Chitosan Hydrogel-Capped Porous SiO2 as a pH Responsive Nano-Valve for Triggered Release of Insulin

A pH responsive, chitosan-based hydrogel film is used to cap the pores of a porous SiO2 layer. The porous SiO2 layer is prepared by thermal oxidation of an electrochemically etched Si wafer, and the hydrogel film is prepared by reaction of chitosan with glycidoxypropyttrimethoxysilane (GPTMS). Optical reflectivity spectroscopy and scanning electron microscopy (SEM) confirm that the bio-polymer only partially infiltrates the porous SiO2 film, generating a double layer structure. The optical reflectivity spectrum displays Fabry-Perot interference fringes characteristic of a double layer, which is characterized using reflective interferometric Fourier transform spectroscopy (RIFTS). Monitoring the position of the RIFTS peak corresponding to the hydrogel layer allows direct, real-time observation of the reversible volume phase transition of the hydrogel upon cycling of pH In the range 6.0-7.4. The swelling ratio and response time are controlled by the relative amount of GPTMS in the hydrogel. The pl-l-dependent volume phase transition can be used to release insulin trapped in the porous SiO2 layer underneath the hydrogel film. At pH 7.4, the gel In the top layer effectively blocks insulin release, while at pH 6.0 Insulin penetrates the swollen hydrogel layer, resulting in a steady release into solution.