Dimitrios A. Koutsouras

High-performance transistors for bioelectronics through tuning of channel thickness

Transistors with tunable transconductance allow high-quality recordings of human brain rhythms. Despite recent interest in organic electrochemical transistors (OECTs), sparked by their straightforward fabrication and high performance, the fundamental mechanism behind their operation remains largely unexplored. OECTs use an electrolyte in direct contact with a polymer channel as part of their device structure. Hence, they offer facile integration with biological milieux and are currently used as amplifying transducers for bioelectronics. Ion exchange between electrolyte and channel is believed to take place in OECTs, although the extent of this process and its impact on device characteristics are still unknown. We show that the uptake of ions from an electrolyte into a film of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) leads to a purely volumetric capacitance of 39 F/cm3. This results in a dependence of the transconductance on channel thickness, a new degree of freedom that we exploit to demonstrate high-quality recordings of human brain rhythms. Our results bring to the forefront a transistor class in which performance can be tuned independently of device footprint and provide guidelines for the design of materials that will lead to state-of-the-art transistor performance.

Neuromorphic device architectures with global connectivity through electrolyte gating.

Information processing in the brain takes place in a network of neurons that are connected with each other by an immense number of synapses. At the same time, neurons are immersed in a common electrochemical environment, and global parameters such as concentrations of various hormones regulate the overall network function. This computational paradigm of global regulation, also known as homeoplasticity, has important implications in the overall behaviour of large neural ensembles and is barely addressed in neuromorphic device architectures. Here, we demonstrate the global control of an array of organic devices based on poly(3,4ethylenedioxythiophene):poly(styrene sulf) that are immersed in an electrolyte, a behaviour that resembles homeoplasticity phenomena of the neural environment. We use this effect to produce behaviour that is reminiscent of the coupling between local activity and global oscillations in the biological neural networks. We further show that the electrolyte establishes complex connections between individual devices, and leverage these connections to implement coincidence detection. These results demonstrate that electrolyte gating offers significant advantages for the realization of networks of neuromorphic devices of higher complexity and with minimal hardwired connectivity.

Orientation selectivity in a multi-gated organic electrochemical transistor

Neuromorphic devices offer promising computational paradigms that transcend the limitations of conventional technologies. A prominent example, inspired by the workings of the brain, is spatiotemporal information processing. Here we demonstrate orientation selectivity, a spatiotemporal processing function of the visual cortex, using a poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) organic electrochemical transistor with multiple gates. Spatially distributed inputs on a gate electrode array are found to correlate with the output of the transistor, leading to the ability to discriminate between different stimuli orientations. The demonstration of spatiotemporal processing in an organic electronic device paves the way for neuromorphic devices with new form factors and a facile interface with biology.

Neurospheres on Patterned PEDOT:PSS Microelectrode Arrays Enhance Electrophysiology Recordings

Microelectrode arrays (MEAs) are a versatile diagnostic tool to study neural networks. Culture of primary neurons on these platforms allows for extracellular recordings of action potentials. Despite many advances made in the technology to improve such recordings, the recording yield on MEAs remains sparse. Here, enhanced recording yield is shown induced by varying cell densities on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)‐coated MEAs. It is demonstrated that high cell densities (900 cells mm−2) of primary cortical cells increase the number of recording electrodes by 53.1% ± 11.3%, compared with low cell densities (500 cells mm−2) with 6.3% ± 1.4%. To further improve performance, 3D clusters known as neurospheres are cultured on the MEAs, significantly increasing single unit activity recordings. Extensive spike sorting is performed to analyze the unit activity recording multiple neurons with a single microelectrode. Finally, patterning of polyethylene glycol diacrylate through laser ablation is demonstrated, as a means to more precisely confine neurospheres on top of the electrodes. The possibility of recording single neurons with multiple neighboring electrodes is shown. Overall, a total recording yield of 21.4% is achieved, with more than 90% obtained from electrodes with neurospheres, maximizing the functionality of these planar MEAs as effective tools to study pharmacology‐based effects on neural networks.

Simultaneous monitoring of single cell and of micro-organ activity by PEDOT:PSS covered multi-electrode arrays

Continuous and long-term monitoring of cellular and micro-organ activity is required for new insights into physiology and novel technologies such as Organs-on-Chip. Moreover, recent advances in stem cell technology and especially in the field of diabetes call for non-invasive approaches in quality testing of the large quantities of surrogate pancreatic islets to be generated. Electrical activity of such a micro-organ results in single cell action potentials (APs) of high frequency and in low frequency changes in local field potentials (slow potentials or SPs), reflecting coupled cell activity and overall organ physiology. Each of them is indicative of different physiological stages in islet activation. Action potentials in islets are of small amplitude and very difficult to detect. The use of PEDOT:PSS to coat metal electrodes is expected to reduce noise and results in a frequency-dependent change in impedance, as shown here. Whereas detection of high-frequency APs improves, low frequency SPs are less well detected which is, however, an acceptable trade off in view of the strong amplitude of SPs. Using a dedicated software, recorded APs and SPs can be automatically diagnosed and analyzed. Concomitant capture of the two signals will considerably increase the diagnostic power of monitoring islets and islet surrogates in fundamental research as well as drug screening or the use of islets as biosensors.

Organic neuromorphic devices based on electrochemical concepts (Conference Presentation)

Neuroinspired device architectures offer the potential of higher order functionalities in information processing beyond their traditional microelectronic counterparts. In the actual neural environment, neural processing takes place in a complex and interwoven network of neurons and synapses. In addition, this network is immersed in a common electrochemical environment and global parameters such as ionic concentrations and concentrations of various hormones regulate the overall behaviour of the network. Here, various concepts of organic neuromorphic devices are presented based on organic electrochemical transistors (OECTs). Regarding the implementation of neuromorphic devices, the key properties of the OECT that resemble the neural environment are also presented. These include the operation in liquid electrolyte environment, low power consumption and the ability of formation of massive interconnections through the electrolyte continuum. Showcase examples of neuromorphic functions with OECTs are demonstrated, including short-, long-term plasticity and spatiotemporal or distributed information processing.

Organic electrochemical transistors for BioMEMS applications

A visible trend over the past few years involves the application of organic electronic materials to the interface with biology, with applications both in sensing and actuation. Examples include biosensors, artificial muscles and neural interface devices. These materials offer an attractive combination of properties, including mechanical flexibility, enhanced biocompatibility, and capability for drug delivery. Most importantly, high ionic mobilities in organic films enable new ways of signal transduction. An example of a device that takes advantage of these properties is the organic electrochemical transistor (OECT). In this device, ions from an electrolyte enter a conducting polymer channel and change its conductivity, hence the drain current. As such OECTs offer a convenient and powerful way to transduce signals of biological origin. Here we report high performance OECTs that are used to record neural activity. As such, they promise to yield a new tool for neuroscience and enhance our understanding on how the brain works.