Precision control of pulse widths for charge balancing in functional electrical stimulation

Maintaining a balance charge is one of the key factors to ensure a safe neural stimulation employing biphasic current stimulus. However, a zero net charge is difficult to achieve due to the fabrication variation of stimulation drivers. Moreover, even with a perfectly matched cathodic and anodic current stimulus, a non-zero residual charge is still built up due to the inter-pulse delay adopted in the stimulation pattern. During this period, the charge injected by the first stimulus pulse is leaked to the surrounding tissue and thus, the remaining charge cannot be completely removed by the following compensating current stimulus even with matched intensity. In this paper, we present a charge-balancing scheme by precisely controlling the pulse width of current stimulus. Charge balance is achieved by using the residual voltage as a feedback signal to control the pulse width of next stimulus. Simulated in Matlab/Simulink, the proposed scheme is shown to mitigate up to 15% intensity mismatch of biphasic current pulses with 1ms inter-pulse delay. For a stimulus with constantly varying intensity, such as retinal and cochlear implants, the proposed scheme is also capable of maintaining a balance charge for a safe neural stimulation.