Kirigami snake robot upgrade, crawling through inflation and deflation cycles

A research team at Harvard University's John A. Paulson School of Engineering and Applied Sciences (SEAS) brought the world a snake-like soft robot based on the kirigami principle. A year later, the team is back with a smarter upgraded version of kirigami.

The first generation of Kirigami snake-like robot

With their sleek bodies, snakes can slither at speeds of up to 14 miles per hour, squeeze into tight spaces, climb trees, and swim. How do they do it? It's all because of scaling. As a snake moves, its scales grip the ground and propel the body forward, much like crampons help hikers gain a foothold on ice. This so-called "friction-assisted locomotion" is possible because of the changing shape and position of a snake's scales.

So, a team of researchers from Harvard's Wyss Institute and Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed a soft robot that can crawl like a snake without hard parts. The soft robot is made using kirigami, an ancient Japanese paper craft that relies on cutting rather than origami folding to change the properties of the material. As the robot stretches, the flat kirigami surface is transformed into a 3D textured surface that grips the ground like snake skin. The research was also published in the journal Science Robotics.

"In recent years, there has been a lot of research into how to make these kinds of deformable, stretchable structures," said Ahmad Rafsanjani, Ph.D., a postdoctoral fellow at SEAS and the paper's first author. "We've shown that kirigami can be integrated into soft robots to achieve locomotion in a way that's simpler, faster, and cheaper than most previous techniques."

The researchers used a laser cutter to carve microscopic cuts into a sheet of plastic , then wrapped the material around an inflatable and deflated silicone tube. When inflated, the cuts pop up and grip the ground. When deflated, the robot moves forward. The tube itself is wrapped with Kevlar fibers to keep the robot in its proper shape.

When the actuator inflates, the kirigami cutouts pop out, creating a rough surface that grips the ground. When the actuator deflates, the cutouts fold flat, propelling the track forward.

Researchers at Harvard University also published a paper on a new type of robot, which mentioned that by utilizing the "anisotropic frictional properties" of snake scales and finding design inspiration from paper-cutting art, they were able to create an inflatable flexible robot that can crawl through a cycle of inflation and deflation.

When all the scales point in the same direction, the available friction increases, making it easier for the snake to move forwards than backwards. While this makes it harder to move backwards, it also means that the snake can first spread all its scales to gain forward momentum, and then pull them together along the abdomen to push the body backwards. This way, as long as the friction of the scales is equal when moving forward and backward, the snake can move back slightly. Furthermore, because the scales are oriented, one side is smooth and the other side is rough. This means that as long as the snake can stay on the surface, it can move forward smoothly. It is worth mentioning that this is also the working principle of bristlebots.

In order to create a scaly skin similar to snake scales, the team made various stretchable plastic sheets, tried a variety of different cut shapes, and each scale was engraved with a unique pattern by laser etching. This structure allows the robot to expand its body and stretch the scale material, and the originally even scales will deform and pop out of the robot body, thereby grabbing the ground and converting the repeated expansion of the body into forward movement. This method is simple, low-cost and very effective.

After cutting with triangles and circles, the team finally found that trapezoidal scales were best suited for this particular snake robot, not because the trapezoidal shape produced more friction, but on the contrary, the trapezoidal shape allowed the scales to fully stretch, thus helping the robot to produce a longer "stride" when expanding its body. As long as the scale design can effectively anchor itself to the ground when the robot moves forward (providing a stronger grip), the robot can quickly convert forward movement into stretching in place.

"We demonstrated that it is possible to exploit the locomotive properties of these kirigami skins by properly balancing the cutting geometry and actuation protocol," Rafsanjani said. "In the future, these components will be further optimized to improve the responsiveness of the system."

Wyss and Harvard researchers have designed a completely untethered soft robot with a tiny tail that integrates onboard control, sensing, actuation, and power.

“We believe we have opened a path for designing a new class of soft crawlers based on the kirigami theory,” said Katia Bertoldi, Ph.D., the paper’s senior author, a Wyss Institute faculty member and the William and Ami Kuan Danforth Professor of Applied Mechanics at SEAS. “These all-terrain soft robots could one day traverse difficult environments for exploration, inspection, monitoring, search and rescue missions, or perform complex laparoscopic medical procedures.”

This isn't the first time a robot has been inspired by snakes. Rescue robots developed by Stanford University and underwater repair robots by Eelume are also related to snakes. Paper-kirigami-style designs have also been used in solar cells and graphene nanotechnology. However, Harvard's snake-like robot is the first to combine the two design studies.

New generation of Kirigami snake-like robots

Now, a research team at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has created a new and improved snake-like soft robot that is faster and more precise than previous robots.

The first generation of robots used flat kirigami sheets that deformed evenly when stretched, but the new robot has a programmable shell, which means the kirigami cuts can pop up as needed, increasing the robot's speed and precision.

The research was published in the Proceedings of the National Academy of Sciences.

"This is the first example of a kirigami structure with nonuniform pop-up deformation," said Ahmad Rafsanjani, a postdoctoral researcher at SEAS and first author of the paper. "In a flat kirigami, the pop-up is continuous, meaning everything appears at once. But in a kirigami shell, the pop-up is discontinuous. This control of shape transformation could be used to design responsive surfaces and smart skins, which require changes in their texture and morphology."

The new study combined two properties of the material, the size of the cutouts and the curvature of the sheet. By controlling these features, the researchers were able to program the dynamic propagation of the pop-ups from one end to the other, or control localized pop-ups.

In previous studies, flat sheets of kirigami were wrapped around elastomeric actuators. In this study, the kirigami surface was rolled into a cylinder, with actuators applying force at both ends. If the cuts are uniform in size, the deformation propagates from one end of the cylinder to the other. However, if the size of the cuts is carefully chosen, the skin can be programmed to deform in a desired order.

"By borrowing ideas from phase-change materials and applying them to kirigami-based building blocks, we demonstrated that elastic and unstretched phases can coexist simultaneously on a cylinder," said Katia Bertoldi, the William and Ami Kuan Danoff Professor of Applied Mechanics at SEAS and senior author of the paper. "By simply combining cuts and curvatures, we can program very different behaviors."

Next, the researchers aim to develop inverse design models for more complex deformations.

"The idea is that if you know how you want the skin to deform, you can just cut it, roll it up and it's done," said Lishuai Jin, a graduate student at SEAS and co-author of the paper.

This research was also partially funded by the National Science Foundation of the United States. With the development of technology, snake-like robots will be better improved and put into more practical applications.