Mechanisms of controlled release from silk fibroin films.

The controlled release of fluorescein-iso-thio-cyanate (FITC)-labeled dextrans from methanol-treated and untreated silk fibroin films was modeled to characterize the release kinetics and mechanisms. Silk films were prepared with FITC-dextrans of various molecular weights (4, 10, 20, 40 kDa). Methanol treatment was used to promote crystallinity. The release data were assessed with two different models, an empirical exponential equation commonly fit to release data and a mechanism-based semiempirical model derived from Fickian diffusion through a porous film. The FITC-dextran release kinetics were evaluated as a function of molecular weight and compared between the untreated- and methanol-treated films. For the empirical model, the estimated values of the model parameters decreased with the molecular weight of the analyte and showed no significant difference between untreated- and methanol-treated films. For the diffusion-based model, the estimated diffusion coefficient was smaller for the methanol-treated films than for the untreated films. Also, the diffusion coefficient was observed to decrease linearly with increasing molecular weight of the analyte. The percent of FITC-dextran loading entrapped and not released was less for the methanol-treated films than for untreated films and linearly increased with molecular weight. A linear regression was fit to the relationship between molecular weight and the percent of entrapped FITC-dextran particles. Using these defined linear relationships, we present an updated version of the diffusion model for simulating release of FITC-dextran of varied molecular weights from methanol-treated and untreated silk films.