Recent Advances in Materials and Flexible Sensors for Arrhythmia Detection

Abstract

Arrhythmias are one of the leading causes of death in the United States, and their early detection is essential for patient wellness. However, traditional arrhythmia diagnosis by expert evaluation from intermittent clinical examinations is time-consuming and often lacks quantitative data. Modern wearable sensors and machine learning algorithms have attempted to alleviate this problem by providing continuous monitoring and real-time arrhythmia detection. However, current devices are still largely limited by the fundamental mismatch between skin and sensor, giving way to motion artifacts. Additionally, the desirable qualities of flexibility, robustness, breathability, adhesiveness, stretchability, and durability cannot all be met at once. Flexible sensors have improved upon the current clinical arrhythmia detection methods by following the topography of skin and reducing the natural interface mismatch between cardiac monitoring sensors and human skin. Flexible bioelectric, optoelectronic, ultrasonic, and mechanoelectrical sensors have been demonstrated to provide essential information about heart-rate variability, which is crucial in detecting and classifying arrhythmias. In this review, we analyze the current trends in flexible wearable sensors for cardiac monitoring and the efficacy of these devices for arrhythmia detection.

Keywords: arrhythmia detection; cardiovascular monitoring; flexible electronics; soft biosensors; wearable sensors.

Figure 1

Examples of flexible sensors and functions for accurate arrhythmia detection [13,14,15,16]. (Figures are adapted or reprinted, clockwise, from the top–left: (1) Sensors Actuators A Phys. 2018, 272, 92–101, Copyright 2019, Elsevier; (2) Proc. Natl. Acad. Sci. 2018, 115, E11015–E11024, Copyright 2018, National Academy of Sciences; (3) Creative Common License by MDPI; (4) Creative Common License by Wiley.

Figure 2

Electrocardiography. (a) Photo of a skin-conformal electrode. (b) Microneedle array-based ECG. Illustration of (i) traditional Ag/AgCl electrode and (ii) microneedle array electrode. (iii) Photo of a microneedle array electrode. (c) Photo of a stretchable hybrid-electronics device. (d) Photo of a soft strain-isolated bioelectric device. (e) Foil micrograph of a flexible ECG patch.

Figure 3

Photoplethysmography. (a) Photo of an ultra-flexible organic optical sensor. (b) Illustration of polymer LED (i) and organic photodiode (ii) pulse oximeters. (c) Photo of a graphene-based flexible sensor in a heart-rate monitoring bracelet. (d) Photos of CNT-based microelectrodes for a fiber optoelectronic device.

Figure 4

(a) Fabrication processes (left) and photo of a fabricated ultrasonic transducer (right). (b) Photo of a finger-worn SCG sensor. (c) Photo of a skin-like SCG sensor with fibers. (d) Flexible strain sensor for heartbeat monitoring.