Comparative adhesion of chemically and physically crosslinked poly(acrylic acid)-based hydrogels to soft tissues

In the design of polymeric biomaterials for soft tissue adhesion, careful regulation of the interactions between the material and soft tissue is critical. In order to improve the efficacy of bioadhesives, a greater understanding of the relationship between their chemical design and resulting adhesion mechanisms is required. In this work, poly(acrylic acid) (PAA) was functionalised either through bromoalkene functionalisation via nucleophilic substitution or via 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM)-mediated conjugation. Novel PAA-based hydrogels were developed through different types (chemistry) of crosslinks and crosslinking mechanisms. Two of these gels were crosslinked through UV initiation using Irgacure 2959 as the photoinitiator, another gel used visible light-mediated crosslinking with eosin Y as the photoinitiator, and the final gel utilised physical crosslinking through the interaction between boronic acid moieties and the polysaccharide mannan. Oscillatory rheometry was used to characterise the mechanical properties of the different gels, including their gelation kinetics. Tensile testing was used to characterise the adhesion of the different gels to hydroxyl and methacrylate self-assembled monolayers (SAMs), as a means of studying adhesion to interfaces with defined surface chemistry. Finally, adhesion to soft tissues (porcine epicardium and keratinized gingiva) was studied through lap shear and tensile testing. Our results indicate a variety of factors responsible for hydrogel/soft tissue adhesion; these include the tissue mechanics, bulk mechanics of the gel, weak non-specific adhesion forces, and strong covalent bonding. Tests confirm the effect of tissue biochemistry on adhesion, with thiyl-bonded gels displaying particularly strong adhesion to tissues (particularly the epicardium). It is hoped that the results presented provide insight for the improved rational design of biomaterials for tissue adhesion.