MicrostimulatorsOne further method of actuation is illustrated by the use of microelectrode devices to electrically stimulate activity of nerves and muscles. Common designs for these devices have already been discussed in the section on chemical sensors (see figure 9 on the chemical sensors page). The use of microelectrode devices facilitates highly specific stimulation of individual nerve fibres compared to other methods of stimulation; this would allow finer control of the stimulation provided enough electrode sites can be inserted into the tissue.As relatively large stimulating currents have to be passed through the electrode sites, microelectrodes for stimulation generally have geometrically larger electrode sites than those for recording (500um.sq up, c.f. 16um.sq up). This is necessary otherwise the currents involved will damage the electrode sites. One area in which silicon microengineering is being applied in the hope that it will result in a considerable improvement over more conventional electrodes is the area of visual prosthesis - providing rudimentary vision for the blind. One project currently in early stages of research involves a "forest" of silicon needles which will be inserted in the visual cortex. Early visual prosthetic devices involved an array of electrodes placed on the surface of the visual cortex (brain). When activated, blind volunteers could see points of light (phosphenes). These devices required relatively high currents to operate, however, and the image was distorted by afterdischarges and interactions between groups of neurons. This lead to the suggestion that a method for more selective stimulation of neurons within the visual cortex was required to provide any functional form of vision. So this is an area where microengineering technology has recently begun to be applied. |
