A highly stretchable conductive material that is transparent and self-healing, and can be electrically activated, has been developed by scientists, including several from the University of California. The material can be used to power artificial muscles and could improve robots, electronic devices and batteries.
The material could potentially be used for applications in a wide range of fields. Biosensors used in environmental monitoring and the medical field could be improved; the lifetime of lithium ion batteries used in electric cars and electronics could be extended; and robots could have the ability to self-heal after mechanical failure.
Chao Wang is an adjunct assistant professor of chemistry and is one of the authors of the paper. He explained that creating a material with all the properties listed has been a challenge for years. Now that this has been achieved, they can begin to explore the possible applications.
This project combines research done in the ionic conductors and self-healing materials fields. Ionic conductors play a key role in solar energy conversion, energy storage, electronic devices and sensors. Self-healing materials are inspired by wound healing in nature and repair damage caused by wear, thus extending the lifetime and lower the cost of devices and materials.
Wolverine, the comic book character who has the ability to self-heal, inspired Wang to develop an interest in self-healing materials.
Christoph Keplinger, an assistant professor at the University of Colorado, Boulder is another author of the paper. Keplinger has previously demonstrated that ionic conductors that are stretchable and transparent can be used to create transparent loudspeakers and to power artificial muscles. Although the devices exhibit several of the key properties of the new material, none of them had the ability to self-heal from mechanical damage.
The identification of bonds that are reversible and stable under electrochemical conditions is a key difficulty. Self-healing polymers normally use non-covalent bonds. This creates a problem because those bonds degrade the performance of the materials as they are affected by electrochemical reactions. Wang used a mechanism called ion-dipole interactions to help solve the problem. Ion-dipole interactions are forces between charged ions and polar molecules that are highly stabile under electrochemical conditions. Wang combined a mobile, high ionic strength salt with a polar, stretchable polymer to create the material with the characteristics the researchers were looking for.
The soft rubber-like material can stretch 50 times its original length, is low cost and easy to produce. It can completely heal, or re-attach, at room temperature within 24 hours after being cut. After only five minutes of healing, the material can in fact be stretched to two times its original length.
Keplinger has two graduate students working with him – Eric Acome and Timothy Morrissey. They showed that the material could be used to power a dielectric elastomer actuator, also called artificial muscle. Artificial muscle is a generic term used for devices or materials that can expand, contract, or rotate reversibly in response to an external stimulus such as pressure, temperature, voltage, or current.
A dielectric elastomer actuator is essentially three individual pieces of polymer that are stacked together. The middle layer is a transparent, non-conductive rubber-like membrane, while the top and bottom layers are the new self-healable, conductive material developed at UC Riverside.
Electrical signals were used to get the artificial muscle to move. The artificial muscle reacts when it receives a signal, much the same as a human muscle (such as a bicep) does when the brain sends a signal to the arm. The most important facet of the research was the team’s ability to demonstrate that the new material can self-heal. This feature mimics wound-healing, a preeminent survival feature of nature. When parts of the artificial muscle were cut into separate pieces, the material was able to heal without using external stimuli. The artificial muscle did in fact return to the same level of performance as before it was cut.
The full study was published in the journal Advanced Material.