Evaluating proteins release from, and their interactions with, thermosensitive poly (N-isopropylacrylamide) hydrogels

Poly (N-isopropylacrylamide) (PNIPAAm) hydrogels possess a lower critical solution temperature (LCST) at around 32 °C. When the external temperature is raised above the LCST, the hydrogels experience abrupt and drastic shrinkage. This unique property makes them very useful for biomedical applications such as on–off switches for modulated drug delivery and tissue engineering. The aim of this work was to study the potential of using PNIPAAm hydrogels for protein delivery, and to obtain basic understandings of the protein–gel interactions as well as their effect on protein loading and release. PNIPAAm gels were synthesized with different crosslinker contents. The effects of crosslinker content, in vitro release temperature, protein loading level and molecular size as well as temperature cycling on protein release from PNIPAAm gels were examined. Greater amount of BSA was loaded using gels fabricated with lower crosslinker contents and loading solution with higher concentrations of BSA. An incomplete release of encapsulated BSA from the gels was observed in all cases. Enhanced mass transfer created by oscillating swelling–deswelling in response to temperature cycling across the LCST and lowering in vitro release temperature did not promote BSA release because of strong BSA–gel interactions. Evidence for the residual BSA in the gels after in vitro release was provided by dyeing the gels with protein determination reagent and shift in the LCST of the gels. In contrast, insulin release was much faster and more complete when compared to BSA because of its smaller size. The protein–gel interactions were analysed by investigating the LCST of, and state of water in, the blank and protein-loaded hydrogels.