Fabrication of a highly stretchable and electrically conductive silicone-embedded composite textile through optimization of the thermal curing process
Graphical abstract
Introduction
Currently, wearable smart electronic devices, such as wearable flexible displays, smart contact lenses, and smart glasses are attracting significant attention. In addition, research into smart textiles, which provide various functionalities and are applicable to general textiles, is being actively conducted [1], [2], [3]. Smart textiles are defined as textile products such as woven, knitted, or non-woven fabrics, in addition to fibers, filaments, and yarns that can interact with the environment and the user [4], [5], [6], [7]. To implement smart textiles, electronic components should be stretchable, soft, and resilient [8], [9]. Accordingly, several studies on functional materials for flexible electrodes have been conducted, such as silicone elastomer of polydimethylsiloxane (PDMS), which is typically used to fabricate such flexible electrodes [10], [11]. PDMS polymer is a synthetic rubber material that can stably adhere to a relatively large area of a substate, even if that surface is uneven. In addition, silicone elastomer typically exhibits strong viscoelastic properties, a good workability, and almost no permanent deformation. As a result, PDMS electrodes have a wide range of potential applications in wearable electronic sensors that can detect human signals. Following in-depth research in the area of fabric-based sensing for biomedical and safety applications [11], [12], flexible electrodes manufactured using PDMS have been reported, and these have been employed in chemical sensors [13], [14], biosensors [15], [16], smart clothing [17], [18], electroencephalograms [19], [20], [21], and thermoelectric devices [22], [23], [24].
However, silicone elastomer is a non-conductive material, and hence, to construct flexible electrodes and strain sensors, it is necessary to introduce conductive materials. More specifically, high-conductivity carbon-based fillers including carbon nanotubes [25], [26], [27] and graphene [28], [29] have been introduced into PDMS elastomer to impart electrical conductivity. In addition, silver nanowires (AgNWs), which are mainly used for transparent and flexible electrodes, were introduced into silicone elastomer to maintain a high conductivity even in a highly elongated state [30], [31]. Meanwhile, the hybrid filler system is advantageous in that it combines the different dimensions and properties of different fillers to generate synergistic effects, and ultimately enhance the electrical conductivities of the conductive polymer composites. For example, combinations of two conductive materials, such as graphene spheres/AgNWs [32], short carbon fibers/whisker carbon nanotubes [33], and silver nanoparticles/AgNWs [34], have been introduced into silicone elastomer to fabricate conductive and flexible polymer electrodes. In addition, a stretchable electrode for application in a wearable display was manufactured by transfer printing of a poly(ethylene terephthalate)/AgNW-based PDMS composite material onto textiles [35]. However, in conventional silicone-based polymer electrodes, the electrical conductivity of non-conductive silicone elastomer has been improved only by controlling the content of the conductive fillers introduced into the silicone polymer, without considering the thermal curing process employed during fabrication of the silicone-based electrode.
Herein, to achieve flexible conductive textile for use in next-generation smart clothing, we propose a facile and effective approach for the fabrication of a conductive composite textile (CCT) containing an excellent conductive network by optimizing the thermal curing process after patterning a silicone polymer composite containing conductive fillers onto a fabric substrate through screen printing process. The screen printing method is mainly used for plating applications on clothes and home appliances, and in the context of large-area smart textiles, conductive silicone polymers can be patterned quickly and easily using this technique [36], [37], [38]. To fabricate a highly stretchable and electrically conductive silicone-embedded CCT, a typical additive-type silicone polymer is employed. After uniformly mixing the curing agent and platinum catalyst with silicone polymer using a planetary mixer, the mixture is printed onto textiles to manufacture CCTs through various thermal curing conditions. Therefore, the silicone polymer existing on the surface of the textile is thermally diffused during the thermal curing process and penetrates the fabric [39], [40]. This will be expected to produce a mechanically interlocked structure between the infiltrated silicone polymer and the fiber matrix that affords stable elastic behavior and electrical performance for CCTs. Compared to conventional silicone-based process, our proposed approach is advantageous because it requires only facile screen printing of silicone polymers without a large amount of solvent and complicated coating equipment. Furthermore, the electrical properties of silicone-embedded CCTs can be enhanced by controlling the thermal diffusion of silicone polymer through simple heat curing process without an intricate design of the conductive fillers. We believe that our simple approach paves a new way to fabricate functional conductive composite textiles for wearable smart electronic devices.
Section snippets
Materials
The additive-type silicone polymer (Mn = 6650 g mol−1, Sylgard 184, Dow Inc., USA) containing 55 wt% of high-conductivity filler and the silicone cross-linking agent of methyl hydrogen silicone fluid (Mn = 2500 g mol−1, wave 803B, Wave Co., Ltd., South Korea) containing 2 wt% of the platinum catalyst were supplied from Wave Company. The conductive filler (GGP Metalpowder AG, Germany) was a dendritic type of copper particles measuring 25–30 μm in size and 0.22–0.26 in the aspect ratio. The
Thermal curing behavior of the conductive silicone composite
Fig. 2(a) shows a representative curing mechanism of the additive-type silicone polymer employed in this study [41], [42]. For effective curing of the conductive silicone composite, it was cured with the aid of a small amount of platinum catalyst. Thermally curable conductive silicone is usually prepared from a mixture of two liquid silicones, namely a silicone polymer and a silicone cross-linker containing a platinum catalyst. The thermal curing process begins when these two liquid silicones
Conclusions
In this study, a highly elastic and electrically conductive silicone-based composite textile was fabricated. During the thermal curing process, the silicone component combined with a conductive filler underwent thermal diffusion to penetrate the fabric, forming a mechanically interlocked stable structure with the fibers to produce a conductive network between the silicone and the conductive filler on the textile surface. The thermal curing process was optimized to form a stable and durable CCT
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by Ministry of Science and ICT (MSIT) in Korean government and Korea Industrial Technology Association (KOITA) as “A study on the programs to support collaborative research among industry, academia and research institutes” (No. KOITA-CLUSTER-2021-02). This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. NRF-2018R1A5A1024127).
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These authors contributed equally to this work.