Tuning hydrogel mechanics using the Hofmeister effect

The mechanical properties of hydrogels are commonly modified by changing the concentration of the molecular components. This approach, however, does not only change hydrogel mechanics, but also the microstructure, which in turn alters the macroscopic properties of the gel. Here, the Hofmeister effect is used to change the thermoresponsiveness of polyisocyanide hydrogels. In contrast to previous Hofmeister studies, the effect is used to change the phase transition temperatures and to tailor the mechanics of the thermoresponsive (semiflexible) polymer gels. It is demonstrated that the gel stiffness can be manipulated over more than two orders of magnitude by the addition of salts. Surprisingly, the microstructure of the gels does not change upon salt addition, demonstrating that the Hofmeister effect provides an excellent route to change the mechanical properties without distorting other influential parameters of the gel. Salts are known to change the interactions between water and polymers and consequently change transition temperatures, an effect known as the Hofmeister effect. It is shown that beyond phase transition temperatures, ions can be used to tailor the linear and nonlinear mechanical behavior of hydrogels. A hundredfold stiffness increase is realized with gels of identical morphology