Credit: © 2008 RSC

Stimuli-responsive hydrogels can be used as chemo–mechanical transducers to convert chemical energy into mechanical motion. By coupling a prototypical nonlinear oscillating reaction with such a hydrogel, Shu-ichi Shinohara and co-workers from Nagoya University and the University of Tokyo have created1 a system that could use the peristaltic motion of the gel to transport cargo.

The Belousov–Zhabotinsky (BZ) reaction can display oscillations in the concentration of its reaction intermediates. It requires a transition metal catalyst, which cycles between an oxidized and reduced state, and various reagents including sodium bromate. The reaction can result in spatial patterns where oscillations travel as waves of 'excitation' with a specific wavelength. By creating a polymeric hydrogel that includes a ruthenium-based BZ catalyst, Shinohara and colleagues were able to couple the BZ reaction to a stimulus response, creating waves of excitation that travelled across the gel causing it to move 'like a worm'.

The excited portion of the gel is swollen when the catalyst is oxidized but shrinks back to its original shape after the wave passes. The wave moves onto the next region of unexcited gel causing it to swell creating a peristalsis-type motion. The period and amplitude of the waves, and thus the motions of the gel, can be controlled by light and also by altering the initial sodium bromate concentration. If harnessed precisely the motion of the gel could be used to deliver cargo using lab-on-a-chip devices or the gels could act as microactuators.