endoSensor

After FDA approval in November, 2005, Endosure sensors are now being implanted in patients a the Michael E. DeBakey VA Medical Center in Houston.

Abdominal aortic aneurysms are the third leading cause of sudden
death in elderly U.S. men and the 13th leading cause of death in the
United States.

The device, made by CardioMEMS Inc., is implanted in the aneurysm sac.

When activated by an external device, it transmits information about pressure inside the aneurysm.

According to Division of Vascular Surgery and Endovascular
Therapy of Emory University, which tested the sensor, aneurysms are
typically repaired by inserting a stent into the affected blood vessel.
This takes the pressure off the aneurysm by creating a new route for
blood to flow.

But since the stents can leak and cause the aneurysm to
rupture, they require regular check-ups. The sensor, which is implanted
into the aneurysm sac, provides a new way to monitor the pressure.

Developer of the Endosure is CardioMEMS (the MEMS stands for MicroElectricoMechanical Systems) in Atlanta. CardioMEMES uses manufacturing techniques from the semiconductor industry to make micron scale devices that incorporates both mechanical and electronic components. Here's a blurb from their web site about the technology:

A key circuit element of the sensor is a capacitor composed of two
conducting plates separated by an air gap. The plates are electrically
interfaced with an inductor/antenna. This circuit has a known resonant
frequency. Pressure changes alter the distance between capacitor
plates which changes the resonant frequency. An electronic monitoring
device detects this change in the sensor's resonant frequency and converts
it into a pressure change. The wireless radiofrequency
(RF) technology in the monitoring device is proprietary.

It looks like they've got a wireless congestive heart failure (CHF) sensor in development.