Soft, Responsive Microactuators and Color-Changing Skins Enabled by Networks of Microgels

Hierarchical systems can overcome operational constraints by balancing the size, geometry, and connectivity of individual elements across multiple length scales. For example, the macroscopic actuation of skeletal muscle is enabled by the microscopic displacements of thousands of individual myofibrils—coordinated motion is facilitated by a hierarchical structure consisting of bundles of muscle fibers (which house the myofibrils), blood vessels, and connective tissue. This construction enables: 1) rapid delivery/removal of chemical fuels and waste by minimizing diffusion pathlengths, 2) alignment of individual contractile elements for concerted motion, and 3) natural damage tolerance. These characteristics are desired in responsive soft systems but that are difficult to produce, especially with bulk materials. Inspired by this challenge, we are developing strategies that mimic the properties of hierarchical systems, like muscle, using synthetic materials. Specifically, we report the design and fabrication of micro-structured hydrogel (microgel) networks with stimuli-responsive contractile capabilities which, when combined with microfluidic encapsulation, provide chemically controlled actuation under ambient conditions. Like biological muscle, these devices provide for active transport of signaling compounds/fuel, programmable motions, and resilience against damage as each microgel operates independently. We will describe two applications enabled by microgel networks: 1) free-standing soft actuators featuring thermal/ionic/molecular control and 2) color-changing skins based on thermally responsive synthetic chromatophores. These demonstrations illustrate the broad potential of microgel networks in the design of new, multifunctional/responsive materials supportive of adaptive to autonomous behaviors broadly applicable to the fields of soft robotics, adaptive surfaces and optics, and human-machine interfaces.