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.