Exploiting gradients in cross-link density to control the bending and self-propelled motion of active gels

Oscillating polymer gels undergoing the Belousov-Zhabotinsky (BZ) reaction provide an ideal medium for probing the interplay between chemical energy and mechanical action. Inspired by recent experiments, we use computational modeling to determine how gradients in crosslink density across the width of a sample can drive long, thin BZ gels to both oscillate and bend, and thereby undergo concerted motion. Free in solution, these samples move forward (in the direction of lower cross-link density) through a rhythmic bending and unbending. Our simulations allow us to not only isolate optimal ranges of parameters for achieving this distinctive behavior but also provide insight into the dynamic coupling between chemical and mechanical energy that is needed to produce the self-sustained motion. We then model samples that are mechanically constrained by their attachment to a flat, rigid surface. By varying the concentration of the reagents in the solution, we show that the undulations of the sample's free end can be significantly modified, so that the overall motion can be directed either upwards or downwards. The findings from these studies provide guidelines for creating autonomously moving objects, which can be used for robotic or microfluidic applications.