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E. Y. Chow, A. Chlebowski, S. Raghunathan, B. N. Fogle, G. Simon, J. M. Clevenger, B. Ziaie, P. P. Irazoqui; A 24-Hour Continuous IOP Monitoring Device Used for the Treatment of Glaucoma in Humans. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2864.
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To develop a fully-wireless miniature implantable device that provides a long-term solution for continuous monitoring of IOP. Since the pressures may not be elevated all the time, continuous monitoring is essential for accurately diagnosing the glaucomatous condition in patients. The device should cause no discomfort, allow for a minimally invasive surgical implantation procedure, and provide accurate pressure readings with 0.5 mmHg accuracy.
The implant consists of an application specific integrated circuit (ASIC), micro-electro mechanical systems (MEMS) capacitive IOP sensor, antenna, capacitive power storage array, and liquid crystal polymer (LCP) based substrate. Our ASIC has been fabricated through Texas Instruments and contains a digitizing interface for our MEMS sensor, on-chip memory, wireless transmitter, and radio frequency (RF) powering module. To obtain sub-nanowatt power constraints, the unit operates on a sleep-wake cycle and obtains one measurement every 5 minutes. The limited power storage and on-chip memory require the implant to be recharged and the data downloaded at the end of every 24-hour period. At that time, the user is required to hold an external device nearby which sends a high power radio frequency wave that is processed by the RF powering module to charge the storage array. After recharging, the 2.4 GHz wireless transmitter transfers all the data from the on-chip memory to the external device.
Every material used in our device was run through long-term chronic in vivo studies to verify biocompatibility. The MEMS capacitive sensor has been thoroughly characterized to produce a linear capacitance change of 2 fF per 0.5 mmHg over the range of 0 to 60 mmHg above atmospheric pressure. The ASIC has been verified in silico to capture these 2 fF capacitance changes over the desired range and successfully digitize and store the data. Successful wireless data transfer and RF powering have been quantified through extensive in vivo studies in the eyes of New Zealand white rabbits.
Our work shows great promise for a tool that clinicians can use to help diagnose and treat glaucomatous patients. Numerous portions of the project have completed in vivo studies while the animal studies for the remaining parts are in progress.Our state-of-the-art device is a culmination of recent advances in the field of biomedical implants and pushing the limits on achieving small-scale and ultra-low-power devices.
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