The next generation of fitness trackers and health monitors may not be little boxes we wear around our necks or on our wrists, they could be flexible wearables or temporary tattoos. But when such devices get damaged, they'll need to be replaced. And when they're no longer needed, they'll end up on the growing mountain of e-waste. Researchers from the University of Colorado Boulder have developed an electronic skin embedded with a host of sensors that can be bent and twisted to custom fit its owner. And if it gets damaged, the e-skin can be healed, and it can be completely recycled at the end of its useful life.

Coming from a team led by Jianliang Xiao of the university's Mechanical Engineering wing, the new e-skin makes use of a special polymer called a polyimine that's been doped with silver nanoparticles to improve mechanical strength, chemical stability and electrical conductivity. The newly-developed thermoset has also been embedded with pressure, temperature, humidity and air flow sensors.

By slightly heating the electronic skin, it can be shaped to comfortably fit the contours of a human limb or irregular surfaces like a robotic hand by applying a little pressure. But its operational lifespan can be extended by repairing damage, and it can be fully recycled.

"What is unique here is that the chemical bonding of polyimine we use allows the e-skin to be both self-healing and fully recyclable at room temperature," said Xiao in a press statement. "Given the millions of tons of electronic waste generated worldwide every year, the recyclability of our e-skin makes good economic and environmental sense."

As mentioned earlier, the e-skin and embedded sensors can be healed in the event of a cut, with the help of terephthalaldehyde, diethylenetriamine, tris(2-aminoethyl)amine and silver nanoparticles mixed in ethanol and the application of heat pressure. The researchers do point to a slight reduction in sensing performance in a rehealed e-skin device compared to a fresh module though.

At the end of its operational life, the e-skin can be soaked in the ethanol solution, which causes the polymers to degrade and the silver nanoparticles to sink to the bottom. "The recycled solution and nanoparticles can then be used to make new, functional e-skin," said Xiao.

The research team noted that the formula used in this project achieved "relatively low stretchability and high strength," and says that fabrication method tweaks and formula modifications could yield electronic skins that are more stretchy, if needed.

Xiao told us that the current device is being powered by an external supply, but "it's also possible to include rechargeable, small button batteries to power the device."

A paper detailing the development has been published in the journal Science Advances.