A Negative Impedance Converter Design to Enhance Capacitive Conduction Through a Neurostimulator Electrode Interface

With the growing interest in electric field-induced neuromodulation for clinical applications, optimizing circuitry and stimulation parameters is crucial for effective therapy. Studies have shown successful neuroregeneration, neuroprotection, and neuronal activation in vitro when electric fields exceed a certain threshold. However, clinical translation remains challenging, as stimulation amplitudes are often constrained by patient tolerance. Strategies that enhance charge delivery per phase within safety limits can improve the efficacy of these techniques.

This paper presents a method to reduce the electrode-tissue interface time constant by incorporating a negative resistance circuit to lower the series resistance and an RC circuit to reduce the equivalent capacitance in a Thevenin model of the interface. By targeting the capacitive conduction phase with higher amplitudes, a voltage-controlled stimulator can deliver greater charge while remaining within tolerance limits. The proposed circuit models are validated in vivo by assessing the stimulation tolerance of rats at the optic nerve.

The proposed circuits connected in series with a two-electrode stimulator effectively reduced the time constant of the current waveform, shifting the magnitude response toward higher frequencies in Bode analysis. In vivo experiments confirmed that animals tolerated, on average, 20% greater charge injection within the first 200 $\mu$s of a rectangular pulse-the interval where capacitive charge transfer dominates over faradaic processes.

Enhancing voltage-controlled neurostimulators with external circuits is a promising approach to overcoming amplitude limitations in clinical neurostimulation. As electrotherapy requires a minimum electric field amplitude for efficacy, increasing patient tolerance can improve treatment success rates.