Dynamically in situ tunable, highly robust and sensitive ionically conductive hydrogels enabled by the Hofmeister effect for potential sensor arrays

For soft electronic applications, regulated mechanical properties and adequate electrical conductivity often help fulfill the specific requirements of flexible materials across diverse applications. Herein, a strategy to broadly and reversibly modulate the mechanical and electrical properties of hydrogels by leveraging the Hofmeister effect is proposed, which alters the aggregation state of polymer chains through the modulation of ionic interactions. By manipulating the effects of cations and anions, the hydrogel's mechanical and electrical parameters can be tuned continuously and reversibly over a wide range. It is also crucial to emphasize that the ions function solely as performance modifiers, primarily facilitating polymer chain aggregation, without participating in the composition or stability of the hydrogel. The dynamic in situ modulation of hydrogel mechanical properties (strength, toughness and elastic modulus) is achieved via a simple cyclic soaking and de-soaking approach, representing enhancements of 36, 184, and 6.9 times, respectively, compared to its initial state. This advancement effectively resolves the long-standing challenge of balancing stiffness, toughness, and conductivity in polymer hydrogels. Furthermore, the sensor can be applied as a wearable device and an array sensor. This work provides a simple and effective route for developing hydrogels with a wide range of dynamically in situ tunable, highly robust and sensitive properties.