Continuous monitoring of blood pressure (BP) is vital in the diagnosis and prevention of cardiovascular diseases (CVDs), which are the leading causes of death worldwide. In clinical settings, the gold standard for continuous BP measurement involves implanting an invasive fiber-based pressure sensor into the artery's center. However, this approach poses risks of patient discomfort, infections, and is too invasive for routine inspections and daily monitoring. Recent advances in flexible electronics and artificial intelligence have paved the way for noninvasive continuous BP monitoring using wearable devices. Despite these advances, current wearable devices suffer from either bulky signal processing instruments (Bio-impedance based [1] and Ultrasound [2] based devices), or poor interfacial instability (photoplethysmography [3] based and pressure sensor-based [4] devices), significantly compromising their wearability and stability. Additionally, existing wearable devices depend on simplified theoretical models or feature-guided data models to convert pulse waves into continuous BP readings, resulting in frequent recalibration and long-term accuracy issues. Consequently, achieving accurate BP measurement with a real wearable device, along with a robust algorithm to convert the measured pulse wave into continuous BP waveforms, remains a significant challenge.

Taking these factors into account, we have developed a comprehensive solution consisting of materials, devices, mechanical designs, data processing methods, and integration strategies for a thin, soft, and miniaturized system known as TSMS (Fig. 1). The TSMS comprises three distinct subsystems: a conformal sensing unit designed to effectively detect skin vibrations resulting from artery deformation, an active pressure adaption unit to improve interfacial stability, and a signal processing/transmission unit responsible for signal preprocessing and wireless transmission.

Figure 1 | Working principle and layouts of the wearable wireless continuous blood pressure monitoring system. a Schematic diagram of signal conversion from piezo response to continuous blood pressure that is presented in a mobile GUI. Physical distance between two sampling sites and time difference in two sensing units were utilized to calculate localized PWV. Pulse wave features, together with localized PWV were transmitted to data for the estimation of beat-to-beat BP. b Explosive view of the wireless wristband, with three subsystems, sensing module, active pressure adaption module and signal processing module. 

One of the primary challenges we encountered during the development of TSMS was how to enhance interfacial robustness while ensuring a positive user experience. Taking inspiration from pulse diagnosis in Traditional Chinese Medicine, we discovered that applying pressure to the sensor with a finger can significantly improve signal quality, resulting in increased amplitude and a higher signal-to-noise ratio. However, excessive pressure resembling that of a cuff caused discomfort to users by impeding artery flow. To address this, we designed micro-airbags to precisely apply backpressure to the sensor units, thereby improving interfacial stability. While the micro-airbags provided valuable support to the sensor units, maintaining optimal pressure levels within the airbags posed a key challenge. Conventionally, a close-looped pressure control between the airbag and the pump is preferred for precise pressure control, which, however, will greatly increase the system bulkiness and power consumption, and thus is not suitable for wearable system. To overcome this, we developed a soft silicone one-way valve to regulate the pressure levels within the airbags while minimizing system complexity and power consumption. Furthermore, we recognized that excessive backpressure could lead to artery closure and user discomfort. Therefore, we proposed a closed-loop multi-pumping phase control strategy to strike a balance between signal quality and user comfort. The demonstration on brachial artery and radio artery under a set of joint angles prove its interfacial robustness. Additionally, a pilot study involving volunteers with different body mass index (BMI) indicated that excellent pulse signal was captured without any discomfort caused to users. 

On the backend, the pretrained data model is responsible for continuously converting the measured pulse wave into continuous BP data. However, current algorithm suffers from poor generalizability and long-term stability. To address these issues, various algorithms have incorporated Pulse Wave Velocity (PWV), that are closely associated with the elastic properties of arteries, to improve estimation accuracy and long-term stability. However, such approaches often sacrifice system wearability due to the requirement for long sampling segments. To overcome this limitation, we adopted local PWV, that calculates PWV in a short segment to improve system wearability, which, in turn, places high demands on sampling rate and data transmission. To tackle this challenge, we proposed a preprocessing strategy in collaboration with a resampling process, to achieve a high sampling rate and low-power wireless data transmission. This strategy allows us to strike a balance between accurate measurements and energy efficiency, ensuring the viability of the wearable system. 

With all these issues addressed, the real wearable TSMS was realized, with all the system components encapsulated in a silicone wristband that weighs only 50 g, and as thin as 4 mm. Upon its implementation, we performed rigorous experiments to study its improvement on interfacial stability against a set of body movements, and overall measurement accuracy on a total number of 87 volunteers. The experimental results indicate that the TSMS demonstrates excellent interface anti-interference capability and measurement accuracy. We envision that the TSMS could play a crucial role in realizing precise BP control in hypertension individuals and cardiovascular disease prevention through continuous BP monitoring.

This study entitled "Thin, soft, wearable system for continuous wireless monitoring of artery blood pressure" has been published in Nature Communications (https://doi.org/10.1038/s41467-023-40763-3).

References

1. Kireev, D. et al. Continuous cuffless monitoring of arterial blood pressure via graphene bioimpedance tattoos. Nat. Nanotechnol. 1–7 (2022) doi:10.1038/s41565-022-01145-w.
2. Wang, C. et al. Monitoring of the central blood pressure waveform via a conformal ultrasonic device. Nat. Biomed. Eng. 2, 687–695 (2018).
3. Elgendi, M. et al. The use of photoplethysmography for assessing hypertension. Npj Digit. Med. 2, 1–11 (2019).
4. Yi, Z. et al. Piezoelectric Dynamics of Arterial Pulse for Wearable Continuous Blood Pressure Monitoring. Adv. Mater. 34, 2110291 (2022).