Dynamics of optical and anisotropic mechanical responses in hydrogels

Materials that perform complex chemical sensing are ubiquitous in living systems, and would transform developments in biomedicine, environmental monitoring, and many other areas. We introduce a continuum mechanical framework that predicts the minimal set of components needed to integrate dynamical signal processing capabilities into simple hydrogels without structural modifications. For a common polyacrylic acid hydrogel, with copper cations and acid as representative chemical stimuli, the theory explains the experimentally observed (i) two-dimensional traveling waves of copper induced blue color selectively indicating a slowly progressing acid stimulus, (ii) anisotropic mechanical responses depending on the direction of acid progression, associated with tilting of an array of embedded passive microplates that report local hydrogel deformations. These results suggest simple hydrogels have a much larger sensing space than is currently made use of.