Scientists develop sustainable material for flexible artificial muscles

UCLA materials scientists and colleagues at the nonprofit scientific research institute SRI International have developed a new material and manufacturing process for creating artificial muscles that are stronger and more flexible than their biological counterparts.

“Creating an artificial muscle to enable work and detect force and touch has been one of the great challenges of science and engineering,” said Qibing Pei, a professor of materials science and engineering at UCLA Samueli School of Engineering and the corresponding author of a study that was recently published in Science†

To a soft material eligibility for use as a artificial muscleit must be able to perform mechanical energy and remain viable under harsh conditions, meaning it does not easily lose its shape and strength after repeated work cycles. While many materials are considered contenders for creating artificial muscles, dielectric elastomers (DE) – lightweight materials with a high elastic energy density – are of particular interest because of their optimal flexibility and toughness.

Dielectric elastomers are electroactive polymers, which are natural or synthetic substances composed of large molecules that can change in size or shape when stimulated by a electric field† They can be used as actuators, allowing machines to work by transforming electrical energy in mechanical work.

Most dielectric elastomers are made of acrylic or silicone, but both materials have drawbacks. While traditional acrylic DEs can achieve a high activation load, they require pre-stretching and lack flexibility. Silicones are easier to make, but they cannot withstand a high load.

Using commercially available chemicals and using an ultraviolet (UV) light curing process, the UCLA-led research team created an improved acrylic-based material that is more pliable, tunable and easier to scale without losing its strength and lose stamina. While the acrylic acid makes more possible hydrogen bonds To make the material more mobile, the researchers also adjusted the crosslinking between polymer chains, making the elastomers softer and more flexible. The resulting thin, processable, high-performance dielectric elastomer film, or PHDE, is then placed between two electrodes to convert electrical energy into motion as an actuator.

Each PHDE film is as thin and light as a piece of human hair, about 35 micrometers thick, and when several layers are stacked on top of each other, they become a miniature electric motor that can act as muscle tissue and produce enough energy to power movement for small robots or sensors. The researchers created stacks of PHDE films ranging from four to fifty layers.

“This flexible, versatile and efficient actuator could open the gates for artificial muscles in new generations of robots, or in sensors and wearable technology that can more accurately mimic or even enhance human movements and capabilities,” Pei said.

Artificial muscles equipped with PHDE actuators can generate more megapascals of force than biological muscles and they also exhibit three to ten times more flexibility than natural muscles.

Multilayer soft films are usually manufactured through a “wet” process where liquid resin is deposited and cured. But that process can result in uneven layers, resulting in a poorly performing actuator. For this reason, many actuators have so far only been successful with single-layer DE films.

The UCLA study involves a “dry” process where the films are layered using a blade and then UV cured to cure, making the layers uniform. This increases the actuator energy output so that the device can support more complex movements.

The simplified process, along with the flexible and durable nature of the PHDE, allows the fabrication of new soft actuators that can bend to jump, such as spider legs, or wind and spin. The researchers also demonstrated the PHDE actuator’s ability to throw a pea-sized ball that is 20 times heavier than the PHDE films. The actuator can also expand and contract like a diaphragm when a voltage is turned on and off, providing a glimpse of how artificial muscles could be used in the future.

Advances could lead to soft robots with improved mobility and endurance, and new wearable and haptic technologies with sense of touch. The production process can also be applied to other soft thin film materials for applications such as microfluidic technologies, tissue engineering or microfabrication.