Am 2200

Electrical stimulation, always in the form of a sinewave, was delivered with an AM 2200 analog current source (AM Systems, Sequim, WA) connected to either two stimulation screws in the skull for transcranial-only stimulation or to the two crocodile clips for transcutaneous-only stimulation. The current source was controlled by an analog voltage waveform generated using an output channel on a data acquisition card (NI USB-6216, National Instruments, Austin, TX) at a sample rate of 30 kHz. The acquisition card was controlled using a custom written MATLAB (Mathworks, Natwick, MA) software.

The voltage transient measurements using a charge-balanced biphasic current-controlled waveform were performed with an analog stimulus isolator (AM 2200, AM Systems, Sequim, WA, USA). A bespoke MATLAB program (R2016a, Mathworks, Natick, MA, USA) was used to generate stimulating waveform with specific pulse width, amplitude, and frequency. The pulses were injected into the electrode-electrolyte test cell, and a data acquisition board (NI USB-6353, National Instruments, Austin, TX, USA) was used to record the voltage transient responses. The charge-balanced biphasic pulse used in the experiments were cathodic-first current pulse with 100 μs duration followed by 100 μs inter-phase delay. The stimulating frequency was set at 50 Hz. The maximum negative potential excursion (E_mc) was esteimated to be the potential immediately after the end of the cathodic pulse (Fig. 5A). The time delay at which the current becomes zero was measured to be approxmiately 12 μs, and E_mc was recorded at 12 μs following the end of the cathodic current pulse. V_dr is the negative driving voltage which is maximum voltage to deliver the cathodic current pulse.

For the optical stimulation, a computer-controlled optical stimulation system developed in a previous study²⁷ was modified to modulate the micro-LED. The light pulse duration in all experiments was set to 5 ms. The sciatic nerve was stimulated at a twitch frequency of 1 Hz. Five twitches were obtained at 20 different illumination intensities. During the tetanic trials, the sciatic nerve was stimulated at frequencies of 10, 30, 50, and 100 Hz with an illumination intensity of 10 mW/mm².
For the electrical stimulation, the microelectrode (563410, A-M Systems, USA) was positioned on the sciatic nerve at a 1-mm distance above the optical nerve cuff electrode with an electrical stimulator (AM 2200, A-M Systems, USA) and computer control using LabVIEW (National Instruments, USA). A balanced biphasic pulse (pulse width of 200 μs and repeat frequency of 20 Hz) with amplitude modulation was induced on the sciatic nerve.