Hofmeister effect induced advancement of the hydrogels by 3D printing

This study explores the cooperative effect of the Hofmeister phenomenon and three-dimensional (3D) printing in synthesizing high-strength hydrogels. We discovered that, when the printed hydrogels were exposed to a high salt concentration solution, the Hofmeister effect instigated robust and more compact interface interactions between the PHEMA/PVP covalent matrix and the gelatin. This significantly improved the mechanical attributes of the hydrogels. Upon comparison with the newly formed hydrogels, the tensile strength, fracture elongation, toughness, and modulus of the hydrogels subjected to the salting-out effect were amplified by factors of 20, 4, 110, and 70 respectively, thus demonstrating their exceptional mechanical performance. The study further reveals that the Hofmeister effect can augment the mechanical anisotropy of the printed hydrogels. In particular, Hofmeister ions trigger a reversible entanglement in the polymer chain. In this condition, adjacent filaments in a single printed layer of hydrogel display more densely entangled chains than the interior of the filaments, resulting in enhanced toughness in different orientations of the hydrogel. The approach described herein is not only applicable to current hydrogel materials but also can be extended to other printable hydrogel materials. By integrating 3D printing and the Hofmeister effect, we can customize the properties of hydrogels to satisfy specific requirements, facilitating the creation of advanced biomedical devices and applications.