Wearable Fiber Optic Technology Based on Smart Textile: a Review

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Review Wearable Fiber Optic Technology Based on Smart Textile: A Review

Zidan Gong 1,* , Ziyang Xiang 1, Xia OuYang 2, Jun Zhang 3, Newman Lau 3 , Jie Zhou 4 and Chi Chiu Chan 1

1 Sino-German College of Intelligent Manufacturing, Shenzhen Technology University, Shenzhen 518118, China; [email protected] (Z.X.); [email protected] (C.C.C.) 2 Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong 999077, China; [email protected] 3 School of Design, The Hong Kong Polytechnic University, Hong Kong 999077, China; [email protected] (J.Z.); [email protected] (N.L.) 4 Apparel & Art Design College, Xi’an Polytechnic University, Xi’an 710048, China; [email protected] * Correspondence: [email protected]; Tel.: +0755-23256330

Received: 9 September 2019; Accepted: 8 October 2019; Published: 11 October 2019

Abstract: Emerging smart textiles have enriched a variety of wearable technologies, including fiber optic technology. Optic fibers are widely applied in communication, sensing, and healthcare, and smart textiles enable fiber optic technology to be worn close to soft and curved human body parts for personalized functions. This review briefly introduces wearable fiber optic applications with various functions, including fashion and esthetics, vital signal monitoring, and disease treatment. The main working principles of side emission, wavelength modulation, and intensity modulation are summarized. In addition, textile fabrication techniques, including weaving and knitting, are discussed and illustrated as combination methods of embedding fiber optic technology into textile fabric. In conclusion, the combination of optical fibers and textiles has drawn considerable interest and developed rapidly. This work provides an overview of textile-based wearable fiber optic technology and discusses potential textile fabrication techniques for further improvement of wearable fiber optic applications.

Keywords: ﬁber optic technology; smart textile; textile fabrication techniques; wearables

1. Introduction Textiles and fibers have been developing over thousands of years with the original function of keeping people warm. Subsequently, people began pursuing fashion and esthetics. Consequently, each historical period has its own textile and apparel characteristics. A new generation of smart textiles has recently emerged along with progress in science, technology, and interdisciplinary fields; these smart textiles enable smart systems to be directly worn on the soft and curved human body [1,2]. With advancements in electronic science and technology in the recent decades, several microelectronic devices have been integrated with textiles for wearable applications; which are equipped with the functions of monitoring, communication, therapy, assistance, and entertainment, among others [3–8]. Continuous explorations in this technical field have created a new market for the wearable technology industry. Many opportunities have also been created, attracting thousands of research institutions and enterprises to develop new technologies and products, including fiber optic technology-based smart textiles. Optical fibers were already known in the 1960s for light transmission but not for signal transmission. In 1966, in the UK, Kao and Hockham used optical fibers for communication in a study that could be a

Materials 2019, 12, 3311; doi:10.3390/ma12203311 www.mdpi.com/journal/materials Materials 2019, 12, 3311 2 of 15 breakthrough in this field. Over time, fiber optic technology has evolved and been gradually applied into textiles in a wearable modality for various implementations, such as communication, display, sensing, and monitoring [9–11]. The fibrous appearance of optical fibers is an advantage over many other devices for application in smart textiles. Optical fibers are quite similar to traditional textile fibers; the former could be ideally processed similarly as standard textile yarns for fabric fabrication, which is the first step to generating smart textile material [12,13]. Optical fibers, especially polymer optical fibers (POFs), are flexible, small-sized, lightweight, durable, cost-effective, and immune to electromagnetic interference. Moreover, fiber optic devices are easily handled with simple connections and have good biocompatibility with the human body. Light transmission or emission could be achieved on any part of a wearable device by integrating optical fibers into suitable textile structures; that is, the functions of fiber optic technology could reach any part of the human body via a wearable modality [1,12,14,15]. These features make optical fibers an ideal material to be embedded into textile structural composites. Today, fiber optic technology not only be exploited for light and signal transmission but also be widely utilized in sensors for good metrological properties [16]. Textile techniques enable such sensors to be wearable, while simultaneously monitoring physiological parameters (e.g., heartbeat and respiratory rate) by measuring mechanical variables [15,17–22]. This work aims to briefly introduce wearable fiber optic applications with various functions, present the main working principles, and discuss certain textile techniques that enable optical fibers to be wearable.

2. Wearable Fiber Optic Technology for New Fashion In recent decades, the use of optical fibers has increasingly expanded to wearable technologies with a wide range of functions. Driven by the demand from young and fashion-conscious consumers for unique apparel, optical fibers have been successfully embedded into textiles for illumination [23]. Unlike traditional optical fibers for signal transmission (light is reflected inside the core, and light is emitted at the end of the fiber) (Figure1a), optical fibers processed for wearable illuminating apparel have micro perforations on the lateral side passing through the cladding to the core, as presented in Figure1b. Light leakage then occurs because light scatters at the perforations, thereby enabling light emission on the fiber surface. [9] The alternative is to achieve the light emission by macro-bending of the optical fiber as shown in Figure1c where the light propagation angle α is more than critical angle αC thus to emit part of the light out of the fiber [23]. Numerous wearable products with attractive and changing colors have been developed by applying this lateral light emission principle of optical fibers. A flexible fiber optic display technology was developed by Koncar (Figure2a) on a two-dimensional surface by applying the textile weaving technique [9]. Therefore, an X-Y network of optical fibers could be achieved in a woven structure acting as a fabric display screen. The display matrix was designed such that different light sources are connected by optical fibers embedded in each designed surface unit. This developed fabric display may have great potential not only in fashion but also in information display, communication, and entertainment. In addition, on the basis of the same working principle, Shenzhen Fashion Luminous Clothing Co., Ltd. (China) has commercialized this fiber optic illuminating technology by fabricating a series of wearable clothing (e.g., jackets, dresses, and underwear) and accessories (e.g., masks, hats, ties, and bags), as presented in Figure2b [ 24]. Jacquard manufacturer Se-yang Textile (Korea) has developed a similar product that can display changing patterns with variable motions and illumination levels [25]. The colors of the designed luminous fiber optic fabrics could be changed because the original color of the emitted light at the micro perforations could be mixed with the color of added irradiated guided light or reflected ambient light to present a new color [23,26]. Materials 2019, 12, x FOR PEER REVIEW 2 of 16

communication, display, sensing, and monitoring [9–11]. The fibrous appearance of optical fibers is an advantage over many other devices for application in smart textiles. Optical fibers are quite similar to traditional textile fibers; the former could be ideally processed similarly as standard textile yarns for fabric fabrication, which is the first step to generating smart textile material [12,13]. Optical fibers, especially polymer optical fibers (POFs), are flexible, small-sized, lightweight, durable, cost-effective, and immune to electromagnetic interference. Moreover, fiber optic devices are easily handled with simple connections and have good biocompatibility with the human body. Light transmission or emission could be achieved on any part of a wearable device by integrating optical fibers into suitable textile structures; that is, the functions of fiber optic technology could reach any part of the human body via a wearable modality [1,12,14,15]. These features make optical fibers an ideal material to be embedded into textile structural composites. Today, fiber optic technology not only be exploited for light and signal transmission but also be widely utilized in sensors for good metrological properties [16]. Textile techniques enable such sensors to be wearable, while simultaneously monitoring physiological parameters (e.g., heartbeat and respiratory rate) by measuring mechanical variables [15,17–22]. This work aims to briefly introduce wearable fiber optic applications with various functions, present the main working principles, and discuss certain textile techniques that enable optical fibers to be wearable.

2. Wearable Fiber Optic Technology for New Fashion In recent decades, the use of optical fibers has increasingly expanded to wearable technologies with a wide range of functions. Driven by the demand from young and fashion-conscious consumers for unique apparel, optical fibers have been successfully embedded into textiles for illumination [23]. Unlike traditional optical fibers for signal transmission (light is reflected inside the core, and light is emitted at the end of the fiber) (Figure 1a), optical fibers processed for wearable illuminating apparel have micro perforations on the lateral side passing through the cladding to the core, as presented in Figure 1b. Light leakage then occurs because light scatters at the perforations, thereby enabling light emission on the fiber surface. [9] The alternative is to achieve the light emission by macro-bending of the optical fiber as shown in Figure 1c where the light propagation angle α is more than critical angle Materials 2019, 12, 3311 3 of 15 훼퐶 thus to emit part of the light out of the fiber [23].

Materials 2019, 12, x FOR PEER REVIEW 3 of 16

Numerous wearable products with attractive and changing colors have been developed by applying this lateral light emission principle of optical fibers. A flexible fiber optic display technology was developed by Koncar (Figure 2a) on a two-dimensional surface by applying the textile weaving technique [9]. Therefore, an X-Y network of optical fibers could be achieved in a woven structure acting as a fabric display screen. The display matrix was designed such that different light sources are connected by optical fibers embedded in each designed surface unit. This developed fabric display may have great potential not only in fashion but also in information display, communication, and entertainment. In addition, on the basis of the same working principle, Shenzhen Fashion Luminous Clothing Co., Ltd. (China) has commercialized this fiber optic illuminating technology by fabricating a series of wearable clothing (e.g., jackets, dresses, and underwear) and accessories (e.g., masks, hats, ties, and bags), as presented in Figure 2b [24]. Jacquard manufacturer Se-yang Textile (Korea) has developed a similar product that can display changing patterns with variable motions Figure 1. Principle of lateral light emission: (a) traditional optical ﬁber; (b) perforation of the cladding; and illuminationFigure 1. Principle levels of [25]lateral. The light colors emission: of the (a )designed traditional lumino optical usfiber; fiber (b) optic perfor fabricsation of could the cladding; be changed (c) macro-bending of the optical ﬁber. because(c) macro the original-bending color of the of optical the emitted fiber. light at the micro perforations could be mixed with the color of added irradiated guided light or reflected ambient light to present a new color [23,26].

FigureFigure 2. 2.Wearable Wearable fiber fiber optic optic illuminating illuminating technology: technology: (a ()a flexible) flexible fabric fabric display; display; [9[9]]( b(b)) luminous luminous fiber fiber opticoptic fabrics fabrics and and sample sample apparel. apparel. [ 24[24]]. .

3. Wearable Fiber Optic Technology for Therapy Light and optical techniques have already been applied in clinical practice for disease treatment, and these methods have brought profound impacts on modern medicine [27]. Light therapy is commonly used for pain relief, tendinopathy injuries, metabolic diseases, and tissue repair by adopting various wavelengths of light [28]. Several fiber optic devices have been integrated with fabric materials and applied close to the human skin with excellent optical and thermal properties in light therapy. Wavelength selection is quite important in light therapy because different light wavelengths have various penetration depths on human tissues, as presented in Figure 3; varying depths of wavelengths are associated with different therapeutic effects. Near-infrared light can influence chromatophores and enhance ATP (Adenosine triphosphate) synthesis in mitochondria to speed up wound healing and stimulate hair growth [29].

Materials 2019, 12, 3311 4 of 15

3. Wearable Fiber Optic Technology for Therapy Light and optical techniques have already been applied in clinical practice for disease treatment, and these methods have brought profound impacts on modern medicine [27]. Light therapy is commonly used for pain relief, tendinopathy injuries, metabolic diseases, and tissue repair by adopting various wavelengths of light [28]. Several fiber optic devices have been integrated with fabric materials and applied close to the human skin with excellent optical and thermal properties in light therapy. Wavelength selection is quite important in light therapy because different light wavelengths have various penetration depths on human tissues, as presented in Figure3; varying depths of wavelengths are associated with different therapeutic effects. Near-infrared light can influence chromatophores and enhance ATP (Adenosine triphosphate) synthesis in mitochondria to speed up wound healing and Materialsstimulate 2019, 12 hair, x FOR growth PEER REVIEW [29]. 4 of 16

Figure 3. Varying depths of light penetration on tissue. Figure 3. Varying depths of light penetration on tissue. Shen et al. developed a textile-based side-emitting polymer optical fiber device by weaving POFs intoShen fabrics et al. developed to emit low-level a textile- redbased and side near-infrared-emitting polymer lights optical (600–950 fiber nm) device for collagenby weaving production POFs intoin fabrics human to fibroblast.emit low-level This red device and near could-infrared continually lights (600 provide–950 stablenm) for optical collagen power production density in and humanoperating fibroblast. temperature This device for 10could h without continually any potential provide stable hazard optical when power in contact density with and human operating skin [30 ]. temperatureAdditionally, for blue-wavelength 10 h without any light potential (430–490 hazard nm) is usuallywhen in adopted contact in with light human therapy s forkin neonatal[30]. Additionally,jaundice. Quandt blue-wavelength et al. designed light a(430 homogeneous–490 nm) is luminoususually adopted textile from in light POF therapy for long-term for neonatal neonatal jaundice.jaundice Quandt treatment. et al. designed This weave a homogeneous production has luminous tries di fftextileerent from arrays POF ofthe for POFslong-term in various neonatal fabric jaundicestructures treatment. to develop This weave a comfortable production and has breathable tries differen fabric;t arrays this fabric of the achieved POFs in positive various treatment fabric structureseffects and to develop could thus a comfortable be used as a and wearable breathable phototherapy fabric; this device fabric to achieved provide simultaneouspositive treatment care for effectsnewborns and could in home thus treatment be used as [31 a]. wearable Additionally, phototherapy textile-based device fiber to optic provide technology simultaneous could also care be for used newbornsfor photodynamic in home tre therapyatment (PDT).[31]. Additionally, Cochrane et al.textile designed-based a fiber textile optic light technology diffuser (TLD) could by usingalso be POF usedand for Polyester photodynamic yarns therapy to flexibly (PDT) and. Cochrane homogeneously et al. designed irradiate a lighttextile on light human diffuser skin (TLD) to activate by using drugs POFand and off Polyesterer therapeutic yarns e fftoects flexibly in dermatology and homogeneously [32]. Due to irradiate the complexities light on ofhuman the human skin to anatomy, activate the drugswearable and offer capability therapeutic and tunability effects in for dermatology different wavelengths [32]. Dueof to fiber the optic complexities technology of wouldthe human make it anatomy,prospective the wearable in medical capabi andlity healthcare and tunability fields. for different wavelengths of fiber optic technology would make it prospective in medical and healthcare fields. 4. Wearable Fiber Optic Sensor for Monitoring 4. Wearable Fiber Optic Sensor for Monitoring 4.1. Applications for Real-time Vital Signal Detection 4.1. Applications for Real-time Vital Signal Detection With the continuous improvement of living standards and the increased pursuit for healthcare, opticalWith the fiber continuous technology improvement has been developed of living intostandards a variety and ofthe wearable increased monitoring pursuit for applications healthcare, to opticalachieve fiber real-time technology sensing has andbeen to developed increase medical into a andvariety healthcare of wearable diagnosis monitoring accuracy. applications [18] Such optical to achievefiber real sensors-time havesensing high and biocompatibility to increase medical and and do healthcare not produce diagnosis heat; theyaccuracy. are also [18] notSuch susceptible optical fiberto sensors magnetic have resonance high biocompatibility and electrical dischargesand do not [ 19produce]. Table heat;1 summarizes they are also the latestnot susceptible wearable fiberto magnetic resonance and electrical discharges [19]. Table 1 summarizes the latest wearable fiber optic monitoring applications. The table shows that respiration is one of the most commonly monitored biological signals. Continuous respiratory activity monitoring, which is closely associated with many diseases, is an essential parameter in healthcare performance evaluation. For instance, respiration- related diseases include not only respiratory disorders, such as sleep apnea, asthma, and sudden infant death syndrome, but also cardiac and psychological diseases (e.g., heart failure and stress- related panic attacks), which are linked to irregular respiration [21,33]. Therefore, real-time human respiratory data should be collected and recorded. Several studies employed flexible sensors made of optical fibers into wearable modalities and placed them on the chest, back, and abdomen to test a series of physiological parameters, including respiratory rate, respiratory period, and inspiratory and expiratory phase durations [15,18,20,21,34]. Some multi-functional optical sensors have been developed for measuring vital biological parameters other than respiration, such as heartbeat and body temperature. For example, Koyama et al. in addition to Yang et al. adopted wearable macro bending optical sensors that detect tiny body vibrations caused by heartbeat and respiration to record both respiratory and cardiac activities [19,22]. Li et al. and Fajkus et al. developed wearable fiber

Materials 2019, 12, 3311 5 of 15 optic monitoring applications. The table shows that respiration is one of the most commonly monitored biological signals. Continuous respiratory activity monitoring, which is closely associated with many diseases, is an essential parameter in healthcare performance evaluation. For instance, respiration-related diseases include not only respiratory disorders, such as sleep apnea, asthma, and sudden infant death syndrome, but also cardiac and psychological diseases (e.g., heart failure and stress-related panic attacks), which are linked to irregular respiration [21,33]. Therefore, real-time human respiratory data should be collected and recorded. Several studies employed flexible sensors made of optical fibers into wearable modalities and placed them on the chest, back, and abdomen to test a series of physiological parameters, including respiratory rate, respiratory period, and inspiratory and expiratory phase durations [15,18,20,21,34]. Some multi-functional optical sensors have been developed for measuring vital biological parameters other than respiration, such as heartbeat and body temperature. For example, Koyama et al. in addition to Yang et al. adopted wearable macro bending optical sensors that detect tiny body vibrations caused by heartbeat and respiration to record both respiratory and cardiac activities [19,22]. Li et al. and Fajkus et al. developed wearable fiber Bragg grating (FBG) sensor-based devices that are to be placed on the chest to measure body temperature [16,17]. In addition, the function of real-time monitoring is not limited to the trunk of the human body; vital signal could also be sensed on other parts of the body to present different healthcare statuses. For instance, Najafi and his colleagues embedded optical fiber sensors into socks for assessing plantar pressure and temperature in clinical trials (these parameters are predisposing factors for foot ulcers in patients with diabetic peripheral neuropathy) to manage the biomechanical risk factors of diabetic foot disease [35]. Moreover, Arnaldo et al. applied polymer optical fiber sensors in foot-related wearable devices for gait event detection and joint angle measurement for assistance and rehabilitation [36]. However, despite the variety of wearable applications that have been developed for real-time vital signal detection, the integration method of fiber optic technology and textiles for wearable modalities needs further refinement. Table1 shows that in most cases, fiber optic elements are embedded into polydimethylsiloxane (PDMS) and glued by polymeric glue or integrated onto an elastic substrate that is attached directly on a wearable device. This design may limit comfort, usability, and washability. Only a few studies integrated fiber optic technology into wearable modalities through textile techniques. In conclusion, wearable fiber optic monitoring applications play an important role in constant supervision and diagnostic decision-making due to their continuous measurement capability. Wearable monitoring devices would have great potential not only in healthcare management and rehabilitation but also in sports training (enhanced athletic performance) [37]. Additionally, more textile techniques could be adopted in the future by such applications to achieve comfort and flexibility, which will highly improve the usability and accuracy of devices. Materials 2019, 12, 3311 6 of 15

Table 1. Wearable ﬁber optic technology applications in healthcare monitoring.

Reference Working Mechanism Application Integration Method Location on Body Characteristics Koyama et al. (2018) [22] Intensity modulated Heartbeat and respiration Woven with wool fabric Chest surface Comfort; real-time function; high monitoring into garment accuracy; ability to sense minute-load changes 1 Li et al. (2018) [16] Wavelength modulated Wrist pulse, respiration, Embedded in PDMS Wrist, chest, and finger High sensitivity of 0.83 kPa− ; real-time and finger pulse function; flexibility; wearability; monitoring cost-effectiveness Arnaldo et al. (2018) [36] Intensity modulated Gait monitoring Attached to insole, Foot Flexibility; high repeatability; low cost; orthotic device, and simple signal processing; measurement modular exoskeleton of joint angles and detection of gait events for gait assistance and rehabilitation Lo Presti et al. (2017) [34] Wavelength modulated Respiratory monitoring Glued by polymeric glue Chest wall Monitoring in harsh environments; ability to measure in different positions of the human body Najafi et al. (2017) [35] Wavelength modulated Plantar pressure and Embedded into socks Foot Quick feedback; real-time function; temperature monitoring convenience Fajkus et al. (2017) [17] Wavelength modulated Body temperature, Encapsulated inside Chest surface Non-invasiveness; high accuracy; respiration, and heart rate PDMS multichannel hybrid fiber optic sensor monitoring system Hu et al. (2016) [18] Wavelength modulated Respiratory monitoring Attached to seat-back Back Real-time function; high accuracy; low cost; convenient operation Ciocchetti et al. (2015) [20] Wavelength modulated Respiratory monitoring Glued by adhesive silicon Chest surface Non-invasiveness; good linear response rubber to strain; chemical inertness; small size; flexibility; MR compatibility; high accuracy in the estimation of TR,TI, and TE phases and UT volumes. Yang et al. (2015) [19] Intensity modulated Heartbeat respiration Integrated onto an elastic Back or chest Simultaneous measurement in daily monitoring substrate activities; comfort; cost-effectiveness; high sensitivity; non-invasiveness; simple fabrication Zheng et al. (2014) [15] Intensity modulated Respiration monitoring Embedded into belt fabric Chest or abdomen High strain sensitivity; low hysteresis and repeatability; immunity to electromagnetic interference Witt et al. (2012) [21] Wavelength modulated Respiration monitoring Integrated into Abdominal and thoracic Comfort; continuous measurement; textile-based sensing areas testing in MR environment harness

FBG: ﬁber Bragg grating; MR: magnetic resonance; TR: respiratory period; TI: duration of inspiratory; TE: expiratory; UT: upper thorax; PDMS: polydimethylsiloxane polymer

4.2.Materials Working2019 ,Mechanism12, 3311 of Fiber Optic Sensors for Monitoring 7 of 15 Summarized from the working mechanisms of the latest wearable fiber optic technology applications4.2. Working in Mechanism Table 1, of the Fiber wavelength Optic Sensors- and for Monitoringintensity-modulated sensors are most frequently adopted.Summarized For wavelength from modulation, the working Bragg mechanisms gratings-based of the fiber latest sensors wearable, as shown fiber in optic Figure technology 4, have greatapplications potential in Tablein applications1, the wavelength- of smart and textiles, intensity-modulated due to their sensors advantages are most in frequentlyhigh sensi adopted.tivity, miniaturization,For wavelength flexibility modulation, and Bragg electromagnetic gratings-based immunity fiber sensors, [38,39]. as For shown Bragg in Figuregrating4s,, havewhen great an broadbandpotential in incident applications light ofenters smart the textiles, gratings, due the to theirlight advantageswith central in wavelength high sensitivity, of 휆퐵 miniaturization, will be back- reflectedflexibility. 휆 and퐵 can electromagnetic be written as immunity [38,39]. For Bragg gratings, when an broadband incident light

enters the gratings, the light with central wavelength of λB will be back-reﬂected. λB can be written as

휆퐵 = 2푛Λ (1) λB = 2nΛ (1) where n is the effective refractive index of the guided mode in optical fiber, and Λ represents the gratingwhere pitch.n is the When effective variations refractive of temperature index of the or guidedstrain are mode applied in optical to the fiber,gratings, and theΛ representsgrating pitch the Λgrating would pitch.change When due to variations the thermal of temperature expansion or deformation strain are applied of the to fiber the, gratings, and the refractive the grating index pitch nΛ varieswould because change of due the totherm the thermalo-optic and expansion the strain or- deformationoptic effect [40,41] of the. Accordingly, fiber, and the a refractive variation indexof the n Bvariesragg reflection because ofoccurred the thermo-optic, by which andtarget the physical strain-optic parameters effect [ 40(e.g.,,41 ].temperature Accordingly, and a variation strain) can of be the measuredBragg reflection [42]. occurred, by which target physical parameters (e.g., temperature and strain) can be measured [42].

Figure 4. Schematic of Bragg gratings. Figure 4. Schematic of Bragg gratings. For the intensity modulation, Figure5 shows the working mechanism of fiber optic micro bend sensor.For the Microbending intensity modulation, loss is a type Figure of light 5 shows intensity the working loss caused mechanism by defects of fiber and optic small m geometricalicro bend sensorperturbations. Microbending along theloss fiber is a axis,type theof light deformation intensity ofloss which cause isdin by the defects order and of micrometers small geometrical [42,43 ]. perturbationsLight propagating along in the the fiber microbending axis, the deformation optical fiber withof which intensity is inI Ithecan order be modulated of micrometers by external [42,43] load.

Lsignalsight propagat such asing strain, in the pressure microbending and acceleration optical fiber resulting with intensity in a varied 퐼퐼 can light be intensity modulatedIS. Therefore, by external an loadoutput signals intensity such asID strain,is obtained pressure to monitor and acceleration target physical resulting parameters in a varied such light as intensity respiration 퐼푆 rate,. Therefore, plantar anpressure, output orintensity the heartbeat 퐼퐷 is obtained vibration, to which monitor present target great physical potential parameters in the biomedical such as respiration ﬁeld. rate, plantar pressure, or the heartbeat vibration, which present great potential in the biomedical field.

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Figure 5. Schematic of micro bend sensor. Figure 5. Schematic of micro bend sensor 5. Textile Fabrication Techniques for Wearable Fiber Optic Technology 5. TextileSeveral Fabrication textile T fabricationechniques techniques, for Wearable such Fiber as weaving, Optic Technology knitting, and non-weaving methods, can beSeveral used to textile embed fabrication fiber optic techniques, technology such into as textile weaving, fabric knitting, and make and fiber non optic-weaving technology methods, wearable. can be usedComfort, to embed flexibility, fiber usability,optic technology and accuracy into textile of relevant fabric devicesand make could fiber thus optic be technology highly improved. wearable. Comfort, flexibility, usability, and accuracy of relevant devices could thus be highly improved. 5.1. Weaving 5.1. WeavingWarp yarns and weft yarns are interlaced one by one in a basic woven structure, as presented in Figure6a. In most cases, optical fibers are woven into a fabric in unbent condition or with a limited Warp yarns and weft yarns are interlaced one by one in a basic woven structure, as presented in bending angle to ensure effective transmission and sensing functions [9,44,45]. Optical fibers and Figure 6a. In most cases, optical fibers are woven into a fabric in unbent condition or with a limited standard textile yarns are commonly fabricated via a handloom by interlacing in accordance with design bending angle to ensure effective transmission and sensing functions [9,44,45]. Optical fibers and rules (e.g., plain, twill, and sateen (Figure6b). In addition, optical fibers in di fferent woven structural standard textile yarns are commonly fabricated via a handloom by interlacing in accordance with designs would have various mechanical and sensing properties. For instance, Wang et al. reported that design rules (e.g., plain, twill, and sateen (Figure 6b). In addition, optical fibers in different woven the side-emitting properties of optical fibers in sateen woven structures are significantly higher than structural designs would have various mechanical and sensing properties. For instance, Wang et al. those in plain and twill woven structures [46]. Moreover, optical fiber properties could also influence reported that the side-emitting properties of optical fibers in sateen woven structures are significantly the finalized smart textile performance. For instance, commonly used and commercially available higher than those in plain and twill woven structures [46]. Moreover, optical fiber properties could optical fibers have the diameter range from 250 µm to 3000 µm [47], an relatively larger diameter of also influence the finalized smart textile performance. For instance, commonly used and optical fibers may induce high rigidity of smart textile, meanwhile a relatively smaller diameter would commercially available optical fibers have the diameter range from 250 µm to 3000 µm [47], an cause low light intensity and low shear resistance. [11] By adopting weaving techniques, Quandt and relatively larger diameter of optical fibers may induce high rigidity of smart textile, meanwhile a his colleagues (2017) interlaced optical fibers into fabric following a variety of weave designs (e.g., relatively smaller diameter would cause low light intensity and low shear resistance. [11] By adopting plain weave, plain weave alternating with Trevira CS, Satin 2/2(2), Satin 3/3(3), and Satin 6/6(6)) to weaving techniques, Quandt and his colleagues (2017) interlaced optical fibers into fabric following provide wearable phototherapy on skin for Neonatal jaundice (hyperbilirubinaemia) [31]. In addition, a variety of weave designs (e.g., plain weave, plain weave alternating with Trevira CS, Satin 2/2(2), optical fibers could also be embedded into fabric through embroidered techniques on woven substrates. Satin 3/3(3), and Satin 6/6(6)) to provide wearable phototherapy on skin for Neonatal jaundice As shown in Figure6c, textile techniques of embroidering, such as soutache and schi ffli, have been (hyperbilirubinaemia) [31]. In addition, optical fibers could also be embedded into fabric through reported by Selm et al. (2007) and Quandt et al. (2015) to be applied in wearable fiber optic applications embroidered techniques on woven substrates. As shown in Figure 6c, textile techniques of for the use of body-monitoring, health supervision and photodynamic therapy [37,48]. embroidering, such as soutache and schiffli, have been reported by Selm et al. (2007) and Quandt et al. (2015) to be applied in wearable fiber optic applications for the use of body-monitoring, health supervision and photodynamic therapy [37,48].

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Figure 6. Optical fibers in woven structures: (a) basic woven structure; (b) optical fibers in plain, twill, Figure 6. Optical fibers in woven structures: (a) basic woven structure; (b) optical fibers in plain, twill, and sateen structures; (c) optical fibers in woven structure with embroidering techniques (optical fibers and sateen structures; (c) optical fibers in woven structure with embroidering techniques (optical marked yellow). fibers marked yellow). The preceding section discusses monolayer woven structures with the basic sectional view presentedThe preceding in Figure section7a, in which discusses the gray monolayer dots are warp woven yarns, structures and the with black the lines basic are weft sectional yarns. view Woven presentedfabric materials in Figure could 7a, in also whic beh fabricatedthe gray dots into are multilayer warp yarns, structures. and the Koncar black (2005)lines are integrated weft yarns. optical Wovenfiber fabric with textilesmaterials by could adopting also abe two-layer fabricated basic-velour into multilayer fabric structures. to develop Koncar a wearable (2005) fabricintegrated display. opticalThis fiber woven with structure textiles notby onlyadopting help a to two make-layer embedded basic-velour optical fabric fibers to asdevelop visible a as we possible,arable fabric but also displaykeep. thoseThis w fibersoven tostructure be suffi cientlynot only consistent help to make with theembedded whole fabric optical [9 ].fibers The distinguishedas visible as possible, multilayer butstructural also keep configurationthose fibers to can be eliminatesufficient thely consisten delaminationt with problemthe whole of commonfabric [9]. laminated The distinguished composites. multilayerTherefore, structural this type configuration of fabric usually can exhibitseliminate high the strength,delamination large problem stiffness, of high common impact, laminated and damage composites.resistance; Therefore, hence, such this materialstype of fabric are frequently usually exhibits applied high in textile-basedstrength, large wearable stiffness, applications high impact, and andfunctional damage apparel. resistance; A two-layer hence, such woven materials fabric (Figure are frequently7b) consists applied of two layers in textile of warp-based yarns wearable separated applicationsby one layer and of functional weft yarns. apparel. Two sets A oftwo weft-layer yarns woven interlace fabric in (Figure the fabric 7b up) consists and down of two across layers the warpof warpyarns. yarns Layers separated of the by wovenone layer fabric of weft materials yarns. couldTwo sets be freelyof weft designed yarns interlace in accordance in the fabr withic up practical and downuse, across as shown the inwarp Figure yarns.7c. Even Layers if theof the layers woven remained fabric the materials same, the could interlacing be freely methods designed of weftin accordanceyarns could with vary, pra asctical seen use, in Figure as shown7d–f. in In Figureparticular, 7c. a Even three-dimensional if the layers remained woven fabric the issame, formed the by interlacingconnecting methods multilayer of weft fabrics yarns together could by vary, binding as seen yarns, in which Figure are 7d also–f. called In particular, bundled a yarns, three and- dimensionalZ-direction woven yarns. fabric These is yarns formed can by be connecting further divided multilayer into warp fabrics and together weft yarns by binding according yarns, to the whichconnection are also methodcalled bundled for the diyarns,fferent and layers. Z-direction The portion yarns. ofThese the wovenyarns can (latitude) be further yarn divided of the fabric into is warpreferred and weft to asyarns the splicingaccording or to splicing the connection weft. On method the basis for ofthe the different different layers. interlacing The portion manners, of the and woventhe inclination(latitude) yarn angle of of the the fabric binder is referred yarn, the to warp as the layer, splicing and theor splicing weft layer, weft. the On orthogonal the basis structureof the differentis divided interlacing into the manners whole orthogonal, and the inclination (Figure7c) angle and theof the interlayer binder yarn, orthogonal the warp (Figure layer,7d), and and the the weftangular layer, interlocking the orthogonal structure structure is divided is divided into an into integral the angle whole interlock orthogonal (Figure (Figure7e) and 7c an) and interlayer the interlayerangle interlock orthogonal (Figure (Figure7f). A 7d wide), and variety the angular of orthogonal interlocking structures structure and angular is divided interlocking into an in structurestegral angleare interlock obtained (Figure by varying 7e) and the numbersan interlayer of layers angle of interlock warp and (Figure weft yarns, 7f). A and wide the variety length of and orthogonal distribution structuresof the binder and angular yarns. interlocking Multilayer joints structures can also are beobtained connected by varying by warp the yarns. numbers The interlacingof layers of methodswarp andare weft roughly yarns, divided and the into length two types,and distribution namely, binding of the yarns binder (Figure yarns.7g) Multilayer and warp joints yarnself-interlacing can also be connected(Figure 7 byh). warp Yarn density yarns. The in each interlacing layer could methods vary. are For roughly instance, divided in thethree-layer into two types, woven namely, structure bindingin Figure yarns7g, (Figure the yarn 7g) densityand warp of yarn the top self and-interlacing bottom (Figure layers is 7h higher). Yarnthan density that in in each the layer middle could layer, vary.thereby For instance, forming in a hollowthe three structure-layer woven between structure the layers. in Figure Contrary 7g, tothe the yarn binding density yarns of (Figurethe top7 andg), the bottom layers is higher than that in the middle layer, thereby forming a hollow structure between the layers. Contrary to the binding yarns (Figure 7g), the warp yarn self-interlacing method have the

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Materials 2019, 12, x FOR PEER REVIEW 12 of 16 warp yarn self-interlacing method have the same density among layers. Consequently, the mechanical sameproperties density are among uniform. layers. Various Consequently, interlacing methods the mechanical and fabric layers properties can be are customized uniform. in Various design for interlacingdiﬀerent methods sensing applications. and fabric layers can be customized in design for different sensing applications.

Figure 7. Sectional view of monolayer and multilayer wove structures: (a) monolayer woven structure; Figure 7. Sectional view of monolayer and multilayer wove structures: (a) monolayer woven structure; (b) two-layer woven structure; (c) five-layer whole orthogonal structure; (d) three-layer interlayer (b) two-layer woven structure; (c) five-layer whole orthogonal structure; (d) three-layer interlayer orthogonal structure; (e) three-layer integral angle interlock structure; (f) three-layer interlayer angle orthogonal structure; (e) three-layer integral angle interlock structure; (f) three-layer interlayer angle interlock structure; (g) four-layer binding structure; (h) four-layer self-interlacing structure. interlock structure; (g) four-layer binding structure; (h) four-layer self-interlacing structure. Multilayer woven fabric structures offer wide choices for creating wearable optic fiber sensing Multilayer woven fabric structures offer wide choices for creating wearable optic fiber sensing technology. Optical fibers can be fabricated into textile products with low bending situation and technology. Optical fibers can be fabricated into textile products with low bending situation and reduced deformation by adopting the suitable multilayer woven structure in Figure8. Figure8a reduced deformation by adopting the suitable multilayer woven structure in Figure 8. Figure 8a illustrates an optical fiber embedded into a two-layer woven structure acting as a weft yarn between illustrates an optical fiber embedded into a two-layer woven structure acting as a weft yarn between two layers of warp yarns. When the number of layers increase, as presented in Figure8b, optical fibers two layers of warp yarns. When the number of layers increase, as presented in Figure 8b, optical can be flexibly applied between any two warp layers regardless of the adopted interlacing method. fibers can be flexibly applied between any two warp layers regardless of the adopted interlacing Furthermore, in the integration of optical fibers into a multilayer hollow structure, fibers can be inserted method. Furthermore, in the integration of optical fibers into a multilayer hollow structure, fibers can into hollow paths acting as a yarn in the warp, as shown in Figure8c. be inserted into hollow paths acting as a yarn in the warp, as shown in Figure 8c.

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Materials 2019, 12, 3311 11 of 15 Materials 2019, 12, x FOR PEER REVIEW 13 of 16

Figure 8. Optical fibers in multilayer woven structures: (a) optical fiber as a weft yarn between two layers of warp yarns; (b) optical fibers applied between any two warp layers in a multilayer interlacing structure; (c) optical fibers as warp yarns in a multilayer hollow structure. FigureFigure 8. 8.OpticalOptical fibers fibers in inmultilayer multilayer woven woven structures: structures: (a) ( aoptical) optical fiber fiber as asa weft a weft yarn yarn between between two two layersVariouslayers of of structural warp warp yarns; yarns; designs (b (b)) optical optical and fibers layer fibers applied configurations applied between between any can twoany provide warp two layers warpnot only in layers a multilayeradded in a possibilities multilayer interlacing for wearableinterlacingstructure; fiber structure; (c optic) optical technology (c fibers) optical as warp fibers but yarns as also warp in enrich a yarns multilayer in relevant a multilayer hollow applications structure. hollow structure. in a user-friendly and competitive manner. In the development of a textile-based fiber optic material, textile yarns in Various structural designs and layer configurations can provide not only added possibilities for differentVarious layers structural (above, designs below, and or nextlayer toconfigurations the embedded can optical provide fibers) not only could added be designedpossibilities using for wearable fiber optic technology but also enrich relevant applications in a user-friendly and competitive wearabledifferent types fiber of optic materials technology (e.g., conductivebut also enrichand waterproof). relevant applications Thus, the developed in a user wearable-friendly fiber and manner. In the development of a textile-based fiber optic material, textile yarns in different layers competitiveoptic application manner. would In thebe equipped development with of additional a textile - functions,based fiber and optic the material, usability textile is considerably yarns in (above, below, or next to the embedded optical fibers) could be designed using different types of differentimproved. layers Additionally, (above, below,more than or nextone woven to the structure embedded can optical exist in fibers) a single could piece be of designed fabric material. using materials (e.g., conductive and waterproof). Thus, the developed wearable fiber optic application diThefferent woven types fabric of materialsin Figure (e.g.,9 was conductive designed toand cover waterproof). six- and Thus,two-layer the developedstructures. wearableConsequently, fiber would be equipped with additional functions, and the usability is considerably improved. Additionally, optichollow application paths form would on the be surface equipped of the with fabric. additional In this functions,case, optical and fibers the usabilitycould be isembedded considerably into more than one woven structure can exist in a single piece of fabric material. The woven fabric in improved.those paths Additionally, on the fabric more surface than if relevantone woven application structure requirescan exis tsensing in a single fibers piece to beof fabrictightly material. close to Figure9 was designed to cover six- and two-layer structures. Consequently, hollow paths form on Thethe humanwoven body.fabric in Figure 9 was designed to cover six- and two-layer structures. Consequently, the surface of the fabric. In this case, optical fibers could be embedded into those paths on the fabric hollow paths form on the surface of the fabric. In this case, optical fibers could be embedded into surface if relevant application requires sensing fibers to be tightly close to the human body. those paths on the fabric surface if relevant application requires sensing fibers to be tightly close to the human body.

Figure 9. Optical fibers in woven fabric material consisting of different layer structures. Figure 9. Optical fibers in woven fabric material consisting of different layer structures. 5.2. Knitting 5.2. Knitting Knitting is another textile technique used to fabricate smart textiles. Knitted fabric is typically Figure 9. Optical fibers in woven fabric material consisting of different layer structures. constructedKnitting withis another loops thattextile are technique interconnected used byto coursesfabricate and smart wales. textiles. In courses, Knitted threads fabric go is horizontally typically constructedin the fabric; with in wales, loops threads that are run interconnected vertically. Weft knitting by courses is a commonly and wales. used In construction courses, threads for optical go 5.2. Knitting horizontallyfiber integration; in the this fabric; method in increases wales, threads the elongation run vertically. percentage Weft of smart knitting textile is aapplication. commonly [48 used] This constructiotextileKnitting techniquen foris another optical (Figure fibertextile 10) integration; is technique used to fabricate usedthis method to knitted fabricate increases fabric smart material the textiles. elongation where Knitted stiches percentage fabric run is horizontally typicallyof smart constructedtextilefrom application. left to with right. loops Optical[48] This that fibers textile are in interconnected knittedtechnique structures (Fig ure by need 10) courses is to used bend and to more fabricate wales. than In thoseknitted courses, in wovenfabric threads material structures go horizontallywhereto form stiches loops in run in the courses. horizontally fabric; Thus, in wales,from the linear left threads to density right. run andOptical vertically. bending fibers Weftresistance in knitted knitting of structures yarns is a in commonly knitted need structuresto bend used constructiomoreshould than be thosen kept for lowopticalin woven to protect fiber structures integration; the bending to form this areas. loopsmethod When in courses.increases fiber optic Thus, the technologies elongation the linear need densitypercentage to be and incorporated ofbending smart textileresistintoance fabricsapplication. of yarns and applied in[48] knitted This on textile structures the human technique should body, (Fig be a series urekept 10) low of is physical–mechanical toused protect to fabricate the bending knitted properties, areas. fabric When material including fiber where stiches run horizontally from left to right. Optical fibers in knitted structures need to bend more than those in woven structures to form loops in courses. Thus, the linear density and bending resistance of yarns in knitted structures should be kept low to protect the bending areas. When fiber

MaterialsMaterials 2019, 122019, x ,FOR 12, x PEER FOR PEERREVIEW REVIEW 14 of 1614 of 16 Materials 2019, 12, 3311 12 of 15 opticoptic technologies technologies need needto be to incorporated be incorporated into fabricsinto fabrics and appliedand applied on the on human the human body, body, a series a series of of physical–mechanical properties, including bending, stretch rate at break, and tenacity, must be physicalbending,–mechanical stretch rate properties, at break, and including tenacity, bending, must be stretch considered rate at and break, tested and along tenacity, with ergonomic must be considered and tested along with ergonomic factors [49]. consideredfactors [49 and]. tested along with ergonomic factors [49].

Figure 10. FigureFigure 10. Weft Weft10. knittingWeft knitting knitting structure structure. structure Except for the weft knitting technique, where optical fibers act as courses, laid-in structural ExceptExcept for the for weft the weft knitting knitting technique, technique, where where optical optical fibers fibers act as act courses, as courses, laid- in laid structural-in structural knitting designs would be suitable for wearable fiber optic material fabrication, where optical fibers knittingknitting designs designs would would be suitable be suitable for wearable for wearable fiber fiberoptic optic material material fabrication, fabrication, where where optical optical fibers fibers will not bend considerably. Figure3 shows laid-in knitting structural designs, with the optical fibers will notwill bend not bend considerably. considerably. Figure Figure 3 shows 3 shows laid- inlaid knitting-in knitting structural structural designs, designs, with withthe optical the optical fibers fibers marked yellow. Figure 11a presents the single jersey hopsack structure; a 1 1 optical fiber inlay is markedmarked yellow. yellow. Figure Figure 11a presents 11a presents the single the single jersey jersey hopsack hopsack struc structure; ture;a 1 × a×1 1optical × 1 optical fiber fiberinlay inlay is is located between each plain ground course [50]. The inlay rules and positions of optical fibers could be locatedlocated between between each eachplain plain ground ground course course [50]. [50]The. inlayThe inlay rules rules and positionsand positions of optical of optical fibers fibers could could flexibly arranged on a plain weft knitting structure, as shown in Figure 11b,c, or the inlay direction can be flexiblybe flexibly arranged arranged on a plainon a plain weft weftknitting knitting structure, structure, as shown as shown in Figure in Figure 11b,c 11b, or ,thec, or inlay the inlay direction direction be changed to warp, as seen in Figure 11d, to lay optical fibers between the plain ground wales. can becan chang be changed toe warp,d to warp, as seen as seenin Figure in Figure 11d, to11d lay, to optical lay optical fibers fibers between between the plain the plain ground ground wales. wales.

Figure 11. Optical ﬁber laid-in weft knitting designs: (a) single jersey hopsack structure; (b,c) other FigureweftFigure laid-in11: Optical 11: structures; Optical fiber laidfiber (d)- warpin laid weft-in laid-in knittingweft structure.knitting designs: designs: (a) single (a) single jersey jersey hopsack hopsack structure; structure; (b) and (b )( cand) (c) other otherweft laidweft-in laid structures;-in structures; (d) warp (d) warplaid-in laid structure.-in structure. The laid-in structure appears not only in weft-knitted fabric materials; such structure could also be applied in warp-knitted material. Figure 12a shows the warp knitted structure where optical ﬁber The laidThe- inlaid structure-in structure appears appears not only not onlyin weft in -weftknitted-knitted fabric fabric materials; materials; such suchstructure structure could could also also could be horizontally or vertically integrated into by using the laying-in techniques, as illustrated in be appliedbe applied in warp in warp-knitted-knitted material. material. Figure Figure 12a shows 12a shows the warp the warp knitted knitted structure structure where where optical optical fiber fiber

MaterialsMaterials 20192019, 12, ,12 x ,FOR 3311 PEER REVIEW 15 13of of16 15 could be horizontally or vertically integrated into by using the laying-in techniques, as illustrated in FigureFigure 12b,c 12b,c.. As As warp warp knitting knitting is isa knitting a knitting method method in inwhich which yarns yarns go go through through the the fabric fabric lengthwise lengthwise inin a zigzag a zigzag rule, rule, thus, thus, optical optical fibers fibers are are usually usually not not recommended recommended to toact act as asinterlacing interlacing yarns yarns due due to to thethe sharp sharp bending bending at atinterlacing interlacing point. point. However, However, optical optical fibers fibers sometimes sometimes were were designed designed to toachieve achieve sideside-emitting-emitting by by macro macro-bending-bending configurationconfiguration asas mentioned mentioned in in Section section2, in 2, this in case this the case warp-knitting the warp- knittingstructure structure following following zigzag zigzag rules with rules controlled with controlled sharp bendingsharp bend coulding becould applied. be applied.

Figure 12. Warp-knitted structure and fiber optic application: (a) warp-knitted structure; (b) and (c) Figureoptical 12. fibers Warp laid-knitted in warp-knitted structure and structure. fiber optic application: (a) warp-knitted structure; (b) and (c) optical fibers laid in warp-knitted structure. 6. Conclusions 6. ConclusionThis article conducted a comprehensive review of wearable fiber optic applications. The functions of theseThis articledevices conducted could be mainly a comprehensive divided into review three categories: of wearable 1) fashion fiber optic and esthetic applications. purposes The (to functionsmake wearables of these stylish), devices 2)could disease be mainly treatment divided (wearable into threefiber optic categories: lighttherapy 1) fashion can and treat esthetic neonatal purposesjaundice, (to speed make up wearables wound healing,stylish), and2) disease stimulate treatment hair growth), (wearable and fiber 3) healthcareoptic light monitoringtherapy can (to treatdetect neonatal real-time jaundice, vital signals, speed up such wound as respiratory healing, and rate, stimulate plantar pressure, hair growth) and, heartbeat and 3) healthcare vibration). monitoringThe working (to detect mechanisms real-time of vital fiber signals, optic technology, such as respiratory including rate, side plantar emission pressure, and wavelength and heartbeat and vibration).intensity modulation, The working were mechanisms briefly introduced of fiber tooptic understand technology, the operating including principles side emission of di ff anderent wavelengthwearable fiber and optic intensity applications. modulation, Various weretextile briefly fabrication introduced techniques to were understand illustrated, the as combination operating principlesmethods of for different optical fiberswearable and fiber textiles. optic In applications. conclusion, smartVarious textiles textile enable fabrication fiber optic techniques technology were to illustratedbe wearable., as combination Such fabrics methods highly improve for optic theal fibers comfort, and textil flexibility,es. In usability,conclusion, and smart accuracy textiles of enable relevant fiberdevices optic and technology enrich them to be to wearable. be user-friendly Such fabrics and competitive. highly improve the comfort, flexibility, usability, and accuracy of relevant devices and enrich them to be user-friendly and competitive. Author Contributions: Resources, writing—original draft preparation, methodology, data curation, Z.G. and Z.X. Authorcontributed Contributions: equally to Resources, this article; writing project—original administration draft preparation, and supervision, methodology, Z.G., X.O. data and curation, C.C.C.; Z.G. review and Z. and X. editing,contributed visualization, equally to J.Z., this N.L. article; and project J.Z. administration and supervision, Z.G., X.Y. and C.C.; review and editing,Funding: visualization,This research J.Z., received N.L. and no J.Z. external funding. Funding:Acknowledgments: This researchThe received authors no external would like funding to express. thankfulness to the fund support by Chi Chiu Chan’s Optical Fiber Sensor Group of Sino-German College of Intelligent Manufacturing, Shenzhen Technology University. Acknowledgments: The authors would like to express thankfulness to the fund support by Chi Chiu Chan’s Conflicts of Interest: The authors declare no conflict of interest. Optical Fiber Sensor Group of Sino-German College of Intelligent Manufacturing, Shenzhen Technology University. References Conflicts of Interest: The authors declare no conflict of interest. 1. Tao, X. (Ed.) Handbook of Smart Textiles; Springer: Singapore, 2015. References2. Tao, X. Smart Fibres, Fabrics and Clothing; CRC Press: Boca Raton, FL, USA, 2001. 3. Baurley, S. Interactive and experiential design in smart textile products and applications. Pers. Ubiquitous 1. Tao, X. (Ed.) Handbook of Smart Textiles; Springer: Singapore, 2015. Comput. 2004, 8, 274–281. [CrossRef] 2. Tao, X. Smart Fibres, Fabrics and Clothing; CRC Press: Boca Raton, FL, USA, 2001. 4. Coyle, S.; Lau, K.-T.; Moyna, N.; O’Gorman, D.; Diamond, D.; Di Francesco, F.; Costanzo, D.; Salvo, P.; 3. Baurley, S. Interactive and experiential design in smart textile products and applications. Pers. Ubiquitous Trivella, M.G.; De Rossi, D.E.; et al. BIOTEX—Biosensing Textiles for Personalised Healthcare Management. Comput. 2004, 8, 274–281. IEEE Trans. Inf. Technol. Biomed. 2010, 14, 364–370. [CrossRef][PubMed] 4. Coyle, S.; Lau, K.-T.; Moyna, N.; O’Gorman, D.; Diamond, D.; Di Francesco, F.; Costanzo, D.; Salvo, P.; 5. Sibinski, M.; Jakubowska, M.; Sloma, M.; Sibinski, M.; Jakubowska, M.; Sloma, M. Flexible Temperature Trivella, M.G.; De Rossi, D.E.; et al. BIOTEX—Biosensing Textiles for Personalised Healthcare Sensors on Fibers. Sensors 2010, 10, 7934–7946. [CrossRef] Management. IEEE Trans. Inf. Technol. Biomed. 2010, 14, 364–370.

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