Fig. 1. The flexible piezoelectric epidermal electronic device: (a) Explosive view of the structure of the device; (b) overall and detailed photos of the serpentine structured interdigital electrodes and the design; (c) photograph showing the device attached to the skin; (d) photographs showing the device under longitudinal and latitudinal stretching, twisting and bending, respectively; (e) the SEM image of the surface morphology of the graphene/PZT/PDMS ternary piezoelectric rubber; (f) energy dispersive X-ray spectroscopy (EDX) images illustrating the distribution of C, Si, O, Pb, Zr and Ti.

Fig. 2. Electrical performance of the piezoelectric device with varying applied pressure under a hitting frequency of 1 Hz: (a) Real-time measured open-circuit voltage output under varying applied pressure; (b) average peak open-circuit voltage as a function of pressure (inset: OC voltage output at about 1 kPa pressure and below); (c) real-time measured short-circuit current output under varying applied pressure; (d) average peak short-circuit current as a function of pressure (inset: SC current output at about 1 kPa pressure and below).

Fig. 3. Electrical performance of the piezoelectric device with identical applied pressure ((23.17 ± 1.76) kPa) under varying longitu-dinal and latitudinal stretching strain: (a) Real-time measured open-circuit voltage output under varying longitudinal stretching strain; (b) average peak open-circuit voltage as a function of longitudinal strain; (c) real-time measured open-circuit voltage output under varying latitudinal strain; (d) average peak open-circuit voltage as a function of latitudinal strain.

Fig. 4. Electrical performance of the piezoelectric device when attached to human skin: (a) Photos of four different contact method: touching, poking, tapping and hitting; (b) real-time measured open-circuit voltage outputs; (c) average peak open-circuit voltages; (d) real-time measured short-circuit current outputs; (e) average peak short-circuit currents; (f) comparison between open-circuit voltage outputs of devices in original state and after one-month use when used for tactile sensing; (g) comparison between short-circuit current outputs of devices in original state and after one-month use when used for tactile sensing.

Fig. 5. Demonstration test of supplying energy for 15 LED bulbs only by the piezoelectric device: (a), (b) LEDs are blacked out when fingers lift off from the device; (c), (d) 15 LEDs are all lighten up at the moment fingers hit the device.