Marcus Rosenthal

Dielectric elastomer artificial muscle actuators: Toward biomimetic motion

To achieve desirable biomimetic motion, actuators must be able to reproduce the important features of natural muscle such as power, stress, strain, speed of response, efficiency, and controllability. It is a mistake, however, to consider muscle as only an energy output device. Muscle is multifunctional. In locomotion, muscle often acts as an energy absorber, variable-stiffness suspension element, or position sensor, for example. Electroactive polymer technologies based on the electric-field-induced deformation of polymer dielectrics with compliant electrodes are particularly promising because they have demonstrated high strains and energy densities. Testing with experimental biological techniques and apparatus has confirmed that these dielectric elastomer artificial muscles can indeed reproduce several of the important characteristics of natural muscle. Several different artificial muscle actuator configurations have been tested, including flat actuators and tubular rolls. Rolls have been shown to act as structural elements and to incorporate position sensing. Biomimetic robot applications have been explored that exploit the muscle-like capabilities of the dielectric elastomer actuators, including serpentine manipulators, insect-like flapping-wing mechanisms, and insect-like walking robots.

Multifunctional electroelastomer roll actuators and their application for biomimetic walking robots

Electroelastomers (also called dielectric elastomer artificial muscles) have been shown to exhibit excellent performance in a variety of actuator configurations, but making a compact, free-standing, muscle-like actuator capable of obtaining good performance has been a challenge. By rolling highly prestrained electroelastomer films around a compression spring, we have demonstrated Multifunctional Electroelastomer Rolls (MERs) that combine load bearing, actuation, and sensing functions. The MER spring rolls are compact, have a potentially high electroelastomer-to-structure weight ratio, and can be configured to actuate in several ways, including axial extension, bending, and as multiple degree-of-freedom actuators that combine both extension and bending. One degree-of-freedom (1-DOF), 2-DOF, and 3-DOF MERs have all been demonstrated through suitable electrode patterning on a single monolithic substrate. A 1-DOF MER with 9.6 g weight, 12 mm diameter, and 65 mm total length can deliver up to 15 N force and 12 mm stroke. Its capacitance is around 13 nF and changes linearly with strain during axial tension or compression. The MERs are useful in a number of applications where compact and high-stroke actuation is required. The applications as artificial muscles are particularly appealing, as multifunctionality prevails in natural muscles.

Programmable surface deformation: thickness-mode electroactive polymer actuators and their applications

Many different actuator configurations based on SRI International’s dielectric elastomer (DE) type of electroactive polymer (EAP) have been developed for a variety of applications. These actuators have shown excellent actuation properties including maximum actuation strains of up to 380% and energy densities of up to 3.4 J/g, using the planar mode of actuation. Recently, SRI has investigated different configurations of DE actuators that allow complex changes in surface shape and thus the creation of active surface texture. In this configuration, the “active” polymer film is bonded or coated with a thicker passive layer, such that changes in the polymer thickness during actuation of the DE device are at least partially transferred to (and often amplified by) the passive layer. Although the device gives out-of-plane motion, it can nonetheless be fabricated using two-dimensional patterning. The result is a rugged, flexible, and conformal skin that can be spatially actuated by subjecting patterned electrodes on a polymer substrate to an electric field. Using thickness-mode DE, we have demonstrated thickness changes of the order of 0.5 - 2 mm by laminating a passive elastomeric layer to a DE polymer that is only 60 μm in thickness. Such thickness changes would otherwise require a very large number of stacked layers of the DE film to produce comparable surface deformations. Preliminary pressures of 4.2 kPa (0.6 psi) in a direction normal to the plane of the DE film have been measured. However, theoretical calculations indicate that pressures of the order of 100 kPa are feasible using a single layer of DE film. Stacking multiple layers of DE film can lead to a further increase in achievable actuation pressures. Even with current levels of thickness change and actuation pressures, potential applications of such surface texture change are numerous. A thin, compliant pad made from these actuators can have a massaging or sensory augmentation function, and can be incorporated into garments if desired. The bumps and troughs could act as valves or pumping elements in a fluidic or microfluidic system. Such a device could also be the basis of a smart skin that controls boundary-layer flow properties in a boat or airplane so as to reduce overall drag. The DE elements of the pad can also be used as sensors to make a touch-sensitive skin for recording human interaction with the environment. By driving a thin, compliant vibrating layer at resonant frequencies, one can also configure these devices as solid or fluidic conveyors that transport material on a macroscopic or microscopic scale.

Applications of dielectric elastomer EPAM sensors

While Electroactive Polymer Artificial Muscle (EPAM) has been presented extensively as a solution for actuation and generation technology, EPAM technology can also be used in multiple novel applications as a discrete or integrated sensor. When an EPAM device, an elastic polymer with compliant electrodes, is mechanically deformed, both the capacitance of the EPAM device, as well as the electrode and dielectric resistance, is changed. The capacitance and resistance of the sensor can be measured using various types of circuitry, some of which are presented in this paper. EPAM sensors offer several potential advantages over traditional sensors including operation over large strain ranges, ease of patterning for distinctive sensing capabilities, flexibility to allow unique integration into components, stable performance over a wide temperature range and low power consumption. Some existing challenges facing the commercialization of EPAM sensors are presented, along with solutions describing how those challenges are likely to be overcome. The paper describes several applications for EPAM sensors, such as an integrated diagnostic tool for industrial equipment and sensors for process and systems monitoring.