(489a) Synthesis and Characterization of Novel, In Situ Cross-Linkable, Biodegradable, Thermoresponsive Hydrogels for Bone Tissue Engineering

In situ forming hydrogels have the potential to be used in critical size bone defects as they are capable of delivering both encapsulated cells and growth factors while also filling a wide range of shaped and sized defects.  Novel thermoresponsive copolymers composed of n-isopropylacrylamide (NiPAAm), monoacryloxyethyl phosphate (MAEP), and acrylamide (AAm) monomers have been synthesized at a variety of compositions and molecular weights, yielding transition temperatures above 37^oC.  MAEP and AAm incorporation resulted in an increase in transition temperature in a concentration dependent manner, with AAm having about twice the effect of MAEP.  The phosphate groups of the MAEP were esterified via a reaction with the hydrophobic, epoxide containing molecule glycidyl methacrylate (GMA) lowering the transition temperature in a concentration dependent manner.  The modified copolymers were dissolved in phosphate buffered saline to form injectable liquid solutions, which undergo rapid thermogelation upon heating to 37^oC.  These hydrogels undergo further chemical cross-linking via free radical initiated polymerization of the attached methacrylate groups, preventing the hydrogel from undergoing post formation syneresis, which often occurs in poly(NiPAAm)-based polymers.  Degradation of these cross-links occurs via both hydrolytic and catalytic (by alkaline phosphatase) breaking of the phosphoester bonds, leaving behind small chains of hydrolyzed oligo(GMA), inorganic phosphate groups with calcium binding potential, and the unmodified copolymer with hydroxyethyl acrylate (HEA) in place of the MAEP groups. The effect of high and low levels of MAEP, AAm, GMA, and molecular weight on swelling, degradation, calcium binding ability, and cytotoxicty were evaluated.  The poly(NiPAAm-co-AAm-co-HEA) degradation products were synthesized and characterized as well to determine solubility at 37^oC.  Through this novel, dual gelation approach, in situ-forming biodegradable hydrogels with soluble degradation products were formed.