A new semiconducting polymer that is stretchable and can heal when damaged could lead to electronic skin that mimics the real thing. The polymer could serve as the base for wearable electronics that need to flex and stretch, and that face mechanical abuse during wear.

The polymer design uses intermolecular hydrogen bonds that can be repeatedly broken and reformed. An international team of scientists and engineers from Canada, Japan, South Korea, Taiwan, the United Kingdom, and the United States reported the new material and the design concept in the journal Nature.

Thin-film field-effect transistors—semiconductor devices that form the basic building blocks of logic circuits—are the essential components of electronics. They are traditionally made of hard, brittle silicon. To make them supple for wearable devices and displays, scientists have typically used rubbery polymers that are blended with semiconducting nanofibers or nanowires. “These composites can be highly stretchable but since they mix insulating materials with semiconducting materials, the charge transport gets affected,” says Stanford University chemical engineer Zhenan Bao.

Bao and her colleagues instead took a molecular design approach to make a semiconducting polymer that is stretchable but also has high charge-transport ability, or carrier mobility. Furthermore, it is easy to heal the tiny nanometer-scale cracks that form in the polymer after repeated drastic stretching. All that is required is heating the material in the presence of chloroform vapors, after which the material almost completely recovers its electronic properties.

The researchers started with the semiconducting polymer 3,6-di(thiophen-2-yl)- 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (or DPP). During the synthesis of the polymer, they added monomers of 2,6-pyridine dicarboxamide (PDCA).

The PDCA breaks up the DPP’s crystalline structure, making it softer and more flexible. The resulting polymer has semiconducting crystalline DPP units, which transport charge, connected by amorphous PDCS regions.

Each PDCS unit contains two amide groups bound together by relatively weak hydrogen bonds. These bonds crosslink the PDCS chains and DPP polymers into a network. When the material is stretched, the amorphous PDCS chains elongate and the hydrogen bonds break to dissipate mechanical energy, Bao explains, “and this bonding can partially recover when the strain is released to allow some repair.”

The researchers used the polymer to make high-performance field-effect transistors that have a charge mobility of 1.3 cm²/V/s. The material retains a mobility of over 1 cm²/V/s and the transistor’s performance does not change even after it is stretched and released 500 times by a quarter of its length, which is typical of most practical applications.

The ability to heal themselves is a feature that has not been seen before in stretchable electronic devices, says Siegfried Bauer, a soft matter physicist at Johannes Kepler University in Linz, Austria. There are several challenges ahead before the devices could be used on the human body, he says. They will need to be more durable and stretchable, and will need to be run on much smaller voltages.

This work “is a milestone in the search for electronic skins that behave much like their archetype,” Bauer and his colleague Martin Kaltenbrunner write in a News & Views article accompanying the research paper. “In the shorter term, healable soft electronic devices hold promise for truly bionic and smart electrical appliances, and might revolutionize future generations of wearables.”

Read the abstract in Nature.