A Granular Actuator Made of Electrically Conductive Grains

Phase-changing composites used for soft robotic actuation typically consist of an elastomer matrix containing pockets of low boiling point solvent. Under heat, the solvent undergoes a phase-change from liquid to gas, producing a rapid volumetric expansion of the bulk composite. Recently, such phase-changing composites have been discretized from a bulk composite into sub-millimeter-sized grains, called granular actuators. Granular actuators have multiple solvent cores and individually expand by up 700% when heated, opening possibilities for multi-scale actuation and tuneable actuator bulk properties. A single granular actuator can be used for microscale actuation, while aggregates for macroscale actuation. Granular actuators can also be jammed (solid-like) or flow (liquid-like) by modulating external pressure, and bulk actuator moduli can be tuned via the packing crystallinity of the actuating grains. However, granular actuators currently require a co-located or external heat source, which is difficult to integrate into a field-deployable soft robot and leads to relatively non-uniform heating that is difficult to control. To solve this, we fabricated granular actuators wherein each grain is coated in a conductive shell, imbuing electrical conductivity and Joule heating. We confirmed the conductivity of random packings made of conductive granular actuators, and further observed distributed actuation of individual grains throughout the packing. In this talk, I will discuss how the conductive coating impacts the mechanical and actuation metrics for individual grains, and how varying particle size can enhance or diminish the electrical conductivity of the particles. In the future, this system could be used for addressable granular actuators, with control over the actuation of an assembly on a localized scale. We also foresee this system being used as a tool to study and visualize the distribution of current through an assembly of deformable and active grains.