Characterization of the tactile sensor. (a) Explosive view of the micro-sphere tactile sensor (scale bar is 5 μm). (b) Scanning electron microscopy (SEM) images of the top and bottom layers of the 2B-sensor in top view, with right side showing the schematic of the top and bottom layers 2B-sensor. (c, d) Fabrication procedures of the top and bottom layers of the sensor. (e) FTIR spectra of the PDMS film before and after N₂-plasma treatment. (f, g) XPS survey spectra of spectrum N 1s region of N₂ plasma-treated PDMS film.