Controlled release of lipase from Candida rugosa loaded into hydrogels of N-isopropylacrylamide and itaconic acid

The series of poly(N-isopropylacrylamide-co-itaconic acid) hydrogels, with lipase from Candida rugosa as a model protein, were synthesized by free radical copolymerization. The composition of hydrogels was varied by monomers ratio, crosslinking agent concentration and amounts of lipase, which was loaded by in situ polymerization. All samples were characterized regarding morphology. The investigation of hydrogel swelling properties revealed their pH and temperature sensitive character. Protein loading efficiency, release profiles and the specific activity yield of the released lipase were also investigated as a function of hydrogel composition, protein content and pH, at the physiological temperature of 37°C. Copolymers of N-isopropylacrylamide and itaconic acid presented high lipase loading efficiency. Another very important feature of these copolymers was that the protein release kinetic strongly depended on the pH value of the medium. The diffusion exponents values around 1 denoted that these hydrogel compositions could be adjusted to follow near zero-order kinetics. Namely, hydrogel formulations released low amounts of lipase at pH 2.20, but much higher released protein quantities were observed at pH 6.80 enabling these copolymers to be attractive candidates as site specific protein oral drug delivery systems.