Promising new materials mimic muscle structure and function

Differential interference contrast (DIC) images of strain-crystallized fibers that are 1X-5X the original hydrated length. The image indicates that fiber alignment increases along the fiber axis with increasing aspect ratio. Credit: Penn State

Inspired by the structure of muscles, an innovative new strategy to create fiber actuators could lead to advances in robotics, prosthetics and smart wearables, according to a Penn State-led team of scientists who discovered the process.

“Actuators are any material that will change or deform under external stimuli, such as parts of a machine contracting, bending, or expanding,” said Robert Hickey, assistant professor of materials science and engineering at Penn State. “And for technologies like robotics, we need to develop flexible, lightweight versions of these materials that can essentially act like artificial muscles. Our job is really to find a new way to do that.

The team developed a two-step process to fabricate fiber actuators that mimic the structure of muscle fibers and excel in several respects over other current actuators, including efficiency, actuation stress, and mechanical properties. They reported their findings today (June 2) in the journal Nature’s nanotechnology.

“It’s a huge area and there’s a lot of exciting research, but it’s really focused on engineering materials to optimize properties,” Hickey said. “What makes our work exciting is that we really focus on the connection between chemistry, structure and property.”

Hickey previously led a team that produced self-assembled nanostructured hydrogel materials. Hydrogels are networks of polymers that can swell and retain large amounts of water while retaining their structure.

In the new research, scientists have found that fibers made of this hydrogel material can stretch many times their original length when hydrated and harden and lock into the elongated shape when dried. in the extended state. Adding water or heat causes the material to return to its original size, making it promising for use as an actuator, the scientists said.

“We started to recognize that these fibers contracted and displayed some really fascinating properties,” Hickey said. “When we started to characterize the structure, we realized that there were some fundamentally interesting things going on here. And we started to recognize that in many ways the structure of these muscles mimicked or mirrored natural muscle.

The materials consist of highly aligned nanoscale structures with alternating crystalline and amorphous domains, resembling the ordered, striated pattern of mammalian skeletal muscle, the scientists said.

The exceptional stretching properties of hydrogels are the result of the combination of rigid amorphous nanoscale domains and water-filled microscale pores. When the hydrogels are stretched, they retract like a rubber band. If the stretched fibers are dried in the stretched state, the polymer network will crystallize locking the elongated shape of the fibers.

“We believe that one of the fundamental reasons why we have these exceptional properties is that the fibers are organized very precisely at the nanometer scale, similar to the sarcomere of a human muscle,” Hickey said. “What happens is you have an even contraction. These amorphous domains are all precisely organized along the fiber, which means that they contract in only one direction, which gives rise to this ability to return to this original state.

Applying water or heat to stretched materials melts the crystals and allows the material to return to its original shape. When stretched to five times its original length, the material can shrink back to less than 80% of its size and can do so over many cycles without a drop in performance, the scientists said.

“The fact that we can use two different stimuli, heat and water, to trigger actuation opens up double the possibilities for materials made with this method,” Hickey said. “Most actuators are triggered by a single stimulus. Dual stimuli open up the versatility of our materials.

Technique accelerates thermal actuation for soft robotics

More information:

Chao Lang et al, Block Copolymer Nanostructured Muscles, Nature’s nanotechnology (2022). DOI: 10.1038/s41565-022-01133-0

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Promising new materials mimic muscle structure and function (2022, June 3)
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