Shape memory alloys (SMAs) are popular as actuators for soft bioinspired robots because they are naturally compliant, have high work density, and can be operated using miniaturized on‐board electronics for power and control. However, SMA actuators typically exhibit limited bandwidth due to the long duration of time required for the alloy to cool down and return to its natural shape and compliance following electrical actuation. This challenge is addressed by constructing SMA‐based actuators out of thermally conductive elastomers and examining the influence of electrical current and actuation frequency on blocking force, bending amplitude, and operating temperature. The actuator is composed of a U‐shape SMA wire that is sandwiched between layers of stretched and unstretched thermal elastomer. Based on the studies, the ability is demonstrated to create a highly dynamic soft actuator that weighs 3.7 g, generates a force of ≈0.2 N, bends with curvature change of ≈60 m−1 in 0.15 s, and can be activated with a frequency above 0.3 Hz with a pair of miniature 3.7 V lithium–polymer batteries. Together, these properties allow the actuator to be used as an “artificial muscle” for a variety of tethered and untethered soft robotic systems capable of fast dynamic locomotion.

By using compliant lightweight actuators with shape memory alloy, we created untethered soft robots that are capable of dynamic locomotion at biologically relevant speeds.

Tunable dry adhesion has a range of applications, including transfer printing, climbing robots, and gripping in automated manufacturing processes. Here, a novel concept to achieve dynamically tunable dry adhesion via modulation of the stiffness of sub-surface mechanical elements is introduced and demonstrated. A composite post structure, consisting of an elastomer shell and a core with a stiffness that can be tuned via application of electrical voltage, was fabricated. In the nonactivated state, the core is stiff and the effective adhesion strength between the composite post and contact surface is high. Activation of the core via application of electrical voltage reduces the stiffness of the core, resulting in a change in the stress distribution and driving force for delamination at the interface and a reduction in the effective adhesion strength. The adhesion of composite posts with a range of dimensions were characterized and activation of the core was shown to reduce the adhesion by as much as a factor of six. The experimentally observed reduction in adhesion is primarily due to the change in stiffness of the core. However, the activation of the core also results in heating of the interface and this plays a secondary role in the adhesion change.

Fig. 1. The top 100 simulated 2-by-2-by-2 configurations of passive (cyan) and volumetrically-actuating (red) voxels (a) were manufactured in reality (b).

Like their natural counterparts, soft bioinspired robots capable of actively tuning their mechanical rigidity can rapidly transition between a broad range of motor tasks-from lifting heavy loads to dexterous manipulation of delicate objects. Reversible rigidity tuning also enables soft robot actuators to reroute their internal loading and alter their mode of deformation in response to intrinsic activation. In this study, we demonstrate this principle with a three-fingered pneumatic gripper that contains "programmable" ligaments that change stiffness when activated with electrical current. The ligaments are composed of a conductive, thermoplastic elastomer composite that reversibly softens under resistive heating. Depending on which ligaments are activated, the gripper will bend inward to pick up an object, bend laterally to twist it, and bend outward to release it. All of the gripper motions are generated with a single pneumatic source of pressure. An activation-deactivation cycle can be completed within 15 s. The ability to incorporate electrically programmable ligaments in a pneumatic or hydraulic actuator has the potential to enhance versatility and reduce dependency on tubing and valves.

The ability to tune the mechanical stiffness between a soft state and a rigid state is essential for various living systems to navigate nature. Examples for this range from muscle-powered motor tasks and sexual reproduction, to spontaneous change in shape for predator evasion. [1] Similar to their natural counterparts, engineered materials with tunable properties including mechanical stiffness have the potential to be used in a broad range of engineering applications. [1,2] Structures made with these materials can change their mechanical rigidity in static or dynamic systems to extend their workspace. [1-4] Multiple strategies have been pursued recently to achieve stiffness tunability, including pneumatic jamming, [3,5,6] chemical interactions, [7] opposing actuator structures, [8-10] magnetorheological fluids, [11,12] external/internal heating of materials with phase change [13-24] or glass transition, [25-28] or through combinations of these techniques. [13,29] Among phase-changing materials, low melting point alloys (LMPA) have been used widely as they are highly electrically conductive, rigid as metal at room temperature, and their melting point can be as low as 47.2 C [21] or 62.0 C. [20] LMPA layers, channels, foams, lattices, and particles have been incorporated as fillers into soft elastomers and shape memory polymers to create engineering materials with stiffness tunability. [13,17,18,20-23] In addition to tuning mechanical stiffness, LMPA fillers can also enhance the thermal and electrical properties of the composites. [18,30,31] Another example of smart composites with tunable stiffness containing phase-changing components is conductive propylene-based elastomers (CPBE), [24] which have a propylene-ethylene copolymer elastomer matrix and dispersed

A thin, stretchable (200% linear strain), multiphase (solid–liquid) silicone composite with uniform electrical conductivity, for Joule heating and high-deformation sensing.

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.