Researchers in China have developed a soft exoskeleton for the lower body that is completely motor-free and is driven instead by high-power artificial muscles. The harness reduces the amount of energy a person spends walking by nearly a sixth, surpassing most hip-assistive exoskeletons, according to a study detailing the device published 10 July in the journal Science Advances..

Companies and research groups worldwide are developing soft exoskeletons as wearable harnesses that can assist and augment tasks such as walking, lifting and running while not restricting natural, comfortable movements like rigid exoskeletons do. However, soft exoskeletons often rely on bulky rigid electric motors or pneumatic actuators, which hinder the user’s mobility.

For decades, scientists have sought to create artificial muscles from polymers known as dielectric elastomers. These soft, lightweight, flexible, stretchable materials change shape when a voltage is applied, much like how the human body’s muscles contract or expand because of electricity.

However, dielectric elastomer actuators have faced a number of challenges in exoskeleton applications. For instance, they often experience a tradeoff between their electrical and mechanical properties—either they require a lot of electricity to drive their motions, or they are mechanically weak. In addition, previous artificial muscle fibers made of rolled-up sheets of dielectric elastomer were typically bulky, limiting their ability to conform to body contours.

Soft Hip Exoskeleton Uses No Motors

In the new study, Wei Yu, an associate professor of mechanical engineering at Hebei University of Technology in Tianjin, China, and his colleagues experimented with a dielectric elastomer rubber made of compounds that were highly polar — that is, within them, electric charges were highly separated. They next added a highly polar compound that helped molecular bonds form within the rubber to augment its mechanical strength, Yu says. At the same time, the high polarity of the compounds within this material helped make it more responsive to applied electric fields, he adds.

The scientists rolled stacks of thin films of their new material to form cylindrical fibers as little as 850 micrometers thick and as much as 250 millimeters long. In tests, soft, thread-like fibers of the new material only 1.95 millimeters wide could lift more than 400 grams, or more than 1,300 times their own mass. All in all, these new fibers are thinner and longer than previous artificial muscle fibers made of rolled-up sheets of dielectric elastomer, and nearly 10 times better in terms of output, the researchers say.

A volunteer clad in the new exoskeleton walks on a treadmill wearing sensors to monitor metabolism.Ziqi Zhang, Wei Yu, et al.

The scientists developed multi-fiber bundles that could plug into devices like Lego pieces. They next incorporated two 10-fiber bundles with a mass of just 6 grams into a soft hip exoskeleton. When the leg extended during walking, an applied voltage caused the fibers to lengthen and store elastic energy, which was released to assist the leg swinging. All in all, if a person is walking 4 kilometers per hour, the harness reduces the energy a person spends walking by an average of 13.9 percent compared to no assistance, the researchers found. This outperforms most previous hip-assistive exos.

Limits of Dielectric Elastomer Actuators Tested

Yu notes the team’s new exoskeleton was tested under lab conditions. “Its long-term stability and reliability still need to be further verified in more complex real-world wearable environments, such as under prolonged continuous movement, exposure to sweat, temperature variations, and differences in individual human motion,” he says.

In addition, Yu cautions that dielectric elastomer actuators typically require high driving voltages of more than 1,000 V. “How to further reduce the operating voltage while maintaining high output performance remains an important direction that we are particularly interested in,” he says.