Scientists have developed an aptamer-based biosensor capable of continuously tracking drugs in the bloodstream in real-time. The sensor consists of shape-changing probes, switches that can only be flipped by the right molecule. The greater the concentration of a drug, the more switches will be flipped. The biosensor is versatile; it can be reconfigured to measure different drugs by swapping out the probes with different ones. This video illustrates individual drug molecules at different concentrations flipping switches in the aptamer-based apparatus. This video relates to a paper that appeared in the Nov. 27, 2013, issue of Science Translational Medicine, published by AAAS. The paper, by Dr. B.S. Ferguson at University of California, Santa Barbara in Santa Barbara, Calif., and colleagues was titled, "Real-Time, Aptamer-Based Tracking of Circulating Therapeutic Agents in Living Animals."