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

## TL;DR

This paper explores how different salts affect the strength and performance of protein-based underwater adhesives made from agricultural byproducts.

## Contribution

The study reveals how salt concentration and type can control adhesive properties, offering a novel approach to improve underwater adhesion.

## Key findings

- Moderate salt concentrations increase bond strength, while high concentrations decrease it.
- Salts that stabilize protein structures create stiffer gels but reduce adhesion.
- Silica fillers enhance αLa gel strength but reduce BSA gel strength when combined with salts.

## Abstract

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.