MEMS vendor Integrated Sensing Systems, Inc. (ISSYS) announced, "that the U.S Patent Office has granted a patent entitled “Wireless MEMS Capacitive Sensor for Physiologic Parameter Measurement” (US Patent No. 6,926,670) which covers the design, manufacturing, and anchoring of miniature, wireless, batteryless, implantable sensors."  The summary from the patent:

The invention comprises a telemetric sensing system for noninvasively monitoring pressure and/or pressure gradients in a cardiac conduit. The system includes one or more implantable sensor unit(s) and a companion reader unit. The sensor unit, which is preferably batteryless and wireless is chronically located within the conduit, or around in a close proximity. For valveless conduits, a sensor unit is placed at either end of the conduit, or around it. For valved conduits, one or more sensor units are located both proximal and distal to the valve, allowing the pressure gradient across the valve to be monitored. One sensor unit can indicate occlusion; however, two sensor units will allow the occlusion to be located (e.g.
proximal/middle/distal along the conduit). As well, with two sensor units, flow rates may be deduced or estimated. Furthermore, trend analysis of the pressures and/or flow rate within the conduit can allow a time-to-failure estimate.

New Scientist has a nice description of the clinical application:

The implant, the size of a grain of rice, is one of a new breed of medical devices that requires no batteries. A radio transmitter and receiver held near the body provides the power and interrogates the implant.

The device is designed for people with congestive heart failure, where fluid builds up in organs and limbs because the heart fails to pump enough blood around the body. There are now more than half a million new cases each year in the US alone.

The condition is usually treated with drugs, and to ensure they are working doctors sometimes have to measure the pressure inside the left atrium of the heart.

At the moment, this can only be done by temporarily inserting a catheter into the heart, via an artery in the arm or leg. In some patients this unpleasant and costly operation has to be done three times a year.

But with the new implant a similar operation would only have to be done once, to place the sensor inside the left atrium, says Nader Najafi, head of Integrated Sensing Systems (ISSYS) in Ypsilanti, Michigan, the company developing the implant.

Next thing you know, they'll be generating continuous waveforms from implants like this.

Pictured above is Sensimed’s Triggerfish implantable MEMS intraocular pressure sensor.