Piezoresistive and piezoelectric flow sensors. (a) Conceptual illustration of piezoresistive flow sensors [55]. (b) Schematic diagram of the flexible piezoresistive flow sensor using VGNs [57]. (c) Sensor output as a function of flow velocity (calibration plot) [57]. (d) Working mechanism of three LIG airflow sensors with equivalent circuit diagram and schematic illustration of PLIG, VLIGF, and SLIGF [59]. (e) Digital photos of cotton, caterpillar, and Lepidoptera insect-butterfly, with SEM images of PLIG, VLIGF, and SLIGF from top view. The scale bars are 250, 250 and 500 μm, respectively [59]. (f) Relative current variation within the airflow range of 0.0023–2.35 m/s [59]. (g) Conceptual illustration of piezoelectric flow sensors [60]. (h) Optical image of the hydrodynamic artificial velocity sensor [60]. Distribution of stress in the x direction when pressure is at 0° (i) and at 90° (j) [60]. (k) Results of the directivity detection [60]. (l) Relationship between charge output and velocity [60].