In traditional medical physics and imaging applications, commonly used imaging modalities for physiologic indicators of the human body include X-radiation(X-ray) imaging [1], Computed Tomography(CT) [2], ultrasound imaging [3], and Magnetic Resonance Imaging(MRI) [4]. X-rays and CTs are radioactive and prolonged and frequent measurements increase the risk of cancer. Ultrasound imaging relies too much on manual manipulation by healthcare professionals, which is inflexible, does not allow for long-term monitoring and leads to a poor patient experience. MRI is costly, time-consuming to detect, and is not indicated for patients with metal implants. Wearable piezoelectric sensors can noninvasively monitor the structure and function of deep tissues [5], obtain more comprehensive information about human physiology, and more accurately monitor early lesions, especially blood pressure (BP), a physiological indicator that requires long-term monitoring, which can help to achieve early observation and intervention of hypertension.

Hypertension is a common chronic disease worldwide, and accurate BP measurement is essential for the diagnosis, treatment and management of the disease. The traditional BP measurement tools of auscultation and oscillometric methods have certain limitations, such as not being able to provide continuous and dynamic BP information and requiring specific measurement environments and equipment [6]. Ordinary wearable sensors detect physical quantities (e.g., strain, pressure, temperature, etc.) on the surface of the body based on changes in resistance, capacitance, or inductance, thereby detecting physiological signals from the human epidermis, but do not include imaging technology [7]. Wearable piezoelectric sensors have noninvasive imaging capabilities that allow rapid acquisition of high-resolution images of internal tissue structure and morphology for fast and accurate diagnosis [8]. Wearable piezoelectric sensors, based on the versatility of piezoelectric materials, are capable of deep tissue imaging, blood flow measurements, and BP high and low measurements, which provide a better option for carrying out routine BP measurement imaging. Therefore, it is crucial to investigate high-performance wearable piezoelectric sensors. In this paper, we introduce the structure and working principle of wearable piezoelectric sensors, classify their piezoelectric materials, and discuss the research on the application of wearable piezoelectric sensors in BP measurement. Finally, we look at the prospects and challenges of wearable piezoelectric sensors for BP measurement.

As shown in Figure 1A, wearable piezoelectric sensors measure BP based on the piezoelectric effect. The basic principle is that when the arterial BP of the human body changes, it creates strain on the skin’s surface. The piezoelectric material within the piezoelectric sensor that is right next to the skin generates electrical signals related to pressure when pressurized. These electrical signals, along with a clinically calibrated model or algorithm for converting them into BP as shown in Figure 1A(i), enable the calculation and display of BP values like systolic and diastolic BP [9]. The overall measurement process is illustrated in Figure 1B. The basic structure of the wearable piezoelectric sensor is an encapsulation layer composed of polydimethylsiloxane, a piezoelectric sandwich layer of lead zirconate titanate fork finger electrodes, a substrate layer composed of polyethylene terephthalate, and some adhesive materials, as shown in Figure 1A(ii). Min et al. designed a wearable piezoelectric BP sensor (WPBPS) based on this principle, achieving a high normalized sensitivity (0.062 kPa^-1) and a fast response (23 m) for Continuous Non-invasive Arterial Pressure (CNAP) monitoring [9]. They screened 35 subjects aged 20–80 years for a clinical trial and found mean differences in systolic (SBP) and diastolic (DBP) BP of only −0.89 ± 6.19 and −0.32 ± 5.28 mmHg.

For now, wearable piezoelectric sensors for BP measurement also suffer from the lack of a unified conversion model, uncertainty in arterial pressure transfer, and coupling of multiple physical fields (e.g., the effect of temperature fields, interference from noise fields). The key to solving these problems needs to start with the sensor itself, whose performance is directly determined by the piezoelectric material. This is because the piezoelectric effect of piezoelectric materials is itself a complex physical phenomenon, the charge generated by the piezoelectric material and the applied pressure is not exactly linear, and there are differences in the correspondence between different materials. Furthermore, in addition to the positive piezoelectric effect, piezoelectric materials also have an inverse piezoelectric effect, which causes the crystal of the piezoelectric material to be mechanically deformed by the alternating electric field, thus affecting the results of BP measurements [10]. Therefore, optimising the properties of piezoelectric materials is one of the popular research directions for the future of wearable piezoelectric sensors in the future.

Piezoelectric materials are the core of piezoelectric transducers, and their performance directly determines the sensitivity and accuracy of the transducers. Categorizing piezoelectric materials can show the advantages and disadvantages of various sensors more clearly and point out the direction for the future development of wearable piezoelectric sensors.

At present, the application of wearable piezoelectric sensors to measure BP has made some progress. In terms of technical performance, some sensors have achieved high sensitivity and rapid response, and clinical verification shows that their measurement results are similar to those of traditional BP monitors. The relationship between the sensor and BP based on the arterial pulse piezoelectric dynamics characterization can be explained by the arterial pulse piezoelectric dynamics equation, P ∼ Ψ V + ∫ 0 t V d τ + P * ( P : blood pressure value; Ψ : parameters related to arterial wall characteristics; V : the flow rate of blood through the arteries; P * : t = 0 basal blood pressure value). The pressure wave generated by the arterial pulse is sensed by the piezoelectric sensor near the skin surface. The piezoelectric material releases different electrical signals depending on the amount of pressure, which are amplified and converted to show different blood pressure values. Yi et al. proposed a new method for analyzing human arterial pulse piezoelectric dynamics from the basis of human hemodynamics and flexible piezoelectric dynamics, revealing a well-established mapping correlation between piezoelectric pulse waves and human BP waves, and also developed a piezoelectric sensor for continuous BP monitoring in a wireless wearable, with a measurement schematic shown in Figure 2A [48]. However, piezoelectric sensors based on arterial pulse piezoelectric dynamics are susceptible to interference from muscle movement, skin displacement, and other factors that produce motion artefacts and lead to inaccurate BP measurements, and no study has yet emerged that completely eliminates this effect.

Wearable multi-sensor BP monitoring system integrates a variety of sensors, such as electrocardiogram (ECG) sensor, photoelectric volume pulse wave (PPG) sensor, accelerometer, etc. The use of multi-parameter fusion data can be obtained, which can more accurately monitor BP. For example, Yan et al. developed an information acquisition platform containing ECG, PPG and pressure pulse sensors for non-invasive blood pressure prediction, designed an algorithm for sample data processing, and constructed a BP prediction model using feature selection and feature fusion techniques of random forest regression (RFR) [49]. Finally, the RFR-based blood pressure prediction results were compared with those of other machine learning algorithms. The results show that the average absolute error of systolic blood pressure and diastolic blood pressure can reach 0.90 mmHg and 2.47 mmHg respectively. The results of BP prediction model based on multi-sensor information fusion meet the AAMI and BHS standards. Although multiple sensors can obtain rich data, the data of different sensors are different in time scale, data format, signal strength and other aspects, so it is a major challenge for current research to effectively fuse the data of different dimensions with appropriate fusion algorithms.

Piezoelectric sensors based on pulse wave conduction time (PTT) detection have the advantages of good measurement stability and high accuracy, but usually require two discrete sensors to acquire pulse and ECG waves, resulting in poor wearing experience, circuit design and signal processing complex, dual-mode sensors for blood pressure monitoring can solve this problem. Jiang et al. designed a dual-mode piezoelectric sensor consisting of an ionic gel membrane and an embedded liquid metal (LM) circuit. Ionic gel can be applied to skin to collect ECG signal and used as encapsulation material of LM circuit [50]. The LM circuit can respond to pressure changes, and its resistance changes with pressure changes, so as to detect pulse waves. By analyzing the time difference between ECG and pulse signals, PTT is obtained to predict blood pressure. They have also integrated a dual-mode sensor into a printed circuit board, worn on the wrist, and a Bluetooth-based wireless signal transmission that can simultaneously capture electrocardiograms and arterial pulses (Figure 2B). This design not only reduces the number of wearable piezoelectric sensors, but also realizes its connection with smart phones, which is a mainstream direction for the future development of wearable piezoelectric sensors.

Piezoelectric ultrasound sensors can generate and receive ultrasound waves using the piezoelectric effect of piezoelectric materials, and then detect parameters such as arterial dilatation and Young’s modulus by measuring the propagation time of the ultrasound waves and the properties of the reflecting material, as well as performing BP estimation [51]. These piezoelectric sensors based on acoustic principles are biologically safe, non-invasive and capable of deep tissue BP observation. The limitations of existing noninvasive BP monitoring methods (e.g., photoelectric volumetric tracing and ocular tonometry, etc., which can only detect superficial peripheral vasculature) can be overcome [52, 53]. Li et al. produced a fabric-based ultrasound transducer for continuous noninvasive monitoring of BP waveforms by combining “S”-shaped stretchable island-bridge electrodes with a fabric substrate using thermocompression transfer printing. The results showed that the 4 × 5 array ultrasound transducer could localize arterioles deep in the tissue with an axial resolution of up to 330 μm, and the “S”-shaped electrodes had an elongation at break of up to 35.6% [54]. However, the biggest problem of piezoelectric ultrasonic sensors at present is that their measurement process involves multiple links such as filtering, amplifying and feature extraction of ultrasonic signals, which has high requirements on data processing algorithms and hardware equipment, and individual signal characteristics may also be different, so it is necessary to further optimize and adjust data processing methods to meet the measurement needs of different populations.

This paper introduces the structure and working principle of wearable piezoelectric sensors, summarizes the advantages and disadvantages of inorganic piezoelectric materials, organic piezoelectric materials and composite piezoelectric materials for blood pressure measurement, and discusses the unique characteristics and limitations of wearable piezoelectric sensors in practical cases of blood pressure measurement. (1) In the face of problems such as susceptibility to interference, wearable piezoelectric sensors can be developed by cooperating with material scientists to develop high-performance materials and optimize the material preparation process to meet the high-performance requirements of wearable medical devices for sensor materials. It can also be combined with electronic information engineering and artificial intelligence technology to realize the intellectualization and networking of piezoelectric sensors, and improve the data processing capacity and transmission speed. In cooperation with medical researchers, wearable piezoelectric sensors for physiological signal monitoring are developed to achieve long-term, real-time and accurate monitoring of human physiological parameters, providing a basis for early diagnosis and treatment of diseases. (2) Wearable devices usually need a long-term stable energy supply, and the traditional battery-powered way has problems such as large volume, heavy weight, and short battery life, which limits the convenience and flexibility of wearable piezoelectric sensors. The development of self-powered wearable piezoelectric sensors, which can be converted into electrical energy by collecting human movement and mechanical energy in the environment, is a promising direction in future research. (3) The current research on piezoelectric sensor BP measurement is mostly in the laboratory stage, and cannot be put into production on a large scale, but in the future, the data collected by the sensor can be combined with algorithms, and at the same time, fused with big data to carry out statistical analysis, to achieve intelligent detection, which is expected to accelerate the marketization of piezoelectric sensors. In the future, with the continuous development of materials science, medical physics, artificial intelligence and other multidisciplinary development, piezoelectric sensors are expected to achieve multifunctional integration, personalized customization, and intelligent automation in BP measurement, to better meet people’s needs for daily health monitoring.