Acoustic flow sensors. (a) Conceptual illustration of Doppler flow sensors. (b) Schematic diagram and optical image (inset) of the flexible acoustic wave device [61]. (c) Frequency responses of the acoustic wave device when the flow rate increases from 0 to 0.8 LPM and then decreases from 0.8 to 0 LPM in step of 0.2 LPM [61]. (d) Schematics (left) and exploded view (right) of the device structure. ACF, anisotropic conductive film [62]. (e) Schematic of the DBUD method [62]. (f) Measured PFV versus true PFV curves. Error bars represent ±SD (N = 5) [62]. (g) Typical carotid blood flow spectra during a cardiac cycle (left) and several cycles (right). Feature points are marked in the left image. Inset: an image showing the device mounted on the neck [62]. (h) Conceptual illustration of time-of-flight flow sensors. (i) Ultrasonic air-coupling time-of-flight configuration [66]. (j) Experimental DC steps linearity test [66].