Wireless gizmos in the body, using sonar:

http://www.sciencedaily.com/releases/2013/05/130531151246.htm

“Ultrasounds may be a more efficient way to share information, Melodia said, because roughly 65 percent of the body is composed of water. This suggests that medical devices, such as a pacemaker and an instrument that measures blood oxygen levels, could communicate more effectively via ultrasounds compared to radio waves.”

"Think of how the Navy uses sonar to communicate between submarines and detect enemy ships," Melodia said. "It's the same principle, only applied to ultrasonic sensors that are small enough to work together inside the human body and more effectively help treat diseases."

Another example involves connecting blood glucose sensors with implantable insulin pumps. The sensors would monitor the blood and regulate, through the pumps, the dosage of insulin as needed in real time.”