Illustration of electrical sensing mechanism of a paper pressure sensor. (a) At the initial state, the distance between the adjacent RGO particles is relatively large, which results in the isolation state. (b) Under the pressure, the distance between the adjacent RGO particles is near enough, the tunneling current generates under the electric field. (c) Under a large pressure, the RGO particles connect with each other to form several current paths. The resistance will not response to the external pressure.