Harnessing the power of chemically active sheets in solution

In living organisms, diffusing chemicals react with soft tissue to actuate a range of mechanical actions, such as periodic pulsations or net motion. Inspired by this biological behaviour, researchers have created chemically active ‘microswimmers’ that translate chemical input into mechanical work. Because typical microswimmers are hard spheres (that is, zero-dimensional objects) and rigid or flexible rods (1D objects), their movements are constrained relative to higher dimensional, soft matter. In recent years, these limitations have motivated research on 2D active sheets that undergo shape changes or directed motion in response to chemical cues in solution; the combination of dimensionality, chemical activity and structural flexibility leads to new, rich dynamic behaviour. In one class of materials, chemical reactions generate internal stresses in the 2D sheets that trigger the shape transformations; in the second class, the release of energy from chemical reactions drives a fluid flow, which not only transports but also morphs the shape of the sheet. Consequently, these reconfigurable, self-propelled sheets can perform self-sustained operations that were unattainable with previously studied active particles, including self-morphing and self-oscillating behaviour. The latter functionalities can lead to the creation of portable, chemical machines and the discovery of new modes of dynamic self-assembly.