Wireless passive radio-frequency strain and displacement sensors

A method of sensing strain, the method comprising the steps of:

• (a) transmitting a radio frequency (RF) signal toward a sensor to produce a transmitted or reflected RF signal, wherein the sensor comprises a first sensing layer including a plurality of first sensing elements on a first rigid substrate, a second sensing layer including a second sensing element on a second substrate, and an insulating interlayer disposed between the first and second sensing layers, wherein the plurality of first sensing elements are electromagnetically coupled to the second sensing element, and wherein relative movement between the first and second sensing layers causes a change in a plurality of resonance frequencies associated with the plurality of first sending elements, thereby detecting direction of movement of the first sensing layer relative to the second sensing layer in a plane partitioning the first and second sensing layers;
• and
• (b) detecting the transmitted or reflected RF signal, wherein a center frequency of a resonant peak in the reflected or transmitted RF signal indicates strain exerted on the sensor.