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Swelling and Degradation Characteristics of Novel Biodegradable in Situ Crosslinkable Poly(Lactide-Ethylene Oxide-Fumarate) Terpolymer Networks

Esmaiel Jabbari1, Xuezhong He2, and Alireza S. Sarvestani2. (1) University of South Carolina, Dept. of Chemical Eng., Rm. 2C11, Swearingen Engineering Center, Columbia, SC 29208, (2) Chemical Engineering, University of South Carolina, Swearingen Engineering center Rm 1B30, 301 South Main Street, Columbia, SC 29208

Introduction: Biomaterials with a wide range of degradation times are attractive as scaffolds in tissue engineering applications. Our laboratory has synthesized a novel poly(lactide-ethylene oxide-fumarate) (PLEOF) terpolymer composed of short lactide and ethylene oxide chains linked by unsaturated fumarate units. The ULMW PLGA blocks are FDA approved for certain clinical applications, fumaric acid occurs naturally in the Kreb's cycle, and poly(ethylene oxide) units do not elicit an immune response. The objective of this work was to investigate the swelling and degradation characteristics of the PLEOF terpolymer networks.

Methods: Difunctional hydroxyl terminated short lactide chains were first synthesized by melt ring-opening polymerization of L-lactide (LA) monomer with diethylene glycol (DEG) as the initiator and tin II-ethyl hexanoate as the catalyst. The molar ratio of LA to DEG was varied from 10 to 30 to produce low molecular weight PLA (LMWPLA) chains with number average molecular weights (Mn) in the range of 1000 to 4000 Dalton. The synthesized LMWPLA was characterized by 1H-NMR, FTIR, and gel permeation chromatography (GPC). The polydispersity index of PLA was 1.5-1.6 independent of the PLA molecular weight. The degree of crystallinity of PLA was also independent of PLA molecular weight in the Mn range of 1000 to 4000 Dalton. The melting point of the semi-crystalline PLA, measured by DSC, depended on the molecular weight of the LMW PLA. PLEOF was synthesized by condensation polymerization of low MW PLA, poly(ethylene glycol) (PEG), and fumaryl chloride (FuCl) with triethylamine (TEA) as the catalyst. FuCl was purified by distillation at 161„aC and PEG was dried by azeotropic distillation from toluene. The molar ratio of FuCl:PLA+PEG and TEA:PLA+PEG were 0.9:1.0 and 1.8:1.0, respectively. PLEOF macromer was synthesized using PEG with Mn ranging from 1 to 5 kD and PLA with Mn ranging from 1 to 7 kD. The weight ratio of PEG to PLA was varied from 100/0 to 85/15 to produce hydrophilic water-soluble terpolymers. In a typical reaction, the dried PEG and LMW PLA were dissolved in methylene chloride under dry nitrogen atmosphere in a three-neck reaction flask. The reaction vessel was placed in an ice bath to limit the temperature rise of the exothermic reaction. Next, FC and TEA each dissolved were added dropwise to the reaction with stirring. After the addition of FC and TEA, reaction was continued for 6 h under ambient conditions. After completion of the reaction, solvent was removed by and residue was dissolved in anhydrous ethyl acetate. The mixture was kept at 5„aC for 12 h for complete precipitation of the by-product triethylamine hydrochloride and the salt was removed by filtration. Ethyl acetate was removed by vacuum distillation at 30„aC. The macromer was re-dissolved in methylene chloride, precipitated twice in ice cold ethyl ether, and dried before use. The structure of PLEOF macromer was characterized by 1H-NMR and FTIR. The presence of peaks at 6.90 ppm in the NMR spectrum attributable to hydrogens of the fumarate group, and presence of a band due to the ester carbonyl stretching vibration centered at 1725 cm-1 in the FTIR spectra, confirmed the incorporation of fumarate monomers into the PLEOF macromer. The ratio of the peaks in the NMR spectrum of PLEOF due to chemical shifts centered at 5.1 ppm (due to the one hydrogen attached to the methine group of the lactide monomer) and 3.6 ppm (due to the four methylene hydrogens (CH2-CH2-O-) of ethylene oxide repeat units) was related to the molar ratio of the PLA to PEG in the terpolymer. For 10% by weight PLA in the feed, this ratio was 0.055, corresponding to 5.2% by mole and 8.7% by weight of the PLA in the terpolymer. Therefore, the copolymer reactivity of PLA with fumaryl chloride was slightly less than that of PEG. The PLEOF macromer with PLA and PEG molecular weights of 3.3 kD (PI of 1.6) and 3.4 kD (PI of 1.3) had Mn and PI of 6.3 kD and 2.9, respectively, as determined by gel permeation chromatography (GPC). Polymers were prepared using PLEOF as the degradable macromer, methylenebisacrylamide (MBIS) as the crosslinking agent, and a neutral redox initiation system. The redox system consisted of ammonium persulfate (APS) and tetramethylethylenediamine (TMEDA), respectively. The disk-shaped samples were used for swelling, cell viability, cell function, and degradation studies. Results: Our results demonstrate that the water content, mesh size, and degradation characteristics of these novel terpolymers can be controlled independently by the molecular weight of PEG, the weight ratio of PLA to PEG, and the molecular weight of PLA, respectively. For example, the weight swelling ratio in water ranged from 0.1 to 6 for PLA:PEG ratios of 0.1 to 0.9, respectively. When the PLA content was increased from 10 to 20, 30, 40, 50, 60, 70, 80, and 90%, The weight loss after 21 days changed from 17 to 24, 58, 85, 53, 32, 31, 15, and 10%, respectively. Conclusion: These novel degradable Poly(lactide-ethylene oxide-fumarate) terpolymers are potentially useful as injectable in-situ crosslinkable cell carriers with tunable degradation time in tissue regeneration.