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J. C. Lin, P.-J. Chen, S. Saati, R. Varma, D. Rodger, M. Humayun, Y.-C. Tai; A Minimally Invasive Wireless Intraocular Pressure Sensor Implant. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2871.
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To design, microfabricate, and verify biocompatibility of an implantable flexible-coiled wireless intraocular pressure (IOP) sensor with the capability of maintaining a high resonant quality factor when immersed in saline solution.
A pressure-sensitive electrical LC-tank resonant circuit, fabricated on a foldable parylene C substrate chosen for its biocompatibility, was utilized in the IOP sensor and characterized using an impedance phase-dip wireless measurement technique. A hand-made coil connected to an HP 4195A network analyzer was used as an external reading instrument. A complete suite of benchtop, ex vivo, and in vivo testing were all performed to characterize the device, with a medical manometer used as a reference pressure. Parylene C coatings of varying thickness were also deposited onto the devices to study the relationship between the parylene C thickness and the corresponding resonant quality factor, which degrades significantly when it is immersed in saline.
The size of the final IOP sensor can be as small as 4 mm × 1.5 mm × 1 mm, suitable for minimally invasive implantation, with incisions measured to be less than 2 mm. The sensitivity was measured as 455 ppm/mmHg, and the resolution of phase difference is 0.001°, leading to a maximum sensing distance of 2 cm with a 2.2-cm-diameter external reading coil. In the in vivo test in rabbit eye, no dislocation or post-operative complications were found over a four month observation period. The quality factor reaches its limit with 40 µm of parylene on the metal coil and is successfully restored to 20, which is comparable to the normal value in the air.
An implantable flexible-coiled wireless IOP sensor has been developed, with electrical properties and biocompatibility being verified. With the flexible parylene coil, the IOP sensor achieves a form factor suitable for miminally invasive implantation. By coating the device with a reasonable thickness of parylene, the quality factor of the sensors in saline has been restored to match those in air, demonstrating a reliable method to improve in vivo pressure sensitivity.
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