Recent research trends in textile-based temperature sensors: a mini review

In this review, the current state of research on textile-based temperature sensors is explored by focusing on their potential use in various applications. The textile-based sensors show various advantages including flexibility, conformability and seamlessness for the wearer. Integration of the textile-based sensors into clothes or fabric-based products enables continuous and sensitive monitoring of change in temperature, which can be used for various medical and fitness applications. However, there are lacks of comprehensive review on the textile-based temperature sensors. This review introduces various types of textile-based temperature sensors, including resistive, thermoelectric and fibre-optical sensors. In addition, the challenges that need to be addressed to fully realise their potential, which include improving sensitivity and accuracy, integrating wireless communication capabilities, and developing low-cost fabrication techniques. The technological advances in textile-based temperature sensors to overcome the limitations will revolutionize wearable devices requiring function of temperature monitoring.


Introduction
Temperature sensors are devices that convert various physical changes caused by changes in temperature into electrical signals. Temperature sensors are classified in various ways depending on principles and materials, such as resistance temperature detectors (RTDs), infrared temperature sensors, and thermocouples. Materials such as metal, metal oxide, and ceramic, which have limited characteristics including inflexibility, heavyweight, and fragility, have been mainly used to make temperature sensors [1][2][3]. Recently, carbon-based materials, like carbon black, graphene, and carbon nanotube, and various flexible substrates, like polydimethylsiloxane (PDMS), textile, and polyimide (PI), have been studied to get better mechanical and electrical properties [4][5][6][7][8][9]. Fabrication methods for temperature sensors vary depending on the sensor type and application and they include thin-film deposition, printing, coating, etc [1,10,11]. Temperature sensors are essential components in various fields, including healthcare, sports and the manufacturing industry [12][13][14][15][16][17].
With the unique properties such as flexibility, lightweight, and seamlessness to the human body, textile-based temperature sensors have emerged as a promising technology, especially in wearable electronics [18][19][20][21]. In addition, the potential of textile-based temperature sensors is expected to revolutionise healthcare by enabling continuous and noninvasive monitoring of patients' body temperature, thereby improving patient care and reducing the costs for healthcare process [22][23][24].
The development of textile-based temperature sensors requires the integration of various types of materials, electronic components, and manufacturing technologies [13][14][15]23]. Therefore, developing the innovative technologies in each parts is important to produce the high quality temperature sensors. For examples, the sensitivity and accuracy of the sensors can be improved through the innovative materials including conductive textiles, which can be integrated as temperature sensors with high sensitivity and accuracy [25][26][27]. In addition, advances in electronic components enables the miniaturisation of devices such as low-power temperature sensors that help to realize commercial wearable devices [28][29][30]. The progression of manufacturing technologies can allow the mass production of textile-based temperature sensors with high precision and reproducibility [31][32][33].
The design of textile-based temperature sensors is critical to their performance and reliability. Design considerations include the choice of materials, sensor configuration and dataprocessing techniques. The choice of materials is important because materials determine the sensitivity and accuracy of temperature sensors (figure 1) [34]. Further, the sensor configuration can affect the spatial resolution and response time of a sensor. Finally, data-processing techniques are necessary to filter out noise and extract meaningful information from sensor data.
Fabrication methods for textile-based temperature sensors have also been a focus of research [31,[34][35][36]. Weaving, printing, and embroidery are some of the techniques that have been used to fabricate temperature sensors in textiles [13,15,16,19,23,25,28,32]. Weaving is a popular method because it can produce sensors having high resolution and durability. Printing and embroidery are also attractive options because they can produce sensors having high flexibility and conformability.
The applications of textile-based temperature sensors are diverse and rapidly expanding [37,38]. In healthcare, textilebased temperature sensors have been used to monitor the body temperature of patients, particularly those with fever or hypothermia [39,40]. In addition, textile-based temperature sensors are now widely used in sports, where the core body temperature of athletes during exercise is monitored to help prevent heat exhaustion and other heat-related illnesses [41,42]. Furthermore, textile-based temperature sensors are equipped in various industrial devices that measure the change in temperature of machinery, equipment and materials, thereby improving the controllability of process, safety, and efficiency [43,44]. In addition, apparels with the textilebased temperature sensors can control the thermal properties of clothing and other textiles, thereby improving comfort and energy efficiency of clothes [45,46].
In this review, a comprehensive overview of the recent advances in textile-based temperature sensors is summarized. The recent developments in materials, applications and manufacturing technologies for textile-based temperature sensors is discussed. Each factors including the choice of materials, sensor configuration and data-processing techniques that can improve the properties of the textile-based temperature sensors is summarized. Furthermore, various fabrication methods that have been used to produce textilebased temperature sensors, including weaving, printing and embroidery, are introduced. Finally, the various applications of textile-based temperature sensors in healthcare, sports, industrial applications and smart textiles are discussed, which provides a perspective for the potential of textile-based temperature sensors for future developments in this rapidly evolving field.

Thermoelectric-powered textile-based temperature sensors
There are several types of textile-based temperature sensors [47]. One type is a thermoelectric-powered textile-based temperature sensor (figure 2) [47]. For the sensors, thermoelectric (TE) materials and electrodes are coated or printed on the surface of textiles. Thermoelectric-powered textile-based temperature sensors are flexible and lightweight sensors that can be used in wearable electronic skin because of their threedimensional (3D) conformability and breathability. These sensors comprise thermoelectric segments that convert temperature differences into an electrical voltage. The sensitivity of these sensors is low because of the low integration density of thermoelectric (TE) segments. These sensors typically comprise a thermoelectric generator (TEG) integrated into a textile substrate, along with temperature-sensing elements such as thermistors or resistive temperature sensors. When the temperature of the TEG changes, it generates a voltage that can be used to power the temperature-sensing elements. This approach eliminates the need for batteries or external power sources, making thermoelectric-powered textile-based temperature sensors ideal for wearable technology and other applications in which portability and flexibility are essential.
Various recent researches have explored the development of the textile-based temperature sensors using thermoelectric materials [37,38]. For example, Liu et al, developed a thermoelectric-powered wearable temperature sensor using a   Semiconducting glass of Cu-As-Te-Se system, polyetherimide (PEI) Sealed-ampoule meltquenching High flexibility 3.5 mV at Δ70 K [51] flexible polymer substrate made from PI, a TEG made from silver selenide (Ag 2 Se)-based thermoelectric materials, and polyvinylpyrrolidone (PVP) as an addition of binder [48]. Ag 2 Se/PVP films were deposited by screen printing. The relationship between temperature difference and voltage was linear ( figure 3(A)). The output voltages of TEG were 11.1 mV and 21.6 mV at the temperature differences of 20 K and 40 K respectively ( figure 3(B)). The voltages of 1.7 mV and 7.3 mV were measured at temperature differences of 4 K and 15 K, respectively, which indicates that their film has the potential to be used for wearable devices (figures 3(C)-(D)). In the other work by Liu et al, a flexible, thermoelectricpowered temperature sensor using a TEG made from In 2 O 3 /Cu was demonstrated [49]. The longer the heat treatment was performed, the better performance was shown Landsiedel et al, demonstrated a flexible thermoelectric sensor matrix, which was constructed by electroless deposition of copper layers on cellulose fabric [28]. When the temperature difference was 71 K, the average voltage was 0.4 mV. Using aluminium as the second material, a thermoelectric coefficient of 3-4 μV K −1 was realised, which is sufficient for manufacturing textile-based temperature measurement devices (figures 4(A)-(G)). Further, Li et al, demonstrated a large-area, wearable, self-powered pressuretemperature sensor based on 3D thermoelectric spacer fabric [50]. They used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as an organic thermoelectric material. The sensor can measure pressure and temperature simultaneously with high sensitivity and accuracy. The output voltage at the temperature difference of 40 K reached 203 mV for the device with 100 units. The working stability was so excellent that the output voltage remained stable for a long time. The LED could also be lit by harvesting energy from body temperature (figures 4(H)-(L)).
Zhang et al, demonstrated thermoelectric fibre sensor containing a semiconducting glass of the quaternary Cu-As-Te-Se system as core and a polymer cladding of polyetherimide (PEI) [51]. The semiconducting glass rod was synthesized by a standard sealed-ampoule technique. It showed high mechanical flexibility owing to the presence of PEI cladding. Sensitivity and accuracy were comparable with commercial temperature sensors. The response time was so fast that it was less than 3 s, and the temperature sensing performance did not fall even when wrapped around a glass rod with radius of 3 mm (figures 5(A)-(I)).
Thermoelectric-powered textile-based temperature sensors offer several advantages over traditional battery-powered sensors, including greater portability, flexibility and scalability. Additionally, they are more environment friendly because they do not require disposable batteries. As research in this field continues, thermoelectric-powered sensors are likely to become increasingly common in wearable technology and other applications in which flexibility and portability are essential. The contents of thermoelectric-powered textilebased temperature sensors are summarized in table 1.

Textile-based resistive temperature sensors
Another type of textile-based temperature sensor is a resistive temperature sensor. These sensors are made by coating or printing conductive materials on textiles (figure 6) [52]. The resistance of the conductive material changes with temperature, enabling the sensor to detect temperature changes. Pure metals are commonly used as thermal resistance materials. Since the kinetic energy of free electrons increases, the resistance tends to increase as the temperature increases. As the temperature rises, the movement of electrons changes, which increases the energy required for directional movement. This leads to an increase in resistance. This is described as follows: where R t and R 0 is the resistance value at time t and t 0, respectively, α represents the temperature coefficient of resistance material [53,54]. Thus, the resistance sensitivity can be represented as: The temperature coefficient varies with temperature and only be regarded as a constant within a certain temperature range. A textile-based resistive temperature sensor is a type of sensor that can be integrated into fabrics to measure temperature changes. It comprises conductive threads or yarns of metals such as silver or copper, which are woven into the fabric in a way that generates temperature-dependent resistance. When the temperature changes, the resistance of the conductive thread changes as well, and this change can be measured to determine the temperature. This makes textilebased resistive temperature sensors an excellent choice for applications that require temperature monitoring, such as in wearable technology, medical monitoring and smart home systems.
Lugoda et al, developed a wearable temperature sensor based on conductive polymer composite yarns [52]. The sensor was integrated into an armband and demonstrated high sensitivity to temperature changes, with a response time of less than 10 s (figures 7(a)-(d)) [52]. Dakoco et al, demonstrated a textile-based temperature sensor fabricated through inkjet printing technology (figures 7(E)-(F)) [11]. The sensor exhibited high accuracy, with a resolution of 0.1°C (figure 7(G)), and it could be integrated into fabrics using standard textile-manufacturing processes. In the work of Liu et al, Ni-coated textile was used to fabricate a temperature sensor [55]. The sensor demonstrated high sensitivity to temperature changes, with a linear response over a wide temperature range (figures 8(a)-(d)). Fromme et al, fabricated e-textile with metal nanocoating pattern by using a laser welding technology (figure 8(e)) [56]. They used a copper-welded textile as a resistive temperature    sensor. The temperature sensor exhibited a linear relationship to temperature and the accuracy of the sensor was ±1°C from 25°C-60°C, ±2°C to 100°C, and ±7.5°C for a temperature higher than 140°C ( figure 8(f)). The authors suggested that these sensors could be used in various applications, including smart textiles and wearable electronics. Overall, the textile-based resistive temperature sensors have demonstrated the potential to be used for a wide range of applications, including wearable technology, medical monitoring and smart home systems. The use of conductive fibres and yarns offers a highly scalable and flexible approach to temperature sensing, with the ability to integrate sensors directly into fabrics during the manufacturing process. The features of the textile-based resistive temperature sensors described above are summarized in table 2.

Fibre optic temperature sensors
Fibre optic temperature sensors are also representative temperature sensors that can be incorporated into textiles ( figure 9). In fibre optic temperature sensors, optical fibres that can detect changes in temperature are used. The thermal gradient results in a change in optical properties of the fibre, which can be converted into the degree of temperature change [36]. These sensors typically comprise a length of optical fibre that is coated with a thermally sensitive material or has a   temperature-sensitive element attached to it. As temperature changes, the optical properties of the fibre change, causing changes in light transmission that can be measured to determine the temperature. Fibre optic temperature sensors offer several advantages over traditional temperature sensors, including high accuracy, fast response times and immunity to electromagnetic interference. In addition, they are highly versatile and can be used in a wide range of industrial areas, including aerospace, automotive and medical monitoring. Fibre optic temperature sensors have many applications such as monitoring in nuclear magnetic resonance imaging and radio-frequency energy environments, structural health monitoring, aerospace, metallurgy, fossil fuels, power production, industrial processing plants, bridges, tunnels, mines, buildings and oil and gas pipelines. There are several types of fibre optic temperature sensors, including distributed and point sensors. In distributed sensors, optical fibres that are specially designed to allow temperature changes to be measured along the entire length of the fibre are used. The temperature changes in large structures such as pipelines or bridges can be detected with such temperature sensors, and they are particularly useful for monitoring temperature gradients or hot spots. By contrast, point sensors use a small section of an optical fibre that is coated with a temperature-sensitive material or has a temperature-sensitive element attached to it. The point sensors can measure temperature changes at specific application areas, such as in wearable electronic components or medical implants. Depending on the sensing system, the temperature sensors are categorised as fibre Bragg grating (FBG) sensors, Raman scattering sensors, fluorescence-based sensors, surface plasmon resonance (SPR) sensors or clad modified fibre optic sensor.
FBG sensors use a length of optical fibre that is inscribed with a periodic pattern of refractive index variations. As the temperature changes, the periodicity of the pattern changes, causing a shift in the wavelength of the reflected light. By measuring this wavelength shift, the temperature can be determined. FBG sensors are highly accurate and can be used in a wide range of applications in fields such as aerospace, oil and gas and civil engineering. Raman scattering sensors use a length of optical fibre that is coated with a temperature-sensitive material. As temperature changes, the material undergoes a phase transition, causing a shift in the frequency of the Raman-scattered light. By measuring this frequency shift, the temperature can be determined. Raman scattering sensors are highly sensitive and can be used in applications such as medical monitoring and   [35]. The temperature change, which was measured by using Raman spectroscopy, was correlated with the change in strain value of MWCNTs depending on temperature. Flexible polyimide (PI) substrates were deposited with Cu electrodes by sputtering. Then, the sensing materials were printed to have a thickness of 50 μm ( figure 10(a)). The temperature sensors showed high sensitivity in the range of 20°C-210°C (figures 11(C)-(F)).
Fluorescence-based sensors use a length of optical fibre that is coated with a temperature-sensitive material. As temperature changes, the material emits light at a different wavelength, which can be measured to determine the temperature. Fluorescence-based sensors are highly sensitive and can be used in applications such as biomedical imaging and . The excitation of SPR was possible by both Au and TiO 2 in the triple combination structure of TiO 2 and Au. Thus, they could achieve a highly sensitive SPR temperature sensor exhibiting a sensitivity of 6038.53 nm RIU −1 and the detection temperature sensitivity of −2.40 nm°C −1 (figures 13(g)-(h)). These values were 77.81% higher than those of traditional Au SPR sensors.
Clad modified fibre optic sensors use nanostructures coated cladding modified fibre (CMF). CMF based sensors are easy to fabricate, less weight, and have good durability. CMF is made by removing a small portion of the cladding using the chemical etching method and coating it with semiconductor metal oxide in its place. When light is projected onto the temperature sensor, some of the light passes through because the refractive index of the modified cladding and the core are different. The refractive index of the modified cladding material changes as the temperature changes, and the intensity of the light also changes accordingly. By measuring    15(A)-(G)). It can be seen that the sensitivity of the Al 2 O 3 -MgO (50%-50%) nanocompositebased sensor was higher than other composites. The temperature and sensitivity had linear relationships which measured as 0.62%/°C in the case of the Al 2 O 3 -MgO (50%-50%) nanocomposite-based sensor ( figure 15(H)).
Overall, fibre optic temperature sensors provide several advantages over traditional temperature sensors, including high accuracy, fast response times and immunity to electromagnetic interference. The progression of the fibre optic sensors will facilitate the realisation of the futuristic wearable devices having the function of temperature monitoring. A comparison of the features of fibre optic temperature sensors is shown in table 3.

Conclusion
In summary, textile-based temperature sensors have shown great promise for a wide range of applications. Recent developments have resulted in various sensor types, each offering benefits over traditional temperature sensors, such as flexibility, conformability and comfort for the wearer. The integration of these sensors into fabric-based products enables continuous temperature monitoring, making them ideal for medical and fitness applications. Researchers have explored different types of fibre-based temperature sensors, including resistive, thermoelectric and fibre-optical sensors, all of which demonstrate good sensitivity in temperature monitoring. However, there are still challenges to overcome, such as improving sensitivity and accuracy, integrating wireless communication capabilities and developing low-cost fabrication techniques. In addition, textile-based temperature sensors must have durability and reliability to withstand various mechanical stresses and should be configured so that temperature sensing is not affected by electronic components. Therefore, further research and development is necessary to address the issues and innovate the conventional textile-based temperature sensors. For that, further optimization of the design, fabrication and integration methods to integrate the sensors into textile products and development of novel materials and sensing technologies to improve the accuracy and sensitivity are required. An interdisciplinary approach among material science, textile engineering, and electronics is also needed. In summary, continuous innovation and advancement in materials, fabrication methods, design, calibration methods, and signal processing are essential for overcoming these challenges. The continued efforts and progress to innovate the fibre-based temperature sensors will revolutionize wearable device industry requiring sensitive temperature monitoring system.