Efficient and selective electrical stimulation and recording of neural activity in peripheral, spinal, or central pathways requires multielectrode arrays at micrometer scale. At present, wire arrays in brain, flexible linear arrays in the cochlea and cuff arrays around nerve trunks are in experimental and/or clinical use. Two-and three-dimensional brush-like arrays and sieve arrays, with around 100 electrode sites, have been proposed, fabricated in microtechnology, and/or tested in a number of labs. As there are no "blueprints " for the exact positions of neurons, an insertable multielectrode has to be designed in a redundant way. Even then, the efficiency of a multielectrode will be less than 100 %, as not even, electrode will contact a neural axon or soma. Therefore, "cultured probe" devices are being developed. i.e., cell-cultured planar multielectrode arrays (MEAs). They may. enhance efficiency and selectivity because neural cells have been grown over and around each electrode site as electrode-specific local networks. If after implantation, collateral sprouts branch from a motor fiber (ventral horn area) and if they can be guided and contacted to each "host " network, a very selective and efficient interface will result. Four basic aspects of the design and development of a cultured probe, coated with rat cortical or dorsal root ganglion neurons, are described. First, the importance of optimization of the cell-electrode contact is presented. It turns out that impedance spectroscopy, and detailed modeling of the electrode-cell interface, is a very, helpful technique, which shows whether a cell is covering an electrode and how strong the sealing is. Second, the dielectrophoretic trapping method directs cells efficiently to desired spots on the substrate, and cells remain viable after the treatment. The number of cells trapped is dependent on the electric field parameters and the occurrence of a secondary force, a fluid,flow (cis a result of field-induced heating). It was found that the viability of trapped cortical cells it-as not influenced by, the electric field (3 V-pp, 14 MHz). Third, cells must adhere to the surface of the substrate and form networks, which are locally confined, to one electrode site. For that, chemical modification of the substrate and electrode areas with various coatings, such as polyethyleneimine (PEI) and fluorocarbon monolayers promotes or inhibits adhesion of cells. The optimal diameter of local circular neurophilic areas and the separation distance between them has been investigated. Good results are obtained on wells with a diameter of 150 mum and a separation distance of 90 mum between the wells. Finally, it is shown how PEI patterning, by a stamping technique, successfully guides outgrowth of collaterals from a neonatal rat lumbar spinal cord explant, after six days in culture.
