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ELECTRONIC AND IONIC DEVICES: Semiconductor Chips with Brain Tissue Peter Fromherz Department of Membrane and Neurophysics Max Planck Institute for Biochemistry Martinsried / München, Germany Abstract The electrical interfacing of semiconductor chips with components of the brain is considered. Both directions of signal transduction between the electronic and the ionic system are described for individual electronic devices such as capacitors or transistors and for multi-device arrays, respectively, on the levels of ion channels, of nerve cells and of brain tissue. The investigation relies on the combination of experimental techniques of semiconductor technology, of molecular biology and of neurophysiology. The observations form a basis for the development of sensors and actuators in biotechnology, of advanced instruments in neurophysiology and of improved devices in neuroprosthetics. 1. Introduction Brains rely on the processing of electrical signals. In contrast to computers, those signals are carried by ions in water, not by electrons in semiconductors and metals. In principle, a transduction of ionic signals and of electronic signals is possible as it was originally demonstrated by Luigi Galvani. It may be achieved by a controlled electrochemical reaction as in Ag/AgCl electrodes of common electrophysiological experiments, or by capacitive and less defined Faradaic currents of metal electrodes as used in neuroprosthetic devices. The ultimate goal for a development of hybrid systems is an integration of electronic and ionic circuits on a microscopic level, i. e. an integration of the analog and digital signal processing in semiconductor devices with the dynamics of nerve cells that are connected by axons, synapses and dendrites. In the progress of basic research in that field, results may be achieved step by step that become relevant for applications for sensors and actuators in biotechnology, for instruments in neurophysiology and for devices in neuroprosthetics, long before truly integrated hybrids will be implemented. The principles of microelectronic as well as of microionic devices are well known. On that basis, a research programme on iono-electronic hybrids must focus on the interaction of semiconductor chips with neuronal materials and on the physics of signal transduction. Three different issues must be considered: (i) the interfacing in both directions from ions to electrons and from electrons to ions, (ii) the interfacing on two different functional levels in semiconductor chips - individual electronic devices such as transistors and capacitors and multi- device arrays obtained by VLSI, and (iii) the interfacing on three different levels of brain dynamics - ion channels, nerve cells and brain tissue. Such a research programme was initiated in 1985 (1). The first results were the interfacing of individual nerve cells from the leech with electrolyte-oxide-semiconductor (EOS) field-effect transistors in 1991 and with EOS capacitors in 1995, respectivly (2). Subsequently, the studies proceeded in
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