Biostability of micro-photodiode arrays for subretinal implantation.

Micro-photodiode arrays based on semiconductor chip technology are being developed to replace degenerated photoreceptor cells in the retina. Electric current is generated in tiny micro-photodiodes and delivered to the adjacent tissue by micro-electrodes. One of the main requirements of a sub-retinal implantable device is long-term stability versus corrosion in vivo (biostability). Biostability of micro-photodiode arrays (MPDA) was investigated in vitro and in vivo. No significant damage was found on chips immersed for up to 21 months in saline solution. Under in vivo conditions, however, the silicon oxide passivation layer of the chip was dissolved within a period of about 6-12 months. Subsequently, the underlying silicon was corroded. In contrast, stimulation electrodes consisting of titanium nitride were well preserved both in vitro and in vivo. The deterioration of the electrical properties of the micro-photodiodes correlated with the morphological damage observed. Strategies aiming at the development of an improved biostable encapsulation of neurotechnological implants have to be investigated and will be discussed briefly.

Functional re-establishment of the perforant pathway in organotypic co-cultures on microelectrode arrays.

Co-cultures of entorhinal cortex (EC) and dentate gyrus (DG) explants are a useful model system to study the formation and stabilization of axonal projections. We adapted this model system to EC-DG co-cultures on microelectrode arrays (MEA) for the characterization of axonal projections on a functional level for days and weeks. EC and DG explants were placed on MEA to allow for the reconstitution of perforant pathway projections. Connections formed were characterized by morphological and electrophysiological analyses to verify characteristic features of perforant pathway signal transmission. Morphological analysis reveals proper projection of EC neurons into the molecular layer of the DG. Examination of synaptic transmission after high frequency stimulation imply unidirectional connections that used glutamate receptors of the AMPA/kainate type as main mediators of excitatory signal transmission. The system was evaluated by the introduction of the NCAM binding peptide C3d. In accordance with in vivo and in vitro experiments C3d modulated signal transmission by NCAM-related mechanisms resulting in morphological re-arrangements.