Piezoelectric Micromachined Ultrasonic Transducer for Arterial Wall Dynamics Monitoring

In this article, a
${1.5} \times {1.5}\,\,\text {mm}^{{2}}$
piezoelectric micromachined ultrasonic transducer (PMUT) array is designed and driven with one cycle of a 5-MHz sinusoid at 10
$\text {V}_{\text {pp}}$
for radial artery motion tracking. The transmit and receive performance figure of merit (FOM) of an individual PMUT over operating frequency is modeled and validated using laser Doppler vibrometer (LDV) measurements. Given a fixed cross section, the FOM inversely scales with frequency. The array aperture size is selected to obtain enough pressure and received signal to measure the radial artery wall reflection at a 5-mm depth in tissue. The 2-mm acoustic beamwidth provides enough lateral resolution for radial artery wall motion tracking. Single-line ultrasonic pulse-echo measurements with high time resolution, also called M-mode ultrasound imaging, are demonstrated to reproduce a known target motion profile with a precision of around 0.5
$\mu \text {m}$
.
In vivo
radial artery dynamics are measured by placing the sensor on the wrist of a volunteer. The measured diameter change waveform of the radial artery is consistent with reports in the literature and captures key arterial pulse waveform features, including systolic upstroke, systolic decline, dicrotic notch, and diastolic runoff. The system has sufficient accuracy and precision to measure both the 50
$\mu \text {m}$
overall diameter change and the 5-
$\mu \text {m}$
diameter change due to the dicrotic notch. A heart rate of 70 beats/min is also derived. This demonstrates the great potential of custom PMUT arrays for continuous cardiovascular system monitoring.