This material for soft robots can bend, straighten and heal itself

March 21, 2021

New polymers may allow robots to "heal." (Pixabay/849536)

Researchers have developed a new composite polymer for use in emerging soft robotics technologies that can be programmed to morph in air or underwater when exposed to light and return to its original shape when the light is turned off. The polymer can even heal itself if cut in two — light causes the halves to fuse back together.

In the study, published March 1 in Composites Part B, the team created the new material by combining silver nanowires with a material called poly (ethylene-co-vinyl acetate). Poly (ethylene-co-vinyl acetate) is a kind of thermoplastic, which means it can be repeatedly softened via heating and hardened via cooling. The researchers added silver nanowires because those nanowires are good at absorbing light and turning it into heat.

"EVA originally is thermal-driven, which is, however, unfavorable for control," Lili Yang, a researcher at Donghua University in Shanghai, China, and an author of the study, explained to The Academic Times. "If silver nanowires, which present an excellent photothermal effect, are involved, the composites are then photo-driven, which can be controlled remotely locally and precisely."

Because the thermoplastic they used is semicrystalline, it does not have a single melting point — instead, it has a melting temperature range. The team programmed the composite at a temperature slightly higher than the uppermost point in that range. Specifically, they exposed it to light until it reached the programming temperature, then bent it into the desired U shape. After turning off the light and letting the composite cool, they had created a reversible, light-controlled actuator. At temperatures in the melting range, the U shape expanded, and at temperatures below the melting range, it contracted. Higher intensity light increased its bending angle.

They designed three different versions of the composite, with 0.5%, 1% and 3% silver nanowires by weight, respectively. When tested in air, all three could bend up to 180 degrees, a crucial ability when it comes to using them in the real world.

If the composites were sliced in two, light was all it took to heal them — in fact, light outperformed healing via a coating of thermoplastic or via a coating of the thermoplastic plus a crosslinker called dicumyl peroxide. A broken composite healed by light could be deprogrammed from a U shape to its original I shape, but composites healed the other two ways could not fully recover.

According to Yang, light exposure healed the composite because it caused chemical crosslinking in the thermoplastic. The composite's shape memory also benefited from the presence of silver nanowires, which were absent from the damaged area when the team tried to heal the composite using thermoplastic and dicumyl peroxide.

The authors recommended the composite with 1% silver nanowires by weight, as it was most efficient at self-healing. "Self-healing is also of great significance for increasing service life and saving cost," explained Yang.

She and her colleagues even used their light-driven, self-healing actuators to assemble a "frog," which could open and close its "ear," and a five-petaled "flower," each petal of which could be independently manipulated.

This project grew out of the team's interest in next-generation 4D printing. 4D printing adds the dimension of time to conventional 3D printing, as researchers strive to design objects that change in response to heat, light, electricity or other stimuli. With inspiration from the multibit screwdriver, which can accommodate a variety of drivers on a single handle, they set out to develop a 4D printing device made up of inexpensive components already programmed to perform a specific function. They soon identified the composite of silver nanowires and thermoplastic as a particularly promising material.

In this study, the researchers also tested their composite in water. The 3% silver nanowire version could reversibly bend in that setting, but it needed more intense light to do so — an unsurprising result given that water conducts heat 25 times as well as air.

According to Yang, materials like the ones she tested have considerable potential for underwater robotics. Soft robots could be designed to resemble octopods or other organisms, making them less disruptive to local ecosystems — a major concern given the continuing human impact on marine life. Like octopods, they could traverse complex underwater terrain or fit into very tight spots.

In an octopus-like design, "Each octopus foot can be designed with different functions, so that the octopus can do the collection, detection, photography and other work in different environments at the same time under the non-contact light control," Yang explained.

Yang hopes that soft robots can someday be deployed in emergency responses or disaster relief, as their flexibility would let them enter nooks, crannies and crevasses that neither humans nor hard robots can access. The robots could be controlled from afar by light, meaning they could be used to remotely locate and rescue survivors. If the robots are damaged, light could also be used to heal them.

"In the future, we believe that soft robotics will be intelligent and integrated," said Yang. For now, she and her colleagues are working to commercialize the technology they have already developed.

The study, "A self-healing composite actuator for multidimensional soft robot via photo-welding," published March 1 in Composites Part B, was authored by Mingxia Liu, Shu Zhu, Yanjia Huang, Zihui Lin, Lili Yang, and Dengteng Ge, Donghua University; and Weiping Liu, Donghua University and COMAC Shanghai Aircraft Manufacturing Co.

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