High Contrast Grating Based Mechanical Sensor for Intraocular Applications

Abstract

Purpose : We demonstrate a novel mechanical sensor based on a flexible two-dimensional nanophotonic high contrast gratings (HCG) for the continuous monitoring of the intraocular pressure (IOP) and the strain in the retina through changes in color.

Methods : A prototype of the nanophotonic sensor is fabricated by using e-beam lithography to define the mask pattern, followed by reactive ionized etching of a 180 nm evaporated amorphous silicon layer to define a 10 µm square 2D grating structure consisting of posts with a 250 nm period and 80% duty cycle. Low index elastomer (polydimethylsiloxane, or PDMS) was spin coated over the gratings to facilitate a transfer release, following which another layer of PDMS is spin coated to embed and protect the sensors. The sensor was suspended in a PDMS chamber with an air well and observed under a microscope, in order to replicate the conditions within the eye in a fundus photography setup. Air was pumped to deform the membrane and sensor and the corresponding change in color was recorded.

Results : Fig.1 (a-b) shows the simulated reflectivity sweep, indicating a region of broadband reflectivity for the green wavelengths, for the chosen grating dimensions and an SEM image of the fabricated HCG. Fig 1 (c, i-vi) shows the change in reflected color with increasing pressure on the sensor, mimicking IOP measurements. The increasing displacement of the sensor membrane shifts the reflected color towards the greener wavelengths, as quantified by the decrease in the Euclidean distance and increase in the peak G value.

Conclusions : An implantable nanophotonics based IOP sensor, operating in the visible wavelength spectrum has been demonstrated, showing its ability to measure changes in pressure through changes in the reflected color. The concept can also be extended to measure the strain at the choroid and optic nerve head.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Fig. 1 (a) Simulated plot of reflectivity with respect to grating period and thickness, indicating a region of high reflectivity for 532nm wavelength for the chosen grating dimensions. (b) SEM image of the fabricated grating. (c) Optical images and segmented images of the sensor showing the change in the sensor color with applied pressure from initial state (i) to final state (vi) at top. The change in the Euclidean Distance and peak RGB values of each image is shown below.