Soft Self-Healing Devices Mimic Biological Muscles
Phillip Keane posted on January 18, 2018 |

Bio-inspired design and soft robotics go together like wine and cheese. Which is understandable, really, because nature designs soft, squishy bodies (like the ones that you and I live inside), and it is the goal of softrobotics to design soft, squishy robots.

Researchers at the University of Colorado Boulder’s Engineering Center have discovered a new method of manufacturing soft, muscle-like actuators that will bring robots one step closer to mimicking their human counterparts and a step farther from their hard plastic and metallic origins. And like their organic, fleshy counterparts, these robo-muscles can self-heal.

Soft robotics is currently a very active field of research, due to the ability of soft robots to change shape in response to their environments (much like an octopus) and their ability to interact safely with humans. It’s safer to work around soft robots because the soft structures are less harmful in the event of a collision between the robot and a human operator.

The new soft actuators, referred to as hydraulically amplified self-healing electrostatic (HASEL) actuators, are designed to replace the bulky and firm traditional piston-type actuators and motors used in traditional robotics designs.

Drawing of donut-type HASEL actuator on the left, and the real version on the right. (Image courtesy of University of Boulder Colorado.)
Drawing of donut-type HASEL actuator on the left, and the real version on the right. (Image courtesy of University of Boulder Colorado.)

The soft structures react to applied voltage with a wide range of motions, much like a dissected frog leg responds to electrical stimuli—which you may remember from biology class. The soft actuated devices can perform a variety of tasks, including grasping delicate objects such as a raw egg, as well as lifting heavy objects. In fact, the HASEL actuators exceed or match the strength, speed and efficiency of biological muscle, and their versatility may enable artificial muscles for human-like robots and a next generation of prosthetic limbs.

The details of three different types of HASEL actuator have just been published separately in the journals Science and Science Robotics.

The three types of new actuators are as follows:

The first type of HASEL device is a donut-shaped elastomer shell filled with an electrically insulating liquid (such as canola oil) that is hooked up to a pair of opposing electrodes. When voltage is applied, the liquid is displaced and causes the shape of the soft shell to change. This type of actuator can be used to produce a grip-like effect, which grips when a voltage is applied, and relaxes when the voltage is switched off.

The second kind of HASEL design is made of layers of highly stretchable ionic conductors that sandwich a layer of liquid. This type of actuator 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.

The third design, which is known as a Peano-HASEL actuator, consists of three small rectangular pouches filled with liquid that are rigged together in a series. The polymer shell, which is made from the same low-cost material as a potato chip bag, is thin, transparent and flexible. Peano-HASEL devices contract when a voltage is applied, much like biological muscle, which makes them especially attractive for robotics applications. Their electrically powered movement allows them to be operated at speeds exceeding that of human muscle.

You can see the different actuators in action in the video below.

"We draw our inspiration from the astonishing capabilities of biological muscle," said Christoph Keplinger, senior author of both papers, and assistant professor at the University of Colorado Boulder’s Department of Mechanical Engineering. "HASEL actuators synergize the strengths of soft fluidic and soft electrostatic actuators, and thus combine versatility and performance like no other artificial muscle before. Just like biological muscle, HASEL actuators can reproduce the adaptability of an octopus arm, the speed of a hummingbird and the strength of an elephant.”

The first paper, titled “Hydraulically amplified self-healing electrostatic actuators with muscle-like performance,” can be found in the journal Science, and the second paper, titled “Peano-HASEL actuators: Muscle-mimetic, electrohydraulic transducers that linearly contract on activation,” can be found in the journal Science Robotics.

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