Harnessing H-bonds stretches electronics

Semiconducting polymers could transform electronics from hard, brittle objects to flexible, wearable devices. Now researchers have found a way to use hydrogen bonding to enable the fabrication of stretchable, self-healing semiconducting polymer transistors that could pave the way for wearable electronics [Oh et al., Nature (2016), doi: 10.1038/nature20102].

In recent decades scientists have focused on polymers with delocalized electrons – known as –conjugated molecules. Hydrogen bonds between polymer molecules, which readily break and reform, were thought to be detrimental to electron conduction. But Zhenan Bao and her coworkers at Stanford University and SLAC National Accelerator Laboratory decided to incorporate hydrogen bonding into their semiconducting polymer networks deliberately and the results are impressive.

The team’s polymers contain crystalline regions, to enable charge transport to take place, interspersed with amorphous sections cross-linked by hydrogen bonds (Fig. 1). The hydrogen bonds in the tightly tangled amorphous regions break and reform easily, allowing the material to be stretched without affecting the crystalline conductive parts.

“The design concept of our stretchable semiconductors relies on dynamic chemistry to incorporate chemical units (or moieties) that can form intermolecular hydrogen bonds between the polymer chains,” explain first authors Jin Young Oh, Simon Rondeau-Gagné, and Yu-Cheng Chiu. “The non-conjugated units, based on a pyridine dicarboxamide motif (or PDCA), are introduced directly into the polymers’ backbone via chemical synthesis.”

The result is a semiconducting polymer that can be repeatedly stretched without altering its electronic properties (Fig. 2). But not only that, when the material is stretched to the point of cracking, the polymer can repair itself if treated with heat and solvent vapor, which prompts the hydrogen bonds to reform, restoring the electronic properties.

“Stretchy mechanics and efficient charge transport typically do not go together,” says John A. Rogers of the University of Illinois at Urbana-Champaign. “But the authors describe some clever chemistries that seem to capture both properties in a single material.”

The researchers demonstrate just how useful semiconducting polymers with these properties are by fabricating proof-of-concept stretchable field-effect transistors (FETs). The devices can maintain their electronic properties despite repeated stretching cycles in the lab and in demonstration devices on the skin.

“Our concept is expected to develop skin-inspired electronic devices that can be stretchable and healable,” say Oh, Rondeau-Gagné, and Chiu.

In an accompanying News & Views, Siegfried Bauer and Martin Kaltenbrunner of Johannes Kepler University in Austria point out that the new devices are not as durable or stretchable as those based on other approaches and require higher voltages [Bauer and Kaltenbrunner, Nature 539 (2016) 365]. But if these shortcomings can be overcome,they believe that the new approach holds promise for bionic and smart electrical appliances, and ultimately wearable electronics as soft, flexible, and repairable as human skin.

Rogers echoes their conclusions, commenting: “It will be interesting to see if the same design principles can lead to the development of analogous materials but with the higher performance levels needed to support advanced applications.”

The researchers are now working on improving the mechanical durability of the material under high strains, say Oh, Rondeau-Gagné, and Chiu, as well as developing autonomous self-healing capabilities.

This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.12.003