Bonded foil strain gauges (BFSG) are metal based foil elements that are incorporated into a flexible electrically insulating material which can then be bonded to a surface with adhesives.
As stress is applied to the material, the BFSG will deform and the resistance changes. If a voltage is applied across the BFSG it can be used to measure the amount of strain by monitoring the variation in voltage drop through the BFSG.