Mechanical Sensors for Cardiovascular Monitoring: From Battery-Powered to Self-Powered

Abstract

Cardiovascular disease is one of the leading causes of death worldwide. Long-term and real-time monitoring of cardiovascular indicators is required to detect abnormalities and conduct early intervention in time. To this end, the development of flexible wearable/implantable sensors for real-time monitoring of various vital signs has aroused extensive interest among researchers. Among the different kinds of sensors, mechanical sensors can reflect the direct information of pressure fluctuations in the cardiovascular system with the advantages of high sensitivity and suitable flexibility. Herein, we first introduce the recent advances of four kinds of mechanical sensors for cardiovascular system monitoring, based on capacitive, piezoresistive, piezoelectric, and triboelectric principles. Then, the physio-mechanical mechanisms in the cardiovascular system and their monitoring are described, including pulse wave, blood pressure, heart rhythm, endocardial pressure, etc. Finally, we emphasize the importance of real-time physiological monitoring in the treatment of cardiovascular disease and discuss its challenges in clinical translation.

Keywords: blood pressure; cardiac output; cardiovascular disease; endocardial pressure; heart rhythm; mechanical sensors; pulse wave.

Figure 1

Schematic illustration of four sensing mechanisms: (A) Piezoresistivity; (B) Capacitance; (C) Piezoelectricity; (D) Triboelectricity.

Figure 2

(A) The waveforms of aortic blood pressure versus ventricular and atrial blood pressure. (B) Cardiac cycle: the energy released by ventricular systole can be divided into two main components: the kinetic energy (E_K) that drives the rapid flow of blood and the potential energy (E_P) stored in the vascular wall.

Figure 3

(A) Comparison of normal and distorted signals due to sensor misalignment in epidermal pulse measurement. Reprinted/adapted with permission from Ref. [38]. (B) Sensor arrays for locating arteries and acquiring accurate pulse waveforms. Reprinted/adapted with permission from Ref. [39]. (C) Ultrathin flexible PZT pressure sensor and the piezoelectric potential distribution, the inset indicates a cross-sectional SEM image (scale bar, 5 µm). Reprinted/adapted with permission from Ref. [40]. (D) Schematic diagram of self-powered ultrasensitive pulse sensor (SUPS) and its working principle. Reprinted/adapted with permission from Ref. [41]. (E) Schematic illustration of the ultrathin and flexible sensor (UFS) and its application for finger-touching measurement in single-electrode mode. Reprinted/adapted with permission from Ref. [42].

Figure 4

(A) The pressure transmission mechanism. Upper (PWA): The correlation between blood pressure waveform and arterial pulse piezoelectric response. Down (PTT): Time difference between two arterial pulse sensors to obtain blood pressure. Reprinted/adapted with permission from Ref. [46]. (B) Diagram illustrating a self-powered textile-based triboelectric sensor. The collected signal can be transmitted wirelessly to mobile phones, and the architecture of the supervised feedforward neural network is used for blood pressure prediction. Reprinted/adapted with permission from Ref. [47]. (C) Brachial–fingertip PTT and PWV monitoring via the self-powered ultrasensitive pulse sensors (SUPSs). Reprinted/adapted with permission from Ref. [48]. (D) A sensor system simultaneously monitors the pulse waves from the human fingertips and ears to read the participants’ PWV and BP values in real time. Reprinted/adapted with permission from Ref. [49].

Figure 5

(A) Schematic showing the response of the piezoelectric effect to blood pressure in artificial artery. Reprinted/adapted with permission from Ref. [57]. (B) An implantable, self-powered, and visualized BP monitoring system. Diagram illustrating the circumferential stress in the expanding aortic wall, inducing the charge distribution of the piezoelectric sensor. Reprinted/adapted with permission from Ref. [61]. (C) Diagram of implantable PVDF/DA NF−based sensor for monitoring subtle mechanical pressure changes in vivo. Reprinted/adapted with permission from Ref. [63]. (D) Schematic of the bioabsorbable triboelectric sensor (BTS) monitoring physiological signals in vivo. Reprinted/adapted with permission from Ref. [64].

Figure 6

(A) Schematic of the i−NG system placed in pig, showing the voltage output during normal heartbeat and heart attack by blocking the coronary arteries causing ischemia. Reprinted/adapted with permission from Ref. [68]. (B) Schematic diagram of the mechanism for monitoring blood flow velocity by iTEAS. The red arrows indicate the direction of blood flow. Reprinted/adapted with permission from Ref. [69]. (C) The comparison of working signals from ECG, FAP, and SEPS in different states and the SEPS implanted into an Adult Yorkshire swine’s heart via minimally invasive surgery. Reprinted/adapted with permission from Ref. [67]. (D) Diagram showing a multifunctional pacemaker lead that combines energy harvesting and sensing strategies. Reprinted/adapted with permission from Ref. [70]. (E) Schematic illustration of the implantable piezoelectric generator (iPEG). Reprinted/adapted with permission from Ref. [71].

Figure 7

(A) Schematic illustration of the calibration methods for cardiac output monitoring. (B) Demonstration of the DMP−Life system and Bland–Altman and Pearson’s correlation plots. Reprinted/adapted with permission from Ref. [78].