Self-healing robot recovers from being stabbed then walks off
by Karmela Padavic-Callaghan  /  December 2022

“A flexible quadruped robot can sense when it is damaged and stop moving until it heals. Robots made of soft and deformable materials can change their body shape and imitate biological tissues like muscles for prosthetics. But because they are soft, they can be susceptible to damage. Hedan Bai at Northwestern University in Illinois and her colleagues made a simple soft robot that can detect when it is harmed and then mend itself before continuing to move.

The robot is about 12 centimetres long and shaped like the letter X. It moves using compressed air that is pushed through its body, making it undulate and lift its four legs. The top of the robot is covered in a layer of self-healing sensors made from a transparent rubbery material that track the robot’s motion. If the sensor is cut, its exposed sides become chemically reactive, allowing it to fuse back together.

The researchers tested the robot’s “damage intelligence” by stabbing a sensor on its leg six times. It stopped for about a minute to let the sensor heal after each cut then resumed moving. In another experiment, they stabbed the sensors on the robot’s legs one at a time. After each stab, the robot stopped to heal for a few minutes and then changed its gait in response to the damage. “We really tried to torture these sensors as much as we can,” says Bai.

Bram Vanderborght at the Free University of Brussels in Belgium says that perfecting self-healing components for robots would make them more sustainable since only parts that are damaged too severely to self-heal – like if they are burned or covered in chemicals, for example – will have to be discarded. Eventually, soft robots with self-healing parts could be used to work in hazardous environments, while the self-healing sensors themselves could be integrated into wearable devices, like space suits where they could react to being damaged by space debris.”

“Lego minifig made out of new material can melt through a cage’s
metal bars before reassembling into its solid form on the other side”

Phase-shifting material useful for soldering devices, in vivo drug delivery
by   /  2/16/2023

“One of the many iconic moments in Terminator 2: Judgment Day was seeing the T-1000 briefly morph into a liquid to pass through the metal bars separating him from his target: a teenage John Connor. A team of engineers mimicked that famous scene with a soft robot in the shape of a Lego minifig. The robot “melts” into liquid form in response to a magnetic field, oozing between the bars of its cage before re-solidifying on the other side. The team described its work in a recent paper published in the journal Matter.

As we’ve previously reported, we traditionally think of robots as being manufactured out of hard, rigid materials, but the subfield of soft robotics takes a different approach. It seeks to build robotic devices out of more flexible materials that mimic the properties of those found in living animals. There are huge advantages to be gained by making the entire body of a robot out of soft materials, such as being flexible enough to squeeze through tight spaces to hunt for survivors after a disaster.

Soft robots also hold strong potential as prosthetics or biomedical devices. Even rigid robots rely on some soft components, such as foot pads that serve as shock absorbers or flexible springs to store and release energy. For instance, Harvard researchers built an octopus-inspired soft robot in 2016 that was constructed entirely out of flexible materials. Soft robots are more difficult to control precisely because they are so flexible.

So, for the “octobot” they replaced the rigid electronic circuits with micro-fluidic circuits. Such circuits regulate the flow of water (hydraulics) or air (pneumatics), rather than electricity, through the circuit’s microchannels, enabling the robot to bend and move. In 2021, engineers at the University of Maryland built a three-fingered soft robotic hand that is sufficiently agile to be able to manipulate the buttons and directional pad on a Nintendo controller—even managing to beat the first level of Super Mario Bros. as proof of concept.

This latest robot belongs to a class known as magnetically actuated miniature machines, typically made of soft polymers (like elastomers or hydrogels) embedded with ferromagnetic particles that have programmed magnetization profiles. These kinds of robots can swim, climb, roll, walk, and jump, as well as change their shape simply by altering the corresponding magnetic field. That makes them ideal for several biomedical applications, such as targeted drug delivery and therapy for healing ulcers. But according to the authors of the new paper in Matter, such elastomer-based composites are difficult to steer through very narrow and confined spaces where the openings are smaller than the dimensions of the material because they are essentially solids and thus have limited deformability.

Eager to find a solution, they looked to the humble sea cucumber for inspiration. Sea cucumbers are fascinating creatures with soft cylindrical bodies and mouths surrounded by retractable tentacles. Some species can even vomit toxins as a means of self-defense. But it’s the sea cucumber’s remarkable ability to loosen and tighten at will the collagen that forms the walls of their body that intrigued these engineers. This lets the sea cucumber essentially “liquefy” its body to squeeze through tiny cracks and crevices, hooking all those collagen fibers back together afterward to once again form a solid body.

The new mini-robot is made of magneto-active phase transitional matter (MPTM), capable of switching back and forth between solid and liquid states. When the MPTM is heated with an alternating magnetic field, it melts into a liquid, while ambient cooling lets it resolidify when the magnetic field is removed. MPTMs are composed of ferromagnetic neodymium-iron-boron microparticles embedded in pure gallium. The resulting material has a melting point of 30.6° C (about 87° F), so it remains solid at room temperature. In its solid form, the MPTM has excellent mechanical strength, good for bearing high loads, and versatile mobility. In its liquid phase, the microparticles can rotate and reorient their magnetic polarity to lengthen, divide, and merge as needed.

“Demonstration of MPTM smart-soldering robot for circuit repair.”

“The magnetic particles here have two roles,” said co-author Carmel Majidi, a mechanical engineer at Carnegie Mellon University. “One is that they make the material responsive to an alternating magnetic field, so you can, through induction, heat up the material and cause the phase change. But the magnetic particles also give the robots mobility and the ability to move in response to the magnetic field.”

In addition to the demonstration involving an MPTM minifig escaping from a cage, Majidi and colleagues demonstrated a smart soldering machine capable of manipulating and fusing electronic components for circuit assembly and repair. Aided by MPTM’s wireless functionality and metal-like electrical conductivity, they were able to remotely control an MPTM device to transport light-emitting diodes (LEDs) to specific spots in circuits using an external magnetic field. The device wrapped the LED pins, then melted into liquid phase to form electrical connections between the pins and soldering pads. The soldering process was completed when the device cooled, ultimately creating a fully functional LED circuit.

“Demonstration of MPTM robot used to clear foreign body from artificial stomach.”

The team also made an MPTM “universal screw” capable of assembling parts in hard-to-reach tight spaces. Damaged or dislodged screws can cause havoc to the safe operation of precision hardware. For this proof of concept, two remotely controlled MPTMs passed through a narrow space and settled to the top of threaded holes. Upon heating, the MPTMs turned into liquid and filled the threaded holes to form screws, which solidified when cooled, thereby joining two plastic plates together.

MPTM robots are equally well-suited for certain in vivo biomedical applications, per the authors. For instance, the team demonstrated a minimally invasive miniature machine that removes a foreign object from an artificial model stomach filled with water. Here, one would need to tailor the melting point to be a bit higher than human body temperature (around 38° C, or 100° F) by embedding the microparticles in a gallium-based alloy instead of pure gallium. The MPTM can maneuver through the stomach in its solid form to locate the foreign object (a ball in the demonstration), melt into its liquid phase to encompass the object, then cool back into a solid so the enclosed object can be safely removed as the MPTM exits the body.

“Demonstration of MPTM robot used for drug delivery in model stomach.”

Finally, the team demonstrated an MPTM capsule capable of providing on-demand drug delivery, also tested in an artificial model stomach filled with water. The solid capsule contains the drug (modeled with fluorescein sodium in the experiment) and is maneuvered into place in the stomach. Upon melting, the drug is released. The capsule then cools and solidifies, even speeding up diffusion of the drug by spinning to create turbulence in the water, before being removed from the stomach. That doesn’t mean we’ll all be having MPTM devices injected into our bodies any time soon. “Future work should further explore how these robots could be used within a biomedical context,” said Majidi. “What we’re showing are just one-off demonstrations, proofs of concept, but much more study will be required to delve into how this could actually be used for drug delivery or for removing foreign objects.”

DOI: 10.1016/j.matt.2022.12.003 Magnetoactive liquid-solid phase transitional matter, Matter, 2023

Gallium: liquid metal could transform soft electronics
by Kurt Kleiner  /  05.03.2022

“By harnessing the unusual properties of a liquid metal called gallium, materials scientists aim to create a new generation of flexible devices for virtual reality interfaces, medical monitors, motion-sensing devices and more. The goal is to take the functionality of electronics and make them softer, says Michael Dickey, a chemical engineer at North Carolina State University. “I mean, the body and other natural systems have figured out how to do it. So surely, we can do it.” Bendable electronics can also be made with conventional metals. But solid metal can fatigue and break, and the more that’s added to a soft material, the more inflexible the material becomes.

“Applications of LM in soft electronics.”

Liquid metals don’t have that problem, Dickey says — they can be bent, stretched and twisted with little or no damage. Flexibility turns out to be just one of gallium’s useful properties. Since it’s a metal, it conducts heat and electricity easily. Unlike the better-known liquid metal mercury, it has low toxicity, and low vapor pressure, so it doesn’t evaporate easily. Gallium flows about as easily as water.

But in air it also quickly forms a stiff outer oxide layer, allowing it to be easily formed into semisolid shapes. The surface tension, which is 10 times that of water, can even be varied by submerging the liquid metal in salt water and applying a voltage. “I’m biased, so take this for what it’s worth. But I think this is one of the most interesting materials on the periodic table because it’s got so many unique properties,” says Dickey, coauthor of an overview of gallium in the 2021 Annual Review of Materials Research.

Gallium has many potential applications in materials science. (1) The reactivity of its surface makes it useful for carrying out chemical reactions; (2) Its ability to self-heal and its liquid state could be harnessed for generating or storing energy; (3) It can be easily reconfigured into different shapes for circuits, optics and more; (4) Its liquid state may have acoustic and fluidic uses; (5) It works well for wearable, bendable electronics; and (6) it could be used to create “tactile logic” devices that respond to environmental stimuli such as touch.

Interest in gallium lagged in the past, partly because of the unfair association with toxic mercury, and partly because its tendency to form an oxide layer was seen as a negative. But with increased interest in flexible and, especially wearable electronics, many researchers are paying fresh attention. To make bendable circuits with gallium, scientists form it into thin wires embedded between rubber or plastic sheets. These wires can connect tiny electronic devices such as computer chips, capacitors and antennas. The process creates a device that could wrap around an arm and be used to track an athlete’s motion, speed or vital signs, for instance, says Carmel Majidi, a mechanical engineer at Carnegie Mellon University.

These liquid metal wires and circuits can stand up to significant bending or twisting. As a demonstration, Dickey made earbud wires that can stretch up to eight times their original length without breaking. Other circuits can heal themselves when torn — when the edges are positioned against each other, the liquid metal flows back together. Gallium circuits can also be printed and applied directly to the skin, like a temporary tattoo.

The “ink” works like a conventional electrode, the kind that is used to monitor heart or brain activity, says Majidi, who made such a circuit by printing the metal onto a flexible material. The tattoos are more flexible and durable than existing electrodes, making them promising for long-term use. The shape-shifting quality of the liquid metal opens up other potential uses. When the metal is squeezed, stretched and twisted, its shape changes, and the change in geometry also changes its electrical resistance. So running a small current through a mesh of gallium wires allows researchers to measure how the material is being twisted, stretched and pressed on.

This principle could be applied to create motion-sensing gloves for virtual reality: If a mesh of gallium wires were embedded inside a thin, soft film on the inside of the glove, a computer could detect the changes in resistance as the wearer moves their hand. “You can use it to track your own body’s motion, or the forces that you’re in contact with, and then impart that information into whatever the virtual world is that you’re experiencing,” Majidi says.

This property even raises the possibility of machines that use what Dickey calls “soft logic” to operate. Rather than requiring computation, machines using soft logic have simple reactions based directly on changes in electrical resistance across the grid. They can be designed so that pushing, pulling or bending different parts of the grid activates different responses. As a demonstration, Dickey created a device that can turn motors or lights on and off depending entirely on where the material is pressed. “There’s no semiconductors here. There’s no transistors, there’s no brain, it’s just based on the way the material is touched,” Dickey says.

Low-level tactile-based logic like this could be used to build responsiveness into devices, akin to building reflexes into soft robots — such reactions don’t require a complex “brain” to process information, but can react directly in response to environmental stimuli, changing color or thermal properties or redirecting electricity. And that outer oxide layer that forms when gallium is exposed to air is now being taken advantage of.  The oxide layer allows the metal to hold its shape, and opens up all sorts of possibilities for patterning and fabrication.

Tiny drops of gallium can be stacked high on top of one another. A drop of gallium can be dragged along a surface, leaving a thin trail of oxide that can be used as a circuit. In addition, in water the oxide layer can be made to form and disappear by applying a tiny amount of voltage, causing the beads to form and collapse instantly. By switching back and forth, Dickey can make the beads move a weight up and down. With refinement, this property could form the basis of artificial muscles for robots, he says. Dickey admits that the technology is still in its early stages, and that the work so far merely suggests how it could be commercialized.

But gallium has so many interesting properties it’s bound to be useful in soft electronics and robotics, he says. He compares the field with the early days of computing. Although the earliest experimental computers made with vacuum tubes and mechanical switches are crude by today’s standards, they established principles that gave rise to modern electronics. Majidi says he also expects to see liquid metal used commercially in the near future. “In the next few years, you’re going to see more and more of this transition of liquid metal technologies out in industry, in the marketplace,” he says. “It’s not really so much a technical bottleneck at this point. It’s about finding commercial applications and uses of liquid metal that actually do make a difference.”



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