(Image via: ncsu.edu)

Researchers at North Carolina State University have demonstrated a caterpillar-like soft robot that can move forward, backward, and dip under narrow spaces. Its movement is driven by a novel pattern of silver nanowires that use heat to control the way the robot bends — allowing users to steer the robot in either direction.

“A caterpillar’s movement is controlled by local curvature of its body — its body curves differently when it pulls itself forward than it does when it pushes itself backward,” said corresponding author Dr. Yong Zhu. “We’ve drawn inspiration from the caterpillar’s biomechanics to mimic that local curvature, and use nanowire heaters to control similar curvature and movement in the caterpillar-bot.

“Engineering soft robots that can move in two different directions is a significant challenge in soft robotics. The embedded nanowire heaters allow us to control the movement of the robot in two ways. We can control which sections of the robot bend by controlling the pattern of heating in the soft robot. And we can control the extent to which those sections bend by controlling the amount of heat being applied.”

The caterpillar-bot comprises two layers of polymer that respond differently when exposed to heat: The bottom layer shrinks, or contracts, when exposed to heat, while the top layer expands in the same circumstance. A pattern of silver nanowires is embedded in the expanding layer of polymer — the pattern includes multiple lead points where researchers can apply an electric current. The researchers can control which sections of the nanowire pattern heat up by applying an electric current to different lead points, and they can control the amount of heat by applying more or less current.

“We demonstrated that the caterpillar-bot is capable of pulling itself forward and pushing itself backward,” said first author Shuang Wu. “In general, the more current we applied, the faster it would move in either direction. However, we found that there was an optimal cycle, which gave the polymer time to cool — effectively allowing the ‘muscle’ to relax before contracting again. If we tried to cycle the caterpillar-bot too quickly, the body did not have time to ‘relax’ before contracting again, which impaired its movement.”

The team also demonstrated that the caterpillar-bot’s movement could be controlled to the point where users were able steer it under a very low gap — akin to slipping under a door. In essence, the researchers could control both forward and backward motion as well as how high the robot bent upwards at any point in that process.

“This approach to driving motion in a soft robot is highly energy efficient, and we’re interested in exploring ways that we could make this process even more efficient,” said Zhu. “Additional next steps include integrating this approach to soft robot locomotion with sensors or other technologies for use in various applications — such as search-and-rescue devices.”

Here is a Tech Briefs interview — edited for length and clarity — with Zhu.

Tech Briefs: What inspired your research?

Zhu: Soft robotics have received much interest recently. Most soft crawlers, however, can crawl only in one direction. In contrast, caterpillars can easily crawl in both directions. Therefore, we were interested in learning how caterpillars crawl and realizing bidirectional locomotions in soft robotic devices.

Tech Briefs: What were the biggest technical challenges you faced?

Zhu: Assembly of the nanowire heater and the thermally active materials (PDMS, liquid crystal elastomers) was a major challenge. We tried a few different sandwiching sequences and combinations until we found a functioning structure with strong adhesion and stable heating performance.

Tech Briefs: Can you explain, in very simple terms, how the technology works?

Zhu: Caterpillars realize bidirectional locomotions by adjusting the body curvature distribution. We fabricated a soft crawler, whose locomotion is enabled by joule heating of a patterned soft heater in a thermal bimorph actuator. With patterned and distributed heaters and programmable heating, different temperature and hence curvature distribution along the body of the robot are achieved, enabling bidirectional locomotion.

Tech Briefs: Do you have any set plans for additional next steps?

Zhu: We have some rough plans. For example, our lab has been working on soft sensors such as strain sensors. It would be straightforward to integrate the soft robots with the soft strain sensors to monitor the curvature along the body of the robot, which can provide feedback to optimize the locomotions.

Tech Briefs: Do you have any advice for engineers aiming to bring their ideas to fruition?

Zhu: We can all come up with many ideas. Focus on those useful ideas. When I say “useful,” I mean those that can solve real problems of societal importance. After that, work hard and don’t give up until you bring the ideas to fruition.