A three-dimensional bipolar microneedle electrode array with local ground integrated at each sidewall for enhanced focal electric stimulation.

The need for spatially-confined electrical stimulation is growing in biomedical applications, for example intracortical stimulation and retinal implant, for enhancement of stimulating resolution. Local grounding techniques have been widely explored to suppress undesired current spread. However, in conventional microneedle arrays like the Utah array, grounding is typically achieved by assigning neighboring electrodes as ground or employing grounding wall around stimulating electrode, which compromises spatial efficiency. In this work, we introduce, for the first time, a bipolar microneedle electrode array (BMEA) that integrates two electrically-independent electrodes within each three-dimensional microneedle structure. The microtip electrode, located at the apex of the microneedle, delivers electrical stimulation, while the local ground electrode, embedded on the sidewall below the microtip, serves to locally confine the spread of current. COMSOL Multiphysics simulations and ex vivo experiments using isolated mouse retina demonstrated that activating the local ground electrode effectively restricts current diffusion, enabling more focused and localized stimulation. This approach offers a compact and efficient solution for focal electrical stimulation with enhanced spatial resolution, providing a promising platform for advanced neural interfacing systems in various biomedical fields.