Applications in blood flow monitoring. (a) Illustration of the sensor with an exposed view of the bilayer coil structure for wireless data transmission, the cuff-type pulse sensor wrapped around the artery, a close-up view of the pressure-sensitive region of the sensor with the two variable capacitors C₁ and C₂ before they are wrapped around the artery, and its equivalent electrical circuit [85]. (b) Image of an implant site where the sensor is implanted and the skin stitched; the reader antenna is placed in close proximity to the antenna of the implant for Δf₀ recordings [85]. (c) Plot of measured Δf₀ versus time; the pulse rate is calculated to be 3.47 b.p.s [85]. (d) Schematics of the stretchable ultrasonic device, with key components labelled [86]. (e) A typical pulse waveform measured from the carotid artery, directly correlated to the left atrial and ventricular events [86]. (f) A typical pulse waveform from the internal jugular vein, directly correlated to the right atrial and ventricular activities. Different phases and characteristic morphologies are marked [86]. (g) Schematic illustration of the device layout, with a photograph and an infrared image of a device on the skin over a vein, during application of power to the actuator [34]. (h) (I) Changes in blood flow as measured by a laser speckle contrast imager (LSCI, black) and our device (blue). (II and III) Fourier transform spectrogram determined from (II) LSCI data (FFT length = 128 s, five samples per second; the color bar is the amplitude of the LSCI spectrogram) and (III) presented device (FFT length = 128 s, two samples per second; the color bar is the amplitude of the thermal anisotropy spectrogram) [34]. (i) Illustration of the position of the vein relative to the device and flow field map. The red arrows show the relative magnitudes of the thermal distribution at peak flow [34]. (j) Full thermal distribution map during peak flow as measured by the presented device [34]. (k) and (l) Similar analyses as (i) and (j), except during occluded flow [34]. (m) Illustration of the wireless design and sensing scheme to simultaneously monitor pressure, heart rate (HR), and flow [87]. (n) Summary of wireless flow monitoring comparing the pressure gradients monitored by the wireless sensor and commercial sensors [87].