(190g) Modeling and Gelation Kinetics of Injectable in Situ Crosslinkable Poly(Lactide-Ethylene Oxide-Fumarate) Hydrogel Networks

Hydrogels due to their hydrophilicity and high water content, coupled with minimally invasive arthroscopic techniques are an attractive cell carrier for treating irregularly shaped defects with minimum tissue dissection and retraction. After injection and hardening in-situ, these three dimensional matrices guide the organization, differentiation, proliferation, and development of seeded cells into the desired tissue. Although preliminary results are promising for naturally derived hydrogels, their low mechanical properties, their pathogenicity, and their limited availability has prompted researchers to investigate the use of synthetic biodegradable and injectable hydrogels. An ideal hydrogel as a cell carrier should have controlled swelling ratio, mesh size and crosslink density, and degradation characteristics. In this work, we present synthesis and characterization of a novel degradable terpolymer hydrogel composed of short lactide and ethylene oxide chains linked by unsaturated fumarate units. 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. 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 60/40 to produce hydrophilic water-soluble terpolymers. The structure of PLEOF macromer was characterized by 1H-NMR and FTIR. 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). Hydrogels 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. Our results demonstrate that the water content, mesh size, and degradation characteristics of these novel terpolymer hydrogels can be controlled independently by the molecular weight of PEG, the weight ratio of PLA to PEG, and the molecular weight of PLA, respectively. These novel degradable Poly(lactide-ethylene oxide-fumarate) terpolymers are potentially useful as injectable in-situ crosslinkable cell carriers in tissue regeneration. In this project, the gelation kinetics of the PLEOF/BISAM hydrogel was measured by rheometry and a kinetic model was developed for the gelation process. The gelation process is monitored by following the viscoelastic parameters throughout the in situ crosslinking (in the rheometer). The PEG/PLA ratio of PLEOF had a non-monotonic effect on the ultimate stiffness modulus and the gel time of the hydrogels. Gels prepared in the absence of BISAM crosslinker, with the ratio of PEG/PLA=70/30 had the optimum properties (higher final modulus and lower gel time). The rate of crosslinking and the gel time were strongly dependant on the concentration of initiator and accelerator. However, samples prepared with high APS/TEMED concentration (0.02 M) were found to exhibit lower final storage modulus, possibly due to the cyclization process. When the concentration of BISAM is increased, the cross-over from viscous to elastic behavior occured at an earlier time. The ultimate modulus of the gels were found to increase monotonically with crosslinker concentration. The structure of the network was assumed as a homogeneous distribution of PLEOF chains, crosslinked through the fumarate groups by short radical BISAM chains. The validity of this scheme was examined by a kinetic model for the reaction. Model predictions are consistent with most of the experimental findings (see figure). The extracted kinetic rate constants indicated that the reaction of BISAM chains with fumarates was diffusion controlled due to the low mobility of PLEOF macromolecules as well as lower reactivity of fumarate double bonds.