Self-healing is an increasingly common ability in the world of new materials, with a wide range of gels and polymers potentially lending their talents to electronic circuits, paints and even spaceship hulls. Now, researchers have developed a stretchy, transparent material that can not only repair itself, but act as an ionic conductor, opening the possibility for self-healing artificial muscles.
Apparently inspired by Wolverine, the Marvel character with accelerated healing powers, the new project builds on work by a Harvard team a few years ago. That team, which included Christoph Keplinger, one of the authors of the current paper, developed a rubbery, flexible loudspeaker powered by ionic conductors. In addition to communicating an electric charge through ions instead of electrons, the materials were transparent and could stretch to several times their original length with no loss of function.
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But self-healing is a new trick for an ionic conductor to perform, since electrochemical reactions seen in conduction would normally weaken the bonds between the molecules of a self-healing polymer. The researchers claim that their material is the first ionic conductor that is transparent, stretchable, and self-healing.
"Creating a material with all these properties has been a puzzle for years," says Chao Wang, another author of the paper. "We did that and now are just beginning to explore the applications."
The team overcame the incompatibility of self-healing and conduction by making use of a mechanism known as ion-dipole interaction. Wang and the team used a polar polymer – meaning its molecules have both a positive and negative charge – and combined it with a high-ionic-strength salt, which allows the material to maintain its molecular bonds even while subject to electrochemical reactions.
The result is a material that can be stretched to 50 times its usual size, and can completely heal itself within 24 hours after being cut, with no lasting damage or performance loss. It only takes five minutes for the material to repair itself enough to be stretched to about twice its normal length, and unlike some other materials, it doesn't take any external stimulation to trigger the process: it happens naturally and effectively at room temperature.
The team then put the material to the test in a dielectric elastomer actuator, or an artificial muscle. Made up of a clear, non-conductive membrane sandwiched between two layers of the new material, these manmade muscles move by responding to electrical signals, just like their natural counterparts. As the researchers demonstrated, the artificial muscle could be cut into two pieces and heal back to its original state, performing just as well as it had before the injury.
Along with powering artificial muscles, the researchers believe their material, which is relatively low-cost and easy to make, could be used to build better biosensors for medical and environmental monitoring, longer-lived batteries and even self-healing robots.
The research was published in the journal Advanced Materials.
The team describes the material in the video below.