Researchers in CU Boulder’s Paul M. Rady Department of Mechanical Engineering recently uncovered new information that could revolutionize the design of electrohydraulic soft actuators to enable robots to perform at faster speeds.

Postdoctoral Research Fellow Philipp Rothemund. 

The experimental setup includes a high-speed camera that tracks a marker at the bottom of the soft actuator. This diagram also shows the weight that hangs from the actuator.

In 2018, the Keplinger Research Group developed a new type of soft actuator that mimics the way muscles move in nature. In addition to being faster, their design was stronger, cheaper and more dexterous than other available technologies at the time. But what exactly contributed to their success? Researchers set out to discover, finding that the right geometry, materials, and applied external loads not only contributed to their original design but could now make their high-performance actuators even faster.

“Soft actuators mimic the way muscles move and need to move quickly, especially when used in robots that jump, run or fly,” said postdoctoral research fellow Philipp Rothemund, the lead researcher on the project. “I was interested in understanding why our electrohydraulic soft actuators, compared to others, are as fast as they are.”

Their findings, described by Rothemund, PhD Student Sophie Kirkman and Assistant Professor Christoph Keplinger, will be released this week in a paper titled in Proceedings of the National Academy of Sciences (PNAS).    

The electrohydraulic soft actuator is composed of a flexible outer shell that is covered with electrodes and houses an oil-based liquid inside. When voltage is applied to the electrodes, the actuator contracts, and when the voltage is turned off, the actuator elongates with the help of a weight—or load—that pulls it into full extension, resulting in a motion that mimics the way muscles move. Researchers in this study measured the time it takes for an actuator to contract and elongate as the actuator’s geometry, properties of the outer shell and the liquid inside, and applied external loads were adjusted.

What they found, Rothemund said, was surprisingly simple.  

“Now, all we need is to measure the speed of two actuators, and based on the result, we can predict how other actuators of the same type are going to react,” Rothemund said.

To measure speed, Rothemund applied a square wave voltage, which turns from off to on and back again, and used a high-speed camera to record the motion of a visual marker he added to the bottom of the actuators. From this recording, he determined exactly how long it took for the actuator to contract and elongate.

Rothemund discovered two dynamic regimes—or states—in which an electrohydraulic soft actuator can exist. Changes in geometry, properties of the liquid inside, and external load affect the actuation speed differently depending on whether the actuator is in the viscous regime, where the liquid inside is thick, or the inertial regime, where the liquid inside flows easily.

In the viscous regime, the thicker liquid resists flow through the actuator, slowing the speed of actuation. For these actuators, Rothemund said a shorter actuator, less-viscous liquid, larger applied voltage and larger applied external load, which hangs at the bottom of the actuator, will significantly increase speed.

In the inertial regime, the liquid flows more easily, making high-speed movement possible. For these actuators, Rothemund said the speed is limited by the inertia of the external load and largely influenced by the length of the actuator—shorter actuators move faster. He said actuators in the inertial regime are faster than those in the viscous regime.

“Other researchers in our group are already using these results to their advantage,” said Rothemund.

Electrohydraulic soft actuators designed to operate in the inertial regime enable unprecedented speeds of motion that he said may one day contribute to a new generation of bio-inspired robots.

[video:https://youtu.be/wGYxmUN7DmU]Speed comparison between electrohydraulic soft actuators in the inertial regime versus the viscous regime. 

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Artimus Robotics, a spinout company of CU Boulder’s Paul M. Rady Department of Mechanical Engineering, recently . This award will enable further research into the unique electromechanical failure mechanism in HASEL actuators, a new class of smart, soft, high-speed robotic hardware. In doing so, the reliability, robustness and efficiency of these actuators can be improved and more widely adopted across highly demanding applications, including industrial automation or underwater marine robotics.

Artimus Robotics was co-founded by multiple researchers and alumni from Rady Mechanical Engineering at CU Boulder. The team includes Assistant Professor Christoph Keplinger, Tim Morrissey (PhDMechEngr’19), Eric Acome (PhDMechEngr’20), and soon-to-graduate PhD students Nicholas Kellaris and Shane Mitchell.

Co-founder and CEO Tim Morrissey answered questions about the SBIR Phase I program, upcoming research and how Artimus Robotics contributes to scientific discovery into products with commercial and societal impact.

Question: What will this award enable?

Answer: This NSF SBIR Phase I Grant enables deep scientific research into the unique electromechanical failure mechanism observed in HASEL actuators. The customers of Artimus Robotics demand highly reliable and robust actuators which can only be achieved by fully and deeply understanding how the HASEL actuators fail. A team of high caliber researchers will commit significant efforts over the next year to perform this deep research and further de-risk the adoption of HASEL actuation technology.

Question: How will Artimus Robotics go about increasing the reliability and robustness?

Answer: To increase the reliability and robustness of HASEL actuators, Artimus will run a series of physical experiments, torture testing the actuators to failure. Artimus will apply high physical and electrical stress, study the way HASEL actuators react to these stresses and determine the resulting failure mechanism. Understanding the failure mechanisms will enable Artimus to improve the reliability of the HASEL actuators and reach the performance metrics required by our customers for the wide adoption of HASEL actuation technology.

Question: How does Artimus Robotics transform scientific discovery into products with commercial and societal impact?

Answer: One of the founding principles of Artimus Robotics is to make a positive impact on the world around us. We believe this goal is best accomplished by transitioning scientific breakthroughs into real-world applications. At Artimus, we are truly built on this belief. As a group of academic engineers that had seen success in academia, we knew in order to truly realize the value of our research, we had to commercialize. We strive every day to balance scientific and engineering discovery with real-world societal needs. 

About America’s Seed Fund

America’s Seed Fund, supported by the National Science Foundation awards $200 million annually to startups and small businesses, transforming scientific discovery into products and services with commercial and societal impact. Startups working across almost all areas of science and technology can receive up to $1.75 million in non-dilutive funds to support research and development, helping de-risk technology for commercial success. America’s Seed Fund is congressionally mandated through the Small Business Innovation Research (SBIR) program. The NSF is an independent federal agency with a budget of about $8.1 billion that supports fundamental research and education across all fields of science and engineering.

About Artimus Robotics

Artimus Robotics is robotic hardware changing the way the world moves. Providing breakthrough HASEL actuation technology, Artimus Robotics actuation solutions offer increased functionality including smart, soft and versatile actuation systems to solve our customers' motion challenges. With applications ranging from consumer robotics to defense, Artimus has realized early customer traction in markets such as industrial automation, underwater marine vehicles and human-machine interfaces. To learn more about Artimus Robotics or how you can try HASEL technology in your application, please  and .

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Faculty at CU Boulder are working toward widespread adoption and practical applications for the soft robotic HASEL actuators demonstrated here earlier this year, through a new $2 million award from the National Science Foundation.

Soft robotics is an emerging field of research that promises substantial advantages over traditional rigid robots, allowing for a new generation of soft, human-like robots that will be able to safely collaborate with people. This technology could also improve wearable or implantable soft robotic components for those with disabilities, such as missing limbs, among many other applications.

Assistant Professor and Mollenkopf Faculty Fellow Christoph Keplinger said the award allows the research team to explore new manufacturing methods and effective designs for 3D printing, new electronic driving systems with distributed microscale power delivery and computation, as well as low- and high-level control strategies.

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Soft, self-healing devices mimic biological muscles, point to next generation of human-like robotics

In the basement of the Engineering Center at the University of Colorado Boulder, a group of researchers is working to create the next generation of robots. Instead of the metallic droids you may be imagining, they are developing robots made from soft materials that are more similar to biological systems. Such soft robots contain tremendous potential for future applications as they adapt to dynamic environments and are well-suited to closely interact with humans.

A central challenge in this field known as “soft robotics” is a lack of actuators or “artificial muscles” that can replicate the versatility and performance of the real thing. However, the in the College of Engineering and Applied Science has now developed a new class of soft, electrically activated devices capable of mimicking the expansion and contraction of natural muscles. 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.

[video:https://youtu.be/YGMyW6AESsQ]

CAPTION: HASEL artificial muscles for next-generation soft robotics. (Footage courtesy of Science/AAAS and Science Robotics/AAAS.)

The newly developed hydraulically amplified self-healing electrostatic (HASEL) actuators eschew the bulky, rigid pistons and motors of conventional robots for soft structures that react to applied voltage with a wide range of motions. The soft devices can perform a variety of tasks, including grasping delicate objects such as a raspberry and a raw egg, as well as 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.

Three different designs of HASEL actuators are detailed today in separate papers appearing in the journals and .

“We draw our inspiration from the astonishing capabilities of biological muscle,” said 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 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.”

One iteration of a HASEL device, described in Science (), 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, 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.

Basic components of a HASEL actuator. A flexible or stretchable shell is filled with insulating liquid. Electrodes are placed on either side of the shell and when voltage is applied, electric forces displace the fluid driving shape change of the muscle. (Image courtesy of Keplinger Research Group and Science/AAAS.)

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.

HASEL artificial muscles lifting a gallon of water. (Image courtesy of Keplinger Research Group and Science/AAAS.)

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. Other soft actuators controlled by high voltage, also known as dielectric elastomer actuators, use a solid insulating layer that fails catastrophically from electrical damage. In contrast, 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. These demonstrations show the exciting potential for HASEL” said 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, detailed in Science Robotics () and 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.

Peano-HASEL actuators rapidly contract upon activation to throw a ball. (Image courtesy of Keplinger Research Group and Science Robotics/AAAS.)

The versatility and simplicity of the HASEL technology lends itself to widespread industrial applications, both now and in the future.

“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.”

Future research will attempt to further optimize materials, geometry and explore advanced fabrication techniques in order to continue improving the HASEL platform and to rapidly enable practical applications.

The researchers have secured patents for the technology and are currently exploring commercial opportunities with the assistance of CU Boulder’s Technology Transfer Office.

“The research coming out of Dr. Keplinger’s lab is nothing short of astounding,” said Bobby Braun, dean of CU Boulder’s 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. This line of research is a core, interdisciplinary strength of our college."

The Science paper was co-authored by Shane Mitchell, Timothy Morrissey, Madison Emmett, Claire Benjamin, Madeline King and Miles Radakovitz of the Department of Mechanical Engineering. The Science Robotics paper was co-authored by Shane Mitchell, Vidyacharan Gopaluni Venkata and Garrett Smith of Mechanical Engineering.

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