Neural Activation Threshold Reduction Using 3D micro-well Electrodes: Towards a High-Resolution Hybrid Retinal Prosthesis

Electrical stimulation of the surviving neurons in a degenerate retina is a promising approach for vision restoration. Our novel Hybrid Retinal Implant (HRI) approach, based on a high-resolution electrode array integrated with glutamatergic neurons, aims at achieving near-normal visual acuity by confining the electrical field in 3D well structures. Here we present the numerical simulation and electrophysiological investigation of this concept.

The induced electrical field confinement effect on the neural activation threshold was investigated by FEM modelling incorporated with biophysical cell properties (COMSOL Multiphysics). This was then validated using cultured cortical cells, which were electrically stimulated in four different modalities: Intracellularly, extracellular current injection using a pipette in close proximity of the cell and two customized MEAs with gold planar electrodes passivated with either 3D micro-well structures or a thin layer of SU8 polymer. The induced electrophysiological signals were then measured using either calcium imaging (OGB) or intracellular recording.

Both micro-well electrodes and extracellular pipette electrodes elicited robust fluorescent calcium signals. Our results reveal that the electrical field confinement induced by the 3D micro-well structures enabled the reduction of activation thresholds compared with extracellular pipette electrode. .Moreover, this field confinement resulted in a higher spatial resolution (as was observed from the lack of cross talk between proximal cells), in agreement with simulation results.

Herein we presented the reduction of neural activation thresholds, using sealed 3D micro-well structures and the increase of spatial resolution. These results lay a foundation for further studies for the realization of the HRI.