Finite Element Analysis Of Electric Field Of Helix Electrode For Stimualtion Of Optic Nerve

ObjectiveVisual prosthesis is an electronic device intended to restore functional vision in those suffering from partial or total blindness. Electrical stimulation applied to the optic nerve can produce visual sensations. Stimulating electrode geometry has very important influence on neural injury and stimulus selectivity. Finite element method is a numerical method for solving electric field distribution of complex volume. A new self-adaptive helix electrode has been designed by finite element software COMSOL Multiphysics. The aim of the design is to reduce nerve injury and to implement selective activation of optic nerve fascicles, so as to provide calculation support for optic nerve stimulation.Methods1. The geometrical model of optic nerve and spiral cuff electrode were developed with finite element software COMSOL Multiphysics. Different electrode stimulation mode (monopolar, bipolar and tripolar) were applied to stimulate optic nerve. By building an appropriate mesh on this model and choosing some boundary conditions, the finite element method was applied to analyze the effect of different electrode stimulation modes on stimulation by computing electric potential, electric field and current density generated by the electrode.2. The optic nerve volume conductor model was developed, including five fiber fascicles with the same radius. Under the tripolar stimulation condition, the finite element software COMSOL Multiphysics was applied to compute the potentials and current densities at positions of different fiber fascicles. The activating function (AF) was introduced to analyze the effect of the distance between electrode and nerve on the stimulation.3. The effect of tissue electrical characteristics around optic nerve on stimulation was studied. The effect of thickness of cerebrospinal fluid (CSF) on stimulation was simulated by means of parametric sweep method.4. A new self-adaptive helix electrode for selective activation of optic nerve has been presented. The new helix electrode consists of silicone helix frame and platinum contacts embedded within the frame. The silicone frame acts as support and insulation. The AF is introduced to characterize the stimulation capability and the selectivity of activating optic nerve fascicle with new helix electrode is simulated by COMSOL Multiphysics, taking into account the variations of electrode contact location.Results1. Under the monopolar structure, the distribution of the current density along the surface of the electrode is inhomogeneous with a local maximum at the contact-nerve sheath and quick decrease away from the electrode. Under biopolar mode, longitudinal current flow between two contacts located on opposite sides of a nerve trunk can be used to excite nerve. It was shown by simulation that the longitudinal tripolar electrode structure could generate larger maximum current density and electric field intensity than that of biopolar electrode structure (located in the opposite of nerve trunk) under the same conditions of stimulation.2. The influence of the distance between electrode and nerve on neural axial electric potential is not obvious with tripolar spiral electrode. The closer to the electrode, the larger the current density value of nerve fascicle is and the more obvious the AF amplitude variation is.3. With the increase in thickness of CSF, the variation of electric potential is not obvious. By means of parametric sweep, the results suggest that, with the increase of the CSF thickness and higher conductivity of CSF than those of other tissues, the distribution of electric field generated by electrodes is extended but scattered, and the diffusion current distribution makes nerve stimulation less effective.4. Assuming normalized AF threshold is0.1V/m2, the difference of ratio of AF over threshold to the whole AF of new helix electrode is larger than that of traditional cuff electrode. With contacts at two ends of helix electrode closer to the middle contact, the large nerve fascicle is first activated and followed by the small one.Conclusions1. Current density is centralized distribution in the electrode-tissue contact. Biopolar electrode structure requires less leads, and is more easily implanted. Tripolar structure has more contacts and easily realizes selective stimulation. According to specific requirements, the most appropriate stimulation modes can be chosen to get the best stimulation results. The study can provide some help to the research on optic nerve prosthesis.2. The distance between nerve and electrode has influence on stimulation by analyzing the stimulation on fiber fascicles at different sites. The activating function along fascicle at the nerve center is relatively small and is difficult to excite.3. The analysis of thickness change of CSF can provide certain theoretical guidance for implanting spiral cuff electrode onto optic nerve in clinical practice.4. New helix electrode decreases the contact area between electrode and nerve. The selectivity of new helix electrode is better than that of traditional cuff electrode. The new helix electrode can selectively activate optic nerve fascicle with variations of electrode contact locations.