Abstract
This paper investigates the efficacy of high frequency switched-mode neural stimulation. Instead of using a constant stimulation amplitude, the stimulus is switched on and off repeatedly with a high frequency (up to 100 kHz) duty cycled signal. By means of tissue modeling that includes the dynamic properties of both the tissue material as well as the axon membrane, it is first shown that switched-mode stimulation depolarizes the cell membrane in a similar way as classical constant amplitude stimulation. These findings are subsequently verified using in vitro experiments in which the response of a Purkinje cell is measured due to a stimulation signal in the molecular layer of the cerebellum of a mouse. For this purpose a stimulator circuit is developed that is able to produce a monophasic high frequency switched-mode stimulation signal. The results confirm the modeling by showing that switched-mode stimulation is able to induce similar responses in the Purkinje cell as classical stimulation using a constant current source. This conclusion opens up possibilities for novel stimulation designs that can improve the performance of the stimulator circuitry. Care has to be taken to avoid losses in the system due to the higher operating frequency.
Highlights
Traditional functional electrical stimulation typically uses a current source with constant amplitude Istim and pulsewidth tpulse to recruit neurons in the target area
A fixed value for Vstim or Istim is used, while the stimulation intensity is controlled by driving the switch with a Pulse Width Modulated (PWM) signal; this is referred to as switched-mode operation
Slices were kept for at least 1 h in Artificial CerebroSpinal Fluid (ACSF) containing the following: 124 NaCl, 5 KCl, 1.25 Na2HPO4, 2MgSO4, 2CaCl2, 26 NaHCO and 20 d-glucose, bubbled with 95% O2, and 5% CO2 at 34◦C. 0.1 mM picrotoxin was added to the ACSF to block the inhibitory synaptic transmission from molecular layer inter-neurons
Summary
Traditional functional electrical stimulation typically uses a current source with constant amplitude Istim and pulsewidth tpulse to recruit neurons in the target area. Stimulator designs consisted of relatively simple programmable current source implementations. Over the years numerous modifications have been proposed to improve important aspects such as power efficiency (Sooksood et al, 2012), safety (Sooksood et al, 2010) and size. Still use constant current at the output. Several studies have investigated the use of alternative stimulation waveforms in an attempt to improve the performance. Some implementations focus on improving the efficiency of the activation mechanism in the neural tissue. In Sahin and Tie (2007) and Wongsarnpigoon and Grill (2010) it was found that Gaussian shaped waveforms increase the neural recruitment
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