A new study has shown that the observed turn-on delay in organic electrochemical transistors (OECTs) is due to a two-step activation process, providing important insights for the design of more efficient and tunable OECTs for a variety of technological and biological applications. Researchers seeking to bridge the gap between biology and technology spend a lot of time thinking about translating between the two very different “languages” of these fields.
“Our digital technology works through a series of electronic switches that control the flow of current and voltage,” said Rajiv Giridharagopal, a researcher at the University of Washington. “But our bodies run on chemistry. Neurons in our brains emit signals electrochemically, by moving ions (charged atoms or molecules) instead of electrons.
Implantable devices, from pacemakers to glucose monitors, rely on components that can speak both languages and bridge the gap. These include 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 the OECT that no one could explain: When the OECT is turned on, there is a delay before the current reaches the desired operating level. When it is turned off, there is no delay. The current drops almost instantly.
UW-led research has solved this mystery and, in the process, paved the way for custom-designed OECTs for a growing number of applications in biosensing, brain-inspired computing and more.
A breakthrough in understanding the workings of the OECT
“How fast you can switch a transistor is important for almost every application,” said project leader David Ginger, a UW chemistry professor, principal scientist at the UW Clean Energy Institute, and faculty member of the UW Institute for Molecular Engineering and Science. “Scientists have noticed the unusual switching behavior of the OECT, but until now we never knew why.”
In a recently published article Nature MaterialsGinger’s team at the UW — along with Professor Christine Luscombe of the Okinawa Institute of Science and Technology in Japan and Professor Chang-Ji Li of Zhejiang University in China — reported that OECTs open via a two-step process that causes a delay. But they appear to close via a simpler, one-step process.
In principle, OECTs work like transistors in electronics: when they are on, they conduct electricity. When they are off, they block it. But OECTs work by coupling the flow of ions to the flow of electrons, making them interesting avenues for interactions with chemistry and biology.
The new study highlights two stages that OECTs go through after they are turned on. First, the ion wave front passes through the transistor. More charge-carrying particles invade the flexible structure of the transistor, causing it to swell slightly and the current to rise to operating levels. Instead, the team found that deactivation is a single-step process: Levels of charged chemicals drop steadily across the transistor, rapidly cutting off the flow of current.
Knowing the cause of the delay will help scientists develop next-generation OECTs for a wider range of applications.
“In the development of technology, there has always been a desire to make components faster, more reliable and more efficient,” Ginger said. “But the ‘rules’ of OECT’s behavior are not yet fully understood. The impetus behind this work is to examine them and apply them in future research and development.”
Whether in devices that measure blood sugar or brain activity, OECTs are most often made of flexible organic semiconductor polymers (repeated units of complex carbon-rich compounds) and operate in liquids containing salts and other chemicals. For this project, the team examined OECTs that change color in response to an electrical charge. The polymer materials were synthesized by Luscombe’s team at the Okinawa Institute of Science and Technology and Li at Zhejiang University, and then developed into transistors by UW doctoral students Jiajie Guo and Shinya “Emerson” Chen, who are co-authors of the paper.
“The challenge in developing materials for OECTs is to create a material that supports efficient ion transport and maintains electronic conductivity,” said Luscombe, who is also a UW-affiliated professor of chemistry, materials science and engineering. “Ion transport requires a flexible material, while providing high electronic conductivity typically requires a more rigid structure, creating a dilemma in the design of such materials.”
Guo and Chen observed under a microscope and recorded on a smartphone camera exactly what happened when the custom-made OECTs were turned on and off. This clearly showed that a two-step chemical process underlies the delay in OECT activation.
Past work, including from Ginger’s group at the UW, has shown that the structure of the polymer, particularly its flexibility, is important for OECTs to function. These devices operate in liquid-filled environments containing chemical salts and other biological compounds that are bulky compared to the electronic base of our digital devices.
Future directions and applications
The new study goes further by more directly relating OECT structure and performance. The team found that the degree of delay in activation should vary depending on what material the OECT is made of, for example, whether its polymers are more regularly or more randomly arranged, according to Giridharagopal. Future work could investigate how to reduce or extend the delay, which was fractions of a second for the OECT in the current study.
“Depending on the type of device you are trying to create, you can adjust the composition, fluid, salts, charge carriers and other parameters to suit your needs,” Giridharagopal said.
OECTs aren’t just used for biosensing. They’re also used to study nerve impulses in muscles, to create artificial neural networks, and to study how our brains process information to understand how it stores and retrieves information. These diverse applications require next-generation OECTs with special features, including rise and fall times, according to Ginger.
“Now that we know the steps needed to implement these applications, development can really accelerate,” Ginger said.
Source: Port Altele
