Review—Textile Based Chemical and Physical Sensors for Healthcare Monitoring

Conductive polymers are organic polymers exhibiting electrical properties similar to those of metals. Polypyrrole, polyaniline, and polythiophene fibers are good examples in practical flexible and stretchable electronic devices, and they can be produced with different sizes and even in nanometer dimensions, with high conductivity (120–130 S cm⁻¹) at room temperature.^10,41,42 To provide the necessary biocompatibility and increased resistance to mechanical stress these polymers can be modified and produced in different shapes such as fiber or nanofiber. Nanofiber is a class of nanomaterials having a diameter in the submicron range (50–500 nm), small pore size, sometimes with high porosity and high surface area with length is in the micron or millimeter range such as polyaniline nanofibers.⁴³ Due to their high surface area, they possess unique properties that make them potential candidates for a wide range of applications such as filtration, barrier fabrics, protective clothing, wipes and biomedical applications such as scaffolds for tissue engineering.⁴⁴ Different methods such spin coating, printing or chemical or electrochemical treatment can be used to synthesize polymer nanofibers.⁴⁵ Among these, the printing provides a convenient route to the deposition of conducting materials such as polymers with spatial resolution in the X,Y plane and makes a layer with thicknesses in nanometer dimensions. In addition, the screen-printing method provides an easily adopted fabrication route for the fabrication of wearable electronics, where all the layers with different functions can be printed on top of the fabric substrate with a layer-by-layer process. Another significant advantage of this method is that they facilitate the use of low-cost patterning techniques at room conditions.^46–48 Generally, the most promising materials are organic polymers because they are intrinsically flexible or can be easily used for making desired composite materials with special chemical, physical, and electronic properties.

Zero, one and two dimensional carbon-based micro/nanomaterials such as graphene (nano nanosheets), carbon nanotube, carbon nanoparticles and its derivatives are promising materials for flexible and wearable electronics, due to their unique properties such as their high electron transfer rate and mechanical properties, large surface area, environmental stability and the potential for production at low cost.⁴⁹

Among the various carbon materials, carbon nanotubes and graphene are two of the most intensively explored carbon allotropes in electrochemical sensors that have been evaluated as the electrode materials with great potential in wearable electronics.⁵⁰ A stretchable, porous, and conductive textile has recently been developed by a simple 'dipping and drying'' process using a carbon nanotubes ink.⁵¹ Interestingly, some efforts have been made to assemble carbon nanotubes into macroscopic fibers in which the carbon nanotubes are highly aligned to maintain high mechanical strength and electrical conductivity of individual carbon nanotube. Particularly, carbon nanotube fibers are anticipated to have a broad range of potential applications, such as fiber-based microelectrodes and (bio) sensors.

Much attention has been paid towards the preparation and applications of nanostructured coatings onto textile substrate due to their promising applications.^52,53 Low dimensional nanostructures of metals, e.g., nanowires, nanorods (ZnO, Ag) or nanoparticles (such as Au, Ag, or Pt), are particularly attractive for fiber-based flexible and wearable electronics because some of these like gold and silver have very high conductivities.^{10,39,53–56} The nanostructured films deposited on textiles have exhibited excellent properties such as conductivity and antimicrobial activity. However, stability, is currently a big challenge and efforts are being made on improving the stability of metallic nanowires and nanoparticles on a textile substrate and utilizing them for inflexibly electronic applications.⁵² Moreover, nanocomposite materials are good choice, as the polymers can be combined with nanostructures that may acquire sensing capability via conductive materials printed onto fibers or fabrics. The various approaches to make fiber-based flexible and wearable electronics by using conductive metal(s) can be grouped into two categories. In the first one, the electronic textile can be fabricated by using conducting fibers made from conductive metal such as steel. While the second category is thin-film-based electronic components where the different methods such as electro-spinning, wet spinning, spin coating, printing or chemical treatment (hydrothermal method) are being used for the preparation of materials. The printing provides a convenient route to the deposition of conducting nanocomposite materials with different thicknesses and the desired materials can be printed on top of the textile substrate through a layer-by-layer process.^10,46,53,57 In all cases, the optimal textile candidate for supporting printable electrochemical sensors have been predicted to have inert properties and yield stable operation for extended periods under normal and heavy wear.

The chemical wearable sensors include (bio) electrochemical and optical and gas sensors where a chemical reaction with the participation of the analyte (ions or (bio) molecules) gives rise to the analytical signal (Figs. 2 and 3). The gas sensors classified into chemical sensors are based on change in the conductometry. Wearable (bio) chemical systems can be used for health monitoring, military applications etc.⁶² Among the different sensors, electrochemical (bio) sensors have a unique position for addressing the needs for field screening and identification of various chemical from ions to gas molecules. They have been found to be suitable for manufacture and miniaturization. Simplicity, low-voltage operation and low-power consumption of electrochemical sensors make them suitable for the requirements of on-body wearable systems into clothes. They are compatible with diverse materials and can operate with inexpensive battery-powered electronic instrumentation.^61,62 Electrochemical wearable sensors based on textile are being used for the detection of ions (Na⁺, H⁺, Ca²⁺, NH₄⁺, etc) (Figs. 2 and 3), biomolecules (glucose, lactate and uric acid) in different body fluids (sweat and saliva) and volatile compounds (such as: CH₄, NO₂, NH₃, CH₃OH, (CH₃)₂CO and body odor) typically present in a body or in our environment (Table I).

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In this kind of sensors, voltammetric, potentiometric, and amperometric techniques can be used. These sensors and their parts can be embedded into or on textiles such as in belt, arm hand, shirt, underwater garment, bandages and jacket and etc.^61,75,76 Numerous sensors based on textiles, cloths or yarns that show potential for integration with clothing can be used as (bio) sensor. Table I shows the characteristics and analytical performances of the textile electrochemical (bio) sensors. The researchers have tried to design the architecture of the removable sensor to eliminate problems commonly associated with the washing of integrated textile sensors. For fabricating clothes with electrochemical sensors, the direct screen-printing of electrodes on the textile substrate is convenient and straightforward technique.^20,46 However, it depends on differences in the hydrophobicity of various textile substrates such as cotton, polyester, and Gore-Tex (waterproof, breathable fabric).²⁰ Owing to the extreme hydrophobicity exhibited by the Gore-Tex fabric in comparison with other natural, synthetic fabrics, electrochemical sensing is facilitated; it has been shown to be a promising substrate for thick-film electrochemical sensors. Another attractive strategy was reported by J. Wang et al.³⁶ wherein the cotton yarns (Fig. 2b) were immobilized with carbon nanotube ink and it was used as a potentiometric sensor for detection of pH, K⁺, and NH₄⁺. Finally, they could fabricate a novel pH-sensitive bandage (Fig. 2g). The potentiometric time-trace of a pH bandage sensor from pH 8.51 to 2.69 along with EMF dependence vs pH for the PANI conducting polymer ISE is shown in Fig. 2h.

In sports (Figs. 3a–3f), bioelectrochemical^21,76,74 textile sensor plays a critical role. Biosensors are closely related to chemical sensors, distinguished by the molecular recognition element which is biological in nature (e.g. enzyme). The glucose⁶⁸ and lactate⁷⁴ sensors based on textiles have also been investigated. Figures 3a–3d are typical chronoamperograms of the tyrosinase-modified neoprene SPE exposed to catechols (such as 4-chlorophenol and phenol) and the stability of the signal over time with respect to the initial measurement for proposed sensors, respectively. As can be seen they developed flexible electrochemical sensors on underwater garments that are able to detect trace levels of marine pollutants and explosive threats in seawater. The electrode response stability was examined over 50 min, relative to the initial measurement for (i) 4-chlorophenol and (ii)phenol. These tests yielded a low RSD for each compound, ranging from 2.07% to 1.73%, respectively.

By using smart textile sensors, it is estimated that 70% of all illnesses are preventable, if suitable screening measurements could be introduced, this could produce dramatic reductions in costs for treatments and medication.^3,87 For example, by adopting suitable exercise and diets, many lifestyle-related illnesses can be prevented. Exercise generates sweat which contains rich information about the physiological condition of the subject as it contains a matrix of essential ions and molecules.^36,78 Real-time sweat analysis (Figs. 3f–3g) during exercise can give valuable information on dehydration and changes in the number of essential biomolecules and ions such as K⁺ and Na⁺ ions. Sweat, saliva, and tears contain multiple physiologically relevant chemical constituents and can be easily obtained continuously for non-invasive real-time monitoring of these analytes. For example, by monitoring level of electrolytes (pH, sodium, ammonium, calcium) and metabolites (glucose, lactate, urea) in sweat and the skin interstitial fluid with suitable sensors can provide valuable health information.^88,89

Despite the extensive progress made in wearable electrochemical (bio)sensors for physiological monitoring, military application, and sports, little attention has been directed at the optical wearable (bio)sensors,^68,67 due to the difficulty for use in optical systems and related instruments into textiles and clothes. These sensors require the desired sample for analysis. Usually, a body fluid such as sweat is delivered to the sensor active surface, after which a reaction occurs, and a signal is generated in a few minutes in these sensors, textiles can act as a micro pump. For example, Schazmann et al.⁸⁸ reported a smart belt that electrode set on textile and it was located on the body to monitor Na⁺ ions in sweat in real time. In this sensor, textile plays the role of sweat pump to the sensor area.

In the optical sensors, a colorimetric chip based on iono-gel (Fig. 4a) has been developed for monitoring pH of sweat.⁶⁸ There are many factors that can be correlated with sweat pH and health. Changes in the pH of the skin are reported to play a role in the pathogenesis of skin diseases like irritant contact dermatitis and acne, and also, showed that inducing metabolic alkalosis through the ingestion of sodium bicarbonate may lead to increased blood and sweat pH. In addition, it has been reported that sweat pH rises in response to an increased sweat rate. Schematic illustration of the micro-fluidic system's performance over and structure of the probe is shown in Figs. 4a–4c. Accordingly, The probe is applied to the monitoring of the pH of sweat during an exercise time. It was the first wearable sweat sensor with an ionogel being an important component in the fabrication of the sensing platform. The ionogel plymer provides an ideal substrate for the organic pH indicator dyes (bromophenol blue, bromocresol green, bromocresol purple and bromothymol blue). It could be attached onto the textile and could be used for monitoring the pH from 0 to 14 in intervals of one pH unit (Fig. 4c). This sensor showed no leaching of the indicators during experiments, the prolonged stability of the device for the monitoring of sweat pH measurements over time and the micro-fluidic structure (Fig. 4a) guarantees that freah sweat is continuously transferrd from the skin to the ionogels.

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Shirley Coyle et al.⁶⁶ reported a new optoelectronic probe on textile using a simple photometer (Fig. 4d) where a textile-based platform with fluid handling properties was used to collect and analyze the sweat samples. A fabricated channel is created by screen printing an hydrophobic acrylic paste on either side of the fabric. At one end of this channel is a window where sweat enters from the skin, based on capillary flow through the fabric. For sensing pH, a sensitive dye (bromocresol purple) is immobilized onto the channel (Fig. 4e). The dye for sweat analysis exhibits color changes from yellow to blue over this pH range A paired emitter-detector LED was used to quantify the color change of the dye. The fabricated optoelectronic textile based sensor was embedded into a waistband as a model and tested on the body during exercise succefully.

Recently, N. Promphet and et. al.⁷² developed a textile-based colorimetric sensor for the detection of lactate and sweat pH and by depositing three different layers (chitosan, sodium carboxymethyl cellulose and indicator dye for pH sensing for lactate assay) onto a cotton fabric (Fig. 4g). The sensing probe for pH was composed of a mixture of two organic dyes (methyl orange and bromocresol green). As the pH of sweat from 1−14, the pH indicator shifted from red to blue, whereas the purple color intensity was enhanced by increasing lactate concentration (reange 0–25 mM) in the sweat (Figs. 4h–4i). The proposed sensor was tested on three human volunteers before and during jogging successfully.

The physical wearable sensors (temperature, pressure, obstacle and strain sensors) can be fabricated using textiles (Fig. 5). In general, a physical sensor, defined as a device that detects a change in a physical stimulus, turns that stimulus into a signal that can be measured or recorded.^34,78

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These sensors have drawn considerable attention, especially in medicine and sports. With the increase in the size of the elderly population, at the same time the emergence of chronic diseases because of the changes in lifestyle, there is a need to monitor the health status of individuals in their daily routine lives. The regular activities of the human body generate various physical stimuli, such as body temperature, blood pressure, the pulse skin and strain. On the other hand, physical sensors are capable of measuring these types of physical parameters or health indicators.³⁴ Generally, the four main vital signs routinely monitored by medical professionals are:

• i.  
  Body temperature (exploration of infection and fever).
• ii.  
  Heart rate (exploration of cardiovascular diseases).
• iii.  
  The rate of breathing or respiration rate (it may increase with fever, illness, and with other medical conditions, the typical respiratory rate for a healthy adult at rest is 12–20 breaths per minute).
• iv.  
  Blood pressure (in the exploration of hypertension, note: blood pressure is not considered a vital sign, but is often measured along with the vital signs).^10,34,78

Researchers have fabricated a smart T-shirt that functions to monitor the wearer's heart rate, respiration, temperature, and other vital signs.⁹ The Wearable Health Care System project has developed a wearable garment that can be worn under regular clothing and is capable of recording biomechanical variables and physiological signals.^33,92,93 In addition, motion signals were monitored because they can be used to extract important physiological information such as the stress level or movement of a person (in some research such as research on Parkinson's disease is very useful). In all of them textile and clothes can play critical roles, and body parameters can be detected with physical wearable sensors based on textiles.

Temperature sensor

The body temperature is regarded as the first vital sign. An increase and decrease of body temperature can be due to an infection or fever and low blood flow rate (due to circulatory shock), respectively. Generally, body temperature is about 32 °C, and for measuring the temperature, the suitable places are the arm or the chest.^3,7 The textile-based sensor can be used as a temperature probe based on thermistors (in thermistor electrical resistivity changes with variation of temperature). Generally, in these sensors the following formula has been used.^94,95

$$\begin{eqnarray}&&\mathrm{ln}\,{R}_{t}=\,\mathrm{ln}\,{R}_{0}\,+\beta \left(\displaystyle \frac{1}{T}-\displaystyle \frac{1}{{T}_{0}}\right)\end{eqnarray} \tag{ 1 }$$

In this equation, a linear relationship between ln (R_t) and 1/T is established. Here, β (in Kelvin) is the material constant for the thermistor (it can be a metal oxide, carbon nanotubes, graphene, nickel oxide).^95–98 R_t is the resistance at temperature T, and R₀ is the resistance at T₀. β with a temperature coefficient of the thermistor have main roles in determining the sensitivity of the thermistor.⁹⁵

The thermistors should have high static and dynamic accuracy in the human skin temperature range (30 °C–45 °C) and can be embedded into or attached on textiles. Detection should be linear with possibly high-temperature efficiency and resistant to humidity is expected as an environmental challenge.^99,100 Jung et al. recently found that combination of poly (3,4-ethylene dioxythiophene) poly (styrene sulfonate) (PEDOT:PSS), silver nanoparticles (AgNPs), and graphene inks can be used to fabricate a highly stretchable self-powered temperature sensor. Also, the printing technique with commercialized thermoelectric materials was utilized on stretchable knitted fabric with excellent elasticity as an affordable and scalable process. In fact, temperature gradients over the printable maters can be fabricated to control the current via the 'Seebeck effect''.¹⁰¹ Hence, temperature difference across the device is detected automatically and without need to any power supplier.¹⁵ In another approach, T. Hughes-Riley et al.⁹⁰ reported a new electronic temperature-sensing yarn that was well-suited for making long-term monitoring of skin temperature when incorporated into a textile with a remote system (Fig. 5a). The miniatured sensor were suitable for wearable applications. They implanted a commercial thermistor chip into the fibers of a yarn, which can be used to produce textile. To protect the thermistor, it was encapsulated in a resin. The fabricated textile showed a short response time over the temperature range of interest (between 0.01–0.35 s). Finally, they could make socks (Figs. 5a–5d) as a prototype temperature-sensor and it was suitable for diabetic patients.

Obstacle detection system

Pressure sensor

A textile-based pressure sensor has been explored for a variety of applications to include sports, caring for the elderly, diagnostics and human motion detection due to their integrability into clothes, socks, shoes, pillow, belt, headband, and even beds^102,104 (Fig. 4c). In all these sensors, the determination of pressure regions or sensor positions for example in health or sports cases are very important. As we know under the human feet or shoes, different pressure regions are available.

For measuring pressure, many types of sensors such as capacitive,¹⁰⁵ piezoresistive,¹⁰⁶ piezoelectric,¹⁰⁷ and optical¹⁰⁸ have been developed. Among these, a capacitive pressure sensor has advantages in terms of its simple design, low power consumption, acceptable sensitivity, and high stability. A capacitive pressure sensor makes use of the principle of a parallel plate capacitor, which consists of two compliant conductive layers (such as conductive fabrics) and flexible spacers (flexible dielectric such as foams, fabric spacers and soft polymers). The amount of electrical charge (Q) that can be stored between the two conductive plates of such a capacitor at a given voltage V is defined by the capacitance C of the capacitor. C can be expressed as a function of the distance d, the given area A and the material between the two plates, therefore, C can be defined as:

$$\begin{eqnarray}&&C=\displaystyle \frac{{\varepsilon }_{0}\,{\varepsilon }_{r}A}{d}\end{eqnarray} \tag{ 2 }$$

where, ε₀ (ε₀ = 8.854 × 10⁻¹² F · m⁻¹) is the vacuum permittivity and ε_r is the relative dielectric constant of the dielectric material. In order to improve the performance and efficacy of the capacitive sensor Many efforts have been made for the selection of a suitable interlayer material or modification with nano-conductive materials (metallic nanoparticles, such as silver nanoparticles as filler or doping agent), and enlarge the deformation space and extend the surface area of the electrodes.¹⁰⁹ A textile capacitor has been fabricated using two electrodes of conductive woven textile separated by compressible spacer material. Meyer,¹¹⁰ developed a pressure sensing pillow with several integrated textile capacitive pressure sensors made from a 3D knit fabric spacer material and conductive textile filaments coated with silver. The developed sensor could be used to detect pressure with pressures up to 10 N cm⁻². An ideal pressure sensor must ensure repeatable measures over time with a low hysteresis value. Therefore, the textile pressure sensors are applicable in areas where high accuracy is not required.¹¹⁰

In a similar work, a smart adhesive bandage²⁴ for monitoring human movement was developed (Figs. 4g–4i). For the fabrication of electrode of the pressure sensor, silver nanowire mixure dispersed in ethanol was drop-coated on to a polyethylene terephthalate film substrate to produce conductive networks. To monitor human movement, the PET film substrate was replaced by an adhesive bandage. Figure 4g shows the schematic of pressure sensor with low weight and high flexibility. The final sensor as a dielectric interlayer demonstrates high sensitivity, as 5.54 kPa−1 (p < 30 Pa) and 0.88 kPa−1 (p > 30 Pa), The adhesive bandage sensor was used as a substrate for detection of articular and muscular movement such as finger and knee bending (Figs. 4h–4i).

Heart rate (pulse) sensor

Traditionally, heart rate can be measured from the radial artery at the wrist and monitoring of blood pressure is performed in a clinic with trained personnel by mounting inflatable pressure cuffs with stethoscopes to the patient's arm, the so-called auscultatory method.^7,10 Heart rate depends on cardithe ac cycle (the cycle is the sequence of events that occurs when the heart beats that deoxygenated blood through the lungs and pumping newly oxygenated blood). The heart rate is the frequency of cardiac cycles, expressed as beats per minute (b.p.m.). To measure the heart rate more accurately, electrical sensors can be used. Generally, skin electrodes are used to pick up the depolarization signal from the heart muscles. This technique is called as electrocardiography.^111,112 A few sensors based on textile have been reported although a lot of research-based on wearaa ble sensor was reported,^99,100,113

Yoo and coworkers(100) designed a wearable electrocardiogram acquisition system implemented with planar-fashionable circuit board on a shirt. The proposed wearable continuous electrocardiography monitoring system is composed of two parts: a group of electrodes and a monitoring chip. The electrodes werescreen-printed directly on fabric. Unlike conventional wet electrode, to overcome these drawbacks of conventional wet electrodes (dry out of electrolyte gel, unpleasant using of gel for a long time and also reduce of toxicity of gel) for long-term monitoring, dry electrodes can be considered as a good alternativece to the wet electrode.

Lee et al.¹¹³ developed a non-contact electrocardiogram measurement system on a bed using conductive textiles for long-term monitoring of heart activities. At first, their goal was to design and develop more user-friendly system by configuring the array and size of electrodes with no constraints of posture and position in a bed, for this purpose they used a foam layer to provide comfort and follow the curvature of the body. Their results suggested that the capacitive coupled textile system was suitable for daily healthcare monitoring and abnormal state of sleeping. Finally, they tested the system to find the potential for monitoring the sleep stages, which is essential for observing the physical and psychological states.

To obtain a continues monitoring of electrocardiogram (ECG) for possible risk of cardiac arrest in babies, Coosemans et al. (114) developed an alternative to conventional ECG electrodes to be used in the long term with no skin irritation and allergic contact reactions. Figures 6a–6c shows the integrated circuitry and secondary coil and a sample set-up with the belt prototype on a three months old baby. General system overview is depicted in Fig. 6a. The measured ECG results of new electrode called Textrode demonstrated the lower frequent drift according to baseline along with a lower signal-to-noise ratio in comparison to conventional gel electrodes. Also, the reported heart rate accuracy is a beat-to-beat detection which suffices the application.¹¹⁴

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Respiration rate sensor