Okinawa Institute of Science & Technology Promotion Corp.

Organic electrochemical transistors (OECTs) 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 immediately. Researchers report that they have discovered the reason for this activation lag, and in the process are paving the way to custom-tailored OECTs for a growing list of applications in biosensing, brain-inspired computation and beyond. Researchers who want to bridge the divide between biology and technology spend a lot of time thinking about translating between the two different "languages" of those realms. "Our digital technology operates through a series of electronic on-off switches that control the flow of current and voltage," said Rajiv Giridharagopal, a research scientist at the University of Washington. "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 UW-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 David Ginger, a UW professor of chemistry, chief scientist at the UW Clean Energy Institute and faculty member in the UW Molecular Engineering and Sciences Institute. "Scientists have recognized the unusual switching behavior of OECTs, but we never knew its cause -- until now." In a paper published April 17 in Nature Materials, Ginger's team at the UW -- along with Professor Christine Luscombe at the Okinawa Institute of Science and Technology in Japan and Professor Chang-Zhi Li 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 UW 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 UW 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 UW, 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 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 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.

Our bodies are made up of trillions of different cells, each fulfilling their own unique function to keep us alive. How do cells move around inside these extremely complicated systems? How do they know where to go? And how did they get so complicated to begin with? Simple yet profound questions like these are at the heart of curiosity-driven basic research, which focuses on the fundamental principles of natural phenomena. Our bodies are made up of trillions of different cells, each fulfilling their own unique function to keep us alive. How do cells move around inside these extremely complicated systems? How do they know where to go? And how did they get so complicated to begin with? Simple yet profound questions like these are at the heart of curiosity-driven basic research, which focuses on the fundamental principles of natural phenomena. An important example is the process by which cells or organisms move in response to chemical signals in their environment, also known as chemotaxis. A constellation of researchers from three different research units at the Okinawa Institute of Science and Technology (OIST) came together to answer basic questions about chemotaxis by creating synthetic droplets to mimic the phenomena in the lab, allowing them to precisely isolate, control and study the phenomena. Their results, which helps answering questions about the principles of movement in simple biological systems, have now been published in the Journal of The American Chemical Society. "We have shown that it is possible to make protein droplets migrate through simple chemical interactions," says Alessandro Bevilacqua, PhD student in the Protein Engineering and Evolution Unit and co-first author on the paper. Professor Paola Laurino, head of the unit and leading author, adds that they "have created a simple system that mimic a very complex phenomenon, and which can be modulated through enzymatic activity." Tensions on the surface While the process of creating droplets might not sound like the most complicated task, mimicking biological processes as close to reality as possible while keeping accurate control over all the variables certainly is. The synthetic, membrane-less droplets contain a very high concentration of the bovine protein BSA to mimic the crowded conditions inside cells, as well as urease, an enzyme that catalyzes the breakdown of urea into ammonia. Ammonia is basic, meaning it has a high pH-value. As the enzyme gradually catalyzes the production of ammonia, it diffuses into the solution, creating a 'halo' of higher pH around the droplet, which in turn enables droplets to detect other droplets and migrate towards each other. The researchers found that the key to understanding the chemotaxis of the droplets is the pH-gradient, as it facilitates the Marangoni effect, which describes how molecules flow from areas of high surface tension to low. Surface tension is the measure of energy required to keep molecules at the surface together, like glue. When pH increases, this glue weakens, causing molecules to spread out and lowering surface tension, which in turn makes it easier for molecules to move. You can see this by adding soap, which has a high pH, to one end of a bathtub of still water: the water will flow towards the end with soap because of the Marangoni effect. When two synthetic droplets are close enough, their halos interact, raising the pH in the environment between them, which makes them move together. Because the surface tension is still strong on the opposite ends of the droplets, they keep their shape until the surfaces touch, and the cohesive forces within the droplets overcome the surface tension, causing them to merge. As larger droplets both produce more ammonia and have a larger surface area (which decreases surface tension), they attract droplets smaller than themselves. Collaborating on ancient soup and future biotech Thanks to the development of these droplets, the researchers have made headway in answering basic questions about biological movement -- and in doing so, they have gained insight into the directed movement of the earliest forms of life in the primordial soup billions of years ago, as well as a lead on creating new biologically inspired materials. Our knowledge of life as it looked billions of years ago is fuzzy at best. A prominent hypothesis is that life originated in the oceans, as organic molecules gradually assembled and became more sophisticated in a 'primordial soup' -- and this could have been facilitated by chemotaxis through the Marangoni effect. "It would have been beneficial for droplets to have this mechanism of migration in the hypothetical origin of life scenario," as Professor Laurino puts it. This migration could have triggered the formation of primitive metabolic pathways whereby enzymes catalyze a variety of substances that ultimately produce a chemical gradient that drives the droplets together, leading to larger and more sophisticated communities. The research also points ahead in time, providing leads on new technology. "One example is the creation of responsive materials inspired by biology," suggests Alessandro Bevilacqua. "We have shown how simple droplets can migrate thanks to a chemical gradient. A future application of this could be technologies that sense or react to chemical gradients, for example in micro-robotics or drug delivery." The work to produce and analyze the synthetic droplets is the result of a combination of deeply integrated interdisciplinarity and the human factors undergirding scientific work. The project began during the coronavirus pandemic, when a member of the Protein Engineering and Evolution Unit was in quarantine with a member of the Complex Fluids and Flows Unit. The two began talking, and though the two units are from two disparate fields -- biochemistry and mechanics, respectively -- the project evolved in tandem. Eventually, members from the Micro/Bio/Nanofluidics Unit joined the project with sophisticated measurements of the droplets' surface tension. The unique non-disciplinary research environment at OIST catalyzed the collaboration. As Professor Laurino puts it, "this project could never have existed if we were separated by departments. It hasn't been an easy collaboration, because we communicate our field in very different ways -- but being physically close made it significantly easier." Alessandro Bevilacqua joins in: "The coffee factor has been very important. Being able to sit down with other unit members made the process much faster and more productive." Their cooperation doesn't stop here -- rather, this paper is the beginning of a fruitful partnership between the three units. "We see a lot of synergy in our work, and we work effectively and efficiently together. I don't see a reason why we should stop," as Professor Laurino states it. It's thanks to the combined efforts of the three units that we now know more about the minute movements of life at the smallest, earliest, and possibly future scale.

Impact of single-cell organisms on sediment oxygen levels and bacterial diversity measured for the first time. Dr. Dewi Langlet, a scientist at the Evolution, Cell Biology and Symbiosis Unit at the Okinawa Institute of Science and Technology (OIST), studies foraminifera, single-cell organisms with shells made of calcium carbonate. He and his collaborators have shown for the first time that the burrowing of single-celled organisms in marine ecosystems affects oxygen distribution and bacterial diversity in sea sediments. Their findings have been published in the journal Biogeosciences. Foraminifera are mostly marine organisms that have been around for about 550 million years and when they die, their shells accumulate on the ocean floor and become part of the ocean sediment. While they are microscopic organisms (between 63 and 500 micrometers in diameter) they are still 'big' compared to other single-cell organisms, with each species having a unique shape. They live and are abundant in all marine sediments, from estuaries to the deep sea. Geologists have studied them for a long time because their shells can fossilize, but we do not know much about their biology. Dr. Langlet is trying to understand how they move in the sediment and how this affects the entire sea bottom ecosystem. Bioturbation occurs when organisms disturb the sediment by moving in it and creating burrows, which affects the mixing of the sediment particles. This affects the size of the particles, regulates water through the sediment, and changes the chemical composition of the sediment. Bioturbators, often referred to as 'tillers of the soil', play an important role in determining nutrient availability and providing food and shelter for many species. They also significantly contribute to many natural processes and outputs, collectively known as 'ecosystem services', that we humans greatly benefit from. "Typically, at the surface of marine sediments oxygen is consumed by the organisms living in the sediment, and gradually the oxygen decreases as you go deeper. We asked the question, 'Does foraminifera affect the oxygen distribution in the sediment when they move or when they create burrows?'" said Dr. Langlet. "It was hypothesized for a long time that they affect the oxygenation and chemistry of the sediment, but it was never proven because they are so small that their impact is very difficult to detect." Previous studies have shown that larger multicellular organisms, such as worms, increase the oxygen penetration in the sediment by creating burrows, but this has never been shown for single-cell organisms such as foraminifera. By creating burrows, foraminifera are engineering their entire ecosystem at a small-scale, allowing them to live deeper in the sediment where there is usually no oxygen. The scientists show that through their burrowing, they affect not just oxygenation but organic matter, bacterial diversity and ultimately how much food there is available in the sediment. Finding enough foraminifera for the experiment was a challenge. "For a big aquarium, we need to study many foraminifera and it is very time consuming to isolate them to know how many there are, so we must work with small amounts of sediment. It's all about miniaturization, working with small systems," Dr. Langlet explained. The effect that these organisms have on their environment is relatively small because of their very small size, so the scientists needed very precise instruments, called microsensors, to accurately measure the distribution of oxygen in the sediment. In his lab at OIST, Dr. Langlet worked with tiny sediment samples, each about 1 cm wide, placed in a tank filled with water. He added foraminifera to these samples and every few days measured how the oxygen levels changed at different depths. He found that with their burrows, foraminifera allow oxygen to go deeper into the sediment, increasing the amount of oxygen by 15 to 20 per cent. This causes a decrease in organic matter which leads to reduced bacterial abundance, which ultimately decreases the movement of oxygen from the water into the sediment. Dr. Langlet's future research will explore the interactions between foraminifera and other organisms of similar size, as well as larger animals such as worms. "Are foraminifera interacting with these worm-created burrows? Could their presence potentially amplify the effects of these burrows?" he asks. These questions will guide his future investigations.