Biodegradable Thermoresponsive Hydrogels for Aqueous Encapsulation and Controlled Release of Hydrophilic Model Drugs

A series of hydrogels with both thermoresponsive and completely biodegradable properties was developed for aqueous encapsulation and controlled release of hydrophilic drugs in response to temperature change. The hydrogels were prepared in phosphate-buffered saline (pH 7.4) through free radical polymerization of N-isopropylacrylamide (NIPAAm) monomer and a dextran macromer containing multiple hydrolytically degradable oligolactate−2-hydroxyethyl methacrylate units (Dex−lactateHEMA). Swelling measurement results demonstrated that four gels with feeding weight ratios of NIPAAm:Dex−lactateHEMA = 7:2, 6:3, 5:4, and 4:5 (w/w) were thermoresponsive by showing a lower critical solution temperature at approximately 32 °C. The swelling and degradation of the hydrogels strongly depended on temperature and hydrogel composition. An empirical mathematical model was established to describe the fast water absorption at the early stage and deswelling at the late stage of the hydrogels at 37 °C. Two hydrophilic model drugs, methylene blue and bovine serum albumin, were loaded into the hydrogels during the synthesis process. The molecular size of the drugs, the hydrophilicity and degradation of the hydrogels, and temperature played important roles in controlling the drug release.