“Our digital technology operates through a series of electronic on-off switches that control the flow of current and voltage,” said , a research scientist at the 91����. “But our bodies operate on chemistry. In our brains, neurons propagate signals electrochemically, by moving ions — charged atoms or molecules — not electrons.”

Implantable devices from pacemakers to glucose monitors rely on components that can speak both languages and bridge that gap. Among those components are OECTs — or organic electrochemical transistors — which allow current to flow in devices like implantable biosensors. But scientists long knew about a quirk of OECTs that no one could explain: When an OECT is switched on, there is a lag before current reaches the desired operational level. When switched off, there is no lag. Current drops almost immediately.

A 91����-led study has solved this lagging mystery, and in the process paved the way to custom-tailored OECTs for a growing list of applications in biosensing, brain-inspired computation and beyond.

“How fast you can switch a transistor is important for almost any application,” said project leader , a 91���� professor of chemistry, chief scientist at the 91���� Clean Energy Institute and faculty member in the 91���� Molecular Engineering and Sciences Institute. “Scientists have recognized the unusual switching behavior of OECTs, but we never knew its cause – until now.”

In a published April 17 in Nature Materials, Ginger’s team at the 91���� — along with Professor at the Okinawa Institute of Science and Technology in Japan and Professor at Zhejiang University in China — report that OECTs turn on via a two-step process, which causes the lag. But they appear to turn off through a simpler one-step process.

In principle, OECTs operate like transistors in electronics: When switched on, they allow the flow of electrical current. When switched off, they block it. But OECTs operate by coupling the flow of ions with the flow of electrons, which makes them interesting routes for interfacing with chemistry and biology.

The new study illuminates the two steps OECTs go through when switched on. First, a wavefront of ions races across the transistor. Then, more charge-bearing particles invade the transistor’s flexible structure, causing it to swell slightly and bringing current up to operational levels. In contrast, the team discovered that deactivation is a one-step process: Levels of charged chemicals simply drop uniformly across the transistor, quickly interrupting the flow of current.

Knowing the lag’s cause should help scientists design new generations of OECTs for a wider set of applications.

“There’s always been this drive in technology development to make components faster, more reliable and more efficient,” Ginger said. “Yet, the ‘rules’ for how OECTs behave haven’t been well understood. A driving force in this work is to learn them and apply them to future research and development efforts.”

Whether they reside within devices to measure blood glucose or brain activity, OECTs are largely made up of flexible, organic semiconducting polymers — repeating units of complex, carbon-rich compounds — and operate immersed in liquids containing salts and other chemicals. For this project, the team studied OECTs that change color in response to electrical charge. The polymer materials were synthesized by Luscombe’s team at the Okinawa Institute of Science and Technology and Li’s at Zhejiang University, and then fabricated into transistors by 91���� doctoral students Jiajie Guo and Shinya “Emerson” Chen, who are co-lead authors on the paper.

“A challenge in the materials design for OECTs lies in creating a substance that facilitates effective ion transport and retains electronic conductivity,” said Luscombe, who is also a 91���� affiliate professor of chemistry and of materials science and engineering. “The ion transport requires a flexible material, whereas ensuring high electronic conductivity typically necessitates a more rigid structure, posing a dilemma in the development of such materials.”

Guo and Chen observed under a microscope — and recorded with a smartphone camera — precisely what happens when the custom-built OECTs are switched on and off. It showed clearly that a two-step chemical process lies at the heart of the OECT activation lag.

Past research, including by Ginger’s group at the 91����, demonstrated that polymer structure, especially its flexibility, is important to how OECTs function. These devices operate in fluid-filled environments containing chemical salts and other biological compounds, which are more bulky compared to the electronic underpinnings of our digital devices.

The new study goes further by more directly linking OECT structure and performance. The team found that the degree of activation lag should vary based on what material the OECT is made of, such as whether its polymers are more ordered or more randomly arranged, according to Giridharagopal. Future research could explore how to reduce or lengthen the lag times, which for OECTs in the current study were fractions of a second.

“Depending on the type of device you’re trying to build, you could tailor composition, fluid, salts, charge carriers and other parameters to suit your needs,” said Giridharagopal.

OECTs aren’t just used in biosensing. They are also used to study nerve impulses in muscles, as well as forms of computing to create artificial neural networks and understand how our brains store and retrieve information. These widely divergent applications necessitate building new generations of OECTs with specialized features, including ramp-up and ramp-down times, according to Ginger.

“Now that we’re learning the steps needed to realize those applications, development can really accelerate,” said Ginger.

Guo is now a postdoctoral researcher at the Lawrence Berkeley National Laboratory and Chen is now a scientist at Analog Devices. Other co-authors on the paper are , a former 91���� postdoctoral researcher in chemistry who is now an assistant professor at the University of Utah; Jonathan Onorato, a 91���� doctoral alum and scientist at Exponent; and Kangrong Yan and Ziqui Shen of Zhejiang University. The research was funded by the U.S. National Science Foundation, and polymers developed at Zhejiang University were funded by the National Science Foundation of China.

For more information contact Ginger at dginger@uw.edu, Luscombe at christine.luscombe@oist.jp and Giridharagopal at rgiri@uw.edu.

That conclusion might crop up during divorce proceedings, or describe a diplomatic row. But scientists designing polymers that can bridge the biological and electronic divide must also deal with incompatible messaging styles. Electronics rely on racing streams of electrons, but the same is not true for our brains.

“Most of our technology relies on electronic currents, but biology transduces signals with ions, which are charged atoms or molecules,” said , professor of chemistry at the 91���� and chief scientist at the 91����’s . “If you want to interface electronics and biology, you need a material that effectively communicates across those two realms.”

Ginger is senior author of a published online June 19 in in which 91���� researchers directly measured a thin film made of a single type of conjugated polymer — a conducting plastic — as it interacted with ions and electrons. They show how variations in the polymer layout yielded rigid and non-rigid regions of the film, and that these regions could accommodate electrons or ions — but not both equally. The softer, non-rigid areas were poor electron conductors but could subtly swell to take in ions, while the opposite was true for rigid regions.

Organic semiconducting polymers are complex matrices made from repeating units of a carbon-rich molecule. An organic polymer that can accommodate both types of conduction — ions and electrons — is the key to creating new biosensors, flexible bioelectronic implants and better batteries. But differences in size and behavior between tiny electrons and bulky ions have made this no easy task.

Their results demonstrate how critical the polymer synthesis and layout process is to the film’s electronic and ionic conductance properties. Their findings may even point the way forward in creating polymer devices that can balance the demands of electronic transport and ion transport.

“We now understand the design principles to make polymers that can transport both ions and electrons more effectively,” said Ginger. “We even demonstrate by microscopy how to see the locations in these soft polymer films where the ions are transporting effectively and where they aren’t.”

Ginger’s team measured the physical and electrochemical properties of a film made out of poly(3-hexylthiophene), or P3HT, which is a relatively common organic semiconductor material. Lead author Rajiv Giridharagopal, a research scientist in the 91���� Department of Chemistry, probed the P3HT film’s electrochemical properties in part by borrowing a technique originally developed to measure electrodes in lithium-ion batteries.

The approach, electrochemical strain microscopy, uses a needle-like probe suspended by a mechanical arm to measure changes in the physical size of an object with atomic-level precision. Giridharagopal discovered that, when a P3HT film was placed in an ion solution, certain regions of the film could subtly swell to let ions flow into the film.

“This was an almost imperceptible swelling — just 1 percent of the film’s total thickness,” said Giridharagopal. “And using other methods, we discovered that the regions of the film that could swell to accommodate ion entry also had a less rigid structure and polymer arrangement.”

More rigid and crystalline regions of the film could not swell to let in ions. But the rigid areas were ideal patches for conducting electrons.

Ginger and his team wanted to confirm that structural variations in the polymer were the cause of these variations in electrochemical properties of the film. Co-author , a 91���� associate professor of materials science and engineering and member of the Clean Energy Institute, and her team made new P3HT films that had different levels of rigidity based on variations in polymer arrangement.

By subjecting these new films to the same array of tests, Giridharagopal showed a clear correlation between polymer arrangement and electrochemical properties. The less rigid and more amorphous polymer layouts yielded films that could swell to let in ions, but were poor conductors of electrons. More crystalline polymer arrangements yielded more rigid films that could easily conduct electrons.

These measurements demonstrate for the first time that small structural differences in how organic polymers are processed and assembled can have major consequences for how the film accommodates ions or electrons. It may also mean that this tradeoff between the needs of ion and electrons is unavoidable. But these results give Ginger hope that another solution is possible.

“The implication of these findings is that you could conceivably embed a crystalline material — which could transport electrons — within a material that is more amorphous and could transport ions,” said Ginger. “Imagine that you could harness the best of both worlds, so that you could have a material that is able to effectively transport electrons and swell with ion uptake — and then couple the two with one another.”

If so, then a bioelectronic divorce may not be on the horizon, but better bioelectronic devices and better batteries should be.

Co-authors were 91���� doctoral students Lucas Flagg, Jeff Harrison, Mark Ziffer and Jon Onorato. The work was funded by the National Science Foundation, the 91���� Clean Energy Institute, the Washington Research Foundation and the Alvin L. and Verla R. Kwiram endowed fund in the 91���� Department of Chemistry.

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For more information, contact Ginger at dginger@uw.edu or 206-685-2331 and Giridharagopal at rgiri@uw.edu or 206-221-4191.

Grant numbers: DMR-1607242, DMR-1533372, DMR-1629369.

DOI: 10.1038/nmat4918