Comparison of Crosslinker Types and Initiation Systems of Thermoresponsive PNIPAM Hydrogels

Abstract

Thermoresponsive hydrogels are three-dimensional polymer networks which undergo conformational changes in aqueous media depending on the external temperature. As the lower critical temperature (LCST) is close to the body temperature, poly(N-isopropylacrylamide) (PNIPAM) is the main thermoresponsive hydrogel used for biomedical applications. Below LCST, PNIPAM hydrogels swell in aqueous media, above LCST they become insoluble and shrink. This behavior makes it possible to design drug release systems controlled by external temperature. Swelling/shrinking response of PNIPAM hydrogel depends on several factors such as crosslinker type, crosslinking density, hydrophobic/hydrophilic balance and initiator type. In this study, the effects of the initiation system and the crosslinker type on different thermoresponsive hydrogels were compared. For this purpose, thermoresponsive hydrogels were synthesized by using ethylene glycol dimethylacrylate (EGDMA) and N,N′-ethylene bisacrylamide (EBAM) as crosslinkers via photo and thermal initiation systems. The hydrogels were characterized by scanning electron microscope (SEM) and FTIR spectroscopy. Effects of the initiation system and the crosslinker type on the release, swelling behavior, morphology and the biocompatibility behavior of the hydrogels were investigated. The hydrogels synthesized with EBAM demonstrated more promising results compared to the one synthesized EGDMA. It was concluded that poly(EBAM-co-NIPAM)-P has the highest swelling ratio and poly(EBAM-co-NIPAM)-T is the most biocompatible hydrogel. In terms of release characteristics, there was not a significant difference between the hydrogels, even though their swelling characteristics differ.