Grad student designs compostable “artificial muscles” 2023

Future robots can bend and bounce off barriers. These machines can be composted after use.

Engineers—including CU Boulder graduate student Ellen Rumley—want that. The researchers presented their “artificial muscle” robotic actuator designs in Science Advances last month.

The group’s actuators, which move fluid in squishy sacs, power lifelike robotic limbs and legs. They break down organically in soil over several months, making them more sustainable than prior models.

“You could dispose of them in an industrial compost bin,” said Rumley, co-first author of the new study and researcher in CU Boulder’s Paul M. Rady Department of Mechanical Engineering. “We hope the project will inspire other engineers to develop robotics with sustainability in mind.”

Baymax, the Big Hero 6 pillowy robot, and a recyclable grocery bag.

Christoph Keplinger, director of the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, directed the study. He and his team introduced Hydraulically Amplified Self-Healing ELectrostatic (HASEL) actuators in 2018. These actuators can squeeze claws and grippers or bend robotic arms like biceps.

They could disappear in a landfill now.

The team created soft robotic actuators using sustainable materials in the latest study. The muscles are flexible and can flex for 100,000 cycles or more.

“It was particularly exciting that we ended up with a materials system that is fully biodegradable and can still match key performance metrics of actuators made from non-biodegradable materials,” said Keplinger, co-founder of Boulder-based HASEL actuator manufacturer Artimus Robotics.

“The sustainability of the new materials system opens up very interesting avenues for applications that require components designed for single- or short-term use, such as food handling or medical applications.”

Robot muscles

Rumley’s search for a different robot spans years. Rumley designed FIRST Tech Challenge robots at a robotics club in Anchorage, Alaska, as a high school student. She hated metal and plastic harsh robots’ inelegance.

“I had felt like the robots’ rigidity was a big limiting factor of their physical capabilities,” said Rumley, who started her graduate studies in Boulder before heading to Germany in 2020 to work as a visiting researcher in Keplinger’s lab. “That was especially jarring when you saw a person walking next to a robot.”

Soft robots could tackle those problems by combining technical breakthroughs with biological species’ squishiness and flexibility.

Transformer oil in plastic pouches with a thin electrical conductor covers HASEL actuators. Rumley claimed the bag will “zip together” when electricity is applied across the conductors, squeezing the fluid from end to end. The pouches alter shape and provide force to devices like robotic limbs.

In 2018, Keplinger, Tim Morrissey, Eric Acome, Shane Mitchell, and Nick Kellaris founded Artimus Robotics to commercialize the actuators with CU Boulder Venture Partners. Kellaris, a 2020 CU Boulder materials science and engineering PhD, co-authored the new research.

Rumley and her colleagues sought sustainable components to replace all those early ideas in the new study.

“We were searching for the perfect combination of compatible materials that would make a high-performance biodegradable muscle,” she stated.

Rumley and her colleagues examined biodegradable options to replace their actuators’ plastic pouches in a methodical materials science endeavor. A biodegradable polyester blend, used in shopping bags, was embossed with carrots.

The team’s final artificial muscles can lift nearly as effectively as HASEL actuators and decay in a composting facility in 6 months.

Future robots

Rumley, a 2020 National Science Foundation Graduate Research Fellow, will study in Europe after earning her doctorate from CU Boulder. She’s learned German and enjoys international living, but she misses Colorado and Alaska’s wide-open spaces.

“I miss wading into rivers and catching salmon,” she added. “This isn’t big in Germany.”

The engineer argues it’s worth it to be part of a research team launching a new robotics era. Keplinger envisions HASEL actuators powering Iron Man-like “exosuits” to assist disabled persons walk.

“The applications are really exciting, especially for devices that can assist folks who are physically impaired,” Rumley said. “Technology that empowers underserved populations drives me.”

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