Introduction
For more than a hundred years now, cardiovascular diseases (CVDs) have been the leading cause of death in the US.1 CVDs include, but are not limited to, coronary heart disease (CHD), stroke, heart failure, arrhythmias (abnormal heart rhythm), and high blood pressure. All CVDs originate from subtle vascular changes that often serve as harbingers of disease long before any prominent physiological symptoms emerge. The ability to monitor these changes continuously and in real-time could dramatically shift the paradigm for early detection, treatment, and management of CVDs. However, traditional diagnostic tools such as cardiac catheterization or blood pressure monitors and cuffs are either too invasive or inadequate for capturing the nuances of cardiovascular health. Enter wearable health technologies—innovations designed to integrate seamlessly into daily life while providing critical health data. Research in wearable health technology is a global endeavor, developing rapidly in the last few years with significant contributions coming from both academic and industry labs. These innovations are poised to redefine how we approach cardiovascular health monitoring and management, offering non-invasive, real-time insights into vital hemodynamic parameters.
Cardiovascular Diseases & Role of Wearable Tech
For diagnosis and prognosis of cardiovascular diseases, healthcare professionals currently use the central blood pressure (CBP) waveform (representing the pressure changes in the central arteries, such as the aorta, over a cardiac cycle), which is an indication of blood pressure in primary arteries near the heart and brain. On the other hand, peripheral blood pressure (PBP) measures blood pressure in peripheral arteries.2 Unlike PBP, CBP reflects the true load or stress on vital organs and the elasticity of central arteries, which play key roles in diagnosing and tracking disease progression.3 This distinction is crucial because CBP provides a more accurate representation of cardiovascular stress and arterial health. As a result, the heart is forced to work harder because of increased load, such as heightened afterload from arterial stiffness. This contributes to conditions like left ventricular hypertrophy and heart failure. Similarly, reduced arterial elasticity amplifies central blood pressure and pulse pressure, accelerating vascular damage and the progression of diseases like hypertension and atherosclerosis. Monitoring CBP offers valuable insights into these factors, enabling better diagnosis and tracking of cardiovascular disease progression3.
However, conventional tools for CBP monitoring face limitations, making them less suitable for routine clinical use. Cardiac catheterization, an invasive procedure where a catheter is inserted into blood vessels to measure CBP directly, provides highly accurate readings but is impractical for regular monitoring due to patient discomfort and risks such as infection.4 Photoplethysmography (PPG) is a non-invasive method that uses light absorption to detect blood volume changes in tissues. It is commonly employed for peripheral vascular assessments, but lacks the depth penetration required for central arteries.5 Despite its ability to monitor arrhythmias and peripheral hemodynamics, PPG is also highly susceptible to motion artifacts (distortions or inaccuracies in the data caused by movement during the measurement process), providing inconsistent results in dynamic environments.3 Similarly, tonometry, which estimates arterial pressure by compressing the skin over a superficial artery, is influenced by tissue stiffness and movement, as well as dependent heavily on operator skill.6 These limitations highlight the need for advanced CBP monitoring technologies. Wearable ultrasound devices, in particular, show promise by addressing these gaps through continuous, non-invasive, and accurate monitoring of central blood pressure.
One such advancement is a wearable ultrasound device developed by Dr. Sheng Xu’s lab at UC San Diego. This device holds transformative potential for cardiovascular health management by enabling continuous, non-invasive monitoring of CBP and other vital hemodynamic parameters. By overcoming the limitations of conventional tools, the Xu Lab’s wearable ultrasound device offers a novel approach to understanding and managing cardiovascular diseases, paving the way for more precise and proactive care.
Highlighting Research at the Xu Lab
To understand the applicability of the wearable ultrasound device, it is first and foremost necessary to learn about its structure. At its core, the device relies on an array of piezoelectric transducers that generate and receive ultrasonic waves. “Piezoelectronic” refers to electricity caused by pressure, and piezoelectric transducers convert the electrical charges produced by certain solid materials into tangible energy. A piezoelectric transducer works similarly to the body of an acoustic guitar. When you pluck a guitar string, the vibrations travel through the guitar’s body, amplifying the sound. Similarly, when pressure is applied to a piezoelectric material, it transforms that mechanical energy into electrical energy.7 These transducers are constructed using multiple piezoelectric microrods embedded within an epoxy resin matrix (providing structural support, adhesion, and durability). This design enhances acoustic coupling, enabling the efficient transfer of sound energy between the ultrasound device and biological tissues. The anisotropic configuration of the microrods—arranged in different directions—further improves the device’s ability to couple with tissues and ensures effective wave transmission for more precise measurements. The device is remarkably thin, measuring only 240 μm (just about the height of 3 sheets of printer paper), and highly stretchable, with mechanical properties that closely mimic human skin. This skin-like flexibility ensures intimate and stable contact with the body, even during intense physical activity. To further enhance usability, the device is encapsulated in elastic silicone elastomers, which not only provide a soft and comfortable interface but also protect the device from moisture and sweat. This encapsulation is equivalent to (and eliminates the need for) a traditional ultrasound gel, making the device more practical for everyday use.
Functionally, the researchers found an optimum balance between resolution and tissue penetration by operating the device at a high frequency of 7.5 MHz. Higher frequencies provide better resolution, allowing for more precise measurements of vessel dynamics, but they are less effective at penetrating deeper tissues. A frequency of 7.5 MHz is optimal for capturing detailed vascular information while still being able to penetrate the skin and underlying tissues effectively. The device utilizes ultrasonic waves to measure changes in blood vessel diameter and calculate blood pressure waveforms with exceptional precision. When placed on the skin, the waves penetrate biological tissues and reflect off interfaces, or the surface where two different materials or structures meet, like vessel walls. The reflected waves are captured by the transducers on the device itself.
Next, the time-of-flight (TOF) data of the reflected waves is then analyzed to determine vessel health with sub-millisecond temporal resolution and micron-scale spatial accuracy. TOF refers to the time taken for an ultrasonic wave to travel from the transducer to a target (such as a vessel wall) and back to the transducer. This time is used to calculate the distance to the target, which is essential for determining vessel dynamics with high precision. Health parameters are processed from the raw data through advanced algorithms. For instance, systolic and diastolic blood pressure values are derived from vessel diameter measurements, while pulse wave velocity (PWV)—a key indicator of arterial stiffness—is calculated by correlating ultrasound data with electrocardiographic (ECG) signals. Additionally, researchers analyze the shape of the waveforms to provide more insights into vascular health. For example, the systolic peak represents when the heart’s left ventricle contracts and pumps blood into the arteries, and other waveform characteristics reflect vascular resistance, elasticity, and cardiac output.
Despite its groundbreaking capabilities, developing this wearable ultrasound device required overcoming several challenges. Motion artifacts posed a significant hurdle since maintaining stable data collection during movement is essential for accurate measurements during regular daily use. The researchers addressed this through the device’s conformal design, which adheres closely to the skin. Biocompatibility was another critical consideration; extensive testing ensured that the materials used in the device were safe for prolonged use without causing irritation or other adverse effects on the skin and underlying tissues.3 Furthermore, energy efficiency was optimized to allow extended use without frequent recharging or excessive power consumption, making it suitable for long-term monitoring applications. By addressing these challenges with innovative solutions, this wearable ultrasound system sets a new standard for continuous health monitoring, offering unprecedented precision and convenience while paving the way for future advancements in personalized healthcare technologies.
Current and Future Applications
In its present form, the wearable ultrasound device revolutionizes hypertension management by enabling CBP monitoring. This real-time data allows clinicians to fine-tune antihypertensive therapies (focusing on lowering high blood pressure), tracking physiological parameters with unprecedented precision and improving patient outcomes with more personalized treatment plans. In post-surgical care, the device’s ability to track vascular dynamics in real-time has proven crucial for early detection of potential complications such as thrombosis or vascular occlusion, significantly improving patient safety during recovery. Its utility extends beyond clinical settings, such as by finding valuable applications in sports medicine by providing detailed insights into cardiovascular responses during exercise to help athletes and trainers make informed decisions about training intensity and recovery.
Looking towards the future, the potential applications of this wearable ultrasound technology are vast and exciting. Researchers are exploring multimodal integration, combining ultrasound with other sensing technologies like PPG and ECG to create comprehensive cardiovascular profiling systems. This integration could provide a more holistic view of cardiovascular health, enabling more accurate diagnoses and treatment strategies. The incorporation of artificial intelligence (AI) and machine learning (ML) algorithms into health data analysis holds promise for bettering its interpretation. While the device collects real-time data, the integration with other sensing technologies, as well as the incorporation of AI and ML algorithms, typically requires external computational power to identify patterns, predict potential health issues, and provide more accurate and actionable insights. These advanced analytics could potentially predict the onset of cardiovascular diseases based on subtle changes in waveform patterns, allowing for earlier interventions and improved patient outcomes.
Furthermore, the continuous data collection enabled by this wearable technology opens up possibilities for extensive longitudinal studies (conducted over an extended period over a fixed group of subjects). These studies could offer unprecedented insights into how vascular parameters evolve over time in response to aging, lifestyle changes, and various medical interventions, potentially reshaping the current understanding of cardiovascular health trajectories.
Beyond cardiovascular applications, researchers in the Xu lab are also exploring the device’s potential in other areas of medicine. There is growing interest in adapting the technology for monitoring liver and kidney function, which could revolutionize the management of chronic diseases affecting these organs. Additionally, the device’s ability to assess blood flow could have significant implications in oncology. Monitoring tumor vasculature for cancer management could provide valuable information about tumor growth and response to treatments. As the technology continues to evolve and miniaturize, it may also find applications in neurological monitoring, fetal health assessment during pregnancy, or even in vascular health monitoring of astronauts during long-term space missions. The non-invasive nature of the device, along with its ability to provide continuous, high-resolution data, positions it as a transformative tool for personalized and preventive medicine.3 By enabling early detection of disease markers and facilitating timely interventions, wearable ultrasound devices have the potential to significantly improve health outcomes and promote more proactive, patient-centered healthcare practices.
Conclusion
The wearable ultrasound device developed by Dr. Xu’s lab exemplifies the power of interdisciplinary innovation within the field of wearable health technology. By addressing limitations in traditional cardiovascular monitoring, this device provides a non-invasive, scalable solution for continuous deep tissue monitoring. As this technology matures, its potential to revolutionize personalized medicine becomes clear: from allowing for early detection to guiding therapy and empowering patients to take control of their health, the impact on cardiovascular care could be groundbreaking. By bridging biology and engineering, wearable technologies like this are paving the way for a healthier, more proactive future–where healthcare is accessible at the individual level.
Sources:
All Leading Causes of Death. 2022. Injury Facts. https://injuryfacts.nsc.org/all-injuries/deaths-by-demographics/all-leading-causes-of-death/.
Ranjan AK, Gulati A. 2023. Controls of Central and Peripheral Blood Pressure and Hemorrhagic/Hypovolemic Shock. Journal of Clinical Medicine. 12(3):1108. doi:https://doi.org/10.3390/jcm12031108. https://www.mdpi.com/2077-0383/12/3/1108.
Wang C, Li X, Hu H, Zhang L, Huang Z, Lin M, Zhang Z, Yin Z, Huang B, Gong H, et al. 2018. Monitoring of the central blood pressure waveform via a conformal ultrasonic device. Nature Biomedical Engineering. 2(9):687–695. doi:https://doi.org/10.1038/s41551-018-0287-x.
Cardiac Catheterization Handbook. 2015. Google Books. [accessed 2025 Jan 8]. https://books.google.co.in/books?hl=en&lr=&id=7GByCgAAQBAJ&oi=fnd&pg=PP1&dq=cardiac+catheterization&ots=pq5xpIR7Nx&sig=KHXl3w13xA68oD89HkwY6_j45LI&redir_esc=y#v=onepage&q=cardiac%20catheterization&f=false.
Alian AA, Shelley KH. 2014. Photoplethysmography. Best Practice & Research Clinical Anaesthesiology. 28(4):395–406. doi:https://doi.org/10.1016/j.bpa.2014.08.006. [accessed 2020 Nov 15]. https://www.sciencedirect.com/science/article/pii/S1521689614000755?via%3Dihub.
Sato T, Nishinaga M, Kawamoto A, Ozawa T, Takatsuji H. 1993. Accuracy of a continuous blood pressure monitor based on arterial tonometry. Hypertension. 21(6_pt_1):866–874. doi:https://doi.org/10.1161/01.hyp.21.6.866.
What Is A Piezo Transducer? americanpiezo. https://www.americanpiezo.com/knowledge-center/piezo-theory/whats-a-transducer/.
