Experimental evaluation of stimulus current generator with Laplacian edge-enhancement for 3-D stacked retinal prosthesis chip

To restore visual sensation of blind patients suffering from age-related macular degeneration (AMD) and retinitis pigmentosa (RP), we have been developing fully-implantable retinal prosthesis consisting of three dimensional (3-D) stacked retinal prosthesis chip with high density through silicon vias (TSVs), flexible cable, and a stimulus electrode array. Using 3-D integration technology, a photoreceptor chip with more than 1000 pixels can be fabricated in the top layer, and stimulus current generator with various image processing functions can also be fabricated in the bottom layer. In this paper, we presented experimental evaluation results of the stimulus current generator with Laplacian edge-enhancement function. The proposed edge enhancement (EE) function used a four-neighbor Laplacian filter circuit as analog signal processing and was implemented in a 0.18-μm 1P6M CMOS technology. A pixel size including the four-neighbor Laplacian filter circuit was 75 × 75 μm², and 37 × 37 pixels occupied a very small active circuit area of 3.2 × 3.2 mm². Experimental results showed the fabricated stimulus current generator completely captured an input image and successfully performed the EE processing for the input image data. Furthermore, total output current from photo-diodes, which became input currents of biphasic pulse generator, was reduced by 87% with the EE circuit. Safer electrical stimulations to the retina cells can be realized with the stimulus current generator with Laplacian EE in the 3-D stacked retinal prosthesis chip.