A charge-metering method for voltage-mode neural stimulation

Abstract

Electrical neural stimulation is the technique used to modulate neural activity by inducing an instantaneous charge imbalance. This is typically achieved by injecting a constant current and controlling the stimulation time. However, constant voltage stimulation is found to be more energy-efficient although it is challenging to control the amount of charge delivered. This paper presents a novel, fully integrated circuit for facilitating charge-metering in constant voltage stimulation. It utilises two complementary stimulation paths. Each path includes a small capacitor, a comparator and a counter. They form a mixed-signal integrator that integrates the stimulation current onto the capacitor while monitoring its voltage against a threshold using the comparator. The pulses from the comparator are used to increment the counter and reset the capacitor. Therefore, by knowing the value of the capacitor, threshold voltage and output of the counter, the quantity of charge delivered can be calculated. The system has been fabricated in 0.18 μm CMOS technology, occupying a total active area of 339 μm × 110 μm. Experimental results were taken using: (1) a resistor-capacitor EEI model and (2) platinum electrodes with ringer solution. The viability of this method in recruiting action potentials has been demonstrated using a cuff electrode with Xenopus sciatic nerve. For a 10 nC target charge delivery, the results of (2) show a charge delivery error of 3.4% and a typical residual charge of 77.19pC without passive charge recycling. The total power consumption is 45 μW. The performance is comparable with other publications. Therefore, the proposed stimulation method can be used as a new approach for neural stimulation.

A 2‐mA charge‐balanced neurostimulator in 0.18‐μm/1.8 V standard CMOS process

“…The charge can be injected either by voltage‐mode stimulation (VMS) or by current‐mode stimulation (CMS). In VMS, charge injection is performed by a constant voltage, which is applied between the stimulating electrode and return electrode 32 . The VMS has the advantage of higher energy efficiency 33,34 but has the disadvantage of a lack of control over the charge delivered to the tissue due to variability of the neurological impedance 35 .…”

“…In VMS, charge injection is performed by a constant voltage, which is applied between the stimulating electrode and return electrode. 32 The VMS has the advantage of higher energy efficiency 33,34 but has the disadvantage of a lack of control over the charge delivered to the tissue due to variability of the neurological impedance. 35 The impedance of neural tissue can significantly change over time and particularly in the period immediately after implantation.…”