A medical device is provided which may include a reservoir which has an opening and contains a sensor; a reservoir cap closing off the opening to isolate the sensor from an environmental component outside the reservoirs, the reservoir cap being impermeable to the environmental component and adapted to selectively undergo a phase change to disintegrate the reservoir cap and thereby expose the sensor to the environmental component. A method of use may include (i) selectively disintegrating a reservoir cap to expose a sensor which is disposed inside a reservoir of a device implanted in a patient, the disintegrating comprising inducing a phase change in the reservoir cap; and (ii) using the sensor to generate an output signal, wherein the output signal is recorded and stored in a writeable computer memory chips, directed to a microprocessor for immediate analysis and processing, or sent to a remote location away from the device.