Iterative electrodes increase neural recruitment for deep brain stimulation

A Review on Implantable Neuroelectrodes

The efficacy of every neuromodulation modality depends upon the characteristics of the electrodes used to stimulate the chosen target. The geometrical, chemical, mechanical and physical configuration of electrodes used in neurostimulation affects several performance attributes like stimulation efficiency, selectivity, tissue response, etc. The efficiency of stimulation in relation to electrode impedance is influenced by the electrode material and/or its geometry. The nature of the electrode material determines the charge transfer across the electrode-tissue interface, which also relates to neuronal tissue damage. Electrode morphology or configuration pattern can facilitate the modulation of extracellular electric field (field shaping). This enables selective activation of neurons and minimizes side effects. Biocompatibility and biostability of the electrode materials or electrode coating have a role in glial formation and tissue damage. Mechanical and electrochemical stability (corrosion resistance) determines the long-term efficacy of any neuromodulation technique. Here, a review of electrodes typically used for implantable neuromodulation is discussed. Factors affecting the performance of electrodes like stimulation efficiency, selectivity and tissue responses to the electrode-tissue interface are discussed. Technological advancements to improve electrode characteristics are also included.

Novel fractal planar electrode design for efficient neural stimulation

Planar electrodes are used in epidural spinal cord stimulation and cortical stimulation. Stimulation efficiency of an electrode is characterized by its ability to activate a volume of neural tissue with lower voltage and power requirements. As current density tends to increase towards the sharp edges of an electrode, we hypothesize that electrode designs involving a greater amount of sharp edges will have higher variations of current density, and therefore increase stimulation efficiency, compared to electrodes with flat or rounded edges. This study uses three Koch snowflake fractal design iterations to compare each voltage and power requirements to those of the traditional square and circular electrode designs. Electrode geometries were constructed in COMSOL Multiphysics, each with an equal surface area. Voltages obtained from the finite element models were interpolated to determine the nodal voltages of 100 axons randomly positioned around the electrode. Threshold voltage and power for activating these model axons were simulated in NEURON. Current density variation was significantly higher on the surface of the 3rd iteration Koch snowflake electrode than the traditional square electrode. This novel electrode also activated a significantly greater number of axons with a lower threshold as compared to the traditional square electrode. The computational models used in this study demonstrated the feasibility of increasing stimulation efficiency through the design of novel planar electrodes with Koch snowflake fractal iterations. Future studies could explore and optimize efficiency when more fractal additions are taken, as well as exploiting other fractal shapes.