Sensitive readout of implantable microsensors using a wireless system locked to an exceptional point

Exceptional points are degeneracies in physical systems at which both the underlying eigenvalues and eigenvectors of the system coalesce. They originated in theoretical explorations of quantum mechanics, but are of increasing value in photonics, acoustics and electronics because their emergence in physical systems with controlled gain and loss can dramatically alter the response of a system. In particular, systems biased at exceptional points can exhibit an amplified response to a small perturbation, enabling greatly enhanced sensitivity for certain resonant sensors. In biomedicine, implanted electronic sensors based on resonant inductor-capacitor (LC) circuits can be used to monitor internal physiological states, but their capabilities are currently limited by the low sensitivity of existing wireless interrogation techniques. Here we show that a reconfigurable wireless system locked to an exceptional point can be used to interrogate in vivo microsensors with a sensitivity 3.2 times the limit encountered by existing schemes. We use a controller that maximizes the abruptness of a parity-time-symmetry phase transition to operate a reconfigurable circuit at an exceptional point and maintain enhanced sensitivity. With this approach, we demonstrate robust readout of LC microsensors (with diameters of 900 mu m) that are subcutaneously implanted in a rat, and show that it can be used for wideband sensor interrogation for measurement of the resonant frequencies of single and multiple sensors.