High-performance transistors for bioelectronics through tuning of channel thickness.

UNLABELLED: 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/cm(3). 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

Emulating homeoplasticity phenomena with organic electrochemical devices

Copyright © Materials Research Society 2018 Biologic neural networks are immersed in common electrolyte environment, and homeoplasticity or global factors of this environment are forcing specific normalization functions that regulate the overall network behavior. In this work, a common electrolyte is used to gate a grid of organic electrochemical devices. The electrolyte functions as a global parameter that controls collectively the device grid. Statistical analysis of the grid and the subsequent definition of global metrics reveal that the grid behaves similarly to a single device. This global control modulates the gain of the device grid, a phenomenon analog to multiplicative scaling in biologic networks. This work demonstrates the potential use of electrolytes as homeostatic media in neuromorphic device architectures

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

Impedance Spectroscopy of Spin-Cast and Electrochemically Deposited PEDOT:PSS Films on Microfabricated Electrodes with Various Areas

We have examined the electrochemical impedance spectroscopy (EIS) of PEDOT:PSS-coated gold electrodes in various electrolytes as a function of temporal frequency (from 0.1 to 104 Hz) by using a custom-designed microfabricated substrate. The electrode areas were systematically varied over a broad range in nominal size from 10×10 μm (100 μm2) to 500×500 μm (2.5×105 μm2). Comparisons were made between spin-cast PEDOT:PSS crosslinked with GOPS and electrochemically deposited PEDOT:PSS films with similar thicknesses. The impedance spectra of the PEDOT:PSS-coated electrodes with various sizes could all be reasonably well described by a two-element equivalent circuit model with a solution resistance Rs and a film capacitance C. These two parameters together define a characteristic temporal frequency fc=1/(2πRsC). By normalizing the impedance with respect to Rs (Zn=|Z|/Rs) and the frequency with respect to fc (fn=f/fc), we found that all of the experimental Bode curves could be collapsed onto a single master plot of Zn vs. fn. In addition, analytical formulas that allow the estimation of the film impedance magnitude |Z| and phase ϕ were derived and experimentally validated. These results are of fundamental interest and are also anticipated to be important for the design, characterization, and optimization of conjugated polymer films for interfacing biomedical devices with living tissue

Electrochemical molecularly imprinted polymers in microelectrode devices

This work demonstrated the possibility to integrate electrochemical molecularly imprinted polymers (e-MIPs) on microelectrodes to detect organic pollutants. e-MIPs are a cross-linked polymer with specific target binding cavities with a redox tracer inside. e-MIPs were obtained by precipitation copolymerization of ferrocenylmethyl methacrylate as a functional monomer and a redox tracer with ethylene glycol dimethacrylate as a cross-linker and bisphenol A as a target molecule. FTIR and elemental analysis confirmed the presence of ferrocene inside the polymers. Nitrogen adsorption/desorption experiments and binding isotherms demonstrated the presence of binding cavities inside the e-MIP. The electrochemical properties of the e-MIP were characterized in organic/aqueous media before their patterned on microelectrode

PEDOT:PSS microelectrode arrays for hippocampal cell culture electrophysiological recordings

© 2017 Materials Research Society. In vitro electrophysiology using microelectrode arrays (MEAs) plays an important role in understanding fundamental biologic processes, screening potential drugs and assessing the toxicity of chemicals. Low electrode impedance and ability to sustain viable cultures are the key technology requirements. We show that MEAs consisting of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) and coated with poly-L-lysine satisfy these requirements. Hippocampal cell cultures, maintained for 3-6 weeks on these MEAs, give high quality recordings of neural activity. This enables the observation of drug-induced activity changes, which paves the way for using these devices in in vitro drug screening and toxicology applications

Neurospheres on Patterned PEDOT:PSS Microelectrode Arrays Enhance Electrophysiology Recordings

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 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

© 2017 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 i s 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