**1. Introduction**

Extreme exposure of the human body to high temperature can cause severe effects such as heat illness. For appropriate heat monitoring, thermometers, metallic electrodes, or sensor chips can measure human temperature directly in the doctor's office. However, their permanent adherence to the skin, especially during outdoor activities, can lead to the wearer´s discomfort and in particular to skin irritation. This can be avoided by using textile-based temperature sensors, which can detect first signs of a heat illness outside the doctor´s office.

Heat-related illness occurs when the body stores more heat than it can release, which is accompanied by symptoms as heat stroke, heat cramps, and heat exhaustion [1,2]. Avoiding heat illness is particularly important in sports and the mining industry [2,3], which can be achieved by monitoring the temperature of the human body by textile sensors [4–8].

Integrated sensors should exhibit significant flexibility and low weight characteristics [9], which is important for health care applications such as body temperature measurement [10]. In health care, the body temperature is detected on the human skin [9]. This data can be used to investigate wound-healing processes, assessing patient comfort, or monitoring temperature development during sleep [9]. The integration of thermocouples into textile structures is a straightforward solution for temperature monitoring [9].

A thermocouple consists of two different conductive materials, which are connected at one point/form a closed circuit. One connection point is termed the measuring junction and the second can be regarded as a reference junction [11]. The thermocouple develops a voltage between two different materials of wires that can be used to measure temperature. Due to temperature changes, a voltage is generated between the different materials. Therefore, output voltage is related to the change in temperature [12]. This effect was first described by Seebeck in 1826. He discovered that a current flowed in a closed circuit between two dissimilar wires when two junctions are exposed to different temperatures [11,13,14]. The output voltage (Δ*U*) is calculated by Equation (1):

$$
\Delta \mathcal{L} I = \alpha \times \Delta T \tag{1}
$$

where α is the difference in the Seebeck coefficient of the two metal conductors, and Δ*T* is the temperature difference between the cold and hot junction [12].

In 1834, Peltier observed a current flow when a junction of two different wires was cooled or heated [11,13,14]. Twenty years later, in 1854, Lord Kelvin (W. Thomson) concluded that the current flow results from a temperature gradient in the conductor [11,13,14]. Figure 1 shows the model of a thermocouple, which uses two different conductors, a signal detection, and signal processing unit. The dotted boxes around the measuring and reference junctions show that these regions are isothermal. Isothermal regions do not contribute to the voltage detection (Δ*U*). The blue and red marked regions are set to different temperatures [13].

**Figure 1.** The model of thermocouple detection and signal processing according to [13].

Based on this model, a thermocouple pair can be constructed. A thermocouple pair generates a voltage when two junctions are set at different temperatures (Figure 2). The change of temperature at one junction leads to a voltage change across the thermocouple pair, which is proportional to the change in temperature.

**Figure 2.** The first junction is heated to T1 while the second junction stays at temperature T2. According to [15], this results in an analog voltage signal.

Temperatures have been measured using various thermocouple assemblies such as thermocouple tips, [16], multipoint thermocouples [17], and a combination of dissimilar metal wires (platinum–rhodium alloy) [18]. Thermocouples can also be formed by printing techniques using iron, nickel, and copper inks [19]. The latest reports show that thermocouples can also be formed using two conductors of the same material with different material thicknesses [20–22].

Recent literature reports the invention of single-metal thermocouples consisting of different conductor widths, which are used for temperature detection [20–22].

Over the last three decades, the interest in the integration of temperature measurement systems into textiles has significantly grown. In Figure 3, this evolution is shown by the number of concepts for "Textile thermocouple", "Temperature measurements in textiles", and "Temperature sensors in textiles" published in the years 1990, 1995, 2000, 2005, 2010, and 2019.

**Figure 3.** Number of concepts for the integration of temperature measurement systems and thermocouples into textiles published until 2019. The SciFinder database was used for the literature search with the key words textile thermocouples, temperature measurements in textiles, and temperature sensors in textiles.

Regarding the concept of textile thermocouples, the slight increase in publications shows that there is no precise definition of textiles thermocouples in literature. In general, thermocouples can be integrated into textiles by various techniques:


For the ease of the reader finding the summary of five techniques, we have summarized them in Table 1, which permit having a quick overview of the methods.

Ziegler and Frydrysiak defined that textile thermocouplesmay bemanufactured from thermoelectrodes consisting of functionalized textiles of woven, non-woven and knitted fabric threads, twisted multifilaments, yarns, and fibers. The functionalization of textiles can be conducted with conductive nanoparticles or electro-conductive polymers [26]. A general definition of textile thermocouples used in this review could be stated as:

• Textile thermocouples detect changes in temperature and consist of an indispensable conductive textile matrix with a textile character.



Textile thermocouples should combine the flexibility and light weight of textiles with the conductive property of the conductor material, which can be defined as a truly textile thermocouple.

In this review, various integration methods and conductor materials for the construction of thermocouples in textiles will be described. This review discusses how textiles serve as appropriate carrier materials for the integration of temperature sensors. Di fferent aspects of manufacturing conductive textiles will be shown. An outlook is given, which emphasizes the advantages and limitations of thermocouples in textiles.

### **2. Concepts of Thermocouple Construction in Textiles**

Di fferent thermocouples have been used to measure temperature on woven, non-woven, and knitted textiles.

Figure 4a shows the construction of five thermocouple pairs, which consist of five aluminum conductor strips and a large copper-coated cellulose fabric as a second conductor. Using the copper-coated cellulose textile as a conductor material makes the thermocouple construction more flexible compared to metal wires. The size of the copper-coated cellulose textile can be varied, which allows the positioning of additional thermocouples independently (Figure 4b). This thermocouple construction needs only one conductor as a sensing line. Figure 4c shows a scheme of electron flow in thermoelectric materials. It describes the formation of a temperature di fference across a conductor when two junctions (regions) are set to di fferent temperatures. The hot junction (region) generates more free electrons compared to the cold junction. Thus, an electron flow occurs from the hot to the cold junction (region) [12].

**Figure 4.** The construction of five thermocouple pairs (**a**), the description of electron flow in thermoelectric materials (**b**) according to [12], and (**c**) an electrical circuit. U0 is the reference junction and U1, U2, U3, U4, and U5 are measuring junctions.

Thermocouples were manufactured from conductive poly(3,4-ethylenedioxythiophene), poly(4styrenesulfonate) (PEDOT-PSS) and polyaniline by screen printing on woven cotton textiles [23]. In addition, thermocouples were used to detect resistivity and temperature as a function of time (up to 35 h). Thermocouple assemblies made from PEDOT-PSS and polyaniline showed a Seebeck coe fficient of 18 μV/K comparing to 15 μV/K copper polymer assemblies [23]. In a further composition, thermocouples were manufactured from several textiles such as polyacrylonitrile staple fibers, steel staple fibers, a silver-coated polyamide thread, a knitted steel fabric, a woven polyacrylonitrile fabric, and a graphite non-woven textile [26]. The electrical signal generated from thermocouples was used to measure the temperature in the range of 30 to 120 ◦C [26]. The thermocouple was constructed from L-shaped copper and constantan (Cu/Ni) stripes on polypropylene textile, which were formed by magnetron sputter deposition. Comparison with a commercial thermocouple indicated no di fference in temperature detection [24]. Temperature sensors were constructed from copper–nickel wire thermocouples, which were soldered onto a firefighter´s glove [25].

Thermocouple sensors have been manufactured from wires to monitor the thermal situation in socks and gloves [9]. The body heat regulation was monitored by a sensor-based platinum array outside of the garmen<sup>t</sup> [9]. Thermocouple sensors were manufactured from copper and constantan wires and were used to detect temperature at 12 di fferent locations in T-shirts [27]. Consequently, a temperature distribution depending on the garment´s size and a distance from the body could be measured [27].

Copper-coated textiles can be used as flexible and lightweight conductor materials in a thermocouple array (Figure 5). The number of conductive lines can be reduced to measuring junctions (red spots), and a reference junction (green spot) can be formed by the attachment of five aluminum conductors (U0, U1, U2, U3, U4, and U5).

**Figure 5.** Copper-coated cellulose textiles used as a conductor matrix for temperature measurement.

Thermocouples were used to measure heat flux through polyester and polyester/cotton fabric with di fferent weaves (plain, satin, and twill [28]. The fabric´s temperature was detected at thermocouple points, which were related to reference points at room temperature [29]. Table 1 discloses various implementations of thermocouples in textiles and their influence on the flexibility of the entire structure, which became sti ffer, especially by gluing and soldering.

The thermocouples shown in Table 1 in this summary were made from metal wires, fibers, and yarns, which increase the textile´s weight and reduce its flexibility. The incorporation techniques mentioned often (Table 1) integrate thermocouples in textiles, thus resulting in discomfort and increased sti ffness. Further scientific work is needed to deal with these inconveniences and to manufacture a truly textile thermocouple.

### **3. Other Strategies for Temperature Measurement in Textiles**

There are several strategies to measure temperature in textiles such as Positive Temperature Coe fficient (PTC), Negative Temperature Coe fficient (NTC), Resistance Temperature Detector (RTD), and fiber Bragg grating (FGB). These strategies determine the shape of temperature sensors, which are manufactured by weaving, lithography, adsorption, screen printing, embroidery, knitting, and gluing, using conductive carbon paints and chemical vapor deposition (CVD).

Temperature measurements were conducted with a PTC resistive temperature sensor. It was manufactured as a thin film capacitor with gas/humidity sensitive polymers on a 50 μm Kapton substrate, on which two electrode lines were formed by lithography. The temperature sensor was woven into a textile (width = 45 mm, length = 200 mm) [30]. PTC sensors also were manufactured from an activated carbon fiber cloth by an electrothermal swing adsorption method [31]. A PTC sensitive polyamide foil (KAPTON) was manufactured by screen printing carbon polymer composites, while polyethylene and rubber were used as binder materials. The PTC-sensitive foil-detected temperature increase from 30 to 42 ◦C as a function of resistance [32]. PTC sensors were used to investigate the heating properties of 40 μm embroidered flexible polyurethane-coated copper filaments. PTC sensors on cotton fabrics recorded the increase in temperature as a function of resistance on embroidered PU-Cu composites. [33].

The temperature in textiles was detected by NTC sensors, which were manufactured from thermosensitive polyvinylidene fluoride (PVDF) fibers of 2 to 6 cm in length and 0.15 mm in diameter. Their active sensor area was formed by thermosensitive, polymer conductive pastes, which were manufactured from multiwall carbon nanotubes (MWCNT) and poly(methylmethacrylate) (PMMA) [34]. In another report, the sensitivity of conductive fabrics to di fferent temperatures was investigated by cotton and silver yarns. The sensitivity was related to the fabric's resistance, which was measured between two brass blocks (500 g each) at a pressing force of 25 N in an oven [35]. Five sensing yarns were incorporated into 2 mm diameter channels on a knitted sock and measured temperature on the skin. One temperature-sensing yarn consisted of a copper wire, six polyester yarns, one NTC thermistor, and a polymer resin, which were processed by a flatbed knitting machine [36].

In a further method, the temperature in textiles was measured by RTD sensors consisting of Kapton and Ti/Au conductors. The sensors were produced in a commercial band weaving process. The sensors were glued with conductive epoxy to metal strips, which were connected to a measurement device [37]. Another RTD detector was manufactured from nylon-6 after electrospinning and functionalized with multiwalled carbon nanotubes (MWCNTs) and polypyrrole (PPy). The samples were treated in the pyrrole vapor for 48 h and connected to two copper electrodes with conductive carbon paint [38]. RTDs were also made from a 100 nm platinum-coated plastic strips of 67.5 mm length and 500 μm width. The strips were woven in Kapton textiles at a distance of 200 μm and were used for temperature measurements [39]. In a further construction, RTD sensors were manufactured from nickel, copper, and tungsten wires on a temperature sensor fabric. These wires were knitted in the middle of a polyester fabric using a flatbed knitting machine. In the design, electrical short circuits were avoided, and a resistance of 3 to 130 ohms was determined during temperature measurements [40]. Flexible temperature and humidity sensors were made from graphene woven fabrics (GWF) on flexible polydimethylsiloxane (PDMS) films, which were deposited by CVD. The change in resistance, which was recorded as a function of temperature from 20 to 60 ◦C, demonstrated the use as a sensor [41].

A further method of temperature detection in textiles is the use of reflected wavelengths, which was caused by angular deformation. This deformation was used during temperature measurement in fiber optical sensors, which are based on the fiber Bragg grating (FBG) method and are woven in socks. The FBG sensors are made of silica core and plastic substrates, which provide the material a durability and light weight [42]. As an example, the body temperature can be detected by FBG sensors, which are woven into fabrics and embedded into a polyester resin [10]. The FBG method was used to investigate the temperature from 30 to 70 ◦C of liquid mixtures (water/glycerin) with a negative thermo-optic coe fficient of −5 × 10−<sup>4</sup> ◦C−1. The FBG fiber was placed in an aqueous solution with the

mixture solution. The Bragg wavelength of the FBG fiber was measured while heating the solution [43]. The structural state of textiles can be monitored when the beam IR laser (1064 nm) impinges on the surface, leading to a thermal gradient of 100 ◦C. The FBG temperature sensors measured radiation on the polymer surface, which can be used for flame or energy attack detection [44]. In addition to the FBG method, plastic optical fibers (POFs) were used as temperature sensors. During that method, the temperature was measured as a function of the intensity, which was caused by the thermal bending of the fiber. The POF sensors consisted of polymethyl methacrylate and fluorinated polymers, which were used as core and cladding [45].

The POF was used in chirped fiber Bragg grating sensors of 10 mm length, which indicated a sensitivity of −191.4 pm/ ◦C. These sensors measured temperatures along the grating length, which were designed for biomedical treatments and thermotherapies [46]. For biomedical application, POF was used due to the rapid production of POF grating devices, which worked below 248 nm and 266 nm UV wavelengths. This led to the manufacture of chirped POF-FBG sensors with a higher sensitivity and better biocompatibility compared to silica-based sensors [47]. In biomechanical investigations, multiple FBG sensors showed a sensitivity of 10.6 pm/ ◦C, which were connected in serial on textiles. This connection formed a temperature sensor network with multiple points, with which temperature values from 20 to 130 ◦C were measured [48].

Di fferent techniques and materials are summarized next, which permit temperature measurements in textiles (Table 2).


**Table 2.** Temperature measurement techniques in textiles.
