An integrated wearable strain, temperature and humidity sensor for multifunctional monitoring
Introduction
Over the past decades, flexible and wearable electronics have been boomingly explored for tremendous applications in personal healthcare, smart consumer electronics, human–machine interfaces, artificial intelligence and so forth[1], [2], [3], [4]. Among various species of wearable electronics, textile-based electronics which integrate conventional fabrics and electronic devices have been one of attractive alternatives because textiles provide great comfort and essentiality to human beings at all times[5], [6]. As a result, enormous efforts have been made to develop conductive textiles with different functions such as energy storage, sensing actuation, wireless communication, heating performance and so on[3], [7], [8], [9]. Researchers are recently commencing to introduce these features into an individual electronic device to achieve a multifunctional integrated electronic system instead of fabricating a device with single function[10], [11]. The trend in multifunctionality for textile-based electronics, therefore, can efficiently reduce fabrication cost and save space[12]. Wearable sensor with strain, temperature and humidity sensing functions, for example, is capable of obtaining human motion, body temperature and respiration.
To fabricate highly conductive textiles, CNT with excellent mechanical and electrical properties has been widely used to decorate textiles[13]. CNT is normally a mixture of semiconductor and metallic in nature and separation from one another is relatively difficult[14]. Hence, combining metals (such as Cu, Ni and Ag) with CNT to obtain hybrid metal/CNT composite may possibly generate synergistic properties from both metals and CNT[15]. Fattah et al. developed a cost effective CNTs-NiNP ink which can be printed as embedded circuits in elastomeric matrix and fabricated to be a strain or oil sensor[16]. Kim et al. presented a nickel coated SWNT reinforced copper matrix composites with improved mechanical and tribological properties[17]. Zhang et al. prepared a thread like composite yarn by twisting Cu filament and CNT yarns, showing an enhanced electrochemical properties and capacitance as a supercapacitor[18]. Meanwhile, different approaches (powder metallurgical, chemical deposition, melting and solidification and spray techniques) have been used to fabricate CNT-metal composites[19], [20], [21]. Chemical deposition techniques including electrodeposition and electroless deposition haves been extensively used due to the merits of low cost and easy operation[22]. Compared to electroplating, the metals in electroless deposition technique are reduced by chemical reaction and deposited on the surface of CNT without electrical current[19]. Since the electroless plating process is not driven by electric current, activation and sensitization of CNT becomes a crucial step[23]. Previous studies report that CNT is sensitized by stannous chloride followed by activated with palladium chloride and then coating activated CNT surface with metal layer, which is conventional but will cause damages to CNT and environment[24]. Moreover, the interfacial bonding strength between CNT and metal coating is not satisfying owing to the hydrophobic nature of CNT, which makes it tough to attach metals to the surface[25]. To solve these issues, surface treatment of CNT is conducted by polymerizing dopamine (DoPA), a biocompatible and eco-friendly biomolecule comprised of amine and catechol groups[26]. A thin polydopamine (PDA) film is self-polymerized on the surface of CNT, introducing abundant hydrophilic functional groups to capture catalytic metals efficiently[27]. In addition, stannous chloride sensitization and palladium chloride activation is substituted with nickel sulfate activation in order to reduce the cost and complexity[28].
In this work, a nickel-CNT layered fabric integrated with strain, temperature and humidity sensing capabilities is prepared by depositing CNT on flax fabric, followed by subsequent electroless nickel deposition involving PDA surface treatment, nickel activation and nickel coating. The whole process is demonstrated in Fig. 1. Microstructures of the composite in each step are investigated by Fourier Infrared, Raman spectroscopy, X-ray diffraction and field emission-scanning electron microscopy. The strain sensing behaviors are evaluated by stretching conductive composites, while temperature and humidity sensing performances are measured in various temperature and relative humidity (RH) environments, respectively. The reliability of the resultant conductive composite is conducted through peeling test and corrosion measurement to validate the adhesion strength and chemical stability in realistic applications. Therefore, the prepared conductive composite with superior mechanical strength and anti-corrosive property is a promising candidate in wearable electronics as multifunctional (strain, temperature and humidity) sensors.
Section snippets
Fabrication of CNT-coated flax fabric
Commercially available plain weave flax fabrics (FF, 120 g·m−2, Taicang Biqi Novel Material Co. Ltd., China) was pre-cleaned with distilled water and acetone, respectively for 5 min to remove impurities and grease, following with rinsing till pH at 7 and drying. CNT aqueous solution (with sodium dodecyl benzene sulfonate as dispersant) was prepared by sonicating for 30 min and the concentration of CNT solution was set as 0.5 mg·ml−1, 1.0 mg·ml−1, 1.5 mg·ml−1 and 2.0 mg·ml−1. Subsequently, the
Structure characterization
The FT-IR spectra obtained for pristine FF (a), CNT/FF (b) and CNT/FF-PDA (c) are shown in Fig. 2(a). In the spectrum of pristine FF, typical band peaks of the main constituents of cellulose, hemicellulose and lignin are occurred at the following locations: –OH stretching vibration at 3343 cm−1, C-H stretching vibration at 2901 cm−1, C = C stretching vibration at 1633 cm−1 and C-O stretching vibration at 1050 cm−1. Furthermore, peaks at 1158 cm−1, 1107 cm−1 and 896 cm−1 are associated with the
Conclusion
NCF sensor with multifunctional capabilities including strain, temperature and humidity response is fabricated by dip-coating CNT and subsequently electroless nickel deposition on flax textile substrate. The unique catechol functional groups in PDA ultrathin film facilitate the chelating capacity towards metal ions and thus enhance the interfacial bonding between CNT and nickel. By virtue of responses in Rrel value to different stimuli, NCF is capable of serving as multifunctional sensors.
CRediT authorship contribution statement
Siyi Bi: Conceptualization, Writing - original draft, Writing - review & editing. Lei Hou: Validation. Yinxiang Lu: .
Declaration of Competing Interest
None.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. U1830108), the Shanghai Natural Science Foundation (20ZR1405000), the exploratory research project of “Yiwu Research Institute of Fudan University “.
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