Development of the Smart T-Shirt for Monitoring Thermal Status of Athletes

2.1. System architecture

The system architecture for monitoring the thermal status of an athlete, which is integrated into the T-shirt, consists of two temperature and humidity sensors SHT-31 [30] acquired from Adafruit, the Wattuino Nanite 85 [31] microcontroller board acquired from Watterott electronic, and a FRAM (Ferroelectric Random Access Memory) [32], which was used for data storage. In addition, a 3.7 V rechargeable lithium ion battery powers the system. Boost converter 3.7 to 5 V was also implemented into the smart T-shirt. The Inter-Integrated Circuit (I²C) communication protocol was used for signal transmission. The circuit schematic that presents the electronics of the smart T-shirt is presented in Figure 1.

Figure 1.

Smart T-shirt circuit schematic.

The performance of temperature and humidity sensors SHT-31 is presented in Table 1. SHT-31 sensors have eight connections (pins), five of which are connected to the corresponding pins on the Wattuino Nanite 85 board. These pins are VDD (voltage supply), VSS (ground), SCL (serial clock), SCA (serial data) and ADDR (address pin). Sensors use the I²C communication protocol, which has an internal IP address, and are directly connected to the I²C SDA and SCL bus. The SDA pin is used for transferring data to and from the sensor. The sensors use communication velocities up to 1 MHz. Data are stored sequentially on FRAM storage via I²C.

The focus of the smart T-shirt is sequential collection of temperature (T) and relative humidity (RH) values, which can be monitored via the Arduino serial monitor. This way the user's thermal state can be monitored. The sensors are connected on a common data bus to send measurements sequentially every 0.5 seconds or every second (the sampling rate can be adjusted) to the main FRAM storage. The system is based on the standard I²C communication protocol. Sensors have two distinctive user-selectable I²C addresses, which are requested by the main program to demand the information from sensors to be updated to the data bus. SHT-31 sensors can operate on wide supply voltage range of 2.4 to 5.5 V, while the Wattuino Nanite 85 microcontroller was chosen to be integrated in to the smart T-shirt because of its small size (Table 1).

The temperature and humidity specifications performance of the SHT-31 sensor are presented in Table 2.

2.2. Integration of electronic components into the T-shirt

Electronic components were integrated into a T-shirt with short sleeves. The T-shirt has a wider cut and a strategically placed porous textile material (net) under the armpits to allow ventilation during physical activities. This T-shirt has been designed to be comfortable for the wearer. It is made of polyester knitted fabric with increased breathability, which allows for a quick evaporation of moisture.

Humidity and temperature sensors, which were integrated in the T-shirt, are positioned according to the locations of two temperature sites (i.e., right spatula and left upper chest) determined by Standard ISO 9886:2004 [33]. The location of sensors and other electronic components integrated in the T-shirt are shown in Figure 2.

Figure 2.

The location of sensors and other electronic components in a smart T-shirt, (a) the front and (b) back.

An acceleration sensor MMA8451 located on the left shoulder is optional and will be the topic of future research.

Conductive threads or ribbons are most commonly used to connect electronic components in smart clothing. The first problem that occurs is the low strength of the conductive threads and ribbons, which can easily break during physical activity. The second problem is the excretion of sweat. In extreme conditions, athletes can produce up to 2.5 liters of sweat during physical activity [34]. For that reason, it is not possible to use ordinary insulated wires, but it is necessary to provide protection of the electronic components from moisture (which is the conductor of electricity) in order to prevent contact. Therefore, it was decided to use single-coated flexible enameled copper wires to make connections between the components of the smart T-shirt.

The experiment was conducted to find an appropriate way of bonding single-coated enamel copper wires to the textile material. Liquid rubber, textile adhesive, and hot welding technology combined with sewing were used to bond copper wires (thickness of 0.12 mm and 0.2 mm) to knitted fabric.

Based on the visual aesthetics as well as the comfort of the garment, it was decided that the wiring in a smart T-shirt would be carried out using thermal welding technique combined with sewing and single-coated enameled copper wires of 0.2 mm thickness (visible on Figures 3 and 4). In addition, the combination of thermal welding and sewing allows the wires to be placed in curved lines with even a distance between several wires placed in parallel position and extra protection from moisture.

Figure 3.

PCB located on back a) during integration on the T-shirt, and b) final look.

Figure 4.

Left shoulder (a) PCB blueprint, (b) PCB integrated in to the T-shirt, and (c) final look.

Boards of SHT31 sensor were integrated and protected in the smart T-shirt. After the sensor was soldered to the copper wires and thus connected to other electronic components in the smart T-shirt, it was necessary to protect the sensor from moisture as well as to protect it from mechanical damage. This was achieved by coating the sensor board with liquid rubber; the only part that remained unprotected was the sensor itself (SH3x), and the next step was the application of nonwoven fabric to the sensor board. This nonwoven fabric covers the surface of the sensor board with the exception of the sensor itself (SH3x). The use of nonwoven fabrics protects the sensors and reduces the direct contact between the sensors and the body. In addition, it enhances the comfort of a smart T-shirt.

The use of liquid rubber and thermal welding technology ensured the permanent and safe connection of electronic components, wires and PBS with knitted fabric, which proved to be true after long-term testing (duration of one year) of smart T-shirts in which several test subjects participated.

Two printed circuit boards (PCBs) were created to connect the electronic components of the smart T-shirt. One was located on the back (Figure 3) and another on the shoulder (Figure 4). The PCB's proper structure was achieved by using the etching technique on a thin layer of copper. The layer was then merged with a flexible foil.

The PCB located on the right shoulder blade (115 mm × 42 mm) had the function of providing a connection to a sensor. On the back of the T-shirt the wires move from the PCB located on the left shoulder to another PCB (65 mm × 25 mm) in a slightly curved line and then move at right angles to the sensor on the right shoulder blade (Figure 3). This arrangement of wires and electronics on the back of the T-shirt provide comfort when worn, so that there is no obstruction of motion during the movement of athlete's hands.

The second PCB is located on the left shoulder of the T-shirt. The Wattuino Nanite 85 and MB85RC256Vw chip were mounted on this PCB and connected to the sensors. The copper wires previously connected to the corresponding pins on the SHT-31 sensors were then soldered to the appropriate sites on the PCB.

The wires on the left shoulder blade that connect to the sensors are carried out in a slightly curved line. Further, the wires move from the sensor to the hip position where boost converter, JST-PH2 pin and battery are placed on the seam that connects the front and back of the shirt. This line is even more curved. In this way, the reduction of the elasticity in the wale and course directions of knitted fabric is minimized.

PCB blueprint and integrated PBC, as well as final look of left shoulder of T-shirt is presented in Figure 4.

The connector near the collar enables the connection of a data cable, so that data stored on the FRAM can be transferred to the personal computer. Data can be monitored via the Arduino serial monitor.

2.3. Wearing comfort of the smart T-shirt

A very important factor when choosing sportswear is wearing comfort. In addition to thermo-physiological comfort both sensory and ergonomic comfort must be taken into consideration when one is designing and making sportswear. The smart T-shirt must not hinder the movements of athletes during physical activities.

One of the main requirements for choosing textile materials for sportswear intended to be worn in warm conditions is that they be light and elastic and that they possess good breathability in addition to enabling fast drying. That is why knitted polyester fabric is most often used for sportswear manufacturing [35]. For that reason, a light and elastic knitted polyester fabric was chosen for the realization of the smart T-shirt prototype.

The problem with the integration of electronic components into extremely elastic and light knitted fabrics is that the integration of electronic components of a certain weight stretches the material in the direction of the rows of the knitted fabrics. Therefore, light electronic components of smaller weight and dimensions must be used in order not to change the shape of the T-shirt and deform the textile material.

Experimental results show that the sewn mesh (positioned under the armpits) significantly increases ventilation between the body and clothing, and so the skin dries faster, resulting in a lower value in the sweat absorbed by the T-shirt [36]. In addition, the design of the T-shirt and use of ventilation holes can improve wearing comfort [37].

The weight of the T-shirt before the electronics components were integrated was 190 g; the weight after integration was 220 g, which means that the total weight of the electronics used including the battery was 30 g. Given these data, we can conclude that this small difference is insignificant and does not negatively affect the wearing comfort of the T-shirt.

A subjective assessment of the T-shirt comfort was performed during physical activity and in a stationary state in which six test subjects participated. For this purpose, a modified questionnaire was designed, which was used for constructing appropriate subjective scales in the assessment and evaluation of the physical environment according to ISO 10551:2019 Standard [38].

The results of the subjective assessments of wearing comfort of T-shirt are presented in Table 3.

All test subjects stated that the smart T-shirt was comfortable and the built-in electronics did not interfere with movement. In addition, all test subjects found that they did not even notice the presence of embedded electronics in the garment.

The battery is charged every four hours, which allows a long period without charging.

Another important feature of a smart T-shirt is the possibility to wash it, which is of great importance for sportswear. The smart T-shirt can be carefully washed and dried after the battery is removed.

2.4. Methodology

Experiments to determine the athletes’ thermal status were conducted in a climate chamber where the ambient temperature and humidity were set as well as the air velocity, which correspond to the conditions when sports competitions are held during the summer period. One professional male athlete participated in the study. The physical activity selected was running on a treadmill, which is a simulation of the activities of football players during a match. A smart T-shirt was used to determine microclimate temperature (T_M) and humidity (RH_M), while MSR 12 (Modular Signal Recorder) from Msr Electronics GmbH, Switzerland, was used to determine skin temperature (T_S) and the skin's relative humidity (RH_S). In Figure 5a a test subject wearing the smart T-shirt had an MSR device placed in a special bag attached to the subject's waist, while in Figure 5b positions of MSR measurements points on body are presented.

Figure 5.

Positions of temperature and relative humidity (a) on the body of test subject wearing the smart T-shirt and (b) the position of measuring skin temperature and relative humidity of skin (positions 3 and 4) defined according to the Standard ISO 9886:2004.

The test subject ran on the treadmill under two different atmospheric conditions. The first atmospheric condition was at T = 20°C and RH = 50%, and the second was at T = 25°C and RH = 50%. The air velocity was 0.5 ms⁻¹ for both atmospheric conditions. The tests’ duration was 55 minutes. Temperature and humidity data from the smart T-shirts were collected each second, and these data were collected twice per minute using MSR. Humidity and temperature sensors from MSR measuring devices were located (attached to the skin) in the same positions as SHT-31 sensors on the smart T-shirts, positions 3 and 4 (Figure 5b).

Figure 6 shows the graphical representation of the testing protocol, representing the velocity of movement of the test subject.

Figure 6.

Graphic representation of the testing protocol.

In addition, in order to verify the accuracy as well as the sensor response an additional experiment was conducted using a Thermo Labo device.

3.1. The test results of the feasibility and response rates of SHT-31 sensors integrated into a smart T-shirt

The aim of this experiment was to investigate the feasibility and response rates of SHT-31 sensors integrated into a smart T-shirt. For this purpose, a KES-F7 Thermo Labo instrument, which contains two hot plates set to T₁ and T₂ was used. The T-shirt has been previously exposed to standard atmosphere in accordance with the requirements of ISO 139 Standard [39].

The total duration of the experiment was one hour; the temperature and humidity data were collected each second. The experiment was conducted at the standard atmosphere in a climate chamber, i.e., where the ambient temperature at 20 ± 2°C and the relative humidity was 65 ± 2%.

Sensor 1, integrated on the front of the T-shirt was left on the plate, which was set to T₁ for half an hour, while sensor 2, which was integrated on the back of the T-shirt, was placed on a plate set at T₂. After 30 minutes, the sensor positions on the plates were replaced. Figure 7a shows the results of temperature measurements of the SHT31 sensor when tested on a KES-F7 Thermo Labo instrument, while Figure 7b shows the change of relative humidity of the SHT31 sensor when tested on a Thermo Labo instrument.

Figure 7.

The results of the temperature and relative humidity of the SHT31 sensor built into the T-shirt, tested on the KES-F7 Thermo Labo instrument. (a) Temperature data and (b) Relative humidity data.

Figure 7a shows that the sensor 1 responded and temperature values began to decline from 27°C to 21°C. A similar change was observed with sensor 2, where the temperature measured increased from 21°C to 27°C.

When we observe at the recorded RH values measured with sensor 1, it is evident that the initial RH value was 44%. During the experiment, there is a slight decrease in the RH values. After 30 minutes when sensor 1 was moved to other plate, there was a slight increase in RH values, which lasted only a few seconds. After one hour, at the end of the experiment, sensor 1 recorded an RH value of 56.30%. Sensor 2 recorded a similar change. At the beginning of the experiment, an RH value of 57.51% was recorded. During the first 30 minutes of the test, the RH values were stable. During the change of the sensor position to the other plate, there was an increase in the RH value, which lasted only a few seconds, and at that time the highest recorded value was 63.68%. After placing the sensors on the plate set at T₂, the RH values decreased, reaching 51.58% at the end of the measurement.

3.2. Monitoring athletes’ thermal status by using a smart T-shirt

Measurement results of skin temperature and relative humidity of skin and microclimate during the physical activities defined in the protocol (Figure 6), performed in two different ambient conditions (20°C and 25°C) are shown in Figures 8 to 11.

Figure 8.

Results of skin temperature obtained with MSR and microclimate temperature determined with the smart T-shirt at 20 °C ambient temperature (a) on front and (b) on back.

Figure 9.

Results of skin relative humidity obtained with MSR and microclimate relative humidity determined with the smart T-shirt at 20°C ambient temperature (a) on the front and (b) on the back.

Figure 10.

Results of skin temperature obtained with MSR and microclimate temperature determined with the smart T-shirt at 25°C ambient temperature (a) on the front and (b) on the back.

Figure 8 shows the results of skin (determined by MSR) and microclimate (determined by SHT-31) temperature obtained at 20°C ambient temperature.

During the performance of physical activities by the test subject at the ambient temperature of 20°C, higher temperature values were obtained on the subject's back, recorded both by MSR and by the smart T-shirt. Significantly, higher values were obtained for skin temperature in comparison with the microclimate temperature: an average of 3.75°C on the test subject's chest and 4.89°C on the back. These results correspond to the experimental data of thermo-physiological studies where higher values of Ts were recorded on upper back in relation to the Ts recorded on chests [40, 41]. Figure 8 shows that temperatures rise slowly at the beginning of the test until the twentieth minute (which is considered to be a warm-up period) and values oscillate with load, i.e., a change in the velocity of movement. This oscillation in temperature values was much more pronounced for those measured on the test subject's back (T_S2 and T_M2) compared to the values obtained on the chest (T_S1 and T_M1).

The microclimate temperature obtained at the ambient temperature of 20°C on the subject's chest ranged from 25.30°C to 28.51°C while these values on the subject's back range from 30.03 °C to 33.16 °C. Moreover, the values of T_s on the subject's chest range from 29.02°C to 31.03°C while temperatures on the subject's back range from 30.03°C to 33.09°C.

Figure 9 shows results of skin and microclimate RH at the ambient temperature of 20°C.

After the fifteenth minute of the test, the body begins to sweat in response to physical exertion, as reflected by a sharp increase in all RH values. Higher RH values were recorded on the subject's back. In a comparison RH_S1 and RH_M1, it can be seen that the smart T-shirt recorded higher RH values compared with the MSR device, namely 18.75% on average. On the test subject's back, the difference in these values was 1.66% on average. This can be explained by the movement of the test subject and the ventilation, which occurs during moving of garment. This ventilation enabled drying of the T-shirt.

Figure 10 presents skin and microclimate temperature obtained at 25°C ambient temperature.

Good consistency was observed between the results of skin and microclimate temperatures on both the subject's chest (Figure 10a) and the back (Figure 10b).

The results obtained and compared for the skin temperature as well as microclimate temperature at the ambient temperature of 20°C (Figure 8) and the ambient temperature of 25°C (Figure 10), indicate clearly that a higher ambient temperature results in greater warming of the body. This was reflected in the results of skin and microclimate temperatures, which were much higher at 25°C ambient temperature. The values of skin and microclimate temperatures increase and decrease with the change of the physical load.

The average difference in the skin and microclimate temperature values on the chest is 2.18°C, while the difference on the back is 0.98°C. The microclimate temperature obtained at an ambient temperature of 25° for the subject's chest range from 28.18°C to 31.05°C while these values for the subject's back range from 28.07°C to 31.51°C. Furthermore, the skin temperature values obtained for the subject's chest range from 30.07°C to 33.28°C, while these values on the subject's back range from 31.02°C to 35.03°C.

Figure 11 shows skin and microclimate RH results obtained at the ambient temperature of 20°C.

Figure 11.

Results of skin relative humidity obtained with the MSR and microclimate relative humidity determined with the smart T-shirt at 25°C ambient temperature (a) on the front and (b) on the back.

During the test, at both ambient temperatures (20°C and 25°C), there is a sharp increase in the RH values after each fifteenth minute of the test. At an ambient temperature of 20°C, after this period there is an oscillation in the RH values on the subject's chest and back with the change of physical load, i.e., the velocity of movement. This can be explained by the drying of the T-shirt caused by the ventilation that occurs during the movement of the garment. At an ambient temperature of 25°C a larger amount of sweat is excreted on the subject's chest (RH_M1 ranging from 43.49% to 99.63% and RH_S1 ranging from 43.03% to 73.01%) and on the subject's back (RH_M2 ranging from 40.79% to 97.95% and RH_S2 ranging from 41.87% to 98.09%) during the same physical activity, which prevents the T-shirt from drying. Increased moisture values (amount of sweat) are the body's response to increased physical exertion and high ambient temperature.