Effect of Salts on the Aggregation and Strength of Protein-Based Underwater Adhesives

While hydrophobic underwater adhesives have often been desired for their ability to remove water from interfaces, their inherent immiscibility with water can also hinder their use. Water-based adhesive systems can lead to improved wetting, lower toxicity, and exhibit dynamic physical responses to aqueous chemistries in the environment. For protein-based adhesives, simple aqueous salts can dramatically alter the intra- and intermolecular forces driving interactions between proteins and with surfaces. Here, we investigate the effect of four main salts found in seawater, NaCl, KCl, MgCl₂, and CaCl₂ on underwater curing adhesives made from two agricultural byproduct proteins, bovine serum albumin (BSA), and bovine α-Lactalbumin (αLa). We demonstrate that salts can significantly impact the adhesion of protein-based adhesives, increasing bond strength at moderate salt concentrations but decreasing at higher concentrations. Calorimetry and rheology experiments show that high ionic strengths hasten gelation time to form weaker materials with lower adhesion, while moderate salt concentrations slow protein aggregation to produce stiffer materials with higher bond strengths. The addition of silica fillers increased the bond strength of salt-containing αLa gels but decreased the bond strength of BSA gels. In general, salts that stabilized native protein structures formed stiffer gel networks but tended to decrease adhesion compared to salts with destabilizing effects. When combining simple salts and protein-based adhesives, we demonstrate control over nearly all attributes of adhesive curing and strength as an effective means to improve underwater adhesion.