Researchers have drawn inspiration from armadillos to create a protective structure that responds to external threats by curling into a protective ball to protect electronic devices or other payloads. The structure is designed to automatically respond when it detects strain and can be tuned to respond to anything from a delicate touch to a significant impact.
"There has been a great deal of growth in the fields of soft robotics and flexible electronics, but those devices are often also fragile," says Yong Zhu, corresponding author of a paper on the work and the Andrew A. Adams Distinguished Professor of Mechanical and Aerospace Engineering at North Carolina State University. "Our goal was to develop a solution that allows these fragile technologies to function but protects them when necessary."
"In its relaxed state, the structure we've developed is fairly flexible, but it can be activated to curve into a rigid external structure," says Jianyu Zhou, first author of the paper and a postdoctoral researcher at NC State. "We could see this technology being used to protect many types of objects – essentially anything it is capable of curving around."
The robo-armadillo, which the researchers call the morpho-interlocking protective module (MIPM), consists of three general layers. The outer layer, or exoskeleton, consists of a series of segmented, curved scales which are made from a 3-D printed resin. The middle, "sensing and actuation" layer, consists of four parts: a liquid-crystal elastomer (LCE), that contracts when heated; a strain sensor made of elastic polymer embedded with silver nanowires; a layer of kapton tape that expands when heated; and then a thin layer of conductive fabric that serves as a "heater" layer. Lastly, there is an endoskeleton layer that consists of heavy-duty paper folded into a series of ridges, which hold a row of rigid polymer "segmental scales" in place.
When the strain sensor detects a touch or impact it signals a control unit, which then sends power to the heater layer. As the heater layer warms up, it causes the LCE layer to contract and the kapton tape layer to expand, causing the entire structure to curve. The end result is that the MIPM structure curls into a protective circle with the exoskeleton facing out.
"As the layers curve into a circle, the segmental scales in the MIPM's endoskeleton lock into each other – creating a robust internal 'skeleton' that contributes to the sturdiness of the structure," says Zhou.
In proof-of-concept testing, the researchers found the MIPM works as intended, with the sensor layer detecting increased strain and triggering the transformation into a protective shell. The researchers also found that increasing the number of segmental scales in the endoskeleton significantly improves the structure's internal rigidity and strength.
"Through mechanics-guided design, we established a trade-off between endoskeleton segmentation and structural lightweighting," says Zhu. "As an example, 10 segmental scales were capable of withstanding around 10 newtons of force.
"We've demonstrated a combination of flexibility and mechanical protection that has a lot of potential, and we welcome collaborations from those who are interested in exploring possible applications," says Zhu. "We're also very interested in pursuing additional opportunities to advance work on flexible yet protective technologies that draw on nature for inspiration."
The paper, "Armadillo-Inspired Active Morphing Skeletons for Soft Machines," will be published May 27 in the open-access journal Science Advances. The paper was co-authored by Weixin Zhou, a postdoctoral researcher at NC State; Seol‐Yee (Jennifer) Lee and Ali Akbari, Ph.D. students at NC State; and Shuang Wu, a former Ph.D. student of NC State who is now an assistant professor of mechanical engineering at the Florida Institute of Technology.
This work was done with support from the National Science Foundation under grant 2134664; and from the Department of Defense under grant W81XWH-21-1-0185.
