Working principle of ultrasonic measurement of blood vessel diameter. (A) Wearing a schematic diagram of the flexible ultrasonic array, which includes four parts: Piezo transducer, serpentine line, PDMS basement, and Cu electrode. (B) At a 2 MHz frequency, the ultrasonic wavelength is about 0.69 mm. It has both good bandwidth (yellow line) and signal quality. (C) A single pulse in the upper and lower walls of arteries will produce a high energy echo due to the difference in acoustic impedance, and then the ultrasonic sensor as the echo receiver receives the echo and converts it into a high level of electrical signal. (D) The array is set to seven ultrasonic arrays of 0.9 mm length and spacing, and excited at a center frequency of 7.5 MHz.