Wearable Optical Fiber Sensors in Medical Monitoring Applications: A Review

1 Heilongjiang Province Key Laboratory of Laser Spectroscopy Technology and Application, Harbin University of Science and Technology, Harbin 150080, China; moc.361@ko99_hxz (X.Z.); moc.kooltuo@krow.gnaynuhcgnaw (C.W.); nc.ude.tsubrh@oow (H.W.)

Chunyang Wang

1 Heilongjiang Province Key Laboratory of Laser Spectroscopy Technology and Application, Harbin University of Science and Technology, Harbin 150080, China; moc.361@ko99_hxz (X.Z.); moc.kooltuo@krow.gnaynuhcgnaw (C.W.); nc.ude.tsubrh@oow (H.W.)

Tong Zheng

2 School of Artificial Intelligence, Beijing Technology and Business University, Beijing 100048, China; nc.ude.ubtb@60211202

Haibin Wu

1 Heilongjiang Province Key Laboratory of Laser Spectroscopy Technology and Application, Harbin University of Science and Technology, Harbin 150080, China; moc.361@ko99_hxz (X.Z.); moc.kooltuo@krow.gnaynuhcgnaw (C.W.); nc.ude.tsubrh@oow (H.W.)

Qing Wu

1 Heilongjiang Province Key Laboratory of Laser Spectroscopy Technology and Application, Harbin University of Science and Technology, Harbin 150080, China; moc.361@ko99_hxz (X.Z.); moc.kooltuo@krow.gnaynuhcgnaw (C.W.); nc.ude.tsubrh@oow (H.W.)

Yunzheng Wang

3 Center for Optics Research and Engineering, Shandong University, Qingdao 266237, China

Peter W. McCarthy, Academic Editor , Zhuofu Liu, Academic Editor , Vincenzo Cascioli, Academic Editor , Lorenzo Scalise, Academic Editor , and Shyqyri Haxha, Academic Editor

1 Heilongjiang Province Key Laboratory of Laser Spectroscopy Technology and Application, Harbin University of Science and Technology, Harbin 150080, China; moc.361@ko99_hxz (X.Z.); moc.kooltuo@krow.gnaynuhcgnaw (C.W.); nc.ude.tsubrh@oow (H.W.)

2 School of Artificial Intelligence, Beijing Technology and Business University, Beijing 100048, China; nc.ude.ubtb@60211202

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Abstract

Wearable optical fiber sensors have great potential for development in medical monitoring. With the increasing demand for compactness, comfort, accuracy, and other features in new medical monitoring devices, the development of wearable optical fiber sensors is increasingly meeting these requirements. This paper reviews the latest evolution of wearable optical fiber sensors in the medical field. Three types of wearable optical fiber sensors are analyzed: wearable optical fiber sensors based on Fiber Bragg grating, wearable optical fiber sensors based on light intensity changes, and wearable optical fiber sensors based on Fabry–Perot interferometry. The innovation of wearable optical fiber sensors in respiration and joint monitoring is introduced in detail, and the main principles of three kinds of wearable optical fiber sensors are summarized. In addition, we discuss their advantages, limitations, directions to improve accuracy and the challenges they face. We also look forward to future development prospects, such as the combination of wireless networks which will change how medical services are provided. Wearable optical fiber sensors offer a viable technology for prospective continuous medical surveillance and will change future medical benefits.

1. Introduction

Optical fiber sensors have been applied in many fields, such as engineering construction monitoring, defense, medicine, industry, and many other fields [1,2]. In agriculture, it can efficiently and quickly detect pesticide residues in farmland on site [3]. Optical fiber sensor has the character electromagnetic interference characteristicsility, small size, flexibility, multiplexing ability, and high accuracy [1]. In addition, because no current is required at the point of measurement, the technology provides the security needed to operate in wet or humid environments, such as sweating or water-based physical therapy procedures [2,4,5,6,7]. These features provide reliable technical support for continuous monitoring devices in physical applications.

Wearable optical fiber sensors, as a specialized category of optical fiber sensors, have garnered increasing attention in medical applications. This is primarily due to their exceptional flexibility, ease of wear, and high precision. These unique features make them highly suitable for continuously monitoring physiological parameters in medical settings [2,4,6,8,9,10,11]. With the continuous improvement of living standards and medical conditions, the life expectancy of human beings continues to rise. An aging population will lead to an increase in geriatric diseases, such as cardiovascular disease [12,13,14,15], stroke [16], Parkinson’s disease [17,18,19], and so on. Young individuals often engage in prolonged sedentary behavior, including extensive overtime work, late-night study sessions, and extended sitting. Unfortunately, adopting improper sitting postures can significantly increase the risk of developing various health issues, such as shoulder ailments [20], neck problems [21], and lower back conditions [22,23,24]. Rehabilitation for some diseases requires continuous monitoring of patient physiological parameters. These conditions have given impetus to the development of medical devices for continuous monitoring.

Rehabilitation for certain conditions necessitates continuous monitoring of patients’ physiological parameters, driving the demand for developing medical devices tailored to continuous monitoring. This focus on continuous monitoring devices has imposed higher requirements on sensors, emphasizing the need for flexibility, compactness, and reliability to align with constant monitoring equipment. This focus on serial monitoring devices has set higher requirements on sensors, emphasizing the need for flexibility, compactness, and reliability to align with continuous monitoring equipment [20,21,23,25]. Portable electrochemical sensors can also be used in biological monitoring [26]. However, traditional sensors are generally electronic or electromechanical, which have low compactness, and need frequent calibration [1,24,27]. In addition, electronic devices are susceptible to electromagnetic interference and have electrical safety problems. By integrating optical fiber sensors into wearable devices, continuous monitoring devices can be designed, resolving the issues above associated with traditional sensors.

Wearable devices based on fiber optic sensors have proven to be effective and comfortable solutions for monitoring vital signs (ankle [23,28,29,30,31,32,33], blood [22,34,35], spine [36], heart rate [22,37,38,39,40], breathing [10,22,41,42,43,44], shoulder and neck [45], joint angle [21,29,30,37,42,46], etc.). The characteristics of optical fibers allow them to be embedded in textiles such as T-shirts and elastic belts for sensing applications, enabling respiratory monitoring [47]. Wearable optical fiber sensors based on fiber Bragg gratings (FBG) can achieve an average error of 0.33% in respiratory monitoring, but they tend to be more expensive. On the other hand, wearable optical fiber sensors based on changes in light intensity have a more straightforward structure and lower cost but slightly lower accuracy in respiratory monitoring. They can also be integrated into elastic gloves to monitor finger joint angles for human–machine interaction [48]. In the field of motion analysis, wearable optical fiber sensors have been applied to monitor joint arches and gait [1]. Wearable optical fiber sensors based on Fabry–Perot interferometry (FPI) have achieved an accuracy of 0.0296 ± 0.001 mw/° in gait analysis. These wearable devices do not interfere with patients’ normal daily life or limit their range of movement. Accuracy of such physical enablers must always be guaranteed, as they are responsible for the overall performance of the wearable device and the data provided by medical staff for diagnosis. Through the wearable fiber optic sensor, the function of fiber optic technology can reach any part of the body and continuously monitor the physiological parameters of the human body [2,22,49,50,51,52,53,54,55]. The use of wearable devices, combined with recent advances in wireless technology, is expected to provide an appropriate solution for remote physical rehabilitation, allowing patients to perform the controlled therapeutic exercise from the comfort of their homes and possibly under constant remote supervision by a doctor or medical staff [23]. In addition, such solutions can be used continuously in daily life to monitor the progress of rehabilitation in daily activities and to continuously monitor disease conditions for prevention. The solution is expected to significantly reduce the influx of patients into hospitals and medical centers [1,6,10,22,56,57,58,59,60]. Therefore, reducing the costs associated with such physical therapy. In addition, storing medical data for further analysis helps to understand a patient’s case and health trends within a particular community [57,61,62,63]. This will significantly improve the level of social public medical care.

This article explores the various applications of wearable optical fiber sensors in medical monitoring. It provides a detailed introduction to the applications of wearable optical fiber sensors in respiratory monitor and joint monitoring, comparing and summarizing the working principles of three f wearable optical fiber sensors. Additionally, it discusses methods to improve accuracy in respiratory monitoring and joint monitoring, as well as the performance optimization of encapsulation materials. The paper begins by discussing the working principle of wearable optical fiber sensors based on fiber Bragg gratings and their wide-ranging applications in the medical field. It then provides an overview of fiber sensors utilizing changes in light intensity and examines their applications in healthcare. Finally, it discusses the working principle of fiber sensors based on Fabry–Perot interferometry and its application in the medical field. Wearable optical fiber sensors can revolutionize the future of medical consultations by enabling continuous monitoring of patients’ physiological parameters through wearable medical devices, eliminating the need for frequent hospital visits for check-ups. These devices are equipped with easily wearable optical fiber sensors, allowing patients to conveniently obtain vital physiological information without leaving their homes. By expediting patients’ recovery and fostering significant improvements in the healthcare environment, these sensors play a crucial role.

2. Wearable Fiber Bragg Grating Sensors

FBG [64] sensor is a thin fiber bundle composed of a fiber core, cladding layer, and buffer layer, and the fiber core is uniformly engraved with a lattice pattern. The carved lattice pattern reflects only specific wavelengths when the core emits broadband light. This reflection wavelength is called the Bragg wavelength and represents the relationship between the effective refractive index of the fiber core and the distance from one lattice to the next [36,65,66].

where λ is the Bragg wavelength, n is the effective refractive index, and Λ is the grating period. When the measured physical quantity (such as temperature, stress, etc.) Used for fiber grating changes, it will cause the corresponding change of n and Λ, resulting in λ drift and, conversely, by detecting the drift of λ [67]. Information about the measured physical quantity can also be obtained. Research on Bragg fiber grating sensors mainly focuses on quasi-distributed measurement of temperature and stress. Λ shift caused by temperature and stress changes can be expressed as: