Soft magnetic metamaterial revolutionizes implantable, ingestible devices

Researchers at Rice University created a soft but sturdy material that can transform in the blink of an eye when exposed to a basic handheld magnet—and then stabilize with no batteries or electricity. The breakthrough holds promise for medical devices that can operate inside the body and soft robots that can adapt on the fly.

On first impression, the material looks like silicone rubber. It is distinguished by the presence of minute hard-magnetic particles that give it a type of built-in memory. On the application of a magnet's stimulus, the structure flips back and forth between two or more structures and freezes. Once set, it stays so until the magnetic field is reversed.

This stability is no mean achievement. Most magnetic soft devices recover their shape as soon as the external field is taken away. Others require heat or light to sustain shape, which can limit where they can operate safely.

The Rice University team, directed by mechanical engineer Yong Lin Kong, eliminated this with the addition of multistability to their design. Multistability is the fact that the material has multiple stable states, as if a snap bracelet that snaps into position and won't move unless deliberately reset.

Instead of relying on stiff pieces, Kong's group kept the material soft throughout. They made it stable through exact geometry, cutting trapezoidal supports and reinforcing zones into each miniaturized unit cell. This geometry trick brought energy barriers that prevent the structure from collapsing back once it snaps into another shape.

"In by incorporating all these features, we encoded multistability into a soft structure," Kong said. "The energy barriers keep it in place even after the actuation force is removed."

The performance is amazing. In tests on the laboratory bench, the material supported loads greater than ten times its weight, endured freezing and boiling temperatures, and continued to function after being submerged in acid or exposed briefly to open flame. Even when stretched almost to twice its original size or smashed by heavy impact, it still changed shapes when magnetized.

Under a magnetic field, the material changes form within a mere 0.15 seconds. Control is simple since no wires or onboard power are needed: rotate the magnet's orientation in one direction to shut the formation and in the opposite direction to open it again.

The researchers also showed that they could "program" different sections of a structure to react at different levels of magnetism. By adjusting measures like beam angle or support, they orchestrated changes row by row. This ability enables complex movements like bending, tilting, or swelling in stages—all controlled from a distance with little more than a magnet.

To show the material could last longer than the lab bench, the researchers put it to the tests. Cylindrical forms held their form in high-speed air flows, whirling water, and direct water spray. Even after being left in replica stomach acid for a week, the material functioned almost as well as at the start.

In another experiment, the memory of magnetic particles was burned by a flame but not that of the silicone matrix. After the particles were reprogrammed, the material regained complete functionality. That level of resilience is essential for medical and robotics applications where failure is not acceptable.

The researchers built several working models to show what this technology would be capable of. One of the highlights was a wireless peristaltic pump built out of eight pieces of metamaterial wrapped around a soft tube. By passing a magnet along the outside, they could compress fluid through the tube in controlled spurts. The pump rejected back pressure without drawing any energy to stay closed.

Additional demonstrations included incorporating lights within the structure to show deployment from beneath a surface and writing sections to incline independently. These proof-of-concepts imply real-world devices for medicine and engineering.

Because the metamaterial is inert and soft, it sidesteps some of the dangers of hard implants, such as puncture or inflammation. Kong and his team envision ingestible systems that expand in the stomach, drug-releasing implants, and wireless pumps or valves that can function deep inside the body.

The metamaterial makes remote control of size and shape of devices in the body possible," Kong said. "We're using this to develop ingestible devices that could potentially treat obesity someday or assist marine mammals. We are working with surgeons at the Texas Medical Center to build wireless fluidic control systems for unmet medical needs."

This study paves the way for a new generation of small devices that expand or reshape as needed, safely. In medicine, that could mean swallowable devices for losing weight, delivering medication, or minimally invasive surgery.

Outside medicine, it could result in shape-shifting soft robots for different tasks or fold-and-unfold antennas without the need for a motor.

Because the material is retained without continuous power, these systems would not only be energy efficient but also be reliable.

Research findings are available online in the journal Science Advances.

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