Microelectrode-Array Neurochip

The recording of cellular electrical activity is most often carried out in culture using saline-filled glass microelectrodes. These electrodes are intra- or extracellularly positioned at the cell membrane using a micromanipulator and can then record the action potentials of an active cell.

Using this method, there is always some risk of rupturing the cell membrane and destroying the cell and the whole network. In addition, it is difficult and very time consuming to work with several glass microelectrodes simultaneously in vivo or in vitro. This limits the number of possible recording sites for simultaneous and long-term intracellular recordings. Voltage-sensitive dyes are also employed in the detection of electrical activity of cells to avoid these problems, but can introduce other problems such as toxicity, which generally limits the life of the culture to generally 30-60 min.

An alternative method is to use extracellular recording electrodes which are located outside the cell and detect the extracellular voltage signal. Since the pioneering efforts by Thomas et al. (1972) and the first successful recording of neuronal action potentials with multi-microelectrode surfaces (Gross et al., 1977), steady progress has brought us to the point where the routine monitoring of the internal dynamics of mammalian neuronal networks is possible.

The growth of networks on beds of high-density microelectrode-arrays (MEA-neurochips) yields cell cultures that allow the long-term continuous and simultaneous monitoring of spike activity from a large number of cells for weeks or even months. The advantage of extracellular MEA-neurochip recording is that the long-term recording from multiple sites in vitro and the monitoring of signal transmission between large numbers of cells are possible.

MEA-neurochips also allow non-invasive, long-term recording and real time analysis of action potential patterns of up to 256 discrete neurons in short- and long-term studies and provide optical access for the observation of network growth.

The MEA-neurochips are fabricated with standard photolithographic techniques. Briefly, commercially-available, sputtered indium tin oxide (ITO) or gold plates are photoetched, insulated with polysiloxane, and de-insulated at the electrode tips. The electrodes are electroplated with a thin layer of gold to lower the interface impedance (at 1 kHz) to approximately 2-4 MOhm.

For extracellular recording, MEA-neurochips are placed into sterilized constant-bath recording chambers and maintained at 37 °C, pH 7.4 and a humidified airflow with 10% CO₂. Recording is performed with the Multichannel Acquisition Processor System, a computer-controlled 64-channel amplifier system from Plexon, Inc., Dallas, TX, USA, which delivers single neuron spike data. Spike identification and separation are accomplished with a template-matching algorithm in real time. The amplitude of routinely recorded signals ranges between 100-500 µV, with a high signal to noise ratio.

NeuroProof enhances MEA-neurochip technology with its own chip engineering and develops new chip designs for specific applications.