Active Pulse-Clamp Stimulation for Rapid Recovery, Charge-Balanced Neural Stimulation

Charge balancing is essential for safe neural stimulation that does not damage tissue. Generally, the two most common stimulation methods are passive recharge - a monophasic pulse followed by a shorting phase that clears accumulated charge - and biphasic stimulation. Passive recharge is safe and power-efficient; however, it has a long recovery time after every pulse that is dictated by the time constant of the neural electrode. Biphasic stimulation rapidly recovers the electrode-tissue interface; however, its main drawback is that it is reliant on precisely matched current sources and electrode linearity to ensure chronic safety. We present a novel stimulation method and system, Active Pulse-Clamp Stimulation (APCS), that achieves guaranteed charge-balance with rapid recovery. Rather than relying on precisely matched current sources or slow, complex compensation techniques, APCS uses discrete-time feedback to ensure that the electrode interface settles rapidly after every pulse. During the recovery period, a clock toggles the state between monitoring and discharging the voltage of the electrode's double-layer capacitance. Unlike passive recharge, the recovery time is fully-customizable. And unlike biphasic stimulation, the interface will always recover to the specified voltage for guaranteed safety. To demonstrate an APCS proof-of-concept, we implemented the stimulator in a 180nm HV CMOS process. We demonstrated both rapid, customizable recovery time and charge balancing using a benchtop electrode model and a clinical electrode in saline.