Abstract
Abstract: :
Purpose: Traditional current sources for neural stimulation waste substantial energy in supplying electrode current. We have built a stimulation system which drives electrodes with a sequence of voltage steps, charging the electrode capacitance with the same total stimulus charge used in traditional current sources, but with much lower energy. During the discharge phase, the system recovers some of the energy stored in the electrode capacitance. The resulting average stimulation power is less than half that used by a current source with the same voltage supplies and only one third that used by typical stimulators today. Methods: This system was implemented in 1.5 µm CMOS, fabricated by MOSIS. The chip was tested alone, then with an RF power coupling system and microfabricated iridium oxide electrodes immersed in physiologic saline solution. To ensure proper stimulator function, the electrode voltage waveform was recorded, and the electrode currents were measured with a series resistor and instrumentation amplifier. To calculate total power consumption, the voltage and current of the secondary coil were likewise measured. The instantaneous coil power was calculated and added to the total. Power measurements were made in standby mode (no electrodes) and while driving 15 electrodes. Results: The system, using supply voltages of ±1.75V generated with Schottky diode rectifiers, delivered 0.68 µC per phase to each of 15 electrodes with a repetition frequency of 100 Hz, using net power of 1.88 mW, or 125 µW per electrode. A traditional current source using the same voltages generated in the same manner uses 271 µW per electrode to deliver the same charge. This represents a power savings of more than 53% over a traditional design using aggressive supply voltages. More typical stimulators use ±2.5V, with the same rectifier style, resulting in 373 µW per electrode, nearly three times our power level. Conclusions: The added circuit complexity of this method is more than made up in the power savings. Furthermore, there is room for improvement in the efficiency of this first–pass prototype. This power–saving idea can be valuable in other implanted stimulators as well, such as cochlear implants and cardiac pacemakers.
Keywords: retina • ganglion cells