Mechanics of stretchable electronics and soft machines

In the emerging field of soft machines, large deformation of soft materials is harnessed to provide functions such as regulating flow in microfluidics, shaping light in adaptive optics, harvesting energy from ocean waves, and stretching electronics to interface with living tissues. Soft materials, however, do not provide all of the requisite functions; rather, soft machines are mostly hybrids of soft and hard materials. In addition to requiring stretchable electronics, soft machines often use soft materials that can deform in response to stimuli other than mechanical forces. Dielectric elastomers deform under a voltage. Hydrogels swell in response to changes in humidity, pH, temperature, and salt concentration. How does mechanics meet geometry, chemistry, and electrostatics to generate large deformation? How do molecular processes affect the functions of transducers? How efficiently can materials convert energy from one form to another? These questions are stimulating intriguing and useful advances in mechanics. This review highlights the mechanics that enables the creation of soft machines.