Optically configurable confinement of electric field with photovoltaic retinal prosthesis

Abstract

Purpose : Prosthetic visual acuity is limited by the pixel size and by crosstalk from the neighboring electrodes. For acuity better than 20/200, pixels should be under 50µm, and local returns are required for the crosstalk suppression. However, small bipolar pixels over-constrain the field penetration and thus limit the efficacy of retinal stimulation. Sequential activation of the photodiode pixels transforms active electrodes into transient returns. We explore this approach for dynamic confinement of electric field in the retina by spatiotemporal control of the images projected onto the photovoltaic array.

Methods : The electric field in the retina generated by a photovoltaic subretinal implant with 425 hexagonal monopolar pixels of 40µm in pitch was modeled using the finite element method and linear combination of the elementary electric fields emanating from each electrode individually. We quantified the spatial coupling among the pixels and calculated the dynamics of the photodiode circuit in the multidimensional form. Electric fields predicted by the model were compared to the potential mapped by micropipette ex-vivo, as well as that recorded from corneas in implanted rats.

Results : Spatiotemporal modeling shows that the electric field generated by active electrode elevates the local potential on neighboring dark pixels and thereby transiently increases their discharge current by up to 10 fold, effectively transforming them into return electrodes. The distance between the active electrode and the transient return defines the penetration depth of the electric field into tissue. Ex-vivo measurements of the electric potential match the model predictions. Amplitude of the corneal signal in rats increases with the width of the grating projected onto the implants, from 40 to 180µm, confirming the configurability of the stimulation depth in-vivo.

Conclusions : Current conducted by photodiodes in an array is affected by other electrodes due to spatial coupling of the electric potential in electrolyte. Therefore, spatiotemporal modulation of light on the array can transform the active electrodes into transient returns. Such optical approach to current steering enables a flexible control of the lateral and axial confinement of electric field, which allows optimization of the stimulation depth and selectivity in every patient, depending on the retinal thickness and its proximity to the electrodes.

This is a 2021 ARVO Annual Meeting abstract.