Design, construction, and fabrication of the stretchable strain sensing system. (a) Exploded view of the strain sensing system. (b) Schematic illustration of the fabrication processes of the amplifying circuit and strain sensor. Optical images of the amplifying circuit in its undeformed (c), bent (d), and twisted (e) states. (f) Reconfiguration of the amplifying circuit. (g) The strain sensing system is attached on an elbow for monitoring its flexion states. When the elbow is straight, the LED is off (top). When the elbow flexes beyond a threshold, the LED turns on (bottom).