Microdevices for Studies of Cultured Neural Networks

A cultured network has the advantages that the network is two-dimensional and easily observed, that the biochemical environment can be controlled, and that conventional electrodes as well as extracellular electrodes incorporated into the cultured substrate can be used to selectively stimulate and record from individual neurons in the network. It is possible to study small numbers of connected neurons, from a few to hundreds. This talk will describe two techniques, the multielectrode array and the silicon neurochip, and their application to long-term communication with a network by means of simultaneous recording or stimulation of many neurons. INTRODUCTION In 1978 I stopped doing research in High Energy Physics and spent a year intensively studying neurobiology. As a physicist, the idea of studying intact nervous systems, with hundreds of millions or even many billions of interconnected cells was very intimidating, so I was intrigued when I learned about cultured neural networks in a dish. In culture, networks can be grown which have from a few to thousands of neurons, in a two-dimensional geometry, easily visible with the phase contrast microscope, and easily accessible to physiological measurements and biochemical manipulations. I decided to learn how to study cultures electrophysiologically, and I had an idea. Instead of just one or two recording electrodes, there could be an array of many, embedded in the bottom of the culture dish, sensing nearby neurons by extracellular recording, and also able to selectively stimulate the culture. Microfabrication using photolithography had become a common electrical engineering technique, so I set out to fabricate arrays of small electrodes on glass substrates that would become the bottom of culture dishes. And I learned to grow neurons in culture, with the help of expert neurobiologists. Figure 1 shows part of what I then called an "electric petri dish" which now would be called a multielectrode array or MEA. The phase contrast photo shows part of a mass culture of sympathetic neurons, a few thousand in the dish, growing on my array of 16 electrodes. The neurons had been dissociated from the superior cervical ganglion of a newborn rat about eight days before the picture was made. They adhered to CP501, Stochastic Dynamics and Pattern Formation in Biological and Complex Systems, edited by S. Kirn, K. J. Lee, and W. Sung © 2000 American Institute of Physics 1-56396-914-9/007$ 17.00 203 Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp FIGURE 1. Cultured rat superior cervical ganglion neurons growing on a multielectrode array. a polylysine treated surface and regrew their axons and dendrites to form a dense synaptically connected network, visible in the photograph. The electrodes in the figure are black dots near the ends of gold leads which go up and down in the picture to terminals at the edge of the glass. The leads are covered with an insulating layer of silicon dioxide, and a horizontal groove which is visible in the figure near the ends of the leads was etched through to expose small 10 micron square gold areas. These were plated with platinum black to provide a low impedance electrical connection to the culture medium. Figure 2a shows a scanning electron micrograph of a platinized electrode, and the spongy deposit effectively increases the electrode surface area by one to two orders of magnitude. Figure 2b illustrates a recording made from the electrode near a cell labeled B with an arrow in Figure 1. The cell was penetrated with a sharp electrode, and short positive stimulus currents, of successively larger amplitude, were passed into