Toughening Wet-Spun Silk Fibers by Silk Nanofiber Templating

Hierarchical design of silkworm silk for functional composites

Silk-reinforced composites (SRCs) manifest the unique properties of silkworm silk fibers, offering enhanced mechanical strength, biocompatibility, and biodegradability. These composites present an eco-friendly alternative to conventional synthetic materials, with applications expanding beyond biomedical engineering, flexible electronics, and environmental filtration. This review explores the diverse forms of silkworm silk fibers including fabrics, long fibers, and nanofibrils, for functional composites. It highlights advancements in composite design and processing techniques that allow precise engineering of mechanical and functional performance. Despite substantial progress, challenges remain in making optimally functionalized SRCs with multi-faceted performance and understanding the mechanics for reverse-design of SRCs. Future research should focus on the unique sustainable, biodegradable and biocompatible advantages and embrace advanced processing technology, as well as artificial intelligence-assisted material design to exploit the full potential of SRCs. This review on SRCs will offer a foundation for future advancements in multifunctional and high-performance silk-based composites.

Self-reinforced silk nanofibrils networks enable ultrafine fibroin monofilament sutures applied in minimally invasive surgery

Superfine recycled regenerated silk fibroin (RRSF) monofilament sutures are designed to minimize surgical incisions, prevent scar formation, and facilitate precise suturing in cosmetic and plastic surgeries. However, balancing fineness and strength remains a challenge because the RRSF extraction process disrupts the hierarchical structure of natural silk fibers, leading to suboptimal mechanical properties. This study employed mechanical peeling and grinding to exfoliate β-sheets-rich silk fibroin nanofibrils (SFNF), which served as homologous reinforcing materials uniformly dispersed in the RRSF spinning system. Water-soluble, biocompatible polyvinyl alcohol (PVA) was introduced to enhance ductility. The fiber underwent extensive water immersion stretching to produce ultrafine fibers and foster a highly oriented internal self-reinforcing SFNF network structure. Monofilament surgical sutures were successfully fabricated following surface coating with chitosan. Our results showed that with 0.1 wt% SFNF and a stretching multiplier of 2.5, the tensile strength of the fibers increased by 33 % compared to those without SFNF. Fluorescence staining confirmed the presence of highly oriented SFNF networks within the fibers. Mechanical simulations validated the pronounced reinforcement effect of this network structure. The sutures, measuring 39.38 μm in diameter and exhibiting a tensile strength of 0.31 N, met USP standards for 9-0 surgical sutures, making them suitable for microsurgery. Additionally, they demonstrated antibacterial properties, biocompatibility, and degraded at a rate of 43.09 % within 30 days. In animal trials, the sutures facilitated wound closure, reduced inflammatory responses, and minimized scar formation.

Recreating Silk's Fibrillar Nanostructure by Spinning Solubilized, Undegummed Silk

The remarkable toughness (>70 MJ m-3) of silkworm silk is largely attributed to its hierarchically arranged nanofibrillar nanostructure. Recreating such tough fibers through artificial spinning is often challenging, in part because degummed, dissolved silk is drastically different to the unspun native feedstock found in the spinning gland. The present work demonstrates a method to dissolve silk without degumming to produce a solution containing undegraded fibroin and sericin. This solution exhibits liquid-liquid phase separation above 10% (wt/wt), a behavior observed in the silk gland but not in degummed silk solutions to date. This partitioning enhances the stability of the undegummed solution, delaying gelation two-fold compared with degummed silk at the same concentration. When spun under identical conditions, undegummed solutions produces fibers 8× stronger and 218× tougher than degummed silk feedstocks. Through ultrasonication, undegummed wet spun fibers are seen to possess hierarchical structure of densely packed ≈20 nm nanofibrils, similar to native silks, although completely absent from fibers wet-spun from degummed silk solutions. This work demonstrates that the preservation of molecular weight, presence of sericin and stimulation of liquid-liquid phase separation underpin a new pathway to recreate a hierarchical fiber with structures akin to native silk.

Continuous Wet Spinning of Regenerated Silk Fibers from Spinning Dopes Containing 4% Fibroin Protein

The wet spinning of fibers from regenerated silk fibroin has long been a research goal. Due to the degradation of the molecular structure of the fibroin protein during the preparation of the regenerated silk fibroin solution, fibroin concentrations with at least 10% protein content are required to achieve sufficient viscosity for wet spinning. In this study, a spinning dope formulation of regenerated silk fibroin is presented that shows a rheological behavior similar to that of native silk fibroin isolated from the glands of B. mori silkworm larvae. In addition, we present a wet-spinning process that enables, for the first time, the continuous wet spinning of regenerated silk fibroin with only 4% fibroin protein content into an endless fiber. Furthermore, the tensile strength of these wet-spun regenerated silk fibroin fibers per percentage of fibroin is higher than that of all continuous spinning approaches applied to regenerated and native silk fibroin published so far.

Advances in Preparation and Properties of Regenerated Silk Fibroin

Over the years, silk fibroin (SF) has gained significant attention in various fields, such as biomedicine, tissue engineering, food processing, photochemistry, and biosensing, owing to its remarkable biocompatibility, machinability, and chemical modifiability. The process of obtaining regenerated silk fibroin (RSF) involves degumming, dissolving, dialysis, and centrifugation. RSF can be further fabricated into films, sponges, microspheres, gels, nanofibers, and other forms. It is now understood that the dissolution method selected greatly impacts the molecular weight distribution and structure of RSF, consequently influencing its subsequent processing and application. This study comprehensively explores and summarizes different dissolution methods of SF while examining their effects on the structure and performance of RSF. The findings presented herein aim to provide valuable insights and references for researchers and practitioners interested in utilizing RSF in diverse fields.

Bioinspired Processing of Keratin into Upcycled Fibers through pH-Induced Coacervation

Keratin is an important byproduct of the animal industry, but almost all of it ends up in landfills due to a lack of efficient recycling methods. To make better use of keratin-based natural resources, the current extraction and processing strategies need to be improved or replaced by more sustainable and cost-effective processes. Here, we developed a simple and environmentally benign method to process extracted keratin, using HCl to induce the formation of a coacervate, a separate aqueous phase with a very high protein concentration. Remarkably, this pH-induced coacervation did not result in the denaturation of keratin, and we could even observe an increase in the amount of ordered secondary structures. The low-pH coacervates could be extruded and wet-spun into high-performance keratin fibers, without requiring heating or any organic solvents. The secondary structure of keratin was largely conserved in these regenerated fibers, which exhibited excellent mechanical performance. The process developed in this study represents a simple and environmentally friendly strategy to upcycle waste keratin into high-performance materials.