Safety of Multi-channel Stimulation Implants: Are Blocking Capacitors Sufficient After Single-Fault Failure?

One reason given for placing capacitors in series with stimulation electrodes is that they prevent direct current flow and therefore tissue damage under fault conditions. We show that this is not true for multiplexed multi-channel stimulators with one capacitor per channel. A test bench of two stimulation channels, two stimulation tripoles and a saline bath was used to measure the direct current flowing through the electrodes under fault conditions. The electrodes were passively discharged between stimulation pulses. For the particular condition used (16mA, 1ms stimulation pulse at 20Hz with electrodes placed 5cm apart), the current ranged from 38μA to 326μA depending on the type of fault. Variation of the fault current with time, stimulation amplitude, stimulation frequency and distance between the electrodes is given. Possible additional methods to improve safety are discussed.