Abstract
Planar electrodes are used in epidural spinal cord stimulation and epidural cortical stimulation. Electrode geometry is one approach to increase the efficiency of neural stimulation and reduce the power required to produce the level of activation required for clinical efficacy. Our hypothesis was that electrode geometries that increased the variation of current density on the electrode surface would increase stimulation efficiency. High-perimeter planar disk electrodes were designed with sinuous (serpentine) variation in the perimeter. Prototypes were fabricated that had equal surface areas but perimeters equal to two, three or four times the perimeter of a circular disk electrode. The interface impedance of high-perimeter prototype electrodes measured in vitro did not differ significantly from that of the circular electrode over a wide range of frequencies. Finite element models indicated that the variation of current density was significantly higher on the surface of the high-perimeter electrodes. We quantified activation of 100 model axons randomly positioned around the electrodes. Input–output curves of the percentage of axons activated as a function of stimulation intensity indicated that the stimulation efficiency was dependent on the distance of the axons from the electrode. The high-perimeter planar electrodes were more efficient at activating axons a certain distance away from the electrode surface. These results demonstrate the feasibility of increasing stimulation efficiency through the design of novel electrode geometries.
Highlights
Most implanted pulse generators (IPGs) used for neural stimulation are powered with primary cell batteries and require surgical replacement when the battery is depleted
The second spatial difference of the extracellular voltages (∝ƒx) was calculated in the region around the electrode using the internodal length of the myelinated model axons as the space step, Δx (=1.2 mm, as in Table 2) to assist in interpretation of the results. We quantified both electrode impedance and stimulation threshold to determine the efficiency of high-perimeter planar electrodes
LOAD IMPEDANCE The load impedance is composed of the series combination of the interface impedance of the stimulating electrode and the tissue resistance between the stimulating electrode and the return electrode
Summary
Most implanted pulse generators (IPGs) used for neural stimulation are powered with primary cell batteries and require surgical replacement when the battery is depleted. High surface area porous electrodes reduce interface impedance and pacing thresholds (Mond and Grenz, 2004), but diffusion limitations prevent accessing the full surface area during short-duration stimulation pulses (Elliott and Owen, 2000; Weiland and Anderson, 2000). The potential advantages of this approach are that novel electrode designs can be implemented with existing manufacturing techniques and will not require the exhaustive biocompatibility testing required for new materials. This approach could be applied in concert with new materials to maximize the efficiency of stimulation
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