Temperature is measured according to a measurement principle based on a particular physical phenomenon. The measurement result is derived from a measurement model based on a measurement principle, that is, from a mathematical relationship between the data collected in a measurement Theoretically, any conductor can be used for a tem perature detector. However, issues such as cost sensitivity, and manufacturing restrictions limit the choice. Wearable sensors have gained considerable interes because of their easy contact with humans Textiles, besides their protective and esthetic functions, are preferable for wearable sensors as they are soft and highly flexible and porous.
In the present paper, CNT-based wearable tem perature sensors fabricated by inkjet printing on taf feta fabric are presented. Prepared CNT-based ink was deposited on the substrate using a commercia desktop inkjet printer. The performance of the pro duced temperature sensor was studied by subjecting the printed sensor to varying temperatures ranging from room temperature to 50 °C. The effect o designed serpentine patterns on the resistance and TCR value was investigated. The impact of mechan ical stresses on sensor performance was also studied to identify potential applications before the integra tion of printed sensors.
The display is used to interact on the computer. We hereby install linux operating system on a raspberry pi controller. This allows the user to use all linux features including file storage and retrieval, tools and browser on the raspberry Pi. We integrate the touch screen display to allow user to easily interact with windows system. We use internal raspberry pi wifi module to connect with the internet.
Components
carbon nanotube powder (8–18 nm (OD), 5–10 nm (ID), 10–20 lm (L)) was purchased from Molchem Company. Dodecyl sulfate sodium salt (SDS) (Product No. 822050), from Sigma-Aldrich Chemical Company, was used as a surfactant. Polyamide based taffeta fabric from Huzhou Hengxin Label Manufacture. was used as the sub strate. The thickness and average weight of taffeta fabric were 0.105–0.115 mm and 62 ± 5 g/m 2 respectively. It has a polyamide coating to improve inkjet printing performance. The taffeta fabric wa chosen as the substrate due to its usability option as a temperature sensor tag on garments. A translucen polyurethane welding tape was used as an encapsu lation layer (Bemis ST604). Pressure and temperature are required to activate the tape. The softening tem perature of the tape is 105 °C. It is possible to provide hydrophobicity of the entire surface and to preven air contact with this tape. The tape is also elastic and washable.
A schematic illustration depicting the fabrication process of the temperature sensor is presented We used inkjet printing technique to prin conductive patterns using temperature-sensitive CNT ink on the fabric surface. Multiple inks were prepared to exist an optimum ratio between the SDS and the carbon nanotube concentration. To produce the temperature-sensitive ink, CNT materia was prepared by dispersing CNT in 50 mL distilled water with the help of a surfactant. After the addition of CNT and SDS into the distilled water, the mixture was sonicated using an ultrasonic probe sonicator (Hielscher UP400S) with an amplitude of 70%, until obtaining a homogeneous mixture. The sonication time was 20 min.the carbon nanotubes before and after sonication. Commercial inkjet printers (For example: Dimatix) are very expensive. Therefore, the printing process was carried out by a Canon office-type printer for low-cost fabrication. The substrate used in this study was a taffeta fabric. The obtained CNT ink was injected into an empty ink tank using a syringe and printed. When carbon nanotubes were printed on the fabric, the ink dried, leaving behind a randomly oriented and tangled CNT network. Multiple prints were passed over the same pattern to achieve conductivity due to the formation of the less porous and more uniform structure. The increase in density of nanomaterials by multiple prints over the same pattern caused electrically conductive CNT patterns.
Polyurethane-containing elastic tape (Bemis ST604 was used to encapsulate printed fabric and provide a high level of protection and insulation due to the combination of chemical and mechanical bonding with substrate. It requires heat and pressure for the activation and penetration on to the fabric. It is usable at 45–75% relative humidity.
The morphology of suspension was characterized with optical microscopy (Motic B1 advanced series) It is necessary to examine the compatibility of the prepared ink with the inkjet printing requirements Thus, the ink’s physical parameters, including vis cosity, density, and surface tension, are essential to check the printability. The surface free energy of the substrate was measured using Owens Wendt method and the surface tension of the ink was measured using the pendant drop method by the Theta Lite optic tensiometer. The contact angle measuremen (which the ink interface meets the solid surface) wa done by Theta Lite optic tensiometer using the sessile drop method Viscosity was studied on a Lamy Reomat viscometer. All measurements were repeated five times to obtain a realistic value and were con ducted at 23 ± 0.5 °C.
Morphology of the printed pattern was examined with the optical microscopy (Motic B1 advanced series) and by scanning electron microscope (SEM (Inovenso IEM 11) to observe the quality of printing SEM images were obtained at 20 kV and 7.5 mm working distance. Electrical resistance measurements of the printed sensors were made by a Keithley 2700 multimeter using four-wire measurement method. The four-wire method enables more accurate measurements by decreasing connecting wire resistance effects.
The measurement system is the data were transferred to the computer through RS232 to the universal serial bus (USB) converter cable. All output data were displayed by using Keithley 2700 Data Acquisition System, which interfaces with PC by Excelinx software. The resistive response of the CNT-printed temperature sensor was evaluated at the temperature range of room temperature to 50 °C on a hotplate (DAIHAN 20D), under the hot water containing beaker and under the fingertip with a special Omega TT-K-30-SLE(ROHS)-type thermo couple with a diameter of 0.6 mm 9 1.0 mm. Here abbreviation ‘‘SLE’’ stands for particular limits o error. Thermocouple wire bids are welded using an Omega Thermocouple and Fine Wire Welder fo improved time response. The current–voltage (I–V characteristics of the printed temperature sensor were measured by using a voltage source (Tektronix PWS4602).
CONCLUSION
In summary, the fabrication and characterizations o sensitive and stable CNT temperature sensors on fabric are demonstrated. A conductive CNT ink wa formulated and deposited on taffeta fabric by using a simple inkjet printing method. The printed tempera ture sensors exhibited NTC behavior. The obtained highest TCR and thermal index values from ou printed temperature sensors were measured as - 1.04%°C -1 and 1135 K, respectively. The presen study also reveals that the calculated temperature fits with the experimentally measured temperature and confirms that the accurate temperature can be detected using the resistance measurement output sensor can be used to measure human body tem- perature in real-time conditions. The prepared CNT ink gives promise for printed sensor applications; however, clogging problems have occurred in the inkjet printing machines used over two months. Therefore, additional processes, including centrifugation filtration for ink formation can be applied, and also adjustments in CNT size can be made to avoid inkjet nozzles from clogging. Continuation of this work will require ink formulation optimization with additional processes to keep the stability of the ink. Besides, studies to improve adhesion between the printed layers and the sub- strate will be emphasized to withstand mechanical stresses at a high level. The next study will also include a detailed investigation into the effects of humidity and washing on the sensor performance.