A light-sensitive, externally powered microchip was surgically implanted subretinally near the macular region of volunteers blind from hereditary retinal dystrophy. The implant contains an array of 1500 active microphotodiodes (‘chip’), each with its own amplifier and local stimulation electrode. At the implant's tip, another array of 16 wire-connected electrodes allows light-independent direct stimulation and testing of the neuron–electrode interface. Visual scenes are projected naturally through the eye's lens onto the chip under the transparent retina. The chip generates a corresponding pattern of 38 × 40 pixels, each releasing light-intensity-dependent electric stimulation pulses. Subsequently, three previously blind persons could locate bright objects on a dark table, two of whom could discern grating patterns. One of these patients was able to correctly describe and name objects like a fork or knife on a table, geometric patterns, different kinds of fruit and discern shades of grey with only 15 per cent contrast. Without a training period, the regained visual functions enabled him to localize and approach persons in a room freely and to read large letters as complete words after several years of blindness. These results demonstrate for the first time that subretinal micro-electrode arrays with 1500 photodiodes can create detailed meaningful visual perception in previously blind individuals.
Electrical activity of electrogenic cells in neuronal and cardiac tissue can be recorded by means of microelectrode arrays (MEAs) that offer the unique possibility for non-invasive extracellular recording from as many as 60 sites simultaneously. Since its introduction 30 years ago, the technology and the related culture methods for electrophysiological cell and tissue assays have been continually improved and have found their way into many academic and industrial laboratories. Currently, this technology is attracting increased interest owing to the industrial need to screen selected compounds against ion channel targets in their native environment at organic, cellular, and sub-cellular level. As the MEA technology can be applied to any electrogenic tissue (i.e., central and peripheral neurons, heart cells, and muscle cells), the MEA biosensor is an ideal in vitro system to monitor both acute and chronic effects of drugs and toxins and to perform functional studies under physiological or induced pathophysiological conditions that mimic in vivo damages. By recording the electrical response of various locations on a tissue, a spatial map of drug effects at different sites can be generated, providing important clues about a drug's specificity. In this survey, examples of MEA biosensor applications are described that have been developed for drug screening and discovery and safety pharmacology in the field of cardiac and neural research. Additionally, biophysical basics of recording and concepts for analysis of extracellular electrical signals are presented.
Visual prostheses such as subretinal implants are intended for electrical multisite excitation of the retinal network. To investigate relevant issues like spatial resolution and operational range, we have developed an in vitro method using microelectrode arrays to stimulate isolated retinae. Ganglion cell activity in the chicken retina evoked by distally applied spatial voltage patterns consisted of fast bursts, transient inhibition and delayed discharges, and depended on the amount, location and spatial pattern of the injected charge. The response was altered or disappeared when synaptic transmission was blocked. Our results indicate that shape perception and object location can be partially achieved with subretinal electrical multisite stimulation.
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