Benchmarking organic mixed conductors for transistors.

Organic mixed conductors have garnered significant attention in applications from bioelectronics to energy storage/generation. Their implementation in organic transistors has led to enhanced biosensing, neuromorphic function, and specialized circuits. While a narrow class of conducting polymers continues to excel in these new applications, materials design efforts have accelerated as researchers target new functionality, processability, and improved performance/stability. Materials for organic electrochemical transistors (OECTs) require both efficient electronic transport and facile ion injection in order to sustain high capacity. In this work, we show that the product of the electronic mobility and volumetric charge storage capacity (µC*) is the materials/system figure of merit; we use this framework to benchmark and compare the steady-state OECT performance of ten previously reported materials. This product can be independently verified and decoupled to guide materials design and processing. OECTs can therefore be used as a tool for understanding and designing new organic mixed conductors

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

La compréhension et l amélioration du transport ionique dans les polymères conducteurs

De nombreux dispositifs pour l électronique organique et la bioélectronique reposent sur le transport mixte (électronique et ionique).Le transport électronique dans les matériaux organique est relativement bien compris, mais une compréhension fondamentale du transport des ions est manquante. J'ai développé un modèle analytique qui décrit le transport d'ions dans une jonction planaire entre un électrolyte et un film de polymère conducteur.Le modèle permet des prédictions de l'évolution temporelle du courant et du drift length des ions.Ces prédictions sont validées par des simulations numériques et en utilisant des paramètres réalistes, je montre que le modèle analytique peut être utilisé pour obtenir la mobilité des ions dans le film. De plus, j'ai développé une méthode expérimentale qui permet l'application du modèle analytique et conduit à une estimation de la mobilité des ions dans les polymères conducteurs. Le PEDOT:PSS offre un transport efficace pour les ions, qui peut être mis en relation avec le gonflement important du film dans l'eau. Je montre que la réticulation du film diminue son gonflement ainsi que la mobilité des ions. Comprendre la forte corrélation entre l'hydratation et la conductivité ionique nous permet de développer des matériaux à mobilité ionique définie et importante. A titre d'exemple, le réglage de la mobilité ionique du PEDOT:TOS est présenté en ajustant le rapport relatif de la phase hygroscopique. Pour finir, j'ai effectué des mesures de spectroscopie d'impédance électrochimique au cours d'une expérience de moving front, afin de proposer une interprétation physique des spectres d'impédance mesurés à une jonction polymère conducteur/électrolyteMany organic electronic and bioelectronics devices rely on mixed (electronic and ionic) transport within a single organic layer. Although electronic transport in these materials is relatively well understood, a fundamental understanding of ion transport is missing. I developed a simple analytical model that describes ion transport in a planar junction between an electrolyte and a conducting polymer film. The model leads to predictions of the temporal evolution of drift length of ions and current. These predictions are validated by numerical simulations and by using realistic parameters, I show that the analytical model can be used to obtain the ion mobility in the film. Furthermore, I developed an experimental method which allows the application of the analytical model and leads to a straightforward estimation of the ion drift mobilities in conducting polymers. PEDOT:PSS was found to support efficient transport of common ions, consistent with extensive swelling of the film in water. Crosslinking the film decreased its swelling and the ion mobility. Understanding the high correlation of hydration and ionic conductivity enables us to engineer materials with high and defined ion mobilities. As an example tuning of ion mobility by adjusting the relative ratio of the hydroscopic phase to PEDOT:TOS is presented. Finally I performed electrochemical impedance spectroscopy during a moving front experiment, in order to give a physical interpretation of the impedance spectra at a conducting polymer/electrolyte junction.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Conducting polymer devices for biolectronics

Pas de résumé en français seulement en anglaisThe emergence of organic electronics a technology that relies on carbon-based semiconductors to deliver devices with unique properties represents one of the most dramatic developments of the past two decades. A rapidly emerging new direction in the field involves the interface with biology. The soft nature of organics offers better mechanical compatibility with tissue than traditional electronic materials, while their natural compatibility with mechanically flexible substrates suits the non-planar form factors often required for implants. More importantly, their ability to conduct ions in addition to electrons and holes opens up a new communication channel with biology. The coupling of electronics with living tissue holds the key to a variety of important life-enhancing technologies. One example is bioelectronic implants that record neural signals and/or electrically stimulate neurons. These devices offer unique opportunities to understand and treat conditions such as hearing and vision loss, epilepsy, brain degenerative diseases, and spinal cord injury.The engineering aspect of the work includes the development of a photolithographic process to integrate the conducting polymer poly(3,4-ethylenedioxythiophene: poly(styrene sulfonate) (PEDOT:PSS) with parylene C supports to make an active device. The technology is used to fabricate electrocorticography (ECoG) probes, high-speed transistors and wearable biosensors. The experimental work explores the fundamentals of communication at the interface between conducting polymers and the brain. It is shown that conducting polymers outperform conventional metallic electrodes for brain signals recording.Organic electrochemical transistors (OECTs) represent a step beyond conducting polymer electrodes. They consist of a conducting polymer channel in contact with an electrolyte. When a gate electrode excites an ionic current in the electrolyte, ions enter the polymer film and change its conductivity. Since a small amount of ions can effectively block the transistor channel, these devices offer significant amplification in ion-to-electron transduction. Using the developed technology a high-speed and high-density OECTs array is presented. The dense architecture of the array improves the resolution of the recording from neural networks and the transistors temporal response are 100 s, significantly faster than the action potential. The experimental transistor responses are fit and modeled in order to optimize the gain of the transistor. Using the model, an OECT with two orders of magnitude higher normalized transconductance per channel width is fabricated as compared to Silicon-based field effect transistors. Furthermore, the OECTs are integrated to a highly conformable ECoG probe. This is the first time that a transistor is used to record brain activities in vivo. It shows a far superior signal-to-noise-ratio (SNR) compare to electrodes. The high SNR of the OECT recordings enables the observation of activities from the surface of the brain that only a perpetrating probe can record. Finally, the application of OECTs for biosensing is explored. The bulk of the currently available biosensors often require complex liquid handling, and thus suffer from problems associated with leakage and contamination. The use of an organic electrochemical transistor for detection of lactate by integration of a room temperature ionic liquid in a gel-format, as a solid-state electrolyte is demonstrated.ST ETIENNE-ENS des Mines (422182304) / SudocGARDANNE-Centre microélec. (130412301) / SudocSudocFranceF

When Bio Meets Technology: Biohybrid Neural Interfaces.

The development of electronics capable of interfacing with the nervous system is a rapidly advancing field with applications in basic science and clinical translation. Devices containing arrays of electrodes can be used in the study of cells grown in culture or can be implanted into damaged or dysfunctional tissue to restore normal function. While devices are typically designed and used exclusively for one of these two purposes, there have been increasing efforts in developing implantable electrode arrays capable of housing cultured cells, referred to as biohybrid implants. Once implanted, the cells within these implants integrate into the tissue, serving as a mediator of the electrode-tissue interface. This biological component offers unique advantages to these implant designs, providing better tissue integration and potentially long-term stability. Herein, an overview of current research into biohybrid devices, as well as the historical background that led to their development are provided, based on the host anatomical location for which they are designed (CNS, PNS, or special senses). Finally, a summary of the key challenges of this technology and potential future research directions are presented.The authors acknowledge funding from EPSRC (EP/S009000/1 and DTP program), European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No. 732032 (BrainCom) (G.G.M.) and from the King Abdullah University of Science and Technology (KAUST) Office of sponsored Research (OSR) under award No. OSR‑2016‑CRG5‑3003

Orientation selectivity in a multi-gated organic electrochemical transistor.

UNLABELLED: 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

Multi-dimensional microwave sensing using graphene waveguides

This paper presents an electrolytically gated broadband microwave sensor where atomically-thin graphene layers are integrated into coplanar waveguides and coupled with microfluidic channels. The interaction between a solution under test and the graphene surface causes material and concentration-specific modifications of graphene's DC and AC conductivity. Moreover, wave propagation in the waveguide is modified by the dielectric properties of materials in its close proximity via the fringe field, resulting in a combined sensing mechanism leading to an enhanced S-parameter response compared to metallic microwave sensors. The possibility of further controlling the graphene conductivity via an electrolytic gate enables a new, multi-dimensional approach merging chemical field-effect sensing and microwave measurement methods. By controlling and synchronizing frequency sweeps, electrochemical gating and liquid flow in the microfluidic channel, we generate multidimensional datasets that enable a thorough investigation of the solution under study. As proof of concept, we functionalize the graphene surface in order to identify specific single-stranded DNA sequences dispersed in phosphate buffered saline solution. We achieve a limit of detection of ~1 attomole per litre for a perfect match DNA strand and a sensitivity of ~3 dB/decade for sub-pM concentrations. These results show that our devices represent a new and accurate metrological tool for chemical and biological sensing

Ferroelectricity and resistive switching in BaTiO3_3 thin films with liquid electrolyte top contact for bioelectronic devices

We investigate ferroelectric- and resistive switching behavior in 18-nm-thick epitaxial BaTiO$_3$ (BTO) films in a model electrolyte-ferroelectric-semiconductor (EFS) configuration. The system is explored for its potential as a ferroelectric microelectrode in bioelectronics. The BTO films are grown by pulsed laser deposition (PLD) on semiconducting Nb-doped (0.5 wt\%) SrTiO$_{3}$ (Nb:STO) single crystal substrates. The ferroelectric properties of the bare BTO films are demonstrated by piezoresponse force microscopy (PFM) measurements. Cyclic voltammetry (CV) measurements in EFS configuration, with phosphate buffered saline (PBS) acting as the liquid electrolyte top contact, indicate characteristic ferroelectric switching peaks in the bipolar current-voltage loop. The ferroelectric nature of the observed switching peaks is confirmed by analyzing the current response of the EFS devices to unipolar voltage signals. Moreover, electrochemical impedance spectroscopy (EIS) measurements indicate bipolar resisitive switching behavior of the EFS devices, which is controlled by the remanent polarization state of the BTO layer. Our results represent a constitutive step towards the realization of neuroprosthetic implants and hybrid neurocomputational systems based on ferroelectric microelectrodes

Integration of Organic Electrochemical Transistors with Implantable Probes

Funder: King Abdullah University of Science and Technology; Id: http://dx.doi.org/10.13039/501100004052Abstract: Organic electrochemical transistors (OECTs) are widely used as amplifying transducers of biological signals due to their high transconductance and biocompatibility. For implantable applications that penetrate into tissue, OECTs need to be integrated onto narrow probes. The scarcity of real estate necessitates the use of small local gate electrodes and narrow interconnects. This work shows that both of these factors lead to a decrease in the maximum transconductance and an increase in gate voltage required to attain this maximum. This work further shows that coating the gate electrode with a thick conducting polymer improves performance. These findings help guide the development of efficient OECTs on implantable probes

Conjugated Polymers in Bioelectronics.

The emerging field of organic bioelectronics bridges the electronic world of organic-semiconductor-based devices with the soft, predominantly ionic world of biology. This crosstalk can occur in both directions. For example, a biochemical reaction may change the doping state of an organic material, generating an electronic readout. Conversely, an electronic signal from a device may stimulate a biological event. Cutting-edge research in this field results in the development of a broad variety of meaningful applications, from biosensors and drug delivery systems to health monitoring devices and brain-machine interfaces. Conjugated polymers share similarities in chemical "nature" with biological molecules and can be engineered on various forms, including hydrogels that have Young's moduli similar to those of soft tissues and are ionically conducting. The structure of organic materials can be tuned through synthetic chemistry, and their biological properties can be controlled using a variety of functionalization strategies. Finally, organic electronic materials can be integrated with a variety of mechanical supports, giving rise to devices with form factors that enable integration with biological systems. While these developments are innovative and promising, it is important to note that the field is still in its infancy, with many unknowns and immense scope for exploration and highly collaborative research. The first part of this Account details the unique properties that render conjugated polymers excellent biointerfacing materials. We then offer an overview of the most common conjugated polymers that have been used as active layers in various organic bioelectronics devices, highlighting the importance of developing new materials. These materials are the most popular ethylenedioxythiophene derivatives as well as conjugated polyelectrolytes and ion-free organic semiconductors functionalized for the biological interface. We then discuss several applications and operation principles of state-of-the-art bioelectronics devices. These devices include electrodes applied to sense/trigger electrophysiological activity of cells as well as electrolyte-gated field-effect and electrochemical transistors used for sensing of biochemical markers. Another prime application example of conjugated polymers is cell actuators. External modulation of the redox state of the underlying conjugated polymer films controls the adhesion behavior and viability of cells. These smart surfaces can be also designed in the form of three-dimensional architectures because of the processability of conjugated polymers. As such, cell-loaded scaffolds based on electroactive polymers enable integrated sensing or stimulation within the engineered tissue itself. A last application example is organic neuromorphic devices, an alternative computing architecture that takes inspiration from biology and, in particular, from the way the brain works. Leveraging ion redistribution inside a conjugated polymer upon application of an electrical field and its coupling with electronic charges, conjugated polymers can be engineered to act as artificial neurons or synapses with complex, history-dependent behavior. We conclude this Account by highlighting main factors that need to be considered for the design of a conjugated polymer for applications in bioelectronics-although there can be various figures of merit given the broad range of applications, as emphasized in this Account