Microwave-assisted fast and efficient dissolution of silkworm silk for constructing fibroin-based biomaterials

Top-down extraction of surface carboxylated-silk nanocrystals and application in hydrogel preparation

Bionanomaterial based hydrogels originated from natural biopolymer have drawn much attention for advanced applications. However, nanosilk-based hydrogels derived from top-down approaches remain in their infancy. First, nanosilks based on existing methods fail to prepare hydrogels; second, both nanosilk extraction and surface modification remain a challenge due to high crystallinity and sophisticated hierarchical structures. To produce nanosilk-based hydrogels, pretreatment and oxidation are necessary. In this work, pretreatments were conducted first to loosen the sophisticated structures of natural silk fibers, NaClO oxidation was utilized in succession to introduce carboxyl groups onto silk fibroin. Combined with moderate mechanical disintegration, silk nanocrystals with additional carboxyl groups were prepared facilely. Finally, silk nanocrystal-based hydrogels were prepared successfully through gas phase coagulation. An optimization of pretreatment approaches and oxidation conditions was carried out. The morphologies, chemical and crystalline structures of original, pretreated and oxidized silk fibroin as well as nanofibrillated silk were investigated. In addition, the silk nanocrystal-based hydrogel exhibited outstanding mechanical properties compared to those of dissolved and regenerated silk fibroin-based hydrogels. Moreover, silk nanocrystal-based aerogels present highly porous, interconnected, and crisscrossed network nanostructures, which are ideal candidates for tissue regeneration and provide new prospects as porous scaffolds for bioengineering applications.

Silk Fibroin-Based Materials for Catalyst Immobilization

Silk fibroin is a widely and commercially available natural protein derived from silkworm cocoons. Thanks to its unique amino acid composition and structure, which lead to localized nanoscale pockets with limited but sufficient hydration for protein interaction and stabilization, silk fibroin has been studied in the field of enzyme immobilization. Results of these studies have demonstrated that silk fibroin offers an important platform for covalent and noncovalent immobilization of enzymes through serving as a stabilization matrix/support with high retention of the biological activity of the enzymes of interest. In the hope of providing suggestions for potential future research directions, this review has been written to briefly introduce and summarize key advances in silk fibroin-based materials for immobilization of both enzymes/biocatalysts (including alkaline phosphatase, β-glucosidase, glucose oxidase, lipase, urease, uricase, horseradish peroxidase, catalase, xanthine oxidase, tyrosinase, acetylcholinesterase, neutral protease, α-chymotrypsin, amylase, organophosphorus hydrolase, β-galactosidase, carbonic anhydrase, laccase, zymolyase, phenylalanine ammonia-lyase, thymidine kinase, and several others) and non-enzymatic catalysts (such as Au, Pd, Fe, α-Fe2O3, Fe3O4, TiO2, Pt, ZnO, CuO, Cu2O, Mn3O4, and MnO2).

Self-Assembled Regenerated Silk Fibroin Microsphere-Embedded Fe3O4 Magnetic Nanoparticles for Immobilization of Zymolyase

Cytoplasm of Saccharomyces cerevisiae yeast cells contains a significant amount of desired intracellular products for both industrial utility and academic research. To recover intracellular compounds, it is necessary to break the yeast cells with high efficiency, which, under certain circumstances, requires the use of the lytic enzyme zymolyase to completely digest the cell walls. A promising strategy for zymolyase immobilization on silk fibroin (SF) was developed. SF/Fe3O4 magnetic microspheres (MMs) were constructed by solvent (ethanol)-induced self-assembly of SF surrounding Fe3O4 magnetic nanoparticles (MNs), which were synthesized by a coprecipitation method. Zymolyase was covalently bonded on the surface of the SF/Fe3O4 MMs by a photochemical cross-linking method to produce robust biocatalysts of zymolyase/SF/Fe3O4. The chemical, magnetic, and morphological properties of the MM supports and the immobilized zymolyase were investigated. Enzymolysis results demonstrated that the immobilized zymolyase showed good activity and stability for digesting yeast cell walls, and the biocatalyst can be readily recycled through convenient magnetic separation for reuse. At the optimum pH = 7.5, the immobilized zymolyase maintained 84% of the activity of the free zymolyase and retained 41% of its initial activity after four times of reuse. At unfavorable acidic pH = 4, the immobilized zymolyase retained 81% of its initial activity, while the free zymolyase showed no significant activity. Consequently, the SF/Fe3O4 MMs exhibit superior performance in terms of immobilizing enzymes, which have a good prospect in the biological application.

Design of Self-Healing and Electrically Conductive Silk Fibroin-Based Hydrogels

Self-healing and electrically conductive silk fibroin (SF)-based hydrogels were developed based on the dynamic assembly/disassembly nature of supramolecular complexes and the conductive nature of polypyrrole (PPy). The self-healing properties of the hydrogels were achieved through host-guest interactions between β-cyclodextrin and amino acid side chains (tyrosine, tryptophan, phenylalanine, and histidine) on SF. PPy deposition was achieved via in situ polymerization of pyrrole using ammonium persulfate as an oxidant and laccase as a catalyst. The PPy-coated hydrogels behaved as an elastomer and displayed excellent electrical properties, with adjustable electrical conductivities ranging from 0.8 ± 0.2 to (1.0 ± 0.3) × 10^-3 S·cm^-1. Furthermore, possibility of potential utilization of the hydrogels in electrochemistry applications as flexible yet self-healable electrode materials was explored. This study not only shows great potential in expanding the role of silk-based devices for various applications but also provides a useful approach for designing multifunctional self-healing protein-based hydrogels.