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Fibrillar adhesives composed of fibers with non-circular cross-sections and contacts, including squares and rectangles, offer advantages that include a larger real contact area when arranged in arrays and simplicity in fabrication. However, they typically have a lower adhesion strength compared to circular pillars due to a stress concentration at the corner of the non-circular contact. We investigate the adhesion of composite pillars with circular, square and rectangular cross-sections each consisting of a stiff pillar terminated by a thin compliant layer at the tip. Finite element mechanics modeling is used to assess differences in the stress distribution at the interface for the different geometries and the adhesion strength of different shape pillars is measured in experiments. The composite fibrillar structure results in a favorable stress distribution on the adhered interface that shifts the crack initiation site away from the edge for all of the cross-sectional contact shapes studied. The highest adhesion strength achieved among the square and rectangular composite pillars with various tip layer thicknesses is approximately 65 kPa. This is comparable to the highest strength measured for circular composite pillars and is about 6.5× higher than the adhesion strength of a homogenous square or rectangular pillar. The results suggest that a composite fibrillar adhesive structure with a local stress concentration at a corner can achieve comparable adhesion strength to a fibrillar structure without such local stress concentrations if the magnitude of the corner stress concentrations are sufficiently small such that failure does not initiate near the corners, and the magnitude of the peak interface stress away from the edge and the tip layer thickness are comparable. 

Many soft robotic components require highly stretchable, electrically conductive materials for proper operation. Often these conductive materials are used as sensors or as heaters for thermally responsive materials. However, there is a scarcity of stretchable materials that can withstand the high strains typically experienced by soft robots, while maintaining the electrical properties necessary for Joule heating ( e.g. , uniform conductivity). In this work, we present a silicone composite containing both liquid and solid inclusions that can maintain a uniform conductivity while experiencing 200% linear strains. This composite can be cast in thin sheets enabling it to be wrapped around thermally responsive soft materials that increase their volume or stretchability when heated. We show how this material opens up possibilities for electrically controllable shape changing soft robotic actuators, as well as all-silicone actuation systems powered only by electrical stimulus. Additionally, we show that this stretchable composite can be used as an electrode material in other applications, including a strain sensor with a linear response up to 200% strain and near-zero signal noise. 

Materials with tunable properties, especially dynamically tunable stiffness, have been of great interest for the field of soft robotics. Herein, a novel design concept of robust three‐component elastomer–particle–fiber composite system with tunable mechanical stiffness and electrical conductivity is introduced. These smart materials are capable of changing their mechanical stiffness rapidly and reversibly when powered with electrical current. One implementation of the composite system demonstrated here is composed of a polydimethylsiloxane (PDMS) matrix, Field's metal (FM) particles, and nickel‐coated carbon fibers (NCCF). It is demonstrated that the mechanical stiffness and the electrical conductivity of the composite are highly tunable and dependent on the volume fraction of the three components and the temperature, and can be reasonably estimated using effective medium theory. Due to its superior electrical conductivity, Joule heating can be used as the activation mechanism to realize ≈20× mechanical stiffness changes in seconds. The performance of the composites is thermally and mechanically robust. The shape memory effect of these composites is also demonstrated. The combination of tunable mechanical and electrical properties makes these composites promising candidates for sensing and actuation applications for soft robotics.