Xenofon Strakosas

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.

Structural control of mixed ionic and electronic transport in conducting polymers.

Poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate), PEDOT:PSS, has been utilized for over two decades as a stable, solution-processable hole conductor. While its hole transport properties have been the subject of intense investigation, recent work has turned to PEDOT:PSS as a mixed ionic/electronic conductor in applications including bioelectronics, energy storage and management, and soft robotics. Conducting polymers can efficiently transport both holes and ions when sufficiently hydrated, however, little is known about the role of morphology on mixed conduction. Here, we show that bulk ionic and electronic mobilities are simultaneously affected by processing-induced changes in nano- and meso-scale structure in PEDOT:PSS films. We quantify domain composition, and find that domain purification on addition of dispersion co-solvents limits ion mobility, even while electronic conductivity improves. We show that an optimal morphology allows for the balanced ionic and electronic transport that is critical for prototypical mixed conductor devices. These findings may pave the way for the rational design of polymeric materials and processing routes to enhance devices reliant on mixed conduction.

PEDOT: gelatin composites mediate brain endothelial cell adhesion

Conducting polymers (CPs) are increasingly being used to interface with cells for applications in both bioelectronics and tissue engineering. To facilitate this interaction, cells need to adhere and grow on the CP surface. Extracellular matrix components are usually necessary to support or enhance cell attachment and growth on polymer substrates. Here we show the preparation of PEDOT(TOS):gelatin composites as a new biocompatible substrate for use in tissue engineering. Gelatin, a derivative of the extracellular matrix protein collagen, was incorporated into poly(3,4 ethylenedioxythiophene)-tosylate (PEDOT(TOS)) films via vapour phase polymerisation (VPP) without changing the electrochemical properties of the CP. Further, gelatin, incorporated into the PEDOT(TOS) film, was found to specifically support bovine brain capillary endothelial cell adhesion and growth, indicating that the functionality of the biomolecule was maintained. The biocompatibility of the composite films was demonstrated indicating the significant future potential of biocomposites of this type for use in promoting cell adhesion in electrically active materials for tissue engineering.

Synaptic plasticity functions in an organic electrochemical transistor

Synaptic plasticity functions play a crucial role in the transmission of neural signals in the brain. Short-term plasticity is required for the transmission, encoding, and filtering of the neural signal, whereas long-term plasticity establishes more permanent changes in neural microcircuitry and thus underlies memory and learning. The realization of bioinspired circuits that can actually mimic signal processing in the brain demands the reproduction of both short- and long-term aspects of synaptic plasticity in a single device. Here, we demonstrate the implementation of neuromorphic functions similar to biological memory, such as short- to long-term memory transition, in non-volatile organic electrochemical transistors (OECTs). Depending on the training of the OECT, the device displays either short- or long-term plasticity, therefore, exhibiting non von Neumann characteristics with merged processing and storing functionalities. These results are a first step towards the implementation of organic-based neuromorphic circuits.

A facile biofunctionalisation route for solution processable conducting polymer devices

For the majority of biosensors or biomedical devices, immobilization of the biorecognition element is a critical step for device function. To achieve longer lifetime devices and controllable functionalization, covalent immobilisation techniques are preferred over passive adhesion and electrostatic interactions. The rapidly emerging field of organic bioelectronics uses conducting polymers (or small molecules) as the active materials for transduction of the biological signal to an electronic one. While a number of techniques have been utilized to entrap or functionalize conducting polymers deposited by electro- or vapor phase polymerization, covalent functionalization of solution processed films, essential for realizing low cost or high throughput fabrication, has not been thoroughly investigated. In this study we show a versatile biofunctionalization technique for the solution processable conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) PEDOT:PSS, which is a commercially available material, and has a record high conductivity. Addition of poly(vinyl alcohol) (PVA) into the solution with PEDOT:PSS provides a handle for subsequent silanization with a well-characterised silane reagent, allowing for covalent linkage of biological moieties onto PEDOT:PSS films. We show homogenous and large-scale biofunctionalization with polypeptides and proteins, as well as maintenance of the biological functionalities of the proteins. In addition, no deleterious effects are noted on the electronic or ionic transport properties of the conducting polymer films due to incorporation of the PVA.

Ionic Liquid Gel‐Assisted Electrodes for Long‐Term Cutaneous Recordings

The integration of an ionic liquid gel on conformal electrodes is investigated for applications in long-term cutaneous recordings. Electrodes made of Au and the conducting polymer PEDOT:PSS coated with the gel show a low impedance in contact with the skin that maintains a steady value over several days, paving the way for non-invasive, long-term monitoring of human electrophysiological activity.

PEDOT:TOS with PEG: a biofunctional surface with improved electronic characteristics

Devices based on conducting polymers offer great promise for interfacing with cells. Here, we use vapour phase polymerisation to create a biofunctional composite material of the conducting polymer poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:TOS) and the biologically relevant poly(ethylene glycol) (PEG). On the addition of PEG, electroactivity of the PEDOT is maintained, conductivity is increased, and its performance as the active material in a transistor is unaffected. Both direct and indirect biocompatibility tests prove that PEDOT:TOS and PEDOT:TOS:PEG are biocompatible and nontoxic to mammalian cells. A functionalised PEG (PEG(COOH)) was additionally introduced into PEDOT:TOS to showcase the potential of this material for use in applications requiring biofunctionalisation.

Organic Transistor Arrays Integrated with Finger-Powered Microfluidics for Multianalyte Saliva Testing.

A compact multianalyte biosensing platform is reported, composed of an organic electrochemical transistor (OECT) microarray integrated with a pumpless finger-powered microfluidic, for quantitative screening of glucose, lactate, and cholesterol levels. A biofunctionalization method is designed, which provides selectivity towards specific metabolites as well as minimization of any background interference. In addition, a simple method is developed to facilitate multi-analyte sensing and avoid electrical crosstalk between the different transistors by electrically isolating the individual devices. The resulting biosensing platform, verified using human samples, offers the possibility to be used in easy-to-obtain biofluids with low abundance metabolites, such as saliva. Based on our proposed method, other types of enzymatic biosensors can be integrated into the array to achieve multiplexed, noninvasive, personalized point-of-care diagnostics.

Taking Electrons out of Bioelectronics: From Bioprotonic Transistors to Ion Channels

From cell‐to‐cell communication to metabolic reactions, ions and protons (H+) play a central role in many biological processes. Examples of H+ in action include oxidative phosphorylation, acid sensitive ion channels, and pH dependent enzymatic reactions. To monitor and control biological reactions in biology and medicine, it is desirable to have electronic devices with ionic and protonic currents. Here, we summarize our latest efforts on bioprotonic devices that monitor and control a current of H+ in physiological conditions, and discuss future potential applications. Specifically, we describe the integration of these devices with enzymatic logic gates, bioluminescent reactions, and ion channels.

High‐Performance Vertical Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are promising transducers for biointerfacing due to their high transconductance, biocompatibility, and availability in a variety of form factors. Most OECTs reported to date, however, utilize rather large channels, limiting the transistor performance and resulting in a low transistor density. This is typically a consequence of limitations associated with traditional fabrication methods and with 2D substrates. Here, the fabrication and characterization of OECTs with vertically stacked contacts, which overcome these limitations, is reported. The resulting vertical transistors exhibit a reduced footprint, increased intrinsic transconductance of up to 57 mS, and a geometry-normalized transconductance of 814 S m-1 . The fabrication process is straightforward and compatible with sensitive organic materials, and allows exceptional control over the transistor channel length. This novel 3D fabrication method is particularly suited for applications where high density is needed, such as in implantable devices.