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J. Aebersold, D. Jackson, M. Crain, J. Naber, T. Roussel, R. Keynton, K. Walsh, P. Van der Pol, G. Tezel, H. Kaplan; Development of an Implantable, RFID-Based Intraocular Pressure Sensing System for Glaucoma Patients. Invest. Ophthalmol. Vis. Sci. 2008;49(13):688.
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Elevated intraocular pressure (IOP) is an important risk factor for the initiation and progression of glaucomatous neurodegeneration. Chronic monitoring of IOP is important for the diagnosis and treatment of patients with glaucoma. Assenti, LLC has developed a prototype implant capable of wirelessly detecting IOP that can be integrated into a psuedophakic intraocular lens (IOL) or phakic IOL.
The IOP measurement system consists of a custom Radio Frequency IDentification (RFID) telemetry Integrated Circuit (IC), a planar coil and a Micro-Electro-Mechanical Systems (MEMS) capacitive pressure transducer encased in silicone (Sylgard 184 by Dow Corning). Testing was performed by implanting the prototype in the anterior chamber of a 4 kg New Zealand white rabbit and changing IOP via saline filled infusion lines in the vitreous cavity. An external coil sourcing a 13.56 MHz magnetic field was placed 1 cm from the prototype, which provided an inductive link of 130µW for the implant circuit. The IOP was varied from 10 to 55 mmHg, which correlated with increasing capacitance of the MEMS transducer and the changing modulation frequency detected by the external circuit. Multiple tests were performed to investigate performance and repeatability of the prototype.
The IOP was increased during testing and the prototype exhibited a linear response of frequency versus pressure (see Figure 1). Response of the prototype at 10 mmHg was 2500 Hz and decreased to 2424 Hz at 53 mmHg. Repeated tests indicated a linear response with a sensitivity of 0.5 mmHg/Hz and standard deviation of 0.11 (n=5).
A wireless, battery free, IOP measuring prototype was successfully developed and acutely tested with good repeatability and sensitivity. Future investigations will include chronic implant data to quantify long-term performance and drift.
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