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Self-Healing Materials Extend Robotic Life Span

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What are the capabilities of a soft, self-healing robot? | Rahkendra Ice, Juwon Song, & Colin O'Connor / AAAS

Scientists have constructed a series of eco-friendly soft robots that can self-heal on demand, leaving almost no traces of weakness at the location of their "scars."

Their self-healing power relies on the material they are composed of: a rubbery polymer that has all the classic characteristics of soft robotic bodies — flexibility, elasticity and overall durability — with the added benefits of self-repair and recyclability.

Similar materials have already been used in real-world situations, such as for shooting stands that seal after a bullet has passed through, or coatings of cars that can smooth out scratches. Incorporating these materials into robots — especially those made to interact with humans — is the next step for translating the skill of "self-healing" into the artificial realm.

"Our research opens up promising perspectives. Robots can not only be made lighter and safer, but will also be able to work longer independently without requiring constant repairs," said Bram Vanderborght, professor at Vrije Universiteit Brussel and co-author of the study appearing in the 16 August issue of Science Robotics.

He mentioned the scene in the Disney movie "Big Hero 6," in which its protagonist, the soft robot Baymax, tries to seal its cuts with tape, as the basic essence of this project.

Soft robots are ideal for a variety of functions, such as moving across rough terrain, entering tiny spaces and shifting their outer shape. These functions can be deployed in many real-world applications, from those that require frequent human interaction, such as medical aid or social assistance, to those in demanding environments, such as industry, construction or search-and-rescue missions.

The softness, flexibility and stretchiness of these bots are often inspired by the tissue of living organisms that offers critical protection as well as quick adaptation to damage. Unfortunately, many soft robot prototypes are missing this latter quality.

"The soft materials used for the robots' construction are highly susceptible to damage, such as cuts and perforations caused by sharp objects present in the uncontrolled and unpredictable environments they operate in," said Vanderborght. They can also be damaged from "overpressuring" caused by too much air flow inside the robotic structure — a typical method for controlling soft robotic movement.

To patch up these recurring issues, Vanderborght and his team looked for ways to help soft robots repair their own injuries as they occur. This capacity to self-heal has barely made an entrance into robotics — at least, not with materials.

Vanderborght mentioned the scene in the Disney movie "Big Hero 6," in which its protagonist, the soft robot Baymax, tries to seal its cuts with tape, as the basic essence of this project.

According to Vanderborght, "Self-healing robotics has been achieved mainly on the software level, like, for example, the work of Jean-Baptiste Mouret , in which a robot learns to adapt to its injuries." (When Mouret's robot breaks its back legs, it reprograms its software so that it learns to walk on its front legs.) By contrast, Vanderborght's team endowed self-healing to the robots with materials that allow the bot to regain all of its original mechanical properties.

Using temperature-sensitive elastomer material, the researchers built a soft robotic gripper, hand and artificial muscles that were able to self-repair both large and tiny cuts in 20 to 40 minutes when exposed to mild heat (around 80 degrees Celsius).

The heat breaks the elastomer's firm structure, comprised of cross-linked chemical bonds, eventually making the material mobile enough to seal the cut. Then, upon cooling to around 25 degrees Celsius, the bonds reform, completely "healing" the rupture.

The robots' recovery efficiency was around 98% to 99%, the researchers calculated.

"We did not develop particularly unique robots; the pneumatic muscles, grippers and hands are already described in literature. Our unique contribution is that we demonstrated these prototypes could be developed with self-healing materials," noted Vanderborght.

What's more, his team showed that the material was recyclable. By dissolving used elastomer in chloroform, it could be repurposed into the same self-healing substance, though with a slightly decreased (81%) recovery efficiency.

Such quick and easy self-healing abilities will help prepare soft robots for the real world. "One of the foremost challenges of soft robotics is that more prototypes need to find their way to the market. With our technology we made a step towards solving this challenge," Vanderborght said. "We think our study is just the start of many potential applications."

[Associated image credit: Terryn et al., Sci. Robot. 2, eaan4268 (2017)]