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A Comprehensive 3D FEM Model of Excitable Tissue and Capacitive Electrode Interface

The interface of excitable cells and stimulation or recording electrodes is essential for bioelectronic applications. Parameters such as the electrode impedance and capacitance, interface electrochemistry, surface structuring and long term in vivo stability have been thoroughly studied. However, in many applications, especially clinical ones, only trivial electrode geometries are used, resulting in increased charge density thresholds. Using optimized electrode geometries and stimulation protocols may overall be more effective, especially in the case of implanted bioelectronic devices with limited current generation capabilities. For highly localized target specific electrostimulation, electrode design and stimulation protocol are crucial parameters to consider. We consider multiple planar and 3D electrode configurations for stimulating excitable cells and tissues, and different stimulation protocols using pulsed and modulated current. A comprehensive finite element method (FEM) model encompassing realistic capacitive photo-electrode (organic electrolytic photocapacitor – OEPC) and tissue model is made in COMSOL Multiphysics® software. Electrodes are characterized by their contact electrical properties, contact capacitance and contact resistance, while the OEPC is characterized by its equivalent circuit model. Realistic cell membranes and action potential propagation are implemented using the Hodgkin–Huxley model which can be tailored to a specific cell type. We show that using the optimized electrode configuration enables multifold current density enhancement at the targeted stimulation area which enables effective cell and tissue excitation while minimising the residual effect on the surrounding tissue. Our numerical findings are validated in vitro using cortical neuron cell cultures and mouse brain slices.

bioelectronic interfaces, organic electronics, neurostimulation, FEM modeling

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