Polymer-Based Implantable Electrodes: State of the Art and Future Prospects
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Polymer-Based Implantable Electrodes: State of the Art and Future Prospects Klaus Peter Koch, Anup Ramachandran, Wigand Poppendieck, Dara Feili, and Klaus-Peter Hoffmann Medical Technology and Neuroprosthetics, Fraunhofer-IBMT, Ensheimer Strasse 48, St. Ingbert, Saarland, 66386, Germany
ABSTRACT Depending on their intended application, the electrical interfaces between technical system and the biological system differ with regard to their materials and shapes. There are different approaches using silicon as basic material for sieve or needle-like electrodes for interfacing the nerve tissue. This paper will focus on polymer-based flexible implantable electrodes. Mechanical interaction between the electrode and soft nerve tissue can induce adverse body reaction such as fibrous tissue encapsulation. Using flexible materials to design the electrobiological interface might reduce this effect. Different designs of such flexible electrodes were proposed for contacting the nerves or as platform for sensors. For example, cuff-like electrodes can be elastically wrapped around the nerve for recording or stimulation of neural signals. Using microtechnology for the structuring of polymer substrates, new fiber-like electrodes with multiple electrode sites were developed that can be sewn into the nerve. Thereby, the possibility of selective nerve recording and stimulation was improved. One major problem of these tiny and flexible electrodes is the connection to the recording and stimulation system. Incorporating electronics such as multiplexers or amplifiers directly on the flexible substrate could reduce the number of connection lines and improve the sensor capabilities. Furthermore, the integration of flexible organic circuitry on the implant allows the design of more flexible and intelligent electrodes. Additionally, the electrodes and sensors can be designed using conductive polymers to create a new generation of “All Polymer” active implants. Not only the chemical and mechanical properties of the materials employed can influence the biocompatibility of an implant, but also the surface morphology in the nanometer range plays a key role in e.g. the growth of cells on the implants. Selective growth of different types of cells on different parts of the implant is a challenge for interdisciplinary research. Moreover, the combination of surface nanostructuring, for example interference laser beam structuring, and organic electronics with microstructured polymer implants offers high potentials for new active implants of the next decades. INTRODUCTION In the last decades, several approaches for the electrical interface between technical systems and biological tissue were designed. Instead of using surface electrodes, many applications have the need for implantable electrodes. There are several reasons to implant electrodes instead of external devices. One is to reach a better selectivity between the electrode and a particular nerve or fascicle of the nerve. Additionally, the amplitude for stimulation can be reduced by
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