Abstract

Planar electrodes are increasingly used in a variety of neural stimulation techniques such as epidural spinal cord stimulation, epidural cortical stimulation, transcranial direct current stimulation and functional electric stimulation. Recently, optimized electrode geometries have been shown to increase the efficiency of neural stimulation by maximizing the variation of current density on the electrode surface. In the present work, a new family of modified fractal electrode geometries is developed to increase the neural activation function and enhance the efficiency of neural stimulation. It is hypothesized that the key factor in increasing the activation function in the tissue adjacent to the electrode is to increase the "edginess" of the electrode surface, a concept that is explained and quantified by fractal mathematics. Rigorous finite element simulations were performed to compute the distribution of electric potential produced by proposed geometries, demonstrating that the neural activation function was significantly enhanced in the tissue. The activation of 800 model axons positioned around the electrodes was also quantified, showing that modified fractal geometries yielded a 22% reduction in input power consumption while maintaining the same level of neural activation. The results demonstrate the feasibility of increasing stimulation efficiency using modified fractal geometries beyond the levels already reported in the literature.

Highlights

  • Electrical stimulation of the nervous system is a technique used to restore function to individuals with neurological impairment

  • More comprehensive studies should be carried out in the future to better understand and verify the performance of fractal electrodes. This contribution presents proof-of-concept and preliminary results of using fractal electrodes to increase the efficiency of neural stimulation

  • The present work demonstrates another perspective for improved electrode geometries based on fractals

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Summary

Introduction

Electrical stimulation of the nervous system is a technique used to restore function to individuals with neurological impairment. Deep brain stimulation (DBS), which involves high frequency electrical stimulation of the thalamic or basal ganglia structures to treat movement disorders, has rapidly emerged as an alternative to surgically created lesions (McIntyre et al, 2004). Another new, rapidly growing application is transcranial direct current stimulation (tDCS), a non-invasive, painless, safe and portable technique that modulates cortical excitability (Nitsche and Paulus, 2000). Stimulation devices have very promising applications in control of bladder function, restoring continence and micturition in some neurological diseases (Grill, 2009). In fractal electrodynamics, fractal geometry has been combined with electromagnetic theory to propose improved designs to control the radiation pattern, wave propagation, and scattering characteristics of radio-frequency devices (Jaggard, 1991, 1995) and a number of patents have been filed using fractal shapes to improve design of antennas or frequency selective surfaces (Cohen, 2000a,b; Uei-Ming, 2008)

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