Tuning Chemical and Physical Cross-Links in Silk Electrogelsfor Morphological Analysis and Mechanical Reinforcement.

Electrochemically controlled, reversibleassembly of biopolymersinto hydrogel structures is a promising technique for on-demand cellor drug encapsulation and release systems. An electrochemically sol–geltransition has been demonstrated in regenerated Bombyxmorisilk fibroin, offering a controllable way togenerate biocompatible and reversible adhesives and other biomedicalmaterials. Despite the involvement of an electrochemically triggeredelectrophoretic migration of the silk molecules, the mechanism ofthe reversible electrogelation remains unclear. It is, however, knownthat the freshly prepared silk electrogels (e-gels)adopt a predominantly random coil conformation, indicating a lackof cross-linking as well as thermal, mechanical, and morphologicalstabilities. In the present work, the tuning of covalent and physicalβ-sheet cross-links in silk hydrogels was studied for programmingthe structural properties. Scanning electron microscopy (SEM) revealeddelicate morphology, including locally aligned fibrillar structures,in silk e-gels, preserved by combining glutaraldehyde-cross-linkingand ethanol dehydration. Fourier transform infrared (FTIR) spectroscopicanalysis of either electrogelled, vortex-induced or spontaneouslyformed silk hydrogels showed that the secondary structure of silk e-gels was tunable between non-β-sheet-dominated andβ-sheet-dominated states. Dynamic oscillatory rheology confirmedthe mechanical reinforcement of silk e-gels providedby controlled chemical and physical cross-links. The selective incorporationof either chemical or physical or both cross-links into the electrochemicallyresponsive, originally unstructured silk e-gel shouldhelp in the design for electrochemically responsive protein polymers. [ABSTRACT FROM AUTHOR]