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. Twoand 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 every 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

Abstract-This paper describes the adhesion and growth of dissociated cortical neurons on chemically patterned surfaces over a time period of 30 days. The presence of neurons was demonstrated by measurement of spontaneous bioelectrical activity on a micropatterned multielectrode array. Chemical patterns were prepared with a combination of neurophobic layers of polyethylenoxide-polypropylenoxide-polyethylenoxide (PEO-PPO-PEO) triblockcopolymers adsorbed onto hydrophobic surfaces and neurophilic microprinted tracks of polyethylenimine (PEI). Results showed that commercially available PEO-PPO-PEO triblockcopolymers F108 and F127 (Synperonics, ICI) significantly reduced the adhesion of neuronal tissue when adsorbed on hydrophobic Polyimide (PI) and Fluorocarbon (FC) surfaces over a time period of eight days. In general, both F108-and F127-coated PI displayed equal or better neurophobic background properties after 30 days. Viability of neuronal tissue after 30 days on PEI microprinted F108-and F127-coated PI was comparable with relatively high viability factors between 0.9 and 1 (scale from 0 to 1). Summarizing, the strategy to combine the neurophobic adsorbed triblock-copolymers F108 and F127 onto hydrophobic surfaces with neurophilic microprinted PEI resulted in relatively long-term neuronal pattern preservation with high numbers of viable neurons present after 30 days.

One type of future, improved neural interfaces is the 'cultured probe'. It is a hybrid type of neural information transducer or prosthesis, for stimulation and=or recording of neural activity. It would consist of a micro-electrode array (MEA) on a planar substrate, each electrode being covered and surrounded by a local circularly confined network ('island') of cultured neurons. The main purpose of the local networks is that they act as bio-friendly intermediates for collateral sprouts from the in vivo system, thus allowing for an effective and selective neuron electrode interface. As a secondary purpose, one may envisage future information processing applications of these intermediary networks.In this chapter, first, progress is shown on how substrates can be chemically modified to confine developing networks, cultured from dissociated rat cortex cells, to 'islands' surrounding an electrode site. Additional coating of neurophobic, polyimide coated substrate by tri-block-copolymer coating enhances neurophilic-neurophobic adhesion contrast. Secondly, results are given on neuronal activity in patterned, unconnected and connected, circular 'island' networks. For connected islands, the larger the island diameter (50, 100 or 150 mm), the more spontaneous activity is seen. Also, activity may show a very high degree of synchronization between two islands. For unconnected islands, activity may start at 22 days in vitro (DIV), which is two weeks later than in unpatterned networks.

Adhesion and patterning of cortical neurons was investigated on isolated islands of neuron-adhesive polyethylenimine (PEI) surrounded by a neuron-repellent fluorocarbon (FC) layer. In addition, the development of fasciculated neurites between the PEI-coated areas was studied over a time period of fifteen days. The patterns consisted of PEI-coated wells (diameter 150 microns, depth 0.5 micron) which were etched in a coating of fluorocarbon (FC) on top of polyimide (PI) coated glass. The separation distance between the PEI-coated wells were varied between 10 and 90 microns. This paper shows that chemical patterns of PEI and FC result in highly compliant patterns of adhering cortical neurons after one day in vitro. Interconnecting neurite fascicles between PEI-coated wells were especially present on patterns with a separation distance of 10 microns after eight days in vitro. A significant lower number of interconnecting neurite fascicles was observed on 20 microns separated patterns. Effective isolation of neurons into PEI-coated wells was achieved on patterns with a separation distance of 80 microns as no interconnecting neurite fascicles were observed.

-In this study adhesion and patterning of cortical neurons on modified glass surfaces was investigated. Patterns of cortical neurons were prepared with a combination of polyethylenimine (PEI) and plasma-deposited fluorocarbon (FC). In addition neurite development and fasciculation of interconnecting neurites between PEI-coated areas was studied. The patterns consisted of PEI-coated circular holes (diameter 150 pm) which were initially etched in a Fluorocarbon (FC) layer. The separation distance between the PEI-coated circular holes was varied from 10 up to 90 pm. This paper shows that the chemical patterns, prepared with a combination of polyethylenimine (PEI) and plasma deposited Fluorocarbon (FC), results in highly compliant patterns of adhering cortical neurons. Furthermore it was shown that interconnecting neurite bundles between neurons on the PEI-coated circular holes were especially present on the pattern with a minimal separation distance (10 pm) between the PEI-coated circular holes. In contrast interconnecting neurite bundles were hardly observed on patterns with a maximal separation distance (90 pm) between the PEI-coated circular holes.