Towards pulse-density modulated functional electrical stimulation of neural cells with passive membranes

This work introduces a functional electrical stimulation (FES) mechanism for implantable neural stimulators (INS). The mechanism is a variation of the switched-capacitor based stimulation (SCS) approach. It is specifically tailored for the stimulation of neural cells incorporating passive membranes. Stimulus pulse widths required for stimulating such cells are also longer than those required to stimulate cells which fire action-potentials. This makes current-controlled stimulation (CCS) (1) a power consuming approach as it requires high voltage compliances for the output driving stages, and (2) dangerous to the stimulation electrodes and tissue as the long stimulus current pulse widths could force the electrode potential to exceed the so-called water window, beyond which any injected charge into the tissue goes into irreversible processes of water oxidation or reduction. The presented approach aims at utilizing the tissue's low-pass behavior by integrating pulse-density modulation (PDM) with SCS to to inject quantized amounts of electric charge into the targeted tissue at a relatively high frequency to the membrane properties. The delivered charge per stimulation phase can be controlled via the modulator sampling frequency and/or the intial and final capacitance voltages. The technique allows using smaller and fewer capacitors than those required by SCS. It also ensures the safety of the electrodes by controlling the electrode potential through the capacitor voltage.