Wearable health monitors have become increasingly sophisticated at measuring heart rate, blood oxygenation and other physiological signals, yet one important source of uncertainty has remained largely hidden beneath the skin: the precise shape and location of the blood vessels being monitored. A new study reports an integrated thermal-sensing method designed to solve that problem by estimating vascular depth and vessel diameter directly from the skin surface. The approach combines a mathematical heat-transfer model with a compact sensor that measures how tissue temperature changes after localized heating. In tests on veins in the human arm, the system reportedly estimated vessel depth with an accuracy of approximately 0.3 millimetres and inner diameter with an accuracy of about 0.2 millimetres, results comparable to those obtained with ultrasound imaging.
The advance addresses a fundamental limitation in wearable haemodynamic monitoring. Many wearable devices infer blood flow and related cardiovascular parameters from electrical, optical or thermal signals that travel through several layers of skin and tissue before reaching the sensor. Those signals are influenced by the thickness of the tissue, the position of nearby vessels and the geometry of the vessels themselves. When these anatomical features are not measured in situ, researchers and engineers must rely on assumed values or population averages. Such assumptions can introduce systematic errors into the multiphysics models used to interpret wearable measurements, potentially reducing the accuracy of estimates such as blood-flow velocity, vascular resistance and pulsatile haemodynamic changes.
The method developed by Tian, Deng, Tan and colleagues uses a controlled thermal stimulus to reveal the structure beneath the skin. A localized heating event changes the temperature field in the superficial tissue, while blood flowing through a vessel acts as a moving thermal sink. Because blood carries heat away from the surrounding tissue, the temperature at the skin surface does not simply rise and cool according to the properties of skin alone. It also reflects the depth, size and thermal interaction of the underlying vessel. By recording the transient response rather than relying only on a single temperature value, the researchers extracted information from the entire heating and cooling process.
A central measurement in the study is the skin’s thermal relaxation time constant, denoted by τ. This parameter describes how quickly the surface temperature returns toward its baseline after the heating stimulus is removed. In a simplified thermal system, a shorter relaxation time indicates faster dissipation of heat, while a longer time indicates that heat remains in the tissue for longer. In living tissue, however, the response is governed by several coupled processes, including thermal conduction through the skin, convection caused by blood flow and the geometric relationship between the sensor and the vessel. The researchers established quantitative relationships linking τ to two morphological parameters: the vessel’s depth beneath the skin, represented as HTube, and its inner diameter, represented as DTube.
The analytical model is important because these parameters can influence the thermal signal in different ways. A vessel located close to the surface may produce a pronounced disturbance in the temperature field because heat reaches it quickly. A deeper vessel may generate a weaker or more delayed effect. Diameter also matters: a larger vessel contains more blood and can provide a greater effective pathway for heat removal, although the precise response depends on flow conditions, tissue properties and the heating geometry. By examining the spatially heterogeneous temperature pattern across the skin, rather than treating the surface as thermally uniform, the researchers were able to separate the contributions of depth and diameter. This decoupling is essential because different combinations of vessel size and position could otherwise produce similar temperature traces.
To capture those patterns, the team built a miniaturized sensor measuring 20 by 2 millimetres. Its narrow form factor is intended to be compatible with wearable devices while still providing measurements at multiple positions across the heated region. The sensor records how temperature varies from one location to another and how the variation evolves over time. These data are then compared with predictions from the analytical model. Instead of producing only a qualitative indication that a vessel is present, the system estimates numerical morphological values that can be incorporated into haemodynamic calculations.
The distinction between a vessel-detection system and a morphology-measurement system is significant. Conventional wearable sensors can often identify changes associated with pulsatile blood flow, but interpreting those changes requires knowledge of the path taken by the signal through the body. Optical signals, for example, are absorbed and scattered by skin, connective tissue and blood, while electrical signals are affected by the conductivity of different tissue layers. Thermal signals likewise depend on tissue thickness and vascular architecture. If a model assumes an incorrect vessel depth or diameter, the resulting haemodynamic estimate may be consistently biased even when the sensor itself is functioning correctly. Measuring morphology at the same site as the wearable device could therefore reduce a major source of calibration error.
In measurements of human arm veins, the reported accuracy approached 0.3 millimetres for HTube and 0.2 millimetres for DTube. The study compares these results with ultrasound imaging, which is widely used to visualize vessel anatomy but generally requires a trained operator and a relatively large external system. Ultrasound remains a powerful clinical technique because it can provide real-time images and flow information, but its size and operating requirements limit continuous, everyday monitoring. The thermal approach is not presented as a replacement for diagnostic ultrasound in every setting. Instead, its potential lies in providing a compact, non-invasive and wearable-compatible measurement that can supply anatomical information continuously or intermittently during routine monitoring.
The researchers describe the approach as a way to reduce uncertainty in multiphysics field-conduction models used for haemodynamic estimation. In practical terms, a wearable system could first characterize the local vascular geometry and then use those measurements to interpret subsequent thermal, electrical or optical signals more accurately. This could improve the reliability of monitoring during changes in posture, exercise or vascular health, when blood flow and tissue conditions may vary. The same principle might eventually support personalized devices that automatically adapt their models to the individual anatomy of each user rather than applying a generic calibration derived from a population average.
The findings also highlight the broader value of transient thermal sensing in biomedical engineering. Temperature changes at the skin surface are often treated as indirect indicators of physiology, but their time-dependent structure can encode information about anatomy as well as blood flow. A compact sensor that combines controlled heating, spatial temperature mapping and analytical modelling may therefore open a route toward wearable systems capable of measuring both vascular morphology and haemodynamic function. Before the technology can become a routine consumer or clinical tool, it will require broader validation across different skin types, vessel classes, anatomical locations, blood-flow conditions and levels of tissue thickness. Even so, the reported results suggest that a brief, localized thermal perturbation could provide a surprisingly precise window onto structures hidden beneath the skin, helping transform wearable monitors from signal collectors into anatomically informed measurement systems.
Subject of Research: Non-invasive thermal sensing of vascular morphology for improved wearable haemodynamic monitoring.
Article Title: Integrated thermal sensing of vascular morphology for haemodynamic monitoring
Article References: Tian, Y., Deng, Y., Tan, J. et al. “Integrated thermal sensing of vascular morphology for haemodynamic monitoring.” Nature Sensors 1, 636–648 (2026). https://doi.org/10.1038/s44460-026-00082-8
Image Credits: AI Generated
DOI: 10.1038/s44460-026-00082-8
Keywords: Wearable sensors, vascular morphology, thermal sensing, haemodynamic monitoring, thermal relaxation time, vessel depth, vessel diameter, biomedical engineering, ultrasound comparison, personalized health monitoring.
