Scientists solve chemical mysteries at the intersection of biology and technology

Researchers who want to bridge the gap between biology and technology spend a lot of time thinking about translating between the two different “languages” of those fields.

“Our digital technology works through a series of electronic on-off switches that control the current and voltage flow,” said Rajiv Giridharagopal, a researcher at the University of Washington. “But our bodies work based on chemistry. In our brains, neurons propagate signals electrochemically, by moving ions (charged atoms or molecules), and not by electrons.”

Implantable devices, from pacemakers to glucose meters, rely on components that can speak both languages ​​and bridge that gap. Among these components are OECTs – or organic electrochemical transistors – which allow current to flow in devices such as implantable biosensors. But scientists have long known about a quirk of OECTs that no one could explain: When an OECT is turned on, it takes a while for the current to reach the desired operational level. When disabled, there is no delay. The current drops almost immediately.

A UW-led study has solved this lingering mystery, paving the way for tailor-made OECTs for a growing list of applications in biosensors, brain-inspired computation, and more.

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

In an article published in Natural materialsGinger’s team at the UW – along with Professor Christine Luscombe of the Okinawa Institute of Science and Technology in Japan and Professor Chang-Zhi Li of Zhejiang University in China – reports that OECTs are enabled through a two-step process, which is causing the delay . But they appear to be disabled via a simpler one-step process.

Basically, OECTs work like transistors in electronics: when they are turned on, they allow electric current to flow. When disabled, they block it. But OECTs work by coupling the flow of ions to the flow of electrons, making them interesting pathways for interacting with chemistry and biology.

The new research highlights the two steps that OECTs go through when they are turned on. First, a wavefront of ions rushes over the transistor. Then, more charge-carrying particles enter the flexible structure of the transistor, causing it to swell slightly and bringing the current up to operational levels. In contrast, the team found that deactivation is a one-step process: the levels of charged chemicals simply drop evenly across the transistor, quickly interrupting the flow of current.

Knowing the cause of the slowdown should help scientists design new generations of OECTs for a wider range of applications.

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

Whether in devices to measure blood glucose or brain activity, OECTs are composed largely 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 UW doctoral students Jiajie Guo and Shinya “Emerson” Chen, who are co-lead authors of the paper .

“A challenge in materials design for OECTs lies in creating a substance that allows effective ion transport and maintains electronic conductivity,” says Luscombe, who is also a UW professor of chemistry and materials science and engineering. “Ion transport requires a flexible material, while ensuring high electronic conductivity typically requires a stiffer structure, posing a dilemma in the development of such materials.”

Guo and Chen observed under a microscope – and captured with a smartphone camera – exactly what happens when the custom-made OECTs are turned on and off. This clearly showed that a two-step chemical process is at the core of the OECT activation delay.

Previous research, including by Ginger’s group at the UW, has shown that polymer structure, and especially its flexibility, is important for how OECTs function. These devices operate in fluid-filled environments that contain chemical salts and other biological compounds, which are bulkier 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 delay should vary based on the material the OECT is made of, such as whether the polymers are more ordered or more randomly arranged, Giridharagopal said. Future research could investigate how to shorten or lengthen delay times, which were fractions of a second for OECTs in the current study.

“Depending on the type of device you are trying to build, you can adjust the composition, fluid, salts, charge carriers and other parameters to suit your needs,” says Giridharagopal.

OECTs are not only used in biosensors. 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 diverse applications necessitate building new generations of OECTs with specialized functions, including ramp-up and ramp-down times, Ginger said.

“As we learn the steps needed to make those applications a reality, development can really accelerate,” says Ginger.

Guo is now a postdoctoral researcher at Lawrence Berkeley National Laboratory and Chen is now a scientist at Analog Devices. Other co-authors on the paper include Connor Bischak, a former UW postdoctoral researcher in chemistry who is now an assistant professor at the University of Utah; Jonathan Onorato, a UW doctoral candidate and scientist at Exponent; and Kangrong Yan and Ziqui Shen from Zhejiang University.