2.1. Thermoelectric generator
Wearable thermoelectric generators that use temperature difference to generate electricity can directly convert heat into electricity. There are two main types of wearable thermoelectric devices. One type is made of flexible thermoelectric materials such as polymers. Due to the low ZT value of polymer-based thermoelectric materials, the output power per unit area of such thermoelectric devices can be on the order of 0.1μW/cm2 or even lower. The other refers to flexible thermoelectric devices made of rigid inorganic thermoelectric materials connected with flexible fillers or flexible electrodes. Thermoelectric materials are printed on glass fiber to develop a power generation device that can generate electricity from body temperature [23]. The power generation device can use glass fiber and inorganic materials to reduce heat loss and add power output. The device is thinner (about 500μm thick) and lighter (about 0.13g/cm2 area density). On this basis, in order to collect human body heat, the researchers developed a flexible wearable thermoelectric generator based on flexible printed circuit board and 52 pairs of thermocouples. The generator has good output performance and can supply power for sensors [24].
In this study, a thermoelectric generator is applied, which contains a prototype of self-powered electronic system in the form of an arm ring tied to the arm. When the body temperature is above 35 degrees, the generator collects heat from the skin, which can be converted into electricity. Thermoelectric generators produce voltages from 2V to 5V. It has three special orders of voltage stabilizing input, temperature measuring bridge and amplifier circuit, which can satisfy the needs of charging devices such as mobile phones and will not cause harm to human body.
2.2 Organic reversible temperature-sensitive powder
Organic reversible temperature-sensitive powder, dominated by electron-transferred organic compounds, refers to an organic chromaticity. It consists of an electron donor, an electron acceptor, a regulator, a sensitizer and other solvents. Between the electron giver and the electron acceptor, the phenomenon of electron transfer occurs due to the change of temperature. In the process of electron transfer, the color change of certain wave is absorbed or radiated. Meanwhile, the long light composed of the hidden color dye, color developer and color-changing temperature control solvent appears to produce the reversible color-changing microcapsule (Table 1). The thermochromic capsule changes color by sensing heat, and the color-changing process is in a range, rather than at a point. The corresponding color-changing colors of different components of the thermochromic pigment are displayed in Table 1. There are many organic reversible thermochromic materials, which can be classified into triarylmethane phthalides, indoline phthalides, fluoranes, triphenylmethane, spiropyran, spiroring, dianthrone, α-naphthoquinone derivatives and so on. According to its composition, it is divided into two categories. One is a single-component temperature-sensitive color material. It's dominated by a single substance; Another is a kind of multi-component compound temperature discoloration material, the temperature change powder used in this study. It is a multi-component compound type temperature discoloration material. Its advantages include wide temperature discoloration range (20 ~ 200℃), obvious discoloration, bright color, high color sensitivity, and low production cost.
Table 1. Temperature change effect
Discoloration material
|
Discoloration change
|
NiNH4PO . 6H2O
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Bright green → Grey blue
|
NH4 . VO3
|
White → Brown
|
Cd(OH)2
|
White → yellow
|
Fe4[Fe(CN6)]2
|
Blue → Brown
|
Pb CO3
|
White → yellow
|
Co C2O4
|
Powder → black
|
2.3. Temperature sensor
The temperature sensor bases on the preparation of NTC thermistor to realize the body temperature recognition and analysis. The different voltage is transmitted to different areas of the garment through the 300D/3 conductive sewing thread. In order to control the discoloration of the material with temperature change, the electrothermal conversion is carried out by the conductive wire.
With NTC thermistor as body temperature sensor, energy conversion can be realized when temperature changes. NTC therminors can work stably for a long time and have a long life after being tested with high precision, high sensitivity, high reliability, super high temperature and high pressure. Featured with light weight, flexibility and low cost, they can be easily integrated on the surface of various flexible substrates or embedded in them. The operating temperature of NTC thermistor is -55℃-125℃, and the normal body temperature of human body is 36℃-37℃(measured in armpit). The highest and lowest body temperature of living human beings recorded are 46.5℃ and 13.7℃, respectively. Obviously, the working temperature of NTC thermistor is suitable for human body temperature measurement. MT series medical NTC thermistor has a diameter of 5mm, and a dissipation coefficient of /2(mW·℃-1). Thermal time constant is 3.5s, which is relatively small. Rated power ≤20mW. B value is 3695-4262, which is cost-effective. The amplifier circuit in the temperature sensor uses THE OP07 chip of TI company, which sends the thermistor into the amplifier circuit through the output voltage of the bridge with the change of the human body temperature. The sensor needs to encode with the computer in advance and preset the voltage value corresponding to the temperature, aiming to prepare for the subsequent control of temperature change and color conversion. Body surface temperature of exposed parts such as wrist and forehead vary with environmental temperature [25]. The temperature sensor should be close to thearea of human body that strongly sweats, the heart position at the intersection of the connectinglines of the centers of forechests and underarms, so as to improve the accuracy of body temperature recognition.
SHN MARK grey 300D/3 sewing thread was used in this study, containing polyester and stainless steel fiber (Figure 1). It is antistatic and has conductive properties. Conductive sewing thread is used as a conductive connection to the sensor. It is an insulated thin wire, with heat prevention, durability, and extreme softness. Safety risks will not appear because of long-time work. In the sewing process, a z-shaped overlap mode (Figure 1) is adopted to sew between the two layers of fabric, and the sensor electrodes are connected at both ends.
Considering the fabric smoothness and physiological data monitoring sensor pin number, straight-line parallel connection is used as wiring connections (Figure 2). 4 silver conductive sewing threads are utilized. Sewing process parameters are set to the best spacing of 1 cm. They are parallelly sewn into the selected fabrics, respectively with a temperature sensor, the corresponding 4 pins and docking. Sensors are connected at one end, and the other end connects control module and power module. The flexible connection between garments and electronic components is realized by the detachable bolt interface.
2.4. Non-woven
Table 2 shows the anti-static performance results of non-woven fabrics. The anti-static performance is tested according to GB/ T12703.1-2008 "Evaluation of Electrostatic Properties of Textiles, Part 1: Static Voltage Half-Life", GB/ T12703.3-2009 "Evaluation of Electrostatic Properties of Textiles, Part 3: Charge Quantity", GB/ T12703.4-2010 "Evaluation of Electrostatic Properties of Textiles, Part 4: Resistivity".
Table 2. Fabric Composition
Fabric
|
Silver fiber
|
Polypropylene
|
Polyester fiber
|
Test result
|
Non woven fabric
|
9.4%
|
84.6%
|
6%
|
The fabric has been washed for many times, and the results meet the corresponding standards
|
2.5 Fashion design
The design of wearable clothing is often limited to the design rationality of clothing materials and wires, ignoring the unity of fashion and function. Based on 2021-2022 Autumn/Winter Fashion Trend Style Guide, this design adopts the deconstruction design style in the sweating areas of human body (breast center, alar the heart on the horizontal line intersection point), aiming to design a asymmetric pocket (removable and convenient for cleaning). Temperature sensors are also placed near the temperature thermoelectric generator (in the form of arm ring, tied to the arm). The length of the wire is shortened. The left and right versions of the clothing are asymmetrical, which is consistent with the overall style (Figure 3). The design of the temperature-changing pattern is inspired by the work of Dutch painter Mathijs Vissers, which is a complicated painting without any specific emotional tendency.
①-④ for the pattern drawn with warm pigment, the four areas correspond to four different emotions, namely relief, fear, laughter and anger (Figure 4).
(This study informed consent was obtained from the picture model.)
2.76Circuit Design
The material components involved in the temperature change area include thermoelectric generator, temperature sensor, some alligator clips, some jumper wires, bread board, diode, transistor, organic reversible temperature change powder, conductive sewing thread and hot melt film.
The specific experimental steps include:
Step 1: After the design drawing of clothing effect drawing is completed, the garment CAD plate making is carried out according to the effect drawing.
Step 2: Print the DXL file of garment CAD plate making, create the plane plate making and cut into slices.
Step 3: The organic reversible temperature-sensitive powder is first added with water to turn into temperature-sensitive pigment, and the pattern is drawn on the surface of the non-woven fabric after being cut.
Step 4: Complete the stitching of the entire garment, including pockets and other parts, and put the temperature sensor in a strap bag on the chest.
Step 5: Finish stitching the arm ring and connect the conducting wires.
Step 6: Calculate the length range of the conductive sewing thread at each position; Sew the conductive sewing thread on the painted back, and the interlayer between the face cloth and the lining cloth. If the sewing thread is exposed from the front because of the thin fabric, sew the sewing thread on another suitable fabric, and then glue the two fabrics together with hot melt film.The connection sequence of conductive wires is shown in Figure 5: port 1 of conductive sewing thread -- thermoelectric generator -- Port 1 of temperature sensor, port 9 of temperature sensor -- breadboard, port 1 of temperature sensor -- breadboard -- port 2 of conductive sewing thread, and temperature sensor -- diode and transistor are connected to the breadboard.
Step 7: The temperature change region is programmed into the temperature sensor program, and the voltage is obtained according to the following algorithm.