Feasibility of hybrid ultrasound-electrical nerve stimulation for electroceuticals

Achieving targeted and efficient neural excitation is one of the major challenges in the design of implantable electroceutical devices. In this study, we explore the potential of using direct-contact ultrasound to localize and enhance the process of electrical current stimulation. The underlying premise of this study is to use ultrasonic pulses to vary the non-linear capacitive element formed by the neural membrane which would result in an ionic charge-pump that would reduce the activation threshold for a subsequent electrical stimulation. We have tested this hypothesis using a phantom experiment where a millimeter-scale ultrasonic crystal was affixed directly to the sciatic nerve of a frog and was driven both by a continuous train and by a 5 ms train of 3 MHz pulses with a monophasic electrical stimulus pulse applied at varying latencies. Compound action potential amplitudes were recorded from the gastrocnemius muscle during dual-mode stimulation, and were compared to baseline amplitudes. The experimental results showed that a downward shift in strength-duration was evident, and that a larger latency between ultrasound and electrical stimulation appeared to produce a larger amplitude with respect to the baseline, while near-simultaneous stimulation showed a suppression of action potential amplitude.