Abstract
Purpose:
To create a wireless system to record and monitor intraocular pressure (IOP) fluctuations in a rat’s eye on a 24-hr basis.
Methods:
The device consists of intraocular and ambient pressure transducers, bioamplifiers, a multichannel microprocessor, a data transmitter, and an energy harvester encased in a 30x25mm plastic box, which can be placed under the scruff of the rat’s neck or worn as a backpack. A fine cannula implanted in the rat’s eye conducts IOP to the device, and a data receiver and RF power source interact with the device from outside the animal’s cage. The energy harvester wirelessly powers the device by transforming the RF signal emitted by the power source into a DC voltage. The system runs in two modalities: sleep and active mode. In sleep mode, the microprocessor consumes close to zero current, the sensors are turned off, and the device stores harvested energy. Periodically, the system temporarily switches to active mode during which the sensors are turned on and the device sends collected pressure data to the receiver before returning to sleep mode. The cycle time allows for recovery of power spent on the burst of data collection and transmission. The system was bench tested by measuring the distribution of RF energy inside the cage along a 3D grid with 2cm spacing. The amount of DC voltage and its spatial homogeneity was assessed with and without reflective walls around the cage. The system was then exposed to variable levels of constant hydrostatic pressure.
Results:
Power distribution data showed that reflective walls around the cage created a more uniform RF field that increased energy available to 5-10V. Device consumption of this power placed limits on IOP data rates. It was found that the system used ~500mV for 1-2s of data collection and transmission and that the system could recoup this amount of power by harvesting energy for 30s or more. By cycling between power usage and harvesting, the system was able to successfully transmit pressure signals wirelessly to a nearby computer for hours on end at a rate of once per minute. The standard deviation of pressure signal measurements was 3.5mmHg.
Conclusions:
A device has been developed for rats that can continually measure IOP and wirelessly transmit the data to the researcher at a rate of ~1Hz. The device is wirelessly rechargeable within the confines of the animal's cage, eliminating IOP measurement inaccuracies caused by battery drainage.