Online Charge Balancing With Active Impedance Monitoring in Programmable Stimulator to Extend Electrode Lifetime

Neurostimulation therapies are often applied as an alternative method to pharmaceutical treatment for pain relief. This paper demonstrates a programmable stimulator for analgesic nerve stimulation through invasive electrodes. It provides two modes of operation: constant-current biphasic and capacitor-coupled biphasic. The stimulator is integrated on a 50 × 30 mm Printed Circuit Board (PCB) and can generate up to ±1.6 mA current pulses in steps of $6.3\mu \text{A}$ . A custom STM32 microcontroller library is used to tune the pulse duration range from $1\mu \text{s}$ to 10 s and stimulation frequency range between 0.1 Hz to 100 kHz. An Active Impedance Monitoring (AIM) unit in the PCB measures the frequency response from 1 Hz to 10 kHz with 8 samples per decade every 30 minutes. The electrode resistance, tissue resistance and RC time constant of the electrode-tissue system could be calculated from the frequency response curve, generated by an input square wave stimulation current at different frequencies. In different measurements with discrete RC circuit components, the PCB could calculate the impedance values with >90% precision. A novel charge-balancing algorithm implemented on the STM32 microcontroller could match the positive and negative charge within a 5% error. Through online adjustment of stimulation parameters using active impedance monitoring to achieve charge balancing, the proposed programmable stimulation system extends the electrode lifetime for more than 288 million continuous pulses in vitro studies. The in vitro studies clearly demonstrated the efficacy of charge balanced stimulation that enable the electrodes to last around 3 times longer with negligible degradation.