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Jeffrey T. Hastings, Sunil Deokule, E. Britt Brockman; An Implantable, All-Optical Sensor for Intraocular Pressure Monitoring. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5039.
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A number of researchers have developed implantable intraocular pressure (IOP) sensors to provide more frequent and longer-term IOP monitoring for patients with glaucoma. However, adoption of many of these sensors has been limited by the size and complexity of their implanted circuitry and power sources. Here we present a simplified approach to IOP monitoring that relies on a near infrared (NIR) image of an implanted micromechanical sensor. The sensor design requires only a few biocompatible materials and enables a straightforward, and reversible, implantation procedure.
The sensor chip contains one or more vacuum reference cavities formed by a flexible membrane, a rigid substrate, and a thin spacer. Both substrate and membrane partially reflect light to form an interference pattern of concentric rings. These rings shift radially as the membrane deflects in response to pressure changes. IOP is measured by analyzing a narrow-band NIR image of the pattern. Sensor chips were fabricated using standard processing techniques including chemical vapor deposition, reactive ion etching, and wafer bonding. The chips were packaged in PMMA anchors consisting of a plate, which is secured in a scleral pocket, and a shaft, which positions the sensor chip in the anterior chamber. We characterized sensor response in-vitro over a pressure range of 660 to 790 mmHg by comparing our optical sensor with an electronic reference sensor. Finally, we implanted sensors in two adult New Zealand rabbits and acquired measurements over two weeks.
RMS measurement errors were found to be <0.5 mmHg in-vitro. The sensors survived implantation, two weeks in-vivo, and retrieval. In one rabbit, the implanted sensor was obscured by a hemorrhage, but in the other rabbit the interference pattern remained visible for the duration of the study. Images of the interference pattern could be obtained from awake animals using a custom-built, hand-held NIR camera.
Pressure measurements can be obtained from interference patterns produced by a passive, optical sensor. Measurement accuracy is suitable for IOP monitoring in glaucoma patients. In-vivo studies demonstrated the effectiveness of the implantation procedure, the durability of the sensor, and the feasibility of acquiring images of the interference pattern through the cornea of an awake rabbit.
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