Researchers from the University of California, Berkeley, and collaborating institutions have developed a novel implantable ultrasound-based blood pressure (BP) monitoring system that continuously tracks arterial diameter changes to derive precise BP values. The study, published in Microsystems & Nanoengineering (DOI: 10.1038/s41378-025-01019-w), presents a 5 × 5 mm² piezoelectric micromachined ultrasonic transducers (PMUTs) array designed for subcutaneous implantation. In in vivo tests with an ambulatory sheep, the device achieved systolic and diastolic measurements within −1.2 ± 2.1 mmHg and −2.9 ± 1.4 mmHg of gold-standard arterial line readings, respectively.
Hypertension remains a leading cause of heart disease, stroke, and premature mortality worldwide. While regular BP monitoring can mitigate risks, conventional cuff-based measurements are intermittent and disrupt daily activities. Wearable alternatives such as photoplethysmography (PPG) and ultrasound patches face limitations including shallow penetration depth, gel dependence, and sensitivity to motion or misalignment. Implantable sensors have been explored but often require intrusive arterial placement or suffer from foreign-body reactions. The new PMUT-based system addresses these challenges by providing a minimally invasive, stable coupling that avoids gel use and environmental interference.
The device features a dense 37 × 45 PMUT array fabricated using CMOS-compatible processes. Each PMUT has a 29-µm diaphragm operating at approximately 6.5 MHz in water, offering high axial resolution and strong echo penetration. The dual-electrode bimorph design enhances acoustic output, and an optimized deep reactive ion etching process ensures structural uniformity. To derive BP, the system measures the time-of-flight between ultrasound echoes from the anterior and posterior arterial walls, converting this into a real-time diameter waveform. Bench-top experiments confirmed a linear relationship between diameter and pressure, while simulations showed that wearable systems can lose up to 60% signal strength with only 1 mm of misalignment—an issue the implanted design avoids.
During in vivo testing in an adult sheep, the subcutaneous PMUT array above the femoral artery captured detailed pressure waveforms, including the dicrotic notch, and closely matched arterial line measurements. The study demonstrates that ultrasound-based implants can achieve the stability and precision needed for continuous BP monitoring without the drawbacks of cuffs or fragile wearables. By capturing arterial diameter changes directly through subcutaneous sensing, the device avoids gel dependence, environmental noise, and misalignment issues.
This implantable system represents a promising alternative for patients requiring continuous, unobtrusive BP measurement. Its stability against tissue growth, motion, and environmental interference makes it suitable for long-term hypertension management, early detection of cardiovascular abnormalities, and integration into digital health platforms. Future advances such as beamforming to mitigate positional shifts and data-driven analytics for individualized risk prediction could further expand its clinical utility. The study was supported in part by BSAC (Berkeley Sensor and Actuator Center) and published in Microsystems & Nanoengineering, a journal by Chuanlink Innovations (http://chuanlink-innovations.com).
