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Jeong Hoan Park, Joonsoo Jeong, Sung June Kim; Feasibility of Liquid Crystal Polymer as an Encapsulation Material of Photodiode-based High-Resolution Retinal Prostheses. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1829.
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While photodiode-based retinal prosthetic devices can achieve high number of stimulating channels with correspondence to eye movement, encapsulation of photo-sensitive area against harsh body environment remains an unsolved challenge for long-term implantation. Liquid crystal polymer (LCP) has been considered as a candidate material for long-term reliable biomedical implants in particular owing to its extremely low moisture absorption rate (<0.04 %), yet its application for optical device encapsulation has not been investigated. Thus, we conducted a preliminary experiment using a CMOS image sensor and identified the feasibility of LCP as optical window packaging for photodiode-array-based artificial retina devices.
We prepared two commercial CMOS image sensors (640×428 pixels, pixel size: 3.5×3.5 μm^2): one covered with 25 μm-thick LCP film (Vecstar FA-25N; Kuraray Co., Ltd., Tokyo, Japan), while the other remained uncovered. A black-and-white striped pattern with 50/50 duty cycle printed on a paper of 4.5×4.5 cm^2 was presented at varying spatial frequencies, logarithmically from 2 to 16 cycles at 17 cm distance from the image sensor (equivalent to 0.1 to 0.8 cycles per degree of visual angle), and corresponding captured images were compared. The minimum distinguishable pattern size was examined through monochromic image analysis of acquired images.
Significant image blurring and darkening have been observed, primarily caused by low transmittance and scattering effects of the LCP film of 25 μm thickness. Still patterns could be resolved up to four cycles per paper. This corresponds to a distinguishable line width of 160 μm on the CMOS sensor.
Although there remain further challenges to use LCP as an encapsulation material of optical devices, we expect that at least 100 stimulating channels are feasible for integration into a macular area of 10 mm^2 with a distinguishable line width of 160 μm. Further enhancement of the resolution may be achievable by thinning of the LCP substrate and/or by applying an effective noise filter.
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