Fabrication and characterization of flexible shear force sensors. (a) A schematic illustration of the sensor fabrication process. (b) Scanning electron microscopy (SEM) images of the PDMS@Ag NWs. (i) SEM image of Ag NWs on PDMS. (ii) Side-view SEM image showing that Ag NWs were uniformly distributed on the top and sidewalls of the spiral microcolumns. (iii) Top-view SEM image. (iv) Energy spectra of the spiral column and the substrate. No Ag NWs existed between the microcolumns. (c) SEM images of PDMS@Ag NWs@Ecoflex. (i) Top-view SEM image. (ii) Side-view SEM image showing that Ecoflex filled the air gap evenly. (d) (i) Photograph showing the flexibility of the sensor. (ii) Photograph of a fabricated sensor. (iii) The actually measured capacitance of the fabricated sensor, which is in accordance with the theoretical capacitance of approximately 11.27 pF.