Silk fibroin degumming affects scaffold structure and release of macromolecular drugs

Silk fibroin (SF) is a natural polymer with tremendous potential as a matrix for drug delivery systems as well as for tissue engineering. Silk sericin (SS) removal (degumming) is a critical step during SF purification, potentially affecting SF integrity and resulting in structural changes such as partial hydrolysis and inhibition of micelle formation. In addition to SF composition itself, the molecular weight and charge of encapsulated drugs may significantly affect drug release from SF matrices. The effect of these parameters on drug release was investigated by varying SF degumming time and charge of the model compound encapsulated in SF films. With increasing degumming time, average SF molecular weight decreased, molecular weight distribution became broader and formation of SF micelles was impaired. However, β-sheet content was not affected by degumming time, suggesting that degradation occurred mainly in hydrophilic domains of SF. The release of differently charged dextran derivatives, used as macromolecular model drugs, was significantly affected by SF degumming. Release of neutral dextran increased with increasing degumming time. In contrast, negatively charged dextran showed an inverse effect potentially due to reduced SF charge density with increased degumming time. Interestingly, positively charged dextran were shown to partly form polyelectrolyte complexes with SF by isothermal titration calorimetry but also exhibited phase separation during film drying resulting in fast burst release. These results demonstrate that both, SF preparation as well as drug charge significantly affect drug release from SF matrices.