A sensor system for sensing a stimulus in vivo includes an implantable sensor. The sensor comprises a passive resonator circuit having a resonant frequency and including at least a pair of generally parallel spirally wound unconnected conductive coils sandwiching a layer of solid dielectric material that manifest a change in property affecting the resonant frequency in response to application of the stimulus to the layer. A resonant frequency of the sensor is modulated by altering the spacing or gap between the coils or altering the overlapping area of the coils. The sensor is energized through application of radiofrequency energy and the responding resonant frequency is detected. The sensor can be advantageously attached to a medical implant to form a practical smart implant for clinical purposes.