We introduce a novel phase-changing particulate that amplifies a composite's modulus change in response to thermal stimulus. This particulate additive consists of a low melting point alloy (Field's Metal; FM) formed into microparticles using a facile fabrication method, which enables its incorporation into polymer matrices using simple composite manufacturing processes. The effect of the solid-liquid phase change of the FM particles is demonstrated in two host materials: a thermally responsive epoxy and a silicone elastomer. In the epoxy matrix, this thermal response manifests as an amplified change in flexural modulus when heated, which is highly desirable for stiffness-changing move-and-hold applications. In the silicone matrix, the stretchability can be switched depending on the phase of the FM particles. This phenomenon allows the silicone to stretch and hold a strained configuration, and gives rise to mechanically programmable anisotropy as the FM inclusions are reshaped. FM particles present many opportunities where on-demand tunable modulus is required, and is particularly relevant to soft robotics. Because the melting temperature of FM is relatively close to room temperature, triggering the phase change, and thereby modulating the modulus, can be accomplished with low power consumption. We demonstrate the utility of these FM particle-containing composites as variable stiffness and variable stretchability elements targeting applications in the field of soft robotics.

Fabrics are ubiquitous materials that have conventionally been passive assemblies of interlacing, inactive fibers. However, the recent emergence of active fibers with actuation, sensing, and structural capabilities provides the opportunity to impart robotic function into fabric substrates. Here we present an implementation of robotic fabrics by integrating functional fibers into conventional fabrics using typical textile manufacturing techniques. We introduce a set of actuating and variable-stiffness fibers, as well as printable in-fabric sensors, which allows for robotic closed-loop control of everyday fabrics while remaining lightweight and maintaining breathability. Finally, we demonstrate the utility of robotic fabrics through their application to an active wearable tourniquet, a transforming and load-bearing deployable structure, and an untethered, self-stowing airfoil.

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Variable stiffness in elastomers can be achieved through the introduction of low melting point alloy particles, such as Field's metal (FM), enabling on‐demand switchable elasticity and anisotropy in response to thermal stimulus. Because the FM particles are thermally transitioned between solid and liquid phases, it is beneficial for the composite to be electrically conductive so the stiffness may be controlled via direct Joule heating. While FM is highly conductive, spherical particles contribute to a high percolation threshold. In this paper, it is shown that the percolation threshold of FM particulate composites can be reduced with increasing particles aspect ratio. Increasing the aspect ratio of phase‐changing fillers also increases the rigid‐to‐soft modulus ratio of the composite by raising the elastic modulus in the rigid state while preserving the low modulus in the soft state. The results indicate that lower quantities of high aspect ratio FM particles can be used to achieve both electrical conductivity and stiffness‐switching via a single solution and without introducing additional conductive fillers. This technique is applied to enable a highly stretchable, variable stiffness, and electrically conductive composite, which, when patterned around an inflatable actuator, allows for adaptable trajectories via selective softening of the surface materials.

microscale actuation. However, these material-based actuators are often slow or only expand or contract axially, preventing use in applications that require volumetric expansions. To address such limitations of existing actuating materials, actuators based on the liquid-to-gas phase change of solvent inclusions encapsulated in a hyperelastic matrix have been realized. Phasechange actuators are capable of inducing rapid volumetric expansion similar to fluidic actuators, without being tethered to an external fluid source. [29][30][31][32] Accepting the advantages of phasechange actuators, we now turn our attention to granular media. Granular assemblies, consisting of discrete grains, can be tuned to accomplish a variety of different responses. For example, favorable self-assembly of granular media can be achieved via modulating interparticle interactions and external stimuli, [33][34][35][36] and granular assemblies with different packing configurations have been shown to yield different bulk properties. [36][37][38][39] Furthermore, granular media can exhibit optimized changes in stiffness during jamming transitions, enabling tunable moduli. [16,[40][41][42][43][44] When unjammed, the bulk material is compliant, and when jammed, the bulk material can achieve the stiffness of the grains. Finally, when mixed into carrier fluids, granular media can impart thixotropic behavior on the carrier fluid due to dynamic jamming, [45][46][47][48] enabling 3D printing of previously unprintable materials. With all the advantageous properties granular media has to offer, the technology has seen application in soft robotics, [49][50][51][52] medical devices, [53][54][55] flexible airfoils, [56] and programmable aggregate architecture. [57] Here, we bring together the unique advantages of phasechange soft actuators and granular media to introduce soft granular actuators made of discrete, volumetrically expanding grains. A single active grain consists of multiple solvent cores encapsulated in a hyperelastic silicone shell (Ecoflex 00-30). At elevated temperatures, the encapsulated solvent vaporizes and increases the internal pressure of the hyperelastic shell, inducing volumetric expansion of the grain. The grains are independently capable of rapid, high-force microscopic actuation, and are also easily arranged into granular assemblies to form larger-scale bulk actuators. Furthermore, agglomerates of active grains can self-assemble from disordered arrangements to conform around objects and exhibit variable moduli. Finally, the use of grains suspended in a carrier solvent or resin enables compatibility with granular self-assembly and 3D printing techniques, offering the potential to print volumetrically expanding actuators into freeform patterns across scales.Recent work has demonstrated the potential of actuators consisting of bulk elastomers with phase-changing inclusions for generating high forces and large volumetric expansions. Simultaneously, granular assemblies have been shown to enable tunable properties via different packing...