1 Introduction
Fibers and textiles are ubiquitous in human civilization. As early as the primitive inhabitants, people began to use plant fibers such as kudzu and hemp to rub and weave the fibers into sheets by hand to cover the body. With the continuous development of human science and technology, various synthetic fibers were developed in the middle of the 20th century. In order to meet the increasing requirements of people's quality of life, fibers and textiles were endowed with new functions such as waterproof, flame retardant and radiation protection through functional treatment
[1][2,3]. In the past two decades, smart fibers and textiles with sensing, communication, memory and other capabilities have been developed, which have attracted wide attention in personal health management
[4,5].
As one of the important research directions of flexible wearable sensors, wearable sensors based on smart fibers and textiles have attracted much attention
[6,7]. Compared with traditional electronic monitoring devices, which are bulky, not easy to carry, inconvenient to sample, and unable to monitor in real time, sensors based on smart fibers and textiles not only have the advantages of flexibility, portability, comfort, and bendability,But also is safer, more environment-friendly and more sustainable, is mostly degradable after being discarded, does not cause harm to the environment, and has great application prospect in the aspect of human health management
[8][9⇓~11][12,13]. They can be used to monitor vital signs such as heartbeat, pulse, respiration, blood pressure, temperature, humidity and body movements, as well as to analyze body fluids such as tears, sweat, saliva and urine, and to detect exhaled matter, which can solve the problem that traditional sensors can not monitor in real time
[14⇓~16]. The physical signal received by the sensor can be output through piezoelectric sensing, resistive sensing, capacitive sensing, friction electrification sensing and other sensing technologies
[17,18]; The received biological signals can be output by electrochemical, colorimetric, fluorescence and other sensing technologies
[19,20].
Here, this paper mainly introduces the application of smart fibers and textiles in wearable biosensors in recent years, including vital signs monitoring, body fluid analysis and exhalation detection.The sensing strategies such as colorimetric sensing, fluorescence sensing and piezoelectric sensing are introduced, the current development achievements and challenges are summarized, and the future development is prospected.
2 Smart fibers and textiles for vital signs monitoring
2.1 Respiration and heartbeat
Breathing and heartbeat are the basic physiological characteristics of the human body, which are directly related to people's life state. It is very necessary to monitor breathing and heartbeat in real time with sensitive and efficient sensors, which can feed back the physiological state in time and warn the occurrence of major diseases or emergencies in advance
[21⇓~23].
2.1.1 Piezoelectric sensing
Piezoelectric sensor is developed based on the working principle of piezoelectric effect, that is, piezoelectric materials will produce surface charges when receiving external pressure, which will convert the pressure signal into an electrical signal. Within the limited range of piezoelectric materials, the piezoelectric output is usually positively correlated with the external pressure
[24]. The Fink team has introduced a fabric that can be used as a sensitive hearing microphone, which weaves heat-stretched composite piezoelectric fibers into the fabric so that it can convert mechanical vibrations into electrical signals
[25]. The active layer of the hot-drawn composite piezoelectric fiber is piezoelectric Poly (vinylidene fluoride-trifluoroethylene) (P (VDF-TrFE)) loaded with barium titanate ceramic particles, and the outer layer is Poly (styrene-b- (ethylene-co-butylene) -b-styrene) (SEBS). The acoustic fabric has a response frequency range of 80 ~ 1000 Hz, and the acoustic shirt made of it can effectively capture the heart signal and act as a stethoscope for the skin interface. The signal-to-noise ratio of the signal is as high as 30 decibels, which exceeds that of the recent membrane acoustic devices, and can clearly detect the loud S1 sound and the weak S2 sound. Using the fabric to detect cardiac mechanical activity allows the wearer to monitor their heart condition in a comfortable, continuous, real-time, and long-term manner.
Here we present a brief summary of fiber microphones (Table 1). Polymer fiber microphones mostly use piezoelectric effect, while metal fibers generally vibrate through the ampere force in a magnetic field, and the frequency response range of different types of fiber microphones varies greatly
[26⇓~28][29]. Textiles are often used as sound-absorbing materials. With the breakthrough of textile technology in recent years, the use of fabrics has been further developed, and fiber microphones have attracted wide attention. However, the preparation of fiber microphones with sensitive response and high fidelity faces the problem of low conversion efficiency of mechanical vibration into electrical signals. Therefore, improving the conversion efficiency is the key to the development of high-performance fiber microphones.
In general, the piezoelectric sensor has the characteristics of high sensitivity, simple structure and the like, and can be used for monitoring the breathing and heartbeat of a human body to directly convert the physical activities of the human body into electrical signal output, so that people can intuitively understand the health condition of the human body.
2.1.2 Resistive sensing
Resistive sensor uses the change of resistance to achieve signal response, that is, when the conductive material is subjected to external pressure, the resistance will change accordingly to produce signal response. He et al. Used electrospinning technology to develop highly sensitive and stretchable electronic skin with multi-modal mechanical sensing performance, which has important research value for health monitoring and wearable electronic devices
[30]. The graphene oxide-doped polyacrylonitrile nanofiber yarn and the in-situ polymerized conductive polypyrrole are wound on the elastic yarn to form a composite yarn, and the composite yarn is woven to form a fabric-shaped sensor, which can detect the mechanical force caused by pressure, strain and bending,High sensitivity, wide pressure sensing range, excellent cycling stability and good repeatability (> 10,000 cycles) for detecting human respiration, facial expression, pulse monitoring and full range of human motion. Because such fabric sensors can be conveniently woven into textiles, they have great potential in wearable human health monitoring electronics. Resistive sensor is a common sensor in the field of wearable sensing because of its simple manufacturing method and easy to read signal
[31].
2.2 Pulse and blood pressure
Wrist pulse is an effective mechanical index to evaluate myocardial contractility. Irregular wrist pulse can reflect heart problems such as sinus arrhythmia, atrial flutter, partial heart block and heart failure
[32⇓~34].
2.2.1 Piezoelectric sensing
To enable precise and real-time pulse monitoring, Han's team developed an artificial, wearable, multimodal gel skin sensor, which was compounded with a medical patch and worn on the wrist to provide continuous pulse monitoring for users
[35]. The sensor can simultaneously monitor four biophysical signals on the wrist, including blood pressure, electrocardiogram, Electromyogram (EMG) and Mechanomyography (MMG). The built-in interdigitated carbon electrodes were used as MMG sensors, and the electrical signals of muscle movement were obtained from the electrodes separated on both sides. The PVDF-TrFe gel is polarized by the pulse or muscle movement in the radial artery, causing a change in the charge distribution in the conductive electrode, which converts the mechanical stimulus change of the skin into an electrical signal. Cardiac activity information can be monitored by looking for the relationship between cardiac and muscle activity. Mokhtari et al. Developed three different energy generators and sensors based on piezoelectric nanocomposite fibers: knitting, weaving and weaving
[36]. Poly (vinylidene fluoride) hybrid piezoelectric fibers containing barium titanate nanoparticles were prepared by melt spinning process. When the mass ratio of barium titanate nanoparticles to poly (vinylidene fluoride) is 1 ∶ 10, the hybrid fiber with high performance can be obtained. This smart textile energy harvesting device provides better mechanical (durability, flexibility and comfort) and piezoelectric properties compared to poly (vinylidene fluoride) fiber. The textile-based piezoelectric sensor is durable, lightweight and flexible, and is expected to be used in the fields of medical care and the like.
Piezoelectric sensor has many advantages, but it is greatly affected by air humidity, so attention should be paid to moisture-proof in the use process. In a word, stable detection signal, good environmental adaptability and green environmental protection after abandonment are the future development trends of piezoelectric sensors.
2.2.2 Resistive sensing
Resistive sensing is the more commonly used sensing method, and Sun's team reported a wearable device based on Carbon nanotube (CNT) -Polydimethylsiloxane (PDMS) with piezoresistive and voltage sensing capabilities to simultaneously capture wrist pulse pressure and cardiac electrical signals
[37]. The strain sensor consists of a sandwich piezoresistive CNT-PDMS strip for sensing and two thin layers of flexible PDMS polymer. Electrocardiographic electrodes were placed in the strain area on the substrate, and after pretreatment, the relationship between resistance change and pressure was calibrated, the device sensitivity was determined to be 0.01 Pa
-1, and the linear pressure range of the device was from 0.4 to 14.0 kPa. Because of its wearable characteristics, it can realize continuous monitoring of electrocardiogram and wrist pulse at the same time, avoid the complexity and inconvenience of traditional cardiac monitoring instruments, and provide a variety of physiological cardiac signals for clinical diagnosis.
Although resistive sensors have been widely used, the preparation of fabric-based flexible sensors with high sensitivity and low cost is still a challenge. Reducing stress consumption and improving sensing efficiency are the key to solve this problem. Inspired by the principle of subcuticular Merkel receptor sensing, the Niu team proposed a strategy of orthogonal knitting thread induced local stress concentration
[38]. High modulus Ag/nylon threads were woven orthogonally onto the low modulus microstructured non-woven fabric, which produced obvious local stress concentration at the intersection and promoted the amplification and transmission of the pressure signal, effectively improving the sensitivity (8.59 kPa
-1) and linear range (0.2 – 400 kPa). This resistive sensing method is relatively simple and has great potential in mass production.
2.2.3 Capacitive sensing
In addition to resistive sensing and piezoelectric sensing, capacitive sensing is also a commonly used sensing method. The sensing mechanism is based on the change of capacitance under external force, which leads to the change of contact area, thickness and dielectric constant of the dielectric layer between electrodes. The Sun team developed a washable capacitive pressure sensing fabric based on a hydrophobic Poly (ionic liquid) nanofibrous membrane (PILNM), which has good mechanical properties
[39]. PILNM's wearable pressure sensing textile has high sensitivity to low pressure and bending chord length changes, with a low pressure detection limit even under severe deformation. Due to its superior performance, the wearable pressure-sensing textile based on PILNM is comfortable to wear and suitable for monitoring pulse vibration in different parts of the body as well as various human movements (Fig. 1). Capacitive sensor has the advantages of high sensitivity, low cost and no hysteresis, but its output impedance is high, its load capacity is poor, and its complex connection circuit will cause nonlinear output, which will affect the measurement accuracy.
2.2.4 Triboelectric sensing
Because the pulse is weak and fast, it is difficult to capture its detailed information when measuring, so the monitoring equipment is required to have high sensitivity and appropriate frequency response. Yang's team reported a Triboelectric all-textile sensor array (TATSA), which is woven with conductive yarn and nylon yarn and has high sensitivity to capture subtle pressure. Two TATSAs are sewn on the cuff and chest, enabling simultaneous dynamic monitoring of pulse and respiration
[40]. The combined effect of triboelectrification and electrostatic induction causes the contact-separation motion between the conductive yarn and the nylon yarn to cause charge transfer. When the two materials are in contact, the charge is mainly accumulated on the friction material, and only a small amount of induced charge exists on the electrode, resulting in a small potential. When the two materials are separated, the induced charge on the electrode increases due to the potential difference, which increases the corresponding potential and causes the signal change. It has low pressure sensitivity (7.84 mV·Pa
-1), fast response time (20 ms), high stability (> 100 000 cycles) and machine washability (> 40 cycles).
The complex and fine structure of the triboelectric nanogenerator is difficult to further improve the sensing ability of the textile within the limited contact area. To solve this problem, Li et al. Developed a triboelectric sensing textile composed of core-shell yarns (Fig. 2)
[41]. The core-shell yarn becomes stronger in mechanical properties, and the formed twisted spiral structure is beneficial to bearing various deformations in practical application, and is easy to fit on the carotid artery to record pulse physiological signals. In the braided structure, the nylon wire and the polytetrafluoroethylene wire are selected as the positive electrode layer and the negative electrode layer respectively, and the spiral stainless steel wire is arranged in the braided structure as the inner electrode layer. In the pressure range of 1.95 ~ 3.13 kPa and 3.20 ~ 4.61 kPa, the sensing sensitivity can reach 1.33 and 0.32 V·kPa
-1, respectively, and the sensor still has good mechanical stability and sensing ability after 4200 continuous cycles. This work provides a new idea for the development of large size pressure sensing textiles with high wear resistance. In conclusion, the textile triboelectric nanogenerator provides a precise, durable, and comfortable method for heart rate measurement, which can be further used for real-time physiological monitoring
[42].
2.3 Temperature
Suitable temperature is very important for the human body. The body temperature is about 36 ~ 37 ℃, and the suitable ambient temperature is usually 18 ~ 24 ℃. Too high or too low body temperature will cause certain damage to human health, and even threaten life. The ambient temperature will affect people's comfort, for people with cardiovascular and cerebrovascular diseases, too high ambient temperature may bring life danger
[43⇓~45].
2.3.1 Resistive sensing
In temperature detection, the resistance of the resistive sensor is usually changed by the temperature change, and then the generated signal is changed, so the sensor needs to be sensitive to temperature. Wang et al. Designed and developed an ultra-light Self-powered fire alarm electronic textile (SFA e-textile) based on conductive gel fiber (Fig. 3A), which contains Calcium alginate (CA), powered fire alarm electronic textile nanoparticles (Fe
3O
4nanoparticles,Fe
3O
4NPs) and Silver nanowires (silver nanowires,Ag NWs), which can realize ultra-sensitive temperature monitoring and energy harvesting in fire-fighting clothing, can sense a wide range of temperature from 100 to 400 ℃, and realize repeatable temperature monitoring and power generation of SFA electronic textiles, respectively.On the basis of SFA e-textile, a self-powered fire self-rescue positioning system (Fig. 3B) is further established, which can help rescuers search and rescue trapped firefighters in fire situations
[46]. Li et al. Reported a flexible sensor based on electrospun nanostructures, which can be used to detect temperature
[47]. Integrating the uniform conductive coating with the interlaced nanofiber structure, the nanofiber skeleton can bring superior flexibility to the sensor, and also provide abundant contact sites and contact area, which can improve the sensitivity and working range of the sensor. The sensor consists of a double-layer electrospun felt coated with conductive components (Figure 3C). To reduce the interference of external stimuli, the temperature-dependent signal is transmitted through a wire connected at both ends of the bottom layer, that is, poly (styrene-block-butadiene-block-styrene)/poly (3,4-ethylenedioxythiophene) (PEDOT/SBS) layer as the main part of temperature sensing. The single-layer temperature sensing path, together with the interlayer pressure sensing path, constitutes a specific circuit topology that facilitates the separation of different signals. The interwoven nanofiber structure and flexibility allow the bottom layer to focus on detecting temperature changes. The current change between electrodes A and B is output as the pressure sensing signal, and the resistance change between electrodes B and C is used as the temperature sensing signal, so that the sensor can detect temperature and pressure at the same time through the signal transmission of multi-level sensing paths between layers and within layers (Fig. 3D), and the two detections do not interfere with each other (Fig. 3E). The obtained sensor shows a high pressure sensitivity of 1185.8 kPa
-1 and superior reliability, capable of rapidly detecting external stimuli down to 2.4 Pa and showing excellent response behavior under 5000 cyclic load tests. Different sensing methods are integrated in the same sensor to detect pressure and temperature at the same time, which is simple and easy to operate, and provides a new idea for the development of multi-functional integrated sensors.
2.3.2 Thermochromic sensing
Thermochromic sensing materials have the characteristics of color change caused by temperature change, which can intuitively monitor the temperature change of human body through naked eyes. Kim's team developed a nanofiber-based thermochromic sensor using electrospinning technology
[48]. A specific amount of thermochromic material is added into the spinning solution, so that the prepared nanofiber has thermochromic characteristics, and the nanofiber is applied to masks, bracelets and patches to prepare wearable thermochromic sensors, thereby realizing real-time monitoring of body temperature changes. Generally speaking, this kind of sensor has high sensitivity and simple preparation, but most of the temperature change materials are not resistant to ultraviolet irradiation, which may lead to the reduction of temperature change effect under long-term ultraviolet irradiation.
2.4 Humidity
Humidity will affect the comfort of the human body. Too high humidity will cause symptoms such as elevated body temperature, rapid heartbeat, dizziness and fatigue. Too low humidity will increase the probability of respiratory diseases. Therefore, the monitoring of humidity is very necessary. Wearable sensors based on smart fibers and textiles enable real-time monitoring of humidity
[49,50].
2.4.1 Resistive sensing
Resistive sensors are not only used in heartbeat, respiration, temperature monitoring, but also in humidity monitoring. Gao et al. of Yangzhou University modified Acidified carbon nanotube (ACNT) with super hydrophilicity and super oleophobicity on the surface of Polyurethane (PU) nanofiber, and prepared a flexible and stretchable Conductive nanofiber composite (CNC) with high sensitivity and low detection limit, which can be used to detect moisture and chemical vapor (Fig. 4A)
[51]. The addition of ACNTs was able to increase the tensile strength of the fibers and almost retained the superelasticity of the PU nanofiber membrane, giving the material excellent flexibility (Fig. 4 B). When used in humidity sensors, ACNTs can absorb surrounding water molecules and increase the resistance. PU can absorb chemical vapor through different swelling methods, resulting in different degrees of internal damage of the conductive network and resistance changes. The material can be prepared into a mask for real-time monitoring of human body humidity (Fig. 4C), which has the advantages of low detection limit, fast response and good selectivity.
For the same purpose, the Koo team in Korea developed an agarose-polymer based composite microfiber, which was prepared by the extrusion method containing Ag NWs and Graphene oxide (GO) as 1 D and 2D nanofiller hydrogel filaments, respectively, with stronger conductivity and fast humidity sensing due to the addition of conductive nanofillers
[52]. The synergistic effect between Ag NWs and Reduced graphene oxide (rGO) further improves the fiber conductivity. Wang's team introduced the application of self-heating graphene-reinforced polyvinyl alcohol nanowires in high-sensitivity humidity detection, which can accurately and reliably measure relative humidity in human body system monitoring
[53].
Different from the heartbeat and pulse monitoring, which relies on the tensile deformation of materials to affect the resistance change, in humidity monitoring, the resistance is changed by moisture absorption, but in order to ensure the accuracy of humidity detection results, it is necessary to eliminate the resistance change caused by external pressure or ambient temperature.
2.4.2 Colorimetric sensing
The most common optical sensors are based on colorimetric or fluorescence changes caused by intermolecular interactions between a chromophore or fluorophore and the analyte
[54]. Zhou's team developed a functional flexible film of nanowire clusters
[55]. They fabricated Nanowire cluster films (Nanowire cluster film, NWCF) using a special disordered anodic aluminum oxide substrate and conventional sputtering technique, and integrated them with PDMS to fabricate flexible devices. When a very small amount of water (0.02 mg·mm
-2) is deposited on the NWCF, its refractive index is high enough to cause appreciable changes in the extinction spectrum. Based on this, the flexible plasma film shows the advantages of high sensitivity and robust resistance to motion interference when used to construct humidity sensors. After the pure water is replaced by sweat, the same phenomenon still exists, and the human body humidity can be monitored intuitively. Similar to thermochromic sensors, the biggest advantage of colorimetric sensors is that they can convert humidity changes into color changes and then analyze them through smartphone photographs, but compared with other sensors, colorimetric sensors face the problem of low sensitivity.
2.4.3 Fluorescence sensing
Due to the Twisted intramolecular charge-transfer (TICT) effect, the molecular rotor with conjugated donor (D) -Acceptor (A) structure is of great significance for the detection of water molecules. Zhu's team used the Aggregation-induced emission (AIE) molecules of D-A and the intramolecular motion of one-dimensional polymer fibers to develop a new type of smart humidity sensor
[56]. When absorbing water vapor, the expanding polymer fiber triggers the binary effect of AIE and TICT through the regulation of intramolecular motion, resulting in different fluorescence colors to quantify and map the change of local humidity, and the sensing mechanism is the change of fluorescence color and intensity induced by intramolecular motion. Fluorescence sensor is one of the most widely studied sensors at present, which has many advantages, such as high sensitivity, high selectivity, wide linear range and so on, but its detection results are affected by background fluorescence, low fluorescence lifetime and other factors.
2.5 Body movement
Real-time monitoring of body movements is of great significance in rehabilitation training after illness, remote rehabilitation, daily training of athletes and monitoring of risk-inducing activities in people's daily life
[57⇓~59].
2.5.1 Resistive sensing
A patent of Shanghai Institute of Silicate, Chinese Academy of Sciences introduces the preparation method and application of a flexible wearable conductive fiber sensor
[60]. The sensor consists of a fiber and a conductive layer coated on the outside of the fiber, and adopts a unique elastic spiral double-coated structure. The conductive fiber sensor is fixed on the surface of an elastic polydimethylsiloxane film by using a medical adhesive tape, and the obtained flexible wearable sensor can be attached to various parts of a human body, is used for sensing pressure changes at finger joints, knee joints, thoracic cavities and the like, and makes different responses to different actions. When the part of the human body to be measured moves, the sensor deforms and produces resistance changes, so as to realize the monitoring of human body activities such as pulse, respiration, pronunciation, joint movement and the like.
Wearable sensors made of conductive fibers have the characteristics of softness, bendability and stretchability, which provide a lot of convenience for bending detection, and can reduce costs, continuous monitoring, remote rehabilitation, gamification and detection of risk-inducing activities in daily life. The new piezoresistive multi-walled carbon nanotube/graphite/styrene-butadiene-styrene copolymer fiber developed by Walter et al. Is a wearable piezoresistive sensor suitable for telerehabilitation applications
[61]. The fiber can be woven directly into the garment to form a strain gauge for remote monitoring of human activity, providing an opportunity for gamification of the orthopedic rehabilitation process to motivate participants, greatly improving the rehabilitation process and reducing the time necessary for medical supervision. Zhang's team used wet packaging technology to prepare coaxial conductive fibers for pressure sensing
[62]. Ionic liquid core and thermoplastic elastomer sheath (IL @ TPE) is used to form a coaxial structure of conductive fibers, which has large stretchability, low hysteresis and fast response time, and can reflect the actual movement of the human body. Optical fiber sensors are attached to the skin of human elbow, wrist, finger and knee joints to monitor the bending movement of human wrist and obtain a series of accurate working curves. Therefore, the IL @ TPE optical fiber strain sensor is able to monitor the bending and recovery movement of the elbow joint well.
Compared with physiological activities such as heartbeat and breathing, the range of body movements is larger, and most of them are accompanied by elbow joint movements. Obvious body changes will cause large resistance changes, which will make the output signal obvious, but it will also test the wear resistance, fatigue resistance and other properties of the sensing material.
2.5.2 Capacitive sensing
Large-scale production of wearable sensors has some limitations. Liu et al. Prepared a stretchable and soluble porous Ag NWs/Polyvinyl alcohol (PVA) nanocomposite hydrogel by a simple combination of foaming and freeze-thaw methods
[63]. The silver nanowires are uniformly embedded in the PVA matrix to ensure that the dielectric constant can be increased when a large number of microcapacitors are introduced. Capacitive tactile sensor can be prepared by combining the composite hydrogel with CNT network electrode. The hydrogel-based sensor dissolves in hot water within minutes, and the electrode material has high sensitivity (1.9 kPa
-1), wide detection range (250 kPa), fast response time (25 ms), and remarkable stability over 10 000 loading/unloading cycles. It can be fixed on gloves and other fabrics to monitor limb movement in real time. The sensor can be fabricated in a large area, and then the sensor wafer can be divided according to the requirements, indicating that it has the potential advantage of large-scale production.
2.5.3 RF harmonic sensing
The Yan team designed, manufactured, and characterized a wireless smart mask that embeds a soft and battery-free RF harmonic transponder that enables continuous, real-time wireless monitoring of cough and mask wear using the second harmonic (Figure 5A)
[64]. Unlike the current smart mask, the antenna and circuit of the RF transponder are manufactured based on spray-printed Ag NWs/PEDOT ∶ PSS and multi-scale porous SEBS substrates, which are fixed inside the mask (Fig. 5B). In addition to being ultra-soft and stretchable, the passive cooling and ventilation functions of this substrate can provide long-term wearing comfort.
3 Smart fibers and textiles for body fluid analysis
3.1 Sweat
There are many biochemical substances in sweat, including ions, glucose, cortisol and so on. Through the detection of biological substances in sweat, the health status of the body can be assessed
[65⇓~67].
3.1.1 Electrochemical sensing
Electrochemical sensing is a common way to analyze and detect sweat components. Inspired by lotus leaves (Fig. 6A), Zhang's team developed a lotus leaf-like structure with one hydrophobic side and one hydrophilic side, namely Janus e-textile, based on silk for managing and analyzing biofluids
[68]. Silk was chosen as the textile substrate and sensing electrode due to its natural biocompatibility. Electronic textiles based on silk can provide a comfortable micro-environment for the skin, which can reduce wet adhesion and avoid skin overheating or overheating. The electrode can be used for non-invasive detection and analysis of multiple targets in human sweat with less detection substances and faster response time (Fig. 6B). This technology combines physiological comfort with sensing technology, providing a new type of intelligent textiles for health detection.
Shu's team developed a wearable electrochemical sensor based on highly stretchable Ni-Co metal-organic framework/Ag/reduced graphene oxide/polyurethane (Ni-Co MOF/Ag/rGO/PU, NCGP) fibers for highly sensitive and accurate continuous monitoring of glucose levels in sweat
[69]. This is a wearable electrochemical sensor based on non-invasive fibrous materials. The rGO/PU fiber was prepared by a modified wet spinning technique, and the NCGP fiber electrode was prepared by coating Ni-Co MOF nanosheets on the surface of rGO/PU fiber. The three-electrode system based on NCGP fiber was sewn together with a water-absorbing fabric and fixed on a stretchable PDMS film substrate to prepare an enzyme-free sweat glucose wearable sensor (Fig. 6 C), which realizes high-precision real-time monitoring of glucose in human sweat. The sensor has a high sensitivity of 425.9μA·mM
-1·cm
-2 and a wide linear range from 10 μM to 0.66 mM. The Kwon team developed a flexible, wearable aptamer sensor (Figure 6D) for simple, rapid, real-time monitoring of cortisol
[70].
Neuropeptide is a neurotransmitter mainly distributed in the central nervous system and peripheral nervous system, which can be used as a potential biomarker of stress and depression. Lee et al. Reported a hollow microfiber patch of stretchable elastic nanocomposite for human biosensing of the stress biomarker neuropeptide Y in sweat
[71]. The patch consists of a stretchable liquid treatment device and an electrochemical impedance immunosensor. The patch sensor has a detection limit as low as 50 fM, requires no labels or reagents, and is easy to form, making it a promising wearable non-invasive immunodetection method. Electrochemical sensing for sweat detection is a method of modifying the sensor to enable it to selectively identify and detect the markers in sweat.It has the advantages of high sensitivity, good repeatability and accuracy, and linear output, but it may be more sensitive to temperature and less affected by the environment, which needs to be developed urgently. At the same time, most fibers and textiles have good moisture absorption effect, and the application of fibers and textiles in the sweat sensor can effectively absorb sweat and provide basic support for the work of the sweat sensor.However, sweat may have problems such as small amount and volatilization, so the use of trace sweat for real-time and accurate detection is also the key direction of future development of sweat sensors.
3.1.2 Colorimetric sensing
There are many components in sweat, and it is difficult to detect many components at the same time. The Xu team proposed a flexible, self-healing, wearable hydrogel patch for on-demand sweat colorimetric detection
[72]. The wearable hydrogel patch can be made by introducing a colorimetric reagent into the PVA/sucrose hydrogel through a solvent displacement method, and the preparation process is simple, and the integration with a smartphone makes it have excellent reliability and stability in detecting glucose in sweat (Figure 6E). It was directly attached to human skin for in situ sweat sampling and colorimetric analysis, and the analyte concentration was effectively converted into RGB digital signals by the wearable hydrogel patch (Figure 6 F), which enabled personalized in situ point-of-care detection. At the same time, a hydrogel substrate is designed and processed, and patterned micropores are physically punched on the hydrogel substrate, so that users can select the wearable hydrogel patch corresponding to the detection target according to their own needs. Most colorimetric sensors can qualitatively monitor the composition of sweat in real time through color changes, so as to have a certain understanding of the composition of sweat, but to obtain the specific composition content, it needs to be quantitatively analyzed by other detection technologies.
3.1.3 Fluorescence sensing
Fluorescence sensing has high selectivity, which can detect glucose in sweat with high sensitivity in sweat detection, and can also be combined with smart phones to achieve non-invasive in situ detection of glucose, lactic acid, chloride and other biomarkers. Golmohammadi et al. Manufactured a cellulose-based wearable patch
[73]. The smart wearable sweat patch sensor includes a highly fluorescent sensing probe embedded in a paper base and a microfluidic channel composed of cotton thread, which enables sweat to be collected from the skin surface and transmitted to the sensing probe. The sensor is a portable, selective, rapid and non-invasive healthcare monitoring device that enables indirect fluorescence monitoring of glucose using the quenching effect of hydrogen peroxide on fluorescein decorated on a paper disc. However, there are few reports on fluorescence sensors in sweat sensing, and fluorescence detection requires external light source excitation, which limits portable applications and increases the cost of detection, but this also provides an idea for our work, and we can consider developing miniaturized optoelectronic integrated sensors to solve this problem in future research.
3.2 Saliva
Saliva is a unique matrix known as the "mirror" of the body because it highly reflects blood drug concentrations
[74⇓~76]. It is mainly composed of water (98%), and the rest is a combination of various components, such as enzymes, electrolytes, antibodies, mucins, cytokines, etc
[77].
3.2.1 Electrochemical sensing
Electrochemical sensing is one of the commonly used methods to detect saliva, and Wang et al. Reported a wearable electrochemical sensing device that can simultaneously detect tetrahydrocannabinol and alcohol in saliva directly and dispersively
[78]. The sensor platform consists of a sensor embedded in the ring cover and a wireless electronic device embedded in the ring box. Tetrahydrocannabinol and alcohol in the same diluted saliva sample can be detected simultaneously within 3 minutes without interference between the two analytes or interference from the salivary matrix. Compared with sweat electrochemical sensors, sensors developed for saliva have obvious advantages, such as higher concentrations of tetrahydrocannabinol and alcohol in saliva, which make saliva detection more convenient, accurate and efficient.
3.2.2 Colorimetric sensing
Colorimetric methods are often used in the sensing field, and Ghorbanizamani et al. Reported a Lateral flow immunoassay (Lateral flow immunoassays, LFIA) based on colorimetric evaluation, using rhodamine B-loaded polymer capsules to detect illicit drugs (cocaine and methamphetamine) in saliva samples
[79]. Polyethylene glycol monomethyl ether and polycaprolactone were used as raw materials to synthesize polymers, which were loaded with dyes, and the analyte was quantitatively measured by smartphone assisted imaging technology combined with LFIA. Illicit drugs such as cocaine and methamphetamine can enter the body through smoking, inhalation or intravenous injection, but regardless of the route of ingestion, saliva is highly reflective of the consumption of these molecules with long half-lives, and most of the molecules detected are the original drugs with traces of their metabolites. Synthetic and real human saliva were tested, and the results showed that the overall accuracy was 97% and 98.4%, respectively, with high reliability. In addition to this, the sensor has specificity against various interferents as well as excellent stability.
3.3 Urine
There are many components in urine, such as glucose and uric acid, which can reflect kidney problems and are of great significance for early kidney disease examination or early monitoring and screening of other pathological changes
[80⇓~82].
3.3.1 Electrochemical sensing
For people with urinary incontinence, it is difficult to collect and detect their urine, and urine is a commonly used detection technology for disease diagnosis in clinic, which can reflect a variety of health information, so for people with urinary incontinence, low-cost in situ urine detection device is very important. Liu's team reported a smartphone-and-diaper-based wearable sensing system for in situ detection of biomarkers in urine
[83]. The system consists of a diaper with an integrated electrochemical biosensor, a portable detection device, and a data processing application on a smartphone. Polyethylene terephthalate (PET) was used as the substrate of the sensor, and the silver/silver chloride (Ag/AgCl) reference electrode, carbon Working electrode (WE) and Counter electrode (CE) of the glucose and uric acid sensor were printed on PET. Glucose oxidase (GOx) and urate oxidase were immobilized on the surface of the working electrode to detect Glucose and uric acid in situ. The sensor has good sensitivity, linearity and selectivity, and has great potential application prospects for revealing certain disease information through urine detection in combination with smart phone detection.
3.3.2 Piezoelectric sensing
Wearable piezoelectric sensors can not only be used in physical sensing such as breathing and heartbeat, but also in chemical composition detection. Pathological jaundice can make the baby's body yellow, even anemia, hepatosplenomegaly and other symptoms at the same time, which has a great impact on the safety of newborns. Severe cases are manifested as listlessness, anorexia and even dyspnea. Xue's team fabricated a self-powered wearable biosensor for real-time monitoring of neonatal jaundice in an infant diaper by using the hydrophilic-biosensing coupling effect of zinc oxide nanoarrays (Figure 7)
[84]. The working mechanism is that urine flows on the ZnO nanoarray and reacts with enzymes on the surface (bilirubin and bilirubin oxidase), which can produce bilirubin-dependent water output. The output can not only provide energy for the sensing process, but also be used as a biosensing signal. Bilirubin in the urine was monitored to determine the possibility of jaundice in the infant. The sensor can be used without external power supply and the sensing information can be transmitted through a wireless transmitter, so that the neonatal jaundice can be treated in time. The device can be embedded in the absorbent layer of the baby diaper, can be well fitted with the diaper, and has the advantages of good flexibility and small volume. The textile-based wearable piezoelectric sensor, which converts the information of biomarker components into voltage signal output, has the advantages of simple structure and convenient operation, and is expected to be popularized in the future research of integrated biosensors for energy supply and detection.
3.4 Tear
Human tears can reflect a lot of health information, especially eye health problems. Tears are rich in proteins, and studies have found that the proteins in tears are similar to those in blood because plasma can penetrate through the tear screen. Tears can be collected non-invasively, and although blood-based testing is still irreplaceable, tears can still provide samples for human health monitoring for continuous monitoring of diabetes and glaucoma
[85].
3.4.1 Electrochemical sensing
With the increasing number of diabetic patients worldwide, the development of a wearable glucose sensor capable of real-time self-monitoring of blood glucose is necessary compared with the traditional finger-prick detection method. Mitsubayashi et al. Reported a contact lens biosensor for in situ continuous monitoring of tear glucose
[86]. The biosensor consists of a flexible platinum working electrode and an Ag/AgCl reference/counter electrode. Biocompatible 2-methacryloyloxyethyl phosphorylcholine polymer and PDMS are used as biosensor materials, and GOx is used to modify the sensing region electrode. The sensor includes an integrated power source, a body wire communication transmitter, a circuit, and a sensing probe. Since the wearable structure of the biosensor is similar to a contact lens, non-invasive ocular biomonitoring with minimal irritation is established. Electrochemical sensors are practical and efficient for real-time detection of tears, but their integration into contact lenses is costly and requires external electrodes, which reduces the portability of wearable sensors to a certain extent.
3.4.2 Colorimetric sensing
In fact, the research and development of wearable biosensors based on tear detection is still in its infancy. Generally, the sensing elements are integrated into contact lenses, and long-term wearing of contact lenses may bring many side effects, such as dry eye, corneal damage, etc., which is not really "non-invasive sampling". To solve this problem, Yang et al. Recently developed a non-invasive flexible eye-patch wearable paper-based sensor, which can be easily and comfortably attached to the skin under the eyes to avoid direct contact with the eyes.Non-invasive detection of biomarkers such as pH, ascorbic acid, human serum albumin and glucose in human tears can be achieved with only one drop of tears and a response time of 30 s (Fig. 8)
[87].
4 Smart fibers and textiles for exhalation detection
The exhalation of the human body will reflect the health status of the body, and timely detection of the exhalation can well understand the physical condition, which is of great significance to the prevention and early treatment of infectious diseases and major chronic diseases. It is worth noting that the patient's exhaled breath may contain bacteria, viruses, or Volatile organic compounds (VOCs). Among them, exhaled VOCs, such as ketones, are important markers of diabetes. In addition, for patients with infectious diseases, the exhaled virus can also provide an important basis for the detection of infectious diseases. Here, we mainly summarize and discuss the use of smart fibers and textiles for VOCs in exhaled breath and the current popular COVID-19 detection research.
4.1 Ammonia gas
Human exhaled breath contains abundant physiological and pathological information, which can be used for health assessment and disease prediction
[88,89]. The Su team developed an integrated flexible triboelectric self-powered respiratory sensor based on cerium-doped zinc oxide nanoparticles for simultaneous monitoring of human respiratory behavior and NH
3 concentration in exhaled breath
[90]. Its working mechanism is chemical adsorption and electrostatic induction, and the sensor has higher sensitivity to low concentrations (0.1 ~ 1 ppm) of NH
3 than high concentrations (1 ~ 10 ppm) of NH
3, which means that it can detect trace amounts of NH
3 biomarkers in human breath gas. When exposed to higher concentrations of NH
3, the output voltage will decrease, and the sensor shows good stability and reliability in human breath ammonia monitoring.
4.2 Acetone
Acetone vapor has strong volatility and toxicity, and acetone in human breath can reflect a lot of human health data
[91⇓⇓~94]. Breath analysis is a rapid, noninvasive, painless, low-cost, and simple alternative to blood analysis and endoscopy that enables early disease diagnosis and real-time physiological monitoring, and is a biomarker that carries a wealth of physiological and pathological information. Acetone concentration in exhaled breath has been reported to correlate with blood glucose levels and thus may reflect the status of prediabetic patients.
Here, based on the coupling effect between triboelectrification and chemisorption, the Su team reports a newly designed self-powered acetone sensor that provides an effective and painless method for the diagnosis of prediabetes for sustainable and continuous respiratory monitoring and prediabetes diagnosis
[95]. This work is expected to provide an economical and convenient method for non-invasive wearable pre-diabetes diagnosis. The self-powered acetone sensor supporting wireless power transfer consists of two main parts: an energy harvesting component and a gas sensing component. The energy harvesting component consists of a flexible Polytetrafluoroethylene (PTFE) membrane, a nylon membrane and a support beam. In this work, PTFE and nylon were chosen as triboelectric layers, and the chemisorption of acetone molecules leads to charge transfer and corresponding dielectric constant change in the sensing film, which changes the shielding effect of the sensing layer and thus the output signal. The fabricated sensor exhibited excellent acetone sensing response, sensitivity, selectivity, and moisture resistance, showing the feasibility of practical respiration monitoring.
4.3 Novel coronavirus detection
COVID-19 is still spreading around the world and causing serious sequelae to patients, so it is very important to prevent and detect COVID-19. Collins team reported a lightweight, flexible substrate and textile functionalized with lyophilized cell-free synthetic circuits based on Clustered regularly interspaced short palindromic repeats,CRISPR) is used to detect metabolites, chemicals and pathogen nucleic acid signals, and is a wearable lyophilized cell-free system sensor for the detection of various small molecules, nucleic acids and toxins, which has good elasticity and flexibility
[96]. Detection limits for nucleic acids are comparable to current laboratory methods such as quantitative PCR methods by reporting the presence of specific molecular targets through colorimetric changes or through a fiber optic network that detects fluorescence and luminescence output. At the same time, the team designed a mask with a lyophilized CRISPR sensor. Piercing the blister reservoir allows water to flow through the wick material, moving the virus particles collected from the wearer's breath from the sample collection area to the μPAD device, which is finally visualized by the Lateral flow assay (LFA) strip. Wearable and non-invasive detection of SARS-CoV-2 within 90 minutes at room temperature, users only need to press the switch, no other operation is required. The sensor can help people detect COVID-19 while doing a good job of protection, which greatly saves the manpower, material resources and time needed for detection.
5 Summary and Prospect
Intelligent fibers and textiles have attracted wide attention in the 21st century
[97]. In this paper, we introduce wearable biosensors based on smart fibers and textiles in combination with sensing technology.It includes vital signs (respiration, pulse, blood pressure, temperature and humidity, limb movement) monitoring, body fluid (urine, saliva, tears, sweat) detection and analysis, and exhalation detection that may be related to human health, and summarizes its advantages and problems.
Wearable biosensors based on smart fibers and textiles are flexible, lightweight, comfortable and flexible, and transmit signals more sensitively, intuitively and sustainably. Through the selection of materials and the design and development of processing methods, intelligent fibers and textiles with different functions can be prepared for target detection. At the same time, numerous sensing technologies, including piezoelectric sensing, resistive sensing, capacitive sensing, triboelectric sensing, colorimetric sensing, fluorescence sensing, electrochemical sensing and so on, provide the possibility for the development of more intelligent and advanced wearable biosensors.
Smart fibers and textiles have many advantages in the application of wearable biosensors, but they also face many challenges: (1) At present, due to the high preparation process requirements and production costs, the biosensors are mostly prepared in the laboratory and have not yet been put into large-scale production. The key to solving this problem lies in the development of low-cost large-scale preparation technology for high-quality intelligent fibers or textiles. (2) Many smart fibers and textiles need surface processing, and the modification materials coated or deposited on the surface of fibers or textiles may fall off if the modification process or materials are not properly selected under the conditions of washing and repeated kneading, resulting in a short service life and affecting the performance of wearable biosensors. To solve this problem, when developing sensors, we should comprehensively consider their use scenarios, reasonably select intelligent fibers and textile materials, and solve the problem of combining sensitive elements with substrate materials to ensure the service life of materials. (3) Due to the non-invasive characteristics, when the biosensor is used for body fluid detection, the detection targets are mostly sweat, tears and saliva, but there is a certain deviation between the detection results and blood detection. Therefore, it is very important to determine the relationship between blood detection and sweat, tears, saliva, etc. At the same time, it is also very important to improve the sensitivity of the sensor and low detection limit. (4) Fibers and textiles are soft and breathable, but the inherent advantages of textiles will be sacrificed when they are functionalized. Therefore, when developing textile-based biosensors, how to maintain the original flexibility and air permeability of the fiber without damaging the sensing sensitivity is also a problem to be considered. From the point of view of the weaving mode of textiles, reasonable weaving modes can be designed and utilized to make up for the flexibility and air permeability of fiber loss in the functionalization process. (5) People know little about wearable biosensors, which makes the formal production and sale of wearable sensors face market demand problems. Manufacturers may price them higher because of their rare market, and consumers may be in a wait-and-see state because of new products and higher prices. Therefore, the sensor should be designed according to people's needs, not only to ensure its high sensitivity and high accuracy, but also to take into account its cost and easy operation.
In addition, single signal detection can no longer meet people's daily needs, so it is urgent to develop multi-functional integrated wearable sensors to achieve a variety of signal output, which is the development trend of wearable sensors
[98⇓~100]. Wang and his team at Tohoku University reported a wearable biosensor based on a stretchable fiber triboelectric nanogenerator
[101]. The stretchable fiber triboelectric nanogenerator is fabricated by sequentially covering multi-walled carbon nanotubes and polyaniline on an ultra-stretched copolyester fiber and then winding an enameled wire. The sensor can accurately sense the state of movement and detect glucose, creatinine and lactic acid in sweat in real time. Based on the triboelectric effect and the coupling effect of enzymatic reaction (surface-triboelectric coupling effect), the wearable biosensor can be self-driven without any external power supply, which can reduce the carbon emissions caused by battery production and maintenance in the current global green energy revolution.
In a word, wearable biosensors based on smart fibers and textiles have many advantages, and how to solve the current problems and challenges is very important, but it has been confirmed that such wearable biosensors have universal stability and reliability in monitoring human health data, and have great potential application prospects in human health monitoring.
National Natural Science Foundation of China(22204089)
National Natural Science Foundation of China(21890742)
Natural Science Foundation of Shandong Province(ZR2020QB092)
China Postdoctoral Science Foundation(2021M691689)
Postdoctoral Applied Research Project of Qingdao(202142)
State Key Laboratory of Bio-Fibers and Eco-Textiles (Qingdao University)(ZKT23)
State Key Laboratory of Bio-Fibers and Eco-Textiles (Qingdao University)(GZRC202025)
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