Development of uniform chitosan thin-film layers on silicon chips

Nature creates selectively refractive materials in butterfly wings and crab shells by alternating layers of chitin with different refractive indices. To mimic this construction, control of both thickness and refractive index of chitosan films is required. Methods were developed for adhering the chit osan layer to glass and silicon, polymerizing the chitosan, stabilizing the chitosan films against environmental changes in temperature and humidity, and depositing a uniform chitosan layer with defined thickness. First, comm ercial chitosan was hydrolyzed to obtain lower molecular weight polymers an d further purified. Then chitosan layers were prepared on clean silicon chi ps pretreated with polyvinyl butyral resin for adhesion. The chitosan solut ion with a polymerizing component and a plasticizer consisted of 2.5% purif ied chitosan/2.5% Resimene/0.25% tetraethylene glycol. Spin-coating at vary ing speeds after an incubation period of 1 h at room temperature produced c hitosan layer thicknesses in the ideal range of 1200-2000 Angstrom. The dif ferent thicknesses were reproducible, with a standard deviation across the film between 3 and 5%. The experimental index of refraction calculated for these layers was 1.59.