Development of Multidimensional/Multifunctional Bioelectronic Devices based on Organic Mixed Ionic - Electronic Conductors

Abstract

Organic mixed ionic – electronic conductor (OMIEC) had been received great attention as an interfacial layer for detection of electrophysiological signals from the cells or organs, and an active layer for bioelectronic devices, neuromorphic devices, and energy storage devices, owing to outstanding electrical/electrochemical performance, operational stability, and decent biocompatibility. Further, due to the unique material composition of OMIECs (e.g., the combination of conjugated polymer backbones and waterswellable polymer side chains/additives), the electronic (i.e., electrons and holes for n- and p-type materials) and ionic charges (i.e., anions and cations) can be transported through the inside of the material structure, and their doping state of each conjugated polymer chain can be controllable by inducing electrochemical bias through the electrolyte.However, due to the solution configuration of the OMIEC materials (i.e., dispersed or solvated with polar or nonpolar solvents), the active layer should be defined by solution casting processes such as spin-/bar- /slit-/dip-coating or inkjet printing methods, so that the resultant devices should be laid on the rigid or flexible substrates. Considering the complex morphologies of organs or skins, i) conventional flat geometry of the bioelectronic devices is not suitable to be directly adhered on the surface, and ii) ultrasoft device structure for the conformal contact of 2-D thin devices on the complex morphology makes controllability poorer during the sample preparation and surgical operation. Regarding to this issue, the fabrication procedure for multidimensional OMIEC structures were introduced in part 1 and 2, which have practical advantages on the materials study/analysis and the wearable/implantable bioelectronic device applications, from one-dimensional microfibers to three-dimensional centimeter-scaled arbitrary structures which were consisted of short microfibers (i.e., microstrands). To prepare uniform microfibers, we adopted a conventional wet spinning process by using polar solvent coagulant, and studied the effect of the strain-engineering on the microstructural/electrical/electrochemical properties of the microfibers. Due to the advantage of 1-dimensional microfiber structure, we could prepare and characterized the microfibers with minimized dimensional complexity, and it is applicated to demonstrate microfiber-based 1-dimensional OECT devices. By the microstructural studies, it could be concluded that the strain on the microfibers makes the molecular orientation shifted toward fiber direction without distortion on grain size of the conducting polymer, and we could achieve high carrier mobility of 12.9 cm2V-1s-1 without trade-off in the volumetric capacitance (122 Fcm-3) and carrier density (5.6 x 1020 cm-3) (chapter 1). Further, we also demonstrated 1-dimensional OECT-based chemical sensor devices to detect the concentration of the electrolytes by using the microfibers. Since the potential difference between the gate electrode and the active layer could be shifted by the change of the electrolyte concentration by following Nernst Equation, we could estimate the concentration of the electrolyte by measuring the potential difference. Further, as defining their electrochemical gate electrodes on the surface of the source electrodes, three-terminal OECT-based sensor devices could be operated with pseudo two-terminal configuration (chapter 2) with identical characteristics. In part 2, the fabrication procedures to prepare twoand three-dimensional architectures are described. To build the higher-dimensional structures with 1- dimensional microfibers, we studied how two microfibers to be adhered without any adhesives by conducting phenomenological studies and ex-situ x-ray analysis, and finally, we suggested ‘crystalline domain-mediated reversible/irreversible bonding method for the PEDOT:PSS. Using this method, we could build microporous 2-D mesh electrodes and 3-D complex architectures, and their mechanical/electrical/electrochemical properties were analyzed (chapter 3). In parallel, this mesh electrodes were demonstrated for electrophysiological probes to acquire extracellular action potentials from the heart of the rodents (chapter 4). Due to the mechanical softening on the wet organs, and high electrical/electrochemical properties, we could record the action potential signals by simply attaching the mesh electrodes on the target organs with a very high signal to noise ratio without motion artifacts. (Chapter 1 and 2 were reproduced from ‘Strain-Engineering Induced Anisotropic Crystallite Orientation and Maximized Carrier Mobility for High-Performance Microfiber-Based Organic Bioelectronic Devices’ and ‘Organic electrochemical transistor-based channel dimension-independent singlestrand wearable sweat sensors’ with permissions from John Wiley and Sons and Springer Nature, respectively.) Owing to the unprecedented on-current and transconductance of the OMIEC-based OECT devices, recent research groups are aiming to replace the conventional semiconducting materials (e.g., (amorphous/polycrystalline) thin-film silicon, metal-oxide, and conjugated polymer-based organic semiconductors) with the OMIECs. Even the transistors could be fully replaceable by using OECT devices for switching and amplifying operations, there is no suitable diode device which is one of the essential device components for the current rectification, circuit protection, and signal processing. However, considering the wet and humid environment in which the bioelectronic devices are operated, conventional heterojunction diodes (e.g., p-/n-type semiconductor or metal/semiconductor materials) cannot be adopted to serve the diodelike operation with organic mixed ionic – electronic conductors, but a novel device architecture is needed to realize unconventional electrical/electrochemical stimuli-responsive current rectifying operation. In part 3, the novel device architecture which could rectify the current signals in aqueous electrolyte is demonstrated. In this work, we introduced localized doping/dedoping behavior on asymmetrically patterned OMIEC layers in regard to the channel region, and unipolar I–V characteristic curves could be acquired. The operation mechanism is carefully studied with numerical analysis, and generalized by showing diode behavior with various OMIEC materials. The underlying mechanism on polarity-sensitive balanced ionic doping/dedoping is elucidated by numerical device analysis and in operando spectroelectrochemical potential mapping, while the general material requirements for electrochemical diode operation are deduced using various types of conjugated polymers. In parallel, analog signal rectification and digital logic processing circuits are successfully demonstrated to show the broad impact of organic electrochemical diode-incorporated circuits. We expect that organic electrochemical diodes will play vital roles in realizing multifunctional soft bioelectronic circuitry in combination with organic electrochemical transistors. (Chapter 5 was reproduced from ‘High-Current-Density Organic Electrochemical Diodes Enabled by Asymmetric Active Layer Design’ with a permission from John Wiley and Sons.)