Simulation, Design, and Application of Micro-Lens Enhanced Multi-Emission Optical Nerve Cuff for Peripheral Nerve Stimulation

For two centuries, electrical stimulation has been the conventional method for interfacing with the nervous system. As interfaces with the peripheral nervous system become more refined and higher-resolution, several challenges appear, including immune responses to invasive electrode application, large-to-small axon recruitment order, and electrode size-dependent spatial selectivity. Optogenetics offers a solution that is less invasive, more tissue-selective, and has small-to-large axon recruitment order. By adding genes to express photosensitive proteins optogenetics provides neuroscientists the ability to genetically select cell populations to stimulate with simple illumination. However, optogenetic stimulation of peripheral nerves uses diffuse light to activate the photosensitive neural cell lines. To increase the specificity of stimulus response, research was conducted to test the hypothesis that multiple, focused light emissions placed around the circumference of optogenetic mouse sciatic nerve could be driven to produce differential responses in hindlimb motor movement depending on the pattern of light presented. A Monte Carlo computer simulation was created to model the number of emitters, the light emission size, and the focal power of accompanying micro-lenses to provide targeted stimulation to select regions within the sciatic nerve. The computer simulation results were used to parameterize the design of micro-lenses. By modeling multiple focused beams, only fascicles within a nerve diameter less than 1 mm are expected to be fully accessible to focused optical stimulation; a minimum of 4 light sources is required to generate a photon intensity at a point in a nerve over the initial contact along its surface. To elicit the same effect in larger nerves, focusing lenses would require a numerical aperture > 1. Microlenses which met the simulation requirements were fabricated and deployed on a flexible nerve cuff which was used to stimulate the sciatic nerve in optogenetic mice. Motor neuron responses from this stimulation were compared to global illumination; stimulation using the optical cuff resulted in fine motor movement of the extensor muscles of the digits in the hindlimb. Increasing optical power resulted in a shift to gross motor movement of hindlimb. Finally, varying illumination intensity across the cuff showed changes in the extension of individual digits.