# Blood Pressure Monitoring Based on Flexible Encapsulated Sensors
## 1 Introduction  
Hypertension is a common chronic disease that gradually causes irreversible damage to the heart, brain, kidneys, and other organs of affected individuals. Currently, there are 245 million hypertensive patients in China, but the awareness, treatment, and control rates among patients are only 51.6%, 45.8%, and 16.8%, respectively [1]. Hypertension is difficult to detect, develops rapidly, and has a high mortality rate, requiring long-term and continuous blood pressure monitoring for disease tracking and treatment [2, 3, 4]. Continuous blood pressure measurement requires high accuracy and sensitivity from the sensor. Pressure sensors are known for their high accuracy and good sensitivity, and they are not affected by changes in light intensity. However, traditional pressure sensor chips are typically made of rigid materials such as a metal or semiconductor. Prolonged wearing of these sensors can lead to corrosion from sweat, which affects measurement accuracy and may even result in damage to the sensor chip. Based on the above situation, [PERSON] et al. discovered that Mxene/rGO/P(VDF-TrFE) hybrid material exhibits excellent electrical performance and mechanical strength. Therefore, they fabricated a flexible sensor based on this material [5]. Furthermore, scholars have utilized polydimethylsiloxane (PDMS) as a flexible material for sensor fabrication. Among them, Sylgard 184 PDMS has been widely employed in the field of chip technology [6, 7, 8, 9, 10, 11]. This material exhibits excellent biocompatibility; and its stiffness, elasticity, and other properties are close to those of human skin [12, 13]. [PERSON] et al. fabricated high-performance flexible sensors using deionized water (DIW) and PDMS [14]. [PERSON] et al. developed pyramid microstructured sensors based on PDMS [15]. [PERSON] et al. prepared a flexible dual interdigital electrode sensor usingPDMS, which exhibited high sensitivity, stretchability, a large measurement range, and outstanding stability [16]. Some researchers have found that poly-para-xylylene (parylene) possesses excellent corrosion resistance, moisture barrier, biocompatibility, and thermal stability [17; 18; 19; 20]. This material is widely utilized in fields such as biology and healthcare. For instance, in references [21; 22], parylene was employed to encapsulate bioelectrodes. References [23; 24; 25] applied parylene in retinal transplantation surgeries. [PERSON] et al. used parylene for the water sealing of micro-electromechanical system (MEMS) sensors and achieved devices with excellent waterproofing performance and high sensitivity [26]. [PERSON] et al. summarized the applications of parylene in MEMS [27]. Continuous blood pressure measurement methods can be classified as invasive or noninvasive, with invasive measurement being the gold standard but requiring high operational standards and potentially causing infections, bleeding, blood slots, and other complications [28]. The noninvasive continuous blood pressure monitoring methods mainly include the arterial tonometry method and the pulse wave parameter method, among others [29; 30]. Of these methods, pulse wave parameter measurement extracts feature parameters from pulse waveforms, uses regression analysis to establish the relationship between blood pressure and pulse wave, and thus realizes continuous noninvasive blood pressure measurement [31; 32]. Many products on the market now use photoplethysmography sensors to collect pulse signals and obtain pulse wave feature parameters to calculate blood pressure values [33], such as the Mi bracelet and HUAWEI smartwatch. Photoplethysmography sensors are small, highly integrated, easy to use and flexible, and can be integrated into portable devices to enable wearable blood pressure measurement. However, photoplethysmography sensors are sensitive to changes in light intensity, skin color, and sweat, which can lead to poor device accuracy based on this monitoring principle, making it difficult to meet medical standards (for example, AAMI standards) [34; 35; 36].