Multi-stimuli-responsive poly(hydroxyethyl methacrylate-co-N-vinyl pyrrolidone-co-methacrylic acid-co-N-isopropylacryl amide) hydrogel: synthesis, characterization and application in drug release

Multi-stimuli-responsive hydrogels are being increasingly studied due to their sensitive response to small stimuli and wide applications in biosensors and drug delivery. In this study, a multi-stimuli-responsive hydrogel, poly(hydroxyethyl methacrylate-co-N-vinyl pyrrolidone-co-methacrylic acid-co-N-isopropylacryl amide) [poly(HEMA-co-NVP-co-MAA-co-NIPA)], was synthesized by radical polymerization and characterized by Fourier transform infrared (FTIR) and C-13 NMR spectroscopy techniques. With the aids of scanning electron microscopy (SEM) characterization, it was confirmed that the sensitive stimuli-responsive behavior of the hydrogel stemmed from its microstructure variation with those external stimulus. Rheological study showed that the hydrogel had rheological feature of typical elastomer. Compression tests revealed that the poly(hydroxyethyl methacrylate-co-N-vinyl pyrrolidone) [poly(HEMA-co-NVP)] played an important role in enhancing the compressive modulus of such hydrogel. More interestingly, the equilibrium swelling ratio (ESR) studies further confirmed that the composite hydrogel displayed response sensitively to the stimulus of temperature, pH, and ionic strength. Herein, theophylline as a drug model was adopted due to the multi-stimulus properties of hydrogels, which were a potential candidate for drug loading and delivering. Releasing drug continuously in a given period was dependent on the characteristics of solution and loading time. The mechanisms for drug release from the hydrogels were studied by Ritger-Peppas model.