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An integrated implantable stimulator that is fail-safe without off-chip blocking-capacitors

To improve the functionality of implanted stimulation devices for nerve, retina or brain, one would like to increase the number of electrodes. This presents several difficulties. One is that for electrical safety, stimulators usually have capacitors in series with electrodes to ensure that there is no mean direct current. However, whereas the stimulator electronics is getting smaller and smaller with the advance of Integrated circuit technology, these blocking capacitors are still relatively large and put a limit on miniaturisation. We used an idea that I suggested - a very small integrated capacitor with high-speed (MHz) switching to make a circuit that is electrically-safe without a separate large capacitor. The paper shows that the circuit was safe but the penalties are power loss, due to the high frequency switching, and a greater area for the stimulator chip. The technique will not always be the best choice but it is now an option for stimulators designers’.