Soft robots are made from soft materials that react to applied voltage with a wide range of motions. Such soft robots contain tremendous potential for future applications, as they adapt to dynamic environments and are well-suited to closely interact with humans. The soft devices can perform a variety of tasks, including grasping delicate objects such as a raw egg, as well as lifting heavy objects.
A challenge in this field is a lack of actuators, or “artificial muscles,” that can replicate the versatility and performance of the real thing. A new class of soft, electrically activated devices capable of mimicking the expansion and contraction of natural muscles was developed. These devices, which can be constructed from a wide range of low-cost materials, are able to self-sense their movements and self-heal from electrical damage, representing a major advance in soft robotics.
The hydraulically amplified self-healing electrostatic (HASEL) actuators eschew the bulky, rigid pistons and motors of conventional robots for soft structures that exceed or match the strength, speed, and efficiency of biological muscle. Their versatility may enable artificial muscles for human-like robots and a next generation of prosthetic limbs.
One iteration of a HASEL device consists of a donut-shaped elastomer shell filled with an electrically insulating liquid (such as canola oil), and hooked up to a pair of opposing electrodes. When voltage is applied, the liquid is displaced and drives shape change of the soft shell. As an example of one possible application, several of these actuators were positioned opposite one another and achieved a gripping effect upon electrical activation. When voltage is turned off, the grip releases. Another HASEL design is made of layers of highly stretchable ionic conductors that sandwich a layer of liquid, and expands and contracts linearly upon activation to either lift a suspended gallon of water or flex a mechanical arm holding a baseball.
In addition to serving as the hydraulic fluid that enables versatile movements, the use of a liquid insulating layer enables HASEL actuators to self-heal from electrical damage. Other soft actuators controlled by high voltage use a solid insulating layer that fails from electrical damage. In contrast, the liquid insulating layer of HASEL actuators immediately recovers its insulating properties following electrical damage. This resiliency allows devices to be scaled up to exert larger amounts of force.
HASEL actuators can also sense environmental input, much like human muscles and nerves. A HASEL actuator was connected to a mechanical arm and demonstrated the ability to power the arm while simultaneously sensing position.
A third design, known as a Peano-HASEL actuator, consists of three small, rectangular pouches filled with liquid, rigged together in series. The polymer shell is made from the same material as a potato chip bag, and is thin, transparent, and flexible. Peano-HASEL devices contract on application of a voltage, much like biological muscle, which makes them especially attractive for robotics applications. Their electrically powered movement allows operation at speeds exceeding that of human muscle.