Dual-Mode Cross-Linking Enhances Adhesion of Silk Fibroin Hydrogels to Intestinal Tissue

Abstract

Compared to conventional wound closure methods like sutures and staples, polymer-based tissue adhesives afford some distinct advantages, such as greater ease of deployment in spatially constrained surgical sites. One way to achieve aqueous adhesion is by introducing catechol functional groups that form coordinate and covalent bonds with a variety of substrates. This approach, inspired by marine organisms, has been applied to biopolymers and synthetic polymers, but one key challenge is that compositions that are soluble in water are often susceptible to high swelling ratios that can result in undesired compression of neighboring tissues. This work sought to synthesize aqueous adhesive gels that are capable of two modes of association: (1) adhesion and covalent cross-linking reactions arising from catechol oxidation and (2) noncovalent cross-linking arising from self-assembly of polymer backbones within the gelled adhesive. The network’s self-assembly after gelation was envisioned to afford control over swelling and reinforce its strength. Bombyx mori silk fibroin was selected as the backbone of the adhesive network because it can be processed into an aqueous solution yet later be rendered insoluble in water through the assembly of its hydrophobic protein core. Distinct from a previous approach to functionalize silk directly with catechol groups, this work investigated in situ generation of catechol on silk fibroin by enzymatically modifying phenolic side chains, where it was found that this enzymatic approach led to conjugates with higher degrees of catechol functionalization and aqueous solubility. Silk fibroin was functionalized with tyramine to enrich the protein’s phenolic side chains, which were subsequently oxidized into catechol groups using mushroom tyrosinase (MT). The gelation of the silk conjugates with MT was monitored by rheology, and the gels exhibited low water uptake. Phenolic enrichment increased the rate of chemical cross-linking leading to gelation but did not interrupt assembly of silk’s secondary structures. Adhesion of the tyramine–silk conjugates to porcine intestine was found to be superior to fibrin sealant, and induction of β sheet secondary structures was found to further enhance adhesive strength through a second mode of cross-linking. Neither the chemical functionalization nor phenol oxidation affected the ability of intestinal epithelial cells (Caco-2) to attach and proliferate. Phenolic functionalization and oxidative cross-linking of silk fibroin was found to afford a new route to water-soluble, catechol-functionalized polymers, which were found to display excellent adhesion to mucosal tissue and whose secondary structure provides an additional mode to control strength and swelling of adhesive gels.

Functional silk fibroin hydrogels: preparation, properties and applications

“…Tyramine can be oxidized in situ by MT to generate the catechol groups. In a related study, Danielle L et al 113 used two methods to combine tyramine with SF, which improved the adhesion of the gel and finally formed the SFbased adhesive. The adhesive could be used for the porcine intestine adhesion, and the strength of the adhesive bond was noted to be stronger than the bond achieved using the commercially available fibrin glue.…”

Mechanistic Insights into Silk Fibroin’s Adhesive Properties via Chemical Functionalization of Serine Side Chains

“…Silk fibroin (SF), a natural polymeric protein, lends itself well to the development of the next generation of adhesive. Performant, biocompatible silk-based irreversible adhesives have previously been reported. ,− This polymer can be naturally sourced from silkworm cocoons (i.e., Bombyx mori) which would translate to a cost-effective raw material and manufacturing . It is easily processable and can be versatile in its physical presentation (i.e., as solution, film, gel, or powder) which would mean product and application versatility .…”

Mussel-Inspired Catechol Functionalisation as a Strategy to Enhance Biomaterial Adhesion: A Systematic Review

“…Using a similar enzyme-based strategy, Heichel et al [ 32 ] described an improved approach for the in situ functionalisation of SF with catechol groups, towards its application in wound healing. Using a multi-step protocol, the authors initially modified purified SF by carboxylation, to increase the carboxylate content of SF for reactions with tyramine.…”