A wireless microsystem for continuous measurement of intraocular pressure

A wireless implantable IOP sensing microsystem was successfully fabricated and characterized. This device consists of a micromachined silicon pressure sensor and a separately processed CMOS IC interface chip embedded in a common silicon substrate. A low power receiver/interface chip consisting of: (1) a CMOS full-wave rectifier, (2) an adaptively biased regulator, and (3) an offset free voltage-to-frequency converter was designed and fabricated through MOSIS. The system generates a pulse train, whose frequency depends on the measured eye pressure. A novel stress reducing package was also developed which included a silicon/PDMS guard-ring to absorb the extrinsic stress. Silicon-ring provides a hard shield/handling periphery while the deformable PDMS provides the major stress absorption capability. An etch release process was used for hybrid integration and wafer level device release. With a uniform radial stress, the silicon/PDMS guard-ring blocks 98% of the applied stress as compared to a conventional structure. In addition, the silicon/PDMS guard-ring prevents base-line drift, which is an important factor for long-term measurement of physiological pressures. A final metal layer was used to connect the sensor to the electronics and the receiver coil (no wire bonding was required). The micromachined pressure sensor showed a sensitivity of 55.5muV/V/mmHg at 25°C. With applied pressure the sensor showed a sensitivity of 2.08KHz/mmHg. The IOP sensor was tested for over 30 day and showed a very good stability (<0.1mV/V). Finally, two sources of tissue temperature increase in implantable wireless microsystems were investigated. These included: (1) the effect of a hot transmitter coil, and (2) the contributions of the receiver electronics and implanted microcoil power dissipations. On the basis of measurement results, it was realized that radiative heat transfer from a hot transmitter coil can be one of the contributing factors in tissue temperature rise. We also clearly demonstrated that in a continuous mode of operation, the implanted device temperature increase due to the combined effects of the receiver chip electronics and microcoil series resistance could easily exceed the 1°C safety limit. In pulsed-mode applications, the thermal time constants are long enough to allow a safe operation in most cases.