The Hofmeister Effect on Agar Hydrogels with Mechanical Tunability and Molecular Mechanism

Owing to their biocompatibility and thermal responsiveness, Agar hydrogels are extensively applied in chemistry and biology fields. However, their fixed water content and rigid sugar ring structure normally exhibit limited mechanical strength, while introducing additional networks possibly deteriorates the intrinsic thermoreversible cross-linking properties of Agar hydrogel. In this work, we achieve the mechanical enhancement and tunability of Agar-based single-network hydrogels based on the Hofmeister effect via a preforming postimmersion method without the need for supplementary networks. After being immersed in different solutions of the Hofmeister salt series, the tensile strength and toughness of Agar hydrogels can be regulated between 54.7–412.1 kPa and 5.5–94.1 kJ m^–3. Macroscopic and microscopic analyses via SEM and SAXS, together with molecular dynamics simulations, were employed to reveal the systematic mechanisms from the number of hydrogen bonds to the aggregation state and ultimately to the mechanical properties. Since the gelation of Agar relies on double-helix formation, the Hofmeister series and regulation behaviors are different from typical synthetic polymer hydrogels. These results further promoted the elucidation of the water state regulation in the hydration layer of Agar hydrogels. This work provides an understanding of the correlation between the cross-linking state of molecular chains and the resultant Agar hydrogel properties based on the Hofmeister effect, which inspires research on the mechanical regulation mechanisms of natural polysaccharide-based hydrogels.