Stretchy Wearables Can Now Heal Themselves

If you’ve ever tried to get a bandage to stick to your elbow, you understand the difficulty in creating wearable devices that attach securely to the human body. Add digital electronic circuitry, and the problem becomes more complicated. Now include the need for the device to fix breaks and damage automatically—and let’s make it biodegradable while we’re at it—and many researchers would throw up their hands in surrender.

Fortunately, an international team led by researchers at Korea University Graduate School of Converging Science and Technology (KU-KIST) persevered, and has developed conductor materials that it claims are stretchable, self-healing, and biocompatible. Their project was described this month in the journal Science Advances.

The biodegradable conductor offers a new approach to patient monitoring and delivering treatments directly to the tissues and organs where they are needed. For example, a smart patch made of these materials could measure motion, temperature, and other biological data. The material could also be used to create sensor patches that can be implanted inside the body, and even mounted on the surface of internal organs. The biocompatible materials can be designed to degrade after a period of time, eliminating the need for an invasive procedure to remove the sensor later.

“This new technology is a glimpse at the future of remote healthcare,” says Robert Rose, CEO of Rose Strategic Partners, LLC. “Remote patient monitoring is an industry still in its early stages, but already we are seeing the promise of what is not only possible, but close on the horizon. Imagine a device implanted at a surgical site to monitor and report your internal healing progress. If it is damaged, the device can heal itself, and when the job is done, it simply dissolves. It sounds like science fiction, but it’s now science fact.”

Self-healing elastics

After being cut a ribbonlike film was able to heal itself in about 1 minute.Suk-Won Hwang

The system relies on two different layers of flexible material, both self-healing: one is for conduction and the other is an elastomer layer that serves as a substrate to support the sensors and circuitry needed to collect data. The conductor layer is based on a substance known by the acronym PEDOT:PSS, which is short for Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. It’s a conductive polymer widely used in making flexible displays and touch panels, as well as wearable devices. To increase the polymer’s conductivity and self-healing properties, the research team used additives including polyethylene glycol and glycol, which helped increase conductivity as well as the material’s ability to automatically repair damage such as cuts or tears.

In order to conform to curved tissues and survive typical body motion, the substrate layer must be extremely flexible. The researchers based it on elastomers that can match the shape of curved tissues, such as skin or individual organs.

These two layers stick to each other, thanks to chemical bonds that can connect the polymer chains of the plastic films in each layer. Combined, these materials create a system that is flexible and stretchable. In testing, the researchers showed that the materials could survive stretching up to 500 percent.

The self-healing function arises from the material’s ability to reconnect to itself when cut or otherwise damaged. This self-healing feature is based on a chemical process called disulfide metathesis. In short, polymer molecules containing pairs of linked sulfur atoms, called disulfides, have the ability to reform themselves after being severed. The phenomenon arises from a chemical process called disulfide-disulfide shuffling reactions, in which disulfide bonds in the molecule break and then reform, not necessarily between the original partners. According to the KU-KIST researchers, after being cut, their material was able to recover conductivity in its circuits within about two minutes without any intervention. The material was also tested for bending, twisting, and its ability to function both in air and under water.

This approach offers many advantages over other flexible electronics designs. For example, silver nanowires and carbon nanotubes have been used as the basis for stretchable devices, but they can be brittle and lack the self-healing properties of the KU-KIST materials. Other materials such as liquid metals can self-heal, but they are typically difficult to handle and integrate into wearable circuitry.

As a demonstration, the team created a multifunction sensor that included humidity, temperature, and pressure sensors that was approximately 4.5 square centimeters. In spite of being cut in four separate locations, it was able to heal itself and continue to provide sensor readings.

Implant tested in a rat

To take the demonstration a step further, the researchers created a 1.8-cm² device that was attached to a rat’s bladder. The device was designed to wrap around the bladder and then adhere to itself, so no adhesives or sutures were required to attach the sensor onto the bladder. The team chose the bladder for their experiments because, under normal conditions, its size can change by 300 percent.

The device incorporated both electrodes and pressure sensors, which were able to detect changes in the bladder pressure. The electrodes could detect bladder voiding, through electromyography signals, as well as stimulate the bladder to induce urination. As with the initial demonstration, intentional damage to the device’s circuitry healed on its own, without intervention.

The biocompatible and biodegradable nature of the materials is important because it means that devices fabricated with them can be worn on the skin, as well as implanted within the body. The fact that the materials are biodegradable means that implants would not need a second surgical procedure to remove them. They could be left in place after serving their purpose, and they would be absorbed by the body.

According to Suk-Won Hwang, assistant professor at KU-KIST, a few hurdles remain on the path to commercialization. “We need to test the biocompatibility of some of the materials used in the conductor and substrate layers. While scalable production appears to be feasible, the high cost of disulfide derivatives might make the technology too expensive, aside from some special applications,” he says. “Biocompatibility testing and material synthesis optimization will take one to two years, at least.”