Design of a Implantable Selective Nerve Stimulator

At University College London implantable stimulators suitable for the selective stimulation of nerve are being developed. We hope these devices will be capable of selectively stimulating either by fibre position, fibre size or to launch action potentials in one direction only, using nerve cuff electrodes. These methods of selective stimulation have been demonstrated by many workers; early papers in each category being McNeal and Bowman[4]; Brindley and Craggs[3]; and van den Honert and Mortimer[5]. However, although the first of these papers appeared in 1979, practical stimulators for chronic use of these methods have not been described. Selection by position (i.e. to distinguish separate fascicles) will use a ‘pentapolar’ electrode cuff with four ‘dot’ cathodes spaced round the central circumference between two ring anodes. To exploit anodal blocking, either for unidirectional action potentials or small-before-large fibre recruitment [2], the cuffs may be tripolar, or pseudo-tripolar (with joined anodes). The stimulator produces quasitrapezoidal or rectangular current pulses of amplitude 20μA to 5mA with duration of 20μs to 1ms. For safety, both active and passive charge balancing is used. A custom analogue integrated circuit has been designed which will be the chief component of future implanted devices. The simulation performance of the IC will be shown in this paper. I. The Stimulator The stimulator IC produces either quasi-trapezoidal or rectangular current pules of amplitude 20μA to 5mA with a duration of 20μs to 1ms. The decay constant is specified using a small external capacitor. The size of the active charge balancing phase can be varied between 1/70 and 1/10 of the pulse amplitude. The stimulator can control either one pentapole (as shown in figure 2a,b), two tripoles or three pseudo-tripoles. A flow diagram of the analogue core of the stimulator is shown in figure 1. The charge injected on each electrode that has an actively-controlled current source is both actively and passively charge balanced. This integrated circuit requires a regulated (±5V) supply. The digital control section has not been implemented with the analogue core in the test devices. The digital control section will be added to the analogue core which will control 3 pentapoles, 6 tripoles or 9 dipoles. The mode of use will be defined during manufacture. The analogue core has been manufactured using a full custom 2.4μ CMOS process and is expected to be evaluated during the summer of 1997.