Resulting from its wide range of beneficial properties, the conjugated conducting polymer poly(3,4‐ethylenedioxythiophene) (PEDOT) is a promising material in a number of emerging applications. These ...material properties, particularly promising in the field of bioelectronics, include its well‐known high‐degree of mechanical flexibility, stability, and high conductivity. However, perhaps the most advantageous property is its ease of fabrication: namely, low‐cost and straight‐forward deposition processes. PEDOT processing is generally carried out at low temperatures with simple deposition techniques, allowing for significant customization of the material properties through, as highlighted in this review, both process parameter variation and the addition of numerous additives. Here we aim to review the role of PEDOT in addressing an assortment of mechanical and electronic requirements as a function of the conditions used to cast or polymerize the films, and the addition of additives such as surfactants and secondary dopants. Contemporary bioelectronic research examples investigating and utilizing the effects of these modifications will be highlighted.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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.
Vertical organic electrochemical transistors are demonstrated with decreased channel length down to 450 nm, while retaining a simple fabrication process. These devices exhibit an intrinsic transconductance of up to 57 mS and a geometry‐normalized transconductance of 814 S m−1. The reduced spatial footprint and improved device performance are particularly suited for biointerfacing applications.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Biointegrated neuromorphic hardware holds promise for new protocols to record/regulate signalling in biological systems. Making such artificial neural circuits successful requires minimal ...device/circuit complexity and ion-based operating mechanisms akin to those found in biology. Artificial spiking neurons, based on silicon-based complementary metal-oxide semiconductors or negative differential resistance device circuits, can emulate several neural features but are complicated to fabricate, not biocompatible and lack ion-/chemical-based modulation features. Here we report a biorealistic conductance-based organic electrochemical neuron (c-OECN) using a mixed ion-electron conducting ladder-type polymer with stable ion-tunable antiambipolarity. The latter is used to emulate the activation/inactivation of sodium channels and delayed activation of potassium channels of biological neurons. These c-OECNs can spike at bioplausible frequencies nearing 100 Hz, emulate most critical biological neural features, demonstrate stochastic spiking and enable neurotransmitter-/amino acid-/ion-based spiking modulation, which is then used to stimulate biological nerves in vivo. These combined features are impossible to achieve using previous technologies.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
Interfacing electronics with neural tissue is crucial for understanding complex biological functions, but conventional bioelectronics consist of rigid electrodes fundamentally incompatible with ...living systems. The difference between static solid-state electronics and dynamic biological matter makes seamless integration of the two challenging. To address this incompatibility, we developed a method to dynamically create soft substrate-free conducting materials within the biological environment. We demonstrate in vivo electrode formation in zebrafish and leech models, using endogenous metabolites to trigger enzymatic polymerization of organic precursors within an injectable gel, thereby forming conducting polymer gels with long-range conductivity. This approach can be used to target specific biological substructures and is suitable for nerve stimulation, paving the way for fully integrated, in vivo-fabricated electronics within the nervous system.
Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronic devices such as sensors and neural interfaces. While the majority of OECTs use simple planar geometry, ...there is interest in exploring how these devices operate with much shorter channels on the submicron scale. Here, we show a practical route toward the minimization of the channel length of the transistor using traditional photolithography, enabling large-scale utilization. We describe the fabrication of such transistors using two types of conducting polymers. First, commercial solution-processed poly(dioxyethylenethiophene):poly(styrene sulfonate), PEDOT:PSS. Next, we also exploit the short channel length to support easy in situ electropolymerization of poly(dioxyethylenethiophene):tetrabutyl ammonium hexafluorophosphate, PEDOT:PF6. Both variants show different promising features, leading the way in terms of transconductance (g m), with the measured peak g m up to 68 mS for relatively thin (280 nm) channel layers on devices with the channel length of 350 nm and with widths of 50, 100, and 200 μm. This result suggests that the use of electropolymerized semiconductors, which can be easily customized, is viable with vertical geometry, as uniform and thin layers can be created. Spin-coated PEDOT:PSS lags behind with the lower values of g m; however, it excels in terms of the speed of the device and also has a comparably lower off current (300 nA), leading to unusually high on/off ratio, with values up to 8.6 × 104. Our approach to vertical gap devices is simple, scalable, and can be extended to other applications where small electrochemical channels are desired.
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IJS, KILJ, NUK, PNG, UL, UM
We report a design strategy that allows the preparation of solution processable n-type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone ...is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol-based side chain. A series of random copolymers was prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90, and 100 with respect to the alkyl side chains. These were characterized to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions, and performance in OECTs when operated in aqueous electrolytes. We observed that glycol side chain percentages of >50% are required to achieve volumetric charging, while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.
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Organic semiconductors can be employed as the active layer in accumulation mode organic electrochemical transistors (OECTs), where redox stability in aqueous electrolytes is important for long‐term ...recordings of biological events. It is observed that alkoxy‐benzo1,2‐b:4,5‐b′dithiophene (BDT) copolymers can be extremely unstable when they are oxidized in aqueous solutions. The redox stability of these copolymers can be improved by molecular design of the copolymer where it is observed that the electron rich comonomer 3,3′‐dimethoxy‐2,2′‐bithiophene (MeOT2) lowers the oxidation potential and also stabilizes positive charges through delocalization and resonance effects. For copolymers where the comonomers do not have the same ability to stabilize positive charges, irreversible redox reactions are observed with the formation of quinone structures, being detrimental to performance of the materials in OECTs. Charge distribution along the copolymer from density functional theory calculations is seen to be an important factor in the stability of the charged copolymer. As a result of the stabilizing effect of the comonomer, a highly stable OECT performance is observed with transconductances in the mS range. The analysis of the decomposition pathway also raises questions about the general stability of the alkoxy‐BDT unit, which is heavily used in donor–acceptor copolymers in the field of photovoltaics.
The redox‐stability of benzodithiophene copolymers is analyzed where it is found that copolymers with large ionization potentials (IPs) degrade during electrochemical oxidation in aqueous electrolytes while copolymers with a small IP can be charged reversibly. The degradation product is identified to be a quinone structure which has a significant impact on the performance of the copolymers in electronic devices.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Organic electrochemical transistors (OECTs) are transistors that can have extrinsic transconductances as high as 400 S m−1, but they typically have response times on the order of 1 ms or longer. ...These response speeds are limited by ion transport. It is shown that OECTs can exceed the ionic response speed by a factor of 30 when operated in a high‐speed bias regime.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Deep brain stimulation (DBS) is a technique commonly used both in clinical and fundamental neurosciences. Classically, brain stimulation requires an implanted and wired electrode system to deliver ...stimulation directly to the target area. Although techniques such as temporal interference (TI) can provide stimulation at depth without involving any implanted electrodes, these methods still rely on a wired apparatus which limits free movement. Herein organic photocapacitors as untethered light‐driven electrodes which convert deep‐red light into electric current are reported. Pairs of these ultrathin devices can be driven using lasers at two different frequencies to deliver stimulation at depth via temporally interfering fields. This concept of laser TI stimulation using numerical modeling, tests with phantom brain samples, and finally in vivo tests is validated. Wireless organic photocapacitors are placed on the cortex and elicit stimulation in the hippocampus, while not delivering off‐target stimulation in the cortex. This laser‐driven wireless TI evokes a neuronal response at depth that is comparable to control experiments induced with deep brain stimulation protocols using implanted electrodes. This work shows that a combination of these two techniques—temporal interference and organic electrolytic photocapacitors—provides a reliable way to target brain structures requiring neither deeply implanted electrodes nor tethered stimulator devices. The laser TI protocol demonstrated here addresses two of the most important drawbacks in the field of DBS and thus holds potential to solve many issues in freely moving animal experiments or for clinical chronic therapy application.
Deep brain stimulation (DBS) is an invasive method of brain stimulation used in clinical neuroscience. Although methods like temporal interference provide noninvasive deep stimulation without implanted electrodes, such methods rely on wires, tethers, and batteries. Herein, it is shown that a combination of temporal interference and organic electrolytic photocapacitors provides reliable DBS, and requires neither deeply implanted electrodes nor tethered stimulator devices.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Future brain–computer interfaces will require local and highly individualized signal processing of fully integrated electronic circuits within the nervous system and other living tissue. New devices ...will need to be developed that can receive data from a sensor array, process these data into meaningful information, and translate that information into a format that can be interpreted by living systems. Here, the first example of interfacing a hardware‐based pattern classifier with a biological nerve is reported. The classifier implements the Widrow–Hoff learning algorithm on an array of evolvable organic electrochemical transistors (EOECTs). The EOECTs’ channel conductance is modulated in situ by electropolymerizing the semiconductor material within the channel, allowing for low voltage operation, high reproducibility, and an improvement in state retention by two orders of magnitude over state‐of‐the‐art OECT devices. The organic classifier is interfaced with a biological nerve using an organic electrochemical spiking neuron to translate the classifier's output to a simulated action potential. The latter is then used to stimulate muscle contraction selectively based on the input pattern, thus paving the way for the development of adaptive neural interfaces for closed‐loop therapeutic systems.
A modular circuit constructed mainly of organic electrochemical transistors is used to classify patterns written on a 4 × 4 pixel array, translate a positive classification to a signal that is interpretable by biological systems, and interface with the ventral nerve cord of a medicinal leech to actuate muscle contraction upon encountering a positive classification.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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