Soft robotics represents a new set of technologies aimed at operating in natural environments and near the human body. To interact with their environment, soft robots require artificial muscles to actuate movement. These artificial muscles need to be as strong, fast, and robust as their natural counterparts. Dielectric elastomer actuators (DEAs) are promising soft transducers, but typically exhibit low output forces and low energy densities when used without rigid supports. Here, we report a soft composite DEA made of strain-stiffening elastomers and carbon nanotube electrodes, which demonstrates a peak energy density of 19.8 J/kg. The result is close to the upper limit for natural muscle (0.4–40 J/kg), making these DEAs the highest-performance electrically driven soft artificial muscles demonstrated to date. To obtain high forces and displacements, we used low-density, ultrathin carbon nanotube electrodes which can sustain applied electric fields upward of 100 V/μm without suffering from dielectric breakdown. Potential applications include prosthetics, surgical robots, and wearable devices, as well as soft robots capable of locomotion and manipulation in natural or human-centric environments.

Dielectric elastomer actuators (DEAs) are soft electromechanical devices that exhibit large energy densities and fast actuation rates. They are typically produced by planar methods and, thus, expand in-plane when actuated. Here, reported is a method for fabricating 3D interdigitated DEAs that exhibit in-plane contractile actuation modes. First, a conductive elastomer ink is created with the desired rheology needed for printing high-fidelity, interdigitated electrodes. Upon curing, the electrodes are then encapsulated in a self-healing dielectric matrix composed of a plasticized, chemically crosslinked polyurethane acrylate. 3D DEA devices are fabricated with tunable mechanical properties that exhibit breakdown fields of 25 V µm −1 and actuation strains of up to 9%. As exemplars, printed are prestrainfree rotational actuators and multi-voxel DEAs with orthogonal actuation directions in large-area, out-of-plane motifs.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201907375.cycling and breakdown behavior [33][34][35] and the presence of a rigid frame limits the geometries that can be achieved. [24,36] Recent attention has been directed toward developing approaches that enable contractile displacements in prestrain-free DEAs, including manual and automated stacking of individual planar layers [37] or sequential deposition of active materials via inkjet printing [38] and spray coating. [39] The fabrication of contractile actuators with vertically oriented electrodes offers a more promising approach (Figure 1b). While arrays of vertical electrodes can be patterned lithographically, new masks must be generated for each device design. [40][41][42] By contrast, 3D printing enables the rapid design and fabrication of soft materials in nearly arbitrary geometries. [43][44][45][46][47] For example, direct ink writing (DIW), an extrusion-based 3D printing method, has been used to pattern soft functional materials, including sensors, [48] stretchable electronics, [49] liquid crystalline elastomers, [50] and soft robots. [51,52] While this method has recently been used to print DEAs, they do not exhibit an in-plane contractile response. [52][53][54] Here, we create 3D DEAs composed of interdigitated vertical electrodes that are printed, cured, and encapsulated in an insulating dielectric matrix (Figure 1c). These prestrain-free contractile DEAs can be produced in nearly arbitrary geometries. During their actuation, the stress generated is given by σ = ε 0 ε r (E) 2 , where ε 0 is the vacuum permittivity, ε r is the dielectric constant, and E is the electric field. For small strains, the actuation strain (s z ) is s z = σ/E Y = ε 0 ε r (E) 2 /E Y , where E Y is the Young's modulus. Their actuation performance is therefore maximized by increasing the breakdown field and dielectric constant, while simultaneously reducing the elastic modulus of the matrix. Since variations in the dielectric thickness can cause localization of the electric field that results in p...

Exceptionally large strains can be produced in soft elastomers by the application of an electric field and the strains can be exploited for a variety of novel actuators, such as tunable lenses and tactile actuators. However, shape morphing with dielectric elastomers has not been possible since no generalizable method for changing their Gaussian curvature has been devised. Here it is shown that this fundamental limitation can be lifted by introducing internal, spatially varying electric fields through a layer-by-layer fabrication method incorporating shaped, carbon-nanotubes-based electrodes between thin elastomer sheets. To illustrate the potential of the method, voltage-tunable negative and positive Gaussian curvatures shapes are produced. Furthermore, by applying voltages to different sets of internal electrodes, the shapes can be re-configured. All the shape changes are reversible when the voltage is removed.

Dielectric elastomer actuators (DEAs) are soft, electrically powered actuators that have no discrete moving parts, yet can exhibit large strains (10%–50%) and moderate stress (∼100 kPa). This Tutorial describes the physical basis underlying the operation of DEA's, starting with a simple linear analysis, followed by nonlinear Newtonian and energy approaches necessary to describe large strain characteristics of actuators. These lead to theoretical limits on actuation strains and useful non-dimensional parameters, such as the normalized electric breakdown field. The analyses guide the selection of elastomer materials and compliant electrodes for DEAs. As DEAs operate at high electric fields, this Tutorial describes some of the factors affecting the Weibull distribution of dielectric breakdown, geometrical effects, distinguishing between permanent and “soft” breakdown, as well as “self-clearing” and its relation to proof testing to increase device reliability. New evidence for molecular alignment under an electric field is also presented. In the discussion of compliant electrodes, the rationale for carbon nanotube (CNT) electrodes is presented based on their compliance and ability to maintain their percolative conductivity even when stretched. A procedure for making complaint CNT electrodes is included for those who wish to fabricate their own. Percolative electrodes inevitably give rise to only partial surface coverage and the consequences on actuator performance are introduced. Developments in actuator geometry, including recent 3D printing, are described. The physical basis of versatile and reconfigurable shape-changing actuators, together with their analysis, is presented and illustrated with examples. Finally, prospects for achieving even higher performance DEAs will be discussed.

Active soft materials that change shape on demand are of interest for a myriad of applications, including soft robotics, biomedical devices, and adaptive systems. Despite recent advances, the ability to rapidly design and fabricate active matter in complex, reconfigurable layouts remains challenging. Here, the 3D printing of core-sheath-shell dielectric elastomer fibers (DEF) and fiber bundles with programmable actuation is reported. Complex shape morphing responses are achieved by printing individually addressable fibers within 3D architectures, including vertical coils and fiber bundles. These DEF devices exhibit resonance frequencies up to 700 Hz and lifetimes exceeding 2.6 million cycles. The multimaterial, multicore-shell 3D printing method opens new avenues for creating active soft matter with fast programable actuation.