Hydrogels made from proteins (e.g., gelatin) are commonly used as drug and nutrient delivery carriers, tissue engineering scaffolds, and soft device matrixes due to the biocompatibility and degradability. Because of the water-rich nature, protein-based hydrogels, like many other hydrogels, also suffer from weak mechanics. In this study, we systematically investigated the Hofmeister effect (i.e., ion-specific effect) on the mechanics of protein-based hydrogels with both fine-stranded and particulate microstructures. It revealed that, by simply soaking fine-stranded protein hydrogels (e.g., gelatin gel) into kosmotropic ion solution, the compressive fracture stress could be enhanced from several hundred kPa to over 10 MPa, with the fracture strain increased from 80% to over 99%. On the other hand, when soaked in chaotropic salt solutions, the gels softened and dissolved. The protein hydrogels with particulate structures, however, were insensitive to the salt treatment. This effect was further confirmed with different types of protein gels and salt ions, which confirmed the generality of the Hofmeister effects on protein gels. Further analysis with Raman spectroscopy and phase diagrams revealed that the hydrophobic interactions between fine-stranded protein chains were significantly enhanced during the salt treatment which was the main reason for the enhanced mechanical strength. At last, we added polysaccharide, chitosan, to freeze these hydrophobic interactions after washing off the salt. A mechanically strong, stable, and salt-free gelatin-chitosan hydrogel was produced with mechanical strength surpassing most of the gelatin-based hydrogels.

Tuning the mechanics of protein hydrogels through the Hofmeister effect

ACS Fall 2021

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