Conductive polymer microwires for single cell bioelectrical stimulation

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Thourson, Scott
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There are currently no bioelectrical probes that are able to provide long-term electrical connections to a large population of single cells in the body. The challenge is multi-faceted: engineering an array of subcellular probes that are long, thin, mechanically flexible, electrically conductive, biochemically stable, and can be routed to individual target cells in a hostile, 3D tissue matrix. Conductive polymers have extraordinary properties that could enable the design of a bioprobe that meets many of these requirements simultaneously. The principal advantage of conductive polymers is the combination of their mechanical flexibility, electron conductivity, and efficiency in converting electrons to electric fields in electrolyte solutions. Conductive polymers have historically always been employed as films, but a synthesis method developed by Flanders et. al. yields high-aspect ratio polymer wires with dimensions ranging from 150 nm to 10 μm in diameter and up to millimeters in length. The research presented here investigates conductive polymer microwires for local electric field generation at the single cell level. This work focuses on experimentally characterizing the electrical properties of these wires and their ability to generate local electric fields for single cell stimulation. The advantages and limitations of their performance are identified in light of the ideal bioelectrical probe. The results are used to build a simulation model that can predict local electric field generation by current conductive polymer wires as well as wires with different shapes and enhanced material properties. The current work concludes that the conductivity of conductive polymers is not sufficient to permit the use of small, flexible wire sizes (smaller than 2 x 27 μm) to electrically stimulate single cells. Although conductive polymers are not suitable for electron conduction, their extremely low surface impedance makes them the best material for converting electron current into ionic electric fields at small scales. Thus, conductive polymers do not overcome all the challenges of electrically wiring every cell in our brain, per se, but they do have a critical niche in enabling subcellular dimensions for the next generation of bioelectrical probes.
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