In this presentation, we will present a general biosensor technology capable of continuously tracking a wide range of molecules in living subjects. Our real-time biosensor requires no exogenous reagents and can be readily reconfigured to measure different target molecules by exchanging molecular probes in a modular manner. Next, we will present the first real-time, closed loop feedback control of drug concentration in live animals using the real-time biosensor and discuss potential applications of our technology. Finally, we will discuss methods for generating synthetic molecular switches that are at the heart of the biosensor.