone) (PCL)/hydroxyapatite (HA) [2], polylactide-co-

glycolide (PLGA) [3–5], poly(hydroxyabutyrate-co-hydrox-

ylvalerate/hydroxyapatite) (PHBV/HA) polymer composites

[6], polyurethane [7], and poly-L-lactide (PLLA)–PCL

blend porous membranes [8].

TPU is a linear segmented multiblock copolymer com-

prised of hard and soft segments. The hard segments are

typically composed of diisocyanates, while the soft seg-

ments are made of flexible polyether or polyester glycols.

By adjusting the weight ratios and compositions of the

two different segments, TPU can exhibit a great range of

properties. The TPU family has been widely used in

industry and tissue engineering applications such as adhe-

sives, coatings, and cardiovascular biomaterials [9, 10].

Because of its excellent physicochemical properties and

biocompatibility, a large number of TPU scaffolds fabri-

cated by solvent casting/particle leaching, foaming, elec-

trospinning, inkjet, salt leaching/freeze drying, and TIPS,

have been used in engineered tissues such as artificial

Correspondence to: Lih-Sheng Turng; e-mail: [email protected] or

Xiang-Fang Peng; e-mail: [email protected]

Contract grant sponsor: National Natural Science Foundation of China;

contract grant numbers: 51073061, 21174044; contract grant sponsor:

Fundamental Research Funds for the Central Universities; contract grant

numbers: 2011ZZ0011, 973 Program (2012CB025902).

DOI 10.1002/pc.22793

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2013 Society of Plastics Engineers

POLYMER COMPOSITES—2014

skin [11, 12], cardiovascular implants [7, 13], nerve con-

duits, cancellous bone graft substitutes [14–16], and artic-

ular cartilage [17].

Graphene, a single layer of sp2-bonded carbon atoms in

a two-dimensional honeycomb lattice, and its derivatives

have attracted considerable attention as potential biomateri-

als because of their unique physico-chemical properties

[18–20]. Graphene oxide (GO), one of the most important

graphene derivatives, has a large number of hydrophilic

groups on its surface such as hydroxyl, carboxyl, and

epoxy [21]. These hydrophilic groups endow it stable dis-

persibility in aqueous or organic solutions by electrostatic

repulsion [22] and possible chemical interactions between

oxygenated surface functionalities in GO nanosheets and

polymer matrices [23]. GO has many potential applications

in catalysis [24], electrodes, transparent films [25], as well

as in biological applications [26].

In this work, GO was chosen to produce TPU/GO

composite scaffolds using TIPS, since the hydrophilic

groups on the surface of the GO sheets were expected to

generate improved surface chemical properties such as

hydrophilicity and surface roughness [27], which may

promote cell attachment and cell growth [28]. The mor-

phological, mechanical; biological properties of the com-

posite scaffolds with different GO contents prepared by

TIPS were investigated. It was found that the mechanical

strength of the composite scaffolds was greatly enhanced

compared with that of the pure TPU. The 3T3 fibroblast

cell culture results indicated that the GO exhibited no par-

ticular cytotoxicity; however, excessive loading levels of

GO might delay cell proliferation and induce a slight

decrease in overall cell viability. The good biocompatibil-

ity of the composite scaffolds allows them to be consid-

ered as promising candidates for tissue engineering

applications.

MATERIALS AND METHODS

Materials

Medical grade TPU (TexinVR

Rx85A) was generously

donated by Bayer Material Science, Inc.1,4-dioxane

(99%) was purchased from Sigma-Aldrich. Ultrapure

Milli-Q water (distilled deionized (DI) water) was used in

the experiment. Graphite, sodium nitrate (NaNO3), con-

centrated sulfuric acid (H2SO4), potassium permanganate

(KMnO4), hydrogen peroxide (H2O2), hydrochloric acid

(HCl) (37%) were purchased from Sigma-Aldrich and

were used as received.

Graphite Oxide Synthesis and Purification

Graphite oxide was prepared using a modification of

the Hummers’ method [29–31]. Briefly, 5 g of graphite, 5

g of NaNO3, 230 ml of H2SO4 were stirred together in a

round-bottom flask in an ice bath. Then, 30 g of KMnO4

were slowly added into the solution to prevent the tem-

perature from exceeding 5�C. Next, the suspension was

transferred to an oil bath and maintained at 35�C for 1 h.

After forming a thick paste, 400 ml of water was slowly

added into the mixture. The solution was then transferred

to a 90�C oil bath, where it stayed for 30 min. Finally,

the suspension was further diluted with 1000 ml of water,

and then 30 ml H2O2 was added slowly, turning the color

from dark brown to bright yellow. After being cooled, the

products were filtered and then washed in succession with

a 3% HCl aqueous solution and ethanol (23). Multiple

washes with DI-water were then performed until the pH

of the product was �7. The solid paste was then dried for

5 days using a freeze drier (Freezone 4.5, Labconco,

USA).

Preparation of TPU/GO Nanocomposite Scaffolds UsingTIPS

The schematic representation of nanocomposite fabri-

cation is presented in Fig. 1. The obtained graphite oxide

was dispersed into a dioxane/water solution by ultrasoni-

cation for 2 h to achieve a homogeneous graphene oxide

(GO) solution. The polymers were then added to solutions

containing 0, 1, 5, 10 wt% GO for a total solute concen-

tration of 10% wt/v. The solutions were then stirred over-

night at 60�C in an oil bath. Solutions were poured into

10 mm glass tubes with filter paper lids and placed in an

ice bath at 0�C for 15 min to induce phase separation and

a subsequent coarsening process [32]. The glass tubes

were then placed in a 280�C freezer for 4 h to com-

pletely freeze the solution. In the next step, the tubes

were lyophilized at 254�C and 1022 mbar for a week to

extract the solvent.

Characterization of the TPU/GO Composite Scaffolds

Fourier Transform Infrared Spectroscopy (FTIR).

Fourier transform infrared spectroscopy (FTIR) measure-

ments were carried out using a Bruker Tensor 27 instrument.

The samples were analyzed in absorbance mode in the range

of 600 to 4,000 cm21 with a resolution of 4 cm21.

Raman Measurements. Raman spectroscopy studies

were performed with a Raman microspectrometer

(Thermo Scientific) on the TPU/GO composite scaffolds

with a 532 nm laser line and a 503 objective.

Graphene Oxide Morphology. A Philips CM-100

transmission electron microscope (TEM) (Philips/FEI

Corperation, Holland) at an accelerating voltage of 100

kV was used to study the microstructures of the graphene

oxide. The samples were prepared by dipping a Formvar-

carbon coated grid into a 1% solution of graphene oxide

in deionized water.

Determining Composite Scaffold Morphology. The

morphologies of the scaffolds were observed by scanning

DOI 10.1002/pc POLYMER COMPOSITES—2014 1409

electron microscopy (SEM) (JEOL 6500, Nikon) with an

accelerating voltage of 10 kV. Prior to SEM imaging, the

specimens were fractured in liquid nitrogen and sputtered

with gold for 40 s. To address the role of GO in the com-

posites more clearly, the porous samples were compressed

into films prior to being fractured in the liquid nitrogen.

This enabled improved observation of the dispersion of

the GO. Image Pro-plus software was used to determine

the average pore size and help in the calculation of pore

density.

Equation 1 was used to convert the surface average

diameter to the volume average diameter.

D351

n

Xn

i51

D3i (1)

Equation 2 was used to calculate the volumetric pore

density,

Pore density5N

A

� �3=2

(2)

where, N was the number of pores and A was the area of

the SEM image.

The samples were cut into 10 mm tall cylinders with a

razor blade, their porosities were determined by weighing

the samples and measuring their dimensions to obtain

their volumes using Eq. 3. The porosity was the mean

value of five samples,

Porosity5Vthq2Wm

Vthq3100% (3)

where Wm was the measured weight, q was the weight

average density of the blends, and Vth was the volume of

the measured samples.

Thermogravimetric Analysis (TGA). Thermal stability

measurements were carried out using a thermogravimetric

analyzer (TGA Q50, TA Instruments, USA). The samples

were heated from 30�C to 700�C at a heating rate of

10�C/min. The flow rate of the protection gas (nitrogen)

was 60 ml/min.

Differential Scanning Calorimetry (DSC) Measure-

ments. Thermal property measurements were performed

with a DSC Q20 (TA Instruments). Samples were heated

to 210�C at a heating rate of 5�C/min and held isother-

mally for 5 min to eliminate any prior thermal history.

Samples were then cooled to 280�C at 5�C/min and then

reheated to 210�C at the same rate. All tests were carried

out under the protection of a nitrogen atmosphere.

Mechanical Properties of the Composite Scaffolds.

The mechanical properties of the scaffolds were character-

ized via compression tests. These tests were performed on

five cylinder specimens (10 mm in diameter, 10 mm in

height) of each type using a universal testing machine (Ins-

tron 5967, USA) with a 100 kN load cell. Each specimen

was compressed to a strain of 250% at a rate of 5 mm/min.

Biocompatibility Characterization

Cell Culture. To investigate their biocompatibility and

explore their potential to be used as tissue engineering

scaffolds, the prepared composite scaffolds were sub-

jected to preliminary cytotoxicity screening. Swiss mouse

NIH 3T3 ECACC (European Collection of Cell Cultures)

fibroblasts were used for the biological assays. Cells were

cultured in high-glucose DMEM (Invitrogen), supple-

mented with 20% fetal bovine serum (WiCell), 1 unit/ml

penicillin (Invitrogen), 1 lg/ml streptomycin (Invitrogen):

2 mM l-glutamine (Invitrogen), and maintained on 6-well

tissue culture-treated polystyrene (TCPS) plates (BD Fal-

con) prior to testing. Medium was replaced every two

days and cells were passaged with ethylene

FIG. 1. Schematic representation of the process flow of thermoplastic polyurethane (TPU)/graphene oxide

(GO) nanocomposite fabrication. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

1410 POLYMER COMPOSITES—2014 DOI 10.1002/pc

diaminetetraacetic acid (EDTA) at a 1 : 40 ratio every 6

days during regular maintenance.

Cell Seeding. The composite scaffolds were exposed to

sterilizing UV light for 60 min on each side and placed

into 12-well TCPS plates before cell seeding. 3T3 cells

were dissociated with ethylene diaminetetraacetic acid

(EDTA) for 5 min prior to seeding. The cell suspension

was then seeded onto the surface of the sterilized scaf-

folds at a density of 1.25 3 105 cells/cm2 in the high-

glucose 3T3 medium. Medium was replaced daily for

screening samples at 1 ml per day.

Live/Dead Assay. To confirm the viability of cells cul-

tured on the chosen scaffolds, cells were stained with a

Live/Dead Viability/Cytotoxicity Kit (Invitrogen). Cell

viability was determined 3 days and 10 days after seed-

ing. This kit allows simultaneous visualization of both

live and dead cells. The stain utilized green fluorescent

Calcein-AM to target esterase activity within the cyto-

plasm of living cells, red fluorescent ethidium

homodimer-1 (EthD-1) to indicate cell death by penetrat-

ing damaged cellular membranes. Blue fluorescent DAPI

stain was additionally applied as a nuclear marker. A

Nikon A1Rsi Ti-E confocal microscope was used to

image the cells.

Cell Fixing for SEM. The same specimens used in cell

viability testing were later imaged using SEM. Samples

were rinsed twice with Hanks’ balanced salt solution

(HBSS; Thermo Scientific). Paraformaldehyde (Electron

Microscopy Sciences) was diluted with Hy Clone Hy

Pure molecular biology grade water (Thermo Scientific)

to make a 4% solution. The rinsed samples were then

immersed in the solution for 30 min. After that, the sam-

ples were dehydrated using a series of ethanol washes

(50, 80, 90; 100% ethanol for 30 min each), and finally

the dehydrated samples were dried in a vacuum desiccator

for 2 h before gold sputtering for SEM.

RESULTS AND DISCUSSION

FTIR Results

The stretching vibrations of NAH bonds located

around 3320 cm21 in the polyurethane was highly sensi-

tive to hydrogen bonding [33, 34]. As can be seen in Fig.

2a, the peak around 3320 cm21 shifted to a higher wave

number with the addition of GO in the scaffolds, indicat-

ing the existence of hydrogen bonding between GO and

polyurethane. The two characteristic peaks at 1701 cm21

and 1727 cm21, which belonged to the H-bonded C@O

and free C@O, were also observed in the FTIR spectra of

pure TPU and TPU/GO composites. The intensity of the

free stretching vibration of C@O became weaker with the

addition of GO, the peak of H-bonded C@O also shifted

to a lower wavenumber [35]. These results indicated that

there was an interaction between the backbone of TPU

and the carbonyl groups of GO. However, the interactions

were not linearly proportional to the content of GO in the

composites, which may be because some of the GO

formed aggregates in the composites.

Raman Spectra of GO and TPU/GO Composites

The Raman spectra of graphite oxide and pristine

graphite are shown in Fig. 3a. The pristine graphite had

three characteristic D, G, 2G bands around 1,360, 1,560,

and 2,700 cm21, respectively [36]. After acid treatment,

the Raman spectra showed an increase in the intensity of

the D band with respect to the G band. This indicates that

the structure of the graphite was altered by the oxidation

treatment. In the spectra of pure TPU (shown in Fig. 3b),

the peaks at 2,920, 2,864, and 2,801 corresponded to the

FIG. 2. (a) FTIR spectra of the TPU and TPU/GO composite scaffolds with different concentrations of GO;

(b) the enlarged part of the FTIR spectra in the range of 1,600–1,800 cm 21. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]


stretching vibrations of –CH2. The strong peak at 1615

cm21 was ascribed to the aromatic breathing mode of

vibration. The peaks located at 1,455 and 1,479 cm21

were due to the bending vibrations of –CH2 [37]. How-

ever, the intensity of these characteristic polymer bands

decreased or even disappeared with increased GO in the

composites. On the contrary, the D and G bands, which

belonged to the GO, could be clearly seen in all spectra

FIG. 3. Raman spectra of (a) graphite and graphite oxide and (b) TPU/GO composites. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 4. SEM results of TPU/GO composites: (a) neat TPU, (b) TPU–GO1%, (c) TPU–GO5%, and

(d) TPU–GO10%.


of the composites. The above observations may help to

define the interactions between TPU and GO, which were

also reported in the references [38].

The Morphology of the Composite Scaffold

The microstructures of the composite scaffolds were

examined using SEM as shown in Fig. 4. All composites

exhibited highly porous and interconnected microstruc-

tures with porosity exceeding 80%, which was desirable

for tissue engineering scaffold applications. As the con-

tent of GO increased, the microstructure became more

intact and the pores of the GO-containing composites had

a more spherical morphology than those of the pure TPU

scaffolds, which presented a more elongated structure.

Moreover, GO particles were embedded in the TPU

matrix, some emerged from the surface of the pores. This

resulted in a rough texture forming on the surface. To

investigate the dispersion of GO in the composites, high

magnification images were taken of the compressed

porous samples. Figure 5a shows the typical ultra large

GO sheets, which were �1 or 2 mm in lateral size. The

thickness of GO was about 1 nm, which is typical for a

GO monolayer. The morphology of TPU/GO composites

with different GO loadings is shown in Fig. 5b–d. As can

be seen in Fig. 5b, the GO sheets were nearly visible (as

indicated in the red ellipses) and the surface of the sam-

ple is quite smooth. For TPU-GO5% (cf. Fig. 5c), the GO

sheets tended to aggregate and increase in size as com-

pared to TPU-GO1%. In TPU-GO10% (cf. Fig. 5d), a

large number of parallel, stacked nanoplatelets were

observed. These results indicated that at higher loading

levels, GO sheets tended to aggregate and assume a more

planar conformation in the composites.

The porosity, average pore diameter, and pore density of

the scaffolds are shown in Fig. 6. It was found that the

porosity and pore density of the scaffolds increased linearly

with the addition of GO to the composites. This relationship

was even more evident in the variation of the pore density.

The pore diameter of the composites, however, showed an

inverse relationship to the content of GO. Similar findings

FIG. 5. The morphology of graphene oxide (GO) sheets and TPU/GO composites. (a) TEM image of virgin

GO. SEM images of (b) TPU-GO1%, (c) TPU-GO5%, (d) TPU-GO10%. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

FIG. 6. Porosity, pore diameter, pore density statistical results of TPU,

TPU–GO1%, TPU–GO5%, and TPU–GO10%. [Color figure can be

viewed in the online issue, which is available at



regarding the influence of fillers on composite microstructure

have been reported in the literature [39–41].

Thermal Properties of the Scaffolds

The thermal degradation behaviors of the specimens

were examined by TGA. Figure 7 shows the typical ther-

mograms of TGA and DTG curves. The initial weight

loss was accelerated in the presence of GO due to the

pyrolysis of the labile oxygen-containing functional

groups [42]. Compared with pristine TPU, the initial and

maximum weight loss degradation temperature (which is

clearly shown in Fig. 7b) of TPU in the composites

shifted to a higher temperature while the weight loss

decreased with an increase in GO loading. This indicated

an improvement in the thermal oxidation stability of the

polymeric material due to the barrier effects of the fillers

[33, 43].

DSC Analysis

DSC tests were carried out to investigate the effects of

GO on the crystallization behavior of TPU. As shown in

Fig. 8, compared with neat TPU, the non-isothermal onset

crystallization temperature (Tc) was a bit higher in the

composites, with a monotonic increase in Tc observed for

increased GO content. An increase in GO loading led to

wider and shallower exothermic curves, thus indicating

that the dispersed GO platelets not only acted as seeds for

faster nucleation, but also as barriers to the formation of

large crystallites. However, due to the low content of

hard segments in TPU, the changes between different

samples were not very evident. Jeong et al. reported that

the heat of fusion decreased as graphene sheet content

increased, which suggests that the crystallization of the

TPU hard segments was inhibited by the graphene sheets

[43].

FIG. 7. (a) Thermogravimetric analysis (TG) curves of TPU, TPU–GO1%, TPU–GO5%; TPU–GO10%.;

(b) DTG curves of TPU, TPU–GO1%, TPU–GO5%, and TPU–GO10%. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

FIG. 8. The DSC results of the neat TPU and TPU/GO composites: (a) cooling segment and (b) second

heating segment. [Color figure can be viewed in the online issue, which is available at



Mechanical Properties of TPU/GO Composite Scaffolds

Compression tests were performed on the specimens to

investigate the effects of GO on the mechanical properties

of TPU. Compression tests were carried out on five speci-

mens of each group using a universal testing machine

(Instron 5967) with a 100 kN load cell. The mechanical

properties of the composites were affected by the content

of GO, as shown in Fig. 9. Overall, the compressive

stress–strain curve exhibited linear elastic deformation,

where the stress sharply increased at small strains due to

the accommodation of material into scaffold pores [41].

In Fig. 9a, a significant increase in the strength range of

TPU/GO composites was observed compared with pure

TPU. The compressive modulus was pronouncedly

increased by nearly 200 and 300% with the addition of 5

and 10% GO, respectively, in comparison to neat TPU.

These results indicated that the incorporation of GO had

a reinforcing effect on the mechanical properties of the

composites. These effects were related to intrinsic proper-

ties of the GO nanolayers, the dispersion state of GO in

the matrix, interfacial interactions [44]. As for the TPU/

GO composites, the excellent mechanical properties of

graphene itself, as well as interactions between the GO

layers and the TPU molecules (as shown in the FTIR

FIG. 9. (a) Representative compressive stress–strain behavior of TPU/GO composite scaffolds with varied

GO loading levels; (b) the compressive modulus of TPU/GO composite scaffolds. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 10. Day 3 3T3 fibroblast cell culture results of freeze dried TPU (a, e), TPU–GO1% (b, f), TPU–

GO5% (c, g): TPU–GO10% (d, h) scaffolds: (a–d) are fluorescence microscope pictures (scale bar5100 lm)

where green indicates living cells and red indicates dead cells, (e–h) are SEM images (scale bar5200 lm).

[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


results), led to better stress transfer in the mechanical

tests. However, due to the limited dispersion of the GO

layers, the improvement for the composites was not as

good as that reported in [45].

Cell Viability Analysis and SEM Results of Cell Culture

Day 3 fluorescent images indicated that cells success-

fully attached onto the surfaces of the composite scaf-

folds (shown in Fig. 10). Compared with pristine TPU,

more live cells were detected on the TPU–GO1% com-

posite scaffold, which indicated that the addition of

small amounts of GO may have been beneficial for cell

attachment and proliferation on the scaffold. However,

the number of cells in the TPU–GO5% and TPU–

GO10% composites decreased sharply compared with

the TPU–GO1% specimen. After 10 days of cell culture,

the fluorescent images in Fig. 11 also showed that the

addition of GO had no obvious effect on the cell growth,

that multiple layers of cells had spread extensively over

the surface of all scaffolds. After 10 days, especially in

the TPU–GO1% specimen, the surface of the scaffold

was almost entirely covered by cell layers. By observing

the levels of EthD-1 in the images, it was noted that

there were no obvious variances for cell viability among

different levels of GO content in the Day 10 results. It

was found that in the cell culture results, a limited

amount of GO was better for cell growth and prolifera-

tion, which was also reported in the literature [23, 28].

However, too high of a concentration of GO may have

delayed cell proliferation, which was also reported in the

literature [46]. The reason for this requires further

investigation.

CONCLUSIONS

In this study, GO was successfully incorporated into

TPU to fabricate 3D highly porous and interconnected

scaffolds by thermally induced phase separation for the

first time. The effects of GO on the thermal properties,

mechanical properties, morphology, biocompatibility of

the composites were investigated in detail. FTIR and

Raman results indicated that there were some interactions

between TPU and GO sheets. Compression tests showed

that the compressive modulus of the composite scaffolds

was increased by about 300% when 10% GO was incor-

porated into the TPU matrix. Biocompatibility tests

showed that GO had no obvious toxicity to cell growth,

and an optimal loading of GO (at 1 wt%) was better for

cell proliferation, while higher concentrations delayed cell

proliferation. According to the above investigation, the

3D porous TPU/GO composite scaffolds prepared using

the TIPS method have the potential to be used in tissue

engineering applications.

ACKNOWLEDGMENTS

The first two authors would like to acknowledge the

China Scholarship Council for their financial support and

the Wisconsin Institutes for Discovery and Polymer Engi-

neering Center for the facilities to enable the first two

authors to study at the University of Wisconsin–Madison.

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