Not your typical robot

Researchers at the University of Colorado Boulder have been working to create the next generation of robots.

Members of the Keplinger Research Group. From right: Assistant Professor Christoph Keplinger, graduate student Eric Acome, undergraduate student Madison Emmett, graduate student Nick Kellaris, graduate student VC Gopaluni Venkata, undergraduate student Madeline King, graduate student Shane Mitchell, PhD candidate Timothy Morrissey and undergraduate student Garrett Smith. Photo credit: Glenn Asakawa/University of Colorado Boulder.

And unlike the metallic humanoid bots you might be imagining, the team is focused on developing a robot using soft material that mimics biological systems — these robots have the potential to adapt to dynamic environments and work closely, interacting with humans.

One of the biggest challenges the team has faced when it comes to soft robotics is the  lack of actuators or “artificial muscles” that can replicate the versatility and performance of the real thing.

This is where a group at Keplinger Research Group in the College of Engineering and Applied Science comes in. The team has developed a new class of soft, electrically activated devices capable of mimicking the expansion and contraction of natural muscles.

The team designed the devices using a wide range of low-cost materials and developed them with the capability to self-sense their movements and self-heal from electrical damage — this represents a major advance in soft robotics.

Instead of the bulky, rigid pistons and motors of conventional robots, the team developed hydraulically amplified self-healing electrostatic (HASEL) actuators and soft structures that react to applied voltage with a wide range of motions.

The soft devices can perform a variety of tasks ranging from extremely delicate gripping to lifting heavy objects. 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 team developed three different designs of HASEL actuators.

“HASEL actuators synergize the strengths of soft fluidic and soft electrostatic actuators, and thus combine versatility and performance like no other artificial muscle before,” explains Christoph Keplinger, senior author of both papers, an assistant professor in the Department of Mechanical Engineering and a Fellow of the Materials Science and Engineering Program.

HASEL actuators can be designed as soft grippers to handle and manipulate delicate objects. Photo courtesy of Keplinger Lab/University of Colorado Boulder

Keplinger adds the team draw inspiration from the capabilities of biological muscles and 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.

One iteration of a HASEL device  consists of a donut-shaped elastomer shell filled with an electrically insulating liquid 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, the researchers positioned several of these actuators opposite of 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 which enables versatile movements, the use of a liquid insulating layer enables HASEL actuators to self-heal from electrical damage. The liquid insulating layer of HASEL actuators immediately recovers its insulating properties following electrical damage. This resiliency allows researchers to reliably scale up devices to exert larger amounts of force.

“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,” adds Eric Acome, a doctoral student in the Keplinger group and the lead author of the Science 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.”

HASEL actuators can also sense environmental input, much like human muscles and nerves. The electrode and dielectric combination in these actuators forms a capacitor. This capacitance, which changes with stretch of the device, can be used to determine the strain of the actuator. The researchers attached a HASEL actuator 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 low-cost 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.

“We can make these devices for around ten cents, even now,” said Nicholas Kellaris, also a doctoral 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.”

The team hopes to further develop these devices to optimize materials, geometry and explore advanced fabrication techniques.

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