Rheology and Biocompatibility of Poly(lactide)-poly(ethylene oxide)-poly(lactide) Hydrogels
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Rheology and Biocompatibility of Poly(lactide)-poly(ethylene oxide)-poly(lactide) Hydrogels Sarvesh K. Agrawal1, Kyuong S. Chin1, Naomi Sanabria-Delong2, Khaled A. Aamer2, Heidi Sardinha1, Gregory N. Tew2, Susan C. Roberts1, and Surita R. Bhatia1 1 Department of Polymer Science and Engineering, University of Massachusetts Amherst 120 Governors Drive, Amherst, MA 01003, U.S.A. 2 Department of Chemical Engineering, University of Massachusetts Amherst 686 North Pleasant Street, Amherst, MA 01003 U.S.A. ABSTRACT We report rheological data on hydrogels formed from triblock copolymers of poly(Llactide) (PLLA) and poly(ethylene oxide) (PEO). We are able to create gels with elastic moduli greater than 10,000 Pa, which is an order of magnitude higher than previously achieved with related physically associated gels of similar chemistry. Moreover, the value of the elastic modulus strongly depends on PLLA block length, offering a mechanism to control the mechanical properties as desired for particular applications. Additionally, we have developed protocols for using these materials for cell encapsulation and present preliminary cell viability studies for encapsulated human liver cells (HepG2 cell line). Our results have implications for the design of new materials for soft tissue engineering, where native tissues have moduli in the kPa range.
INTRODUCTION Copolymers of poly(L-lactide) (PLLA) and poly(ethylene oxide) (PEO) have been studied extensively as biomaterials since the initial report by Cohn [1] because of the biodegradability of the polyester and the biocompatibility of PEO [2-11]. These materials are known to form hydrogels of varying weight %, depending on the exact architecture, with transitions near body temperature. Amphiphilic block copolymers composed of poly(lactide) (PLA) and PEO have been prepared with PEO end- or mid-blocks and characterized as gels [1215] by vial inversion methods. Because these studies measure the sol-gel transition by vial inversion, they do not provide detailed mechanical properties [12-15]. For the triblock architecture of interest here, PLA-PEO-PLA, previous reports involve polymers with end-blocks composed of lactic and glycolic acid (PLGA), with the architechture PLGAx-PEO1000-PLGAx. Here, x, the degree of polymerization (DP), was varied both in block length and lactide to glycolide ratio [12]. These materials exhibited sol-gel transitions between 10-23 °C and gel-sol transitions from 27-48 °C depending on concentration and x [12]. The same group showed that polymers composed of D,L lactide formed gels with a sharper gel-sol transition compared to PLGA polymers [13]. More recently, mechanical rheology on PMG1400-PEO1450-PMG1400 (PMG = poly(D,L-3-methyl-glycolide)) showed the elastic modulus of a 27 wt % sample is less than 500 Pa [16]. Further, Kimura and co-workers reported hydrogel formation from PLLA1300PEO4600-PLLA1300 stereocomplexed with a triblock containing poly(D-lactide) (PDLA), PDLA1090-PEO4600-PDLA1090, in which 10 wt % solutions had an elastic modulus up to 1000
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