University of Washington researchers, working with international collaborators, have discovered why organic electrochemical transistors (OECTs) used in biosensors and other medical devices experience a delay when turned on. OECTs use a two-step process to activate, but the work shows that they can be deactivated in a simpler, faster, one-step way, paving the way for improved designs in medical and computing applications.
A new study reveals that the delay observed when switching on organic electrochemical transistors (OECTs) is due to a two-step activation process, providing important insights for designing more effective and customizable OECTs for a variety of technological and biological applications.
Researchers hoping to bridge the gap between biology and technology spend a lot of time thinking about translation between the two different “languages” of those disciplines.
“Our digital technologies work through a series of electronic on-off switches that control the flow of electric current and voltage,” says Rajiv Giridharagopal, a research scientist at the University of Washington, “but our bodies are run by chemistry. In the brain, neurons transmit signals electrochemically, by moving ions (charged atoms or molecules) rather than electrons.”
From pacemakers to blood glucose monitors, implantable devices rely on components that can speak both languages and bridge the gap. One of these components is the OECT (organic electrochemical transistor), which delivers current to devices such as implantable biosensors. But scientists have long known about a strange property of OECT that no one can explain: When you turn an OECT on, there’s a delay before the current reaches the desired operating level. When you turn it off, there’s no delay; the current drops almost instantly.
UW-led research has solved this long-standing mystery, and in the process paved the way for customized OECTs for a growing number of applications, including biosensing and brain-inspired computing.
The three images shown here are screenshots from a microscope camera, illustrating the one-step turn-off process of an OECT. The numbers on the left indicate time. At the moment of switching off, the OECT appears dark because it is filled with charged particles (also known as being fully “doped”). When the OECT is turned off, the number of charged particles across the transistor rapidly decreases, causing it to lighten in color. Credit: Nature Materials
A breakthrough in understanding the operation of OECTs
“How fast a transistor can switch is important for almost any application,” said project leader David Ginger, a UW professor of chemistry, principal scientist at the UW Clean Energy Institute, and faculty member at the UW Institute for Molecular Engineering and Sciences. “Scientists have been aware of the unusual switching behavior of OECTs, but until now they didn’t know what caused it.”
In a paper recently published in Nature Materials, Ginger’s team at the University of Washington, along with Professor Christine Luscombe of the Okinawa Institute of Science and Technology in Japan and Professor Zhang-Jie Li of Zhejiang University in China, report that OECT turns on in a two-step process that causes the delay, but appears to turn off in a simpler one-step process.
In principle, OECTs behave like transistors in electronic devices: when switched on, current flows, and when switched off, the current is blocked. But OECTs work by coupling the flow of ions with the flow of electrons, making them an interesting means of interfacing with chemistry and biology.
The new study reveals two steps that occur when an OECT is switched on. First, a wave front of ions races across the transistor. Then, more charged particles infiltrate the transistor’s flexible structure, causing it to expand slightly and allowing the current to rise to operating levels. In contrast, the research team found that deactivation is a one-step process: levels of charged chemicals drop uniformly throughout the transistor, quickly interrupting the flow of current.
The six images shown here are screenshots from a microscope camera that show the two-stage turn-on process of an OECT. The numbers on the left indicate time. When the OECT first turns on, a dark front of ions travels from the “S” side of the transistor to the “D” side. As more charged particles then travel, the transistor continues to darken. Credit: Nature Materials
Knowing the cause of the delay should help scientists design a new generation of OECTs that can be used in a wider range of applications.
“In technology development, there has always been a push to make parts faster, more reliable, and more efficient,” Ginger said. “But the ‘rules’ of how OECTs work are not well understood. The driving force behind this research is to learn the rules and apply them to future research and development efforts.”
Both the OECTs in the blood glucose and brain activity devices are largely made of flexible organic semiconducting polymers (repeated units of complex, carbon-rich compounds) that operate while immersed in liquids containing salts and other chemicals. In this project, the team studied OECTs that change color in response to an electric charge. The polymer material was synthesized by Luscombe’s team at the Okinawa Institute of Science and Technology and Li’s team at Zhejiang University, then fabricated into transistors by University of Washington doctoral students Jiajie Guo and Shinya “Emerson” Chen, co-first authors of the paper.
“The challenge in designing materials for OECTs is to create substances that promote effective ion transport and maintain electronic conductivity,” said Luscomb, who is also an associate professor of chemical and materials science and engineering at the University of Washington. “Ion transport requires flexible materials, but high electronic conductivity typically requires more rigid structures, creating a dilemma in developing such materials.”
Guo and Chen used a microscope and a smartphone camera to observe what happens when they switch their custom-built OECT on and off, and their results clearly show that a two-step chemical process lies at the heart of the OECT’s activation delay.
Previous research, particularly from Ginger’s group at the University of Washington, has demonstrated that the polymer structure, and in particular its flexibility, is critical to the functioning of OECTs. These devices operate in environments filled with liquids that contain chemical salts and other biological compounds, which are bulky compared to the electronic foundations of digital devices.
Future directions and applications
The new study goes further by more directly linking an OECT’s structure to its performance. Giridharagopal says the team found that the amount of activation delay should vary depending on what material the OECT is made of, such as whether the polymers are more ordered or more randomly arranged. Future research could explore ways to shorten or extend the delay time that the OECTs in this study achieved, which was less than one second.
“Depending on the type of device you’re trying to build, you can tailor the composition, fluids, salts, charge carriers and other parameters to your needs,” Giridharagopal said.
OECTs aren’t just used for biosensing; they’re also used to study nerve impulses in muscles, create artificial neural networks, and form of computing to understand how the brain stores and retrieves information. These diverse applications require the construction of a new generation of OECTs with specialized features, such as ramp-up and ramp-down times, Ginger says.
“As we learn the steps required to make these applications a reality, we can really accelerate development,” Ginger said.
Reference: Jiajie Guo, Shinya E. Chen, Rajiv Giridharagopal, Connor G. Bischak, Jonathan W. Onorato, Kangrong Yan, Ziqiu Shen, Chang-Zhi Li, Christine K. Luscombe and David S. Ginger, “Understanding asymmetric switching times in accumulation-mode organic electrochemical transistors,” April 17, 2024, Nature Materials.
DOI: 10.1038/s41563-024-01875-3
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 are Connor Vischak, a former University of Washington postdoctoral researcher in chemistry who is now an assistant professor at the University of Utah, University of Washington doctoral graduate and Exponent scientist Jonathan Onorato, and Quanrong Yang and Zhiqi Shen of Zhejiang University. The research was funded by the U.S. National Science Foundation, and the polymer developed at Zhejiang University was funded by the China National Science Foundation.
