Soft robotics actuators are “fast as a hummingbird, strong as an elephant”

The development, from engineers at the University of Colorado at Boulder, is aimed at enabling robots that will work closely with humans and new generations of prosthetic limbs capable of life-like movement and strength. “We draw our inspiration from the astonishing capabilities of biological muscle,” said Dr Christoph Keplinger, a mechanical engineer and director of the lab at the university’s College of Engineering and Applied Science

Like many soft robotics devices, the Keplinger group’s actuators combine stretchy elastomers with liquids. Called HASEL (Hydraulically Amplified Self-healing Electrostatic) actuators, Keplinger’s team has developed three types, which they describe in papers in the journals Science and Science Robotics.

One HASEL device described in the Science paper consists of a doughnut-shaped elastomer shell filled with a slightly viscous liquid like rapeseed oil and hooked to a pair of opposing electrodes; when a voltage is applied across these, the liquid is displaced and the shape of the shell responds accordingly. Such a device could form the basis of grippers that can handle delicate objects like raspberries or raw eggs without damaging them.

Also in Science, the team describes an actuator composed of layers of very stretchy ionic conductors sandwiching a layer of liquid that expands and contracts linearly. This device can be fitted to a mechanical arm and can drive it to lift a gallon of water.

“The ability to create electrically powered soft actuators that lift a gallon of water at several times per second is something we haven’t seen before. These demonstrations show the exciting potential for HASEL,” said PhD student Eric Acome, lead author on the paper.“The high voltage required for operation is a challenge for moving forward. However, we are already working on solving that problem and have designed devices in the lab that operate with a fifth of the voltage used in this paper.”

The third design, known as a Peano-HASEL actuator, is described in Science Robotics, and comprises three small rectangular pouches made of the same material as a potato crisp packet, filled with liquid and linked in series. This also contracts in response to an electrical signal, but can respond faster than human muscles. “We can make these devices for around 10 cents, even now,” said Nicholas Kellaris, also a PhD student in the Keplinger group and the lead author of the Science Robotics study. “The materials are low-cost, scalable and compatible with current industrial manufacturing techniques.”

Using insulating liquids in the devices gives them the ability to self-heal, the group claims. Other soft actuators that work using high voltage employ a solid insulating elastomer that can be catastrophically damaged by the applied voltages, breaking down and becoming conductors. Liquid insulators recover their electrical properties after damage, which has allowed the team to scale up their devices to handle large loads and exert correspondingly large forces. “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,” Keplinger said.

“The research coming out of Dr. Keplinger’s lab is nothing short of astounding,” said Bobby Braun, dean of the College of Engineering and Applied Science. “He and his team of students are helping create the future of flexible, more-humanlike robots that can be used to improve people’s lives and well-being.”