and reversibly change rigidity is also attractive for artificial muscle actuators, [5,6] which are becoming increasingly suitable for wearable devices. The goal of rigidity tuning has been addressed using methods like solvent interactions, [7] pneumatic jamming, [8,9] electrostatic adhesion, [10] antagonistic actuator architectures, [11][12][13] fluidic flexible matrix composites, [14] phase-change materials, [15][16][17][18][19][20][21][22][23][24][25][26][27] and magnetorheological fluids. [28] The diversity of these methods results in an equally diverse range of technical challenges, such as long activation times, high activation voltages, [10,15] limited scalability and structural versatility, [11][12][13]26,27] and a dependence on bulky auxiliary equipment. [8,9,14,17,28] At present, there remains to be an electrically powered method for reversible rigidity tuning that exhibits <5 s, <20 V, actuation in a size-scalable architecture that allows for integration into a wide range of systems.In this work, we introduce a rigiditytuning material architecture that changes stiffness in response to moderate electrical voltage (Figure 1a,b and Video S1 (Supporting Information)). Furthermore, we demonstrate its feasibility in both tensile and flexural applications, via an active tendon in an underactuated robotic finger model [29] and a moldable splint (Figure 1c,d). The tendon consists of a conductive thermoplastic elastomer (cTPE) coated with a ≈10-140 µm layer of spray-deposited eutectic gallium-indium liquid metal alloy (EGaIn) [30] and embedded in a silicone matrix ( Figure 1b). Applying voltage to the EGaIn electrodes causes electric current to travel through the cTPE element. This induces rapid Joule heating in the cTPE, bringing it to the melting temperature, above which it softens and can no longer support a tensile load. When current is removed, the element cools and solidifies, and its stiffness is restored. Previously, shape memory polymers (SMPs) [16,[18][19][20][23][24][25]31] and low-melting-point alloys [20][21][22]26,27,32] have also been incorporated into reversible, stiffness-based adhesives and rigidity-tuning elements. Although some of these methods may have very large stiffness change ratios, they typically require external heating equipment or long activation times ( Table 1). Shan et al. exploited Joule heating to directly electrically activate cTPE; however, this technique required activation voltages of above 100 V. [15] We build on previous work with cTPE by introducing a novel design, in which a pair of liquid metal electrodes is oriented on opposite sides of the cTPE. This configuration minimizes the An electrically responsive composite is introduced that exhibits muscle-like changes in elastic stiffness (≈1-10 MPa) when stimulated with moderate voltages (5-20 V). The stiffness-tuning element contains an embedded layer of conductive thermoplastic elastomer (cTPE), composed of a propylene-ethylene copolymer and a percolating network of carbon black. Two opposite surfaces of the cTPE layer ar...
