Highly stretchable and sensitive strain sensor based on polypyrrole coated bacterial cellulose fibrous network for human motion detection
Graphical abstract
Introduction
In recent years, wearable strain sensors, characterized by high sensitivity, large strain tolerance and long-term stability, have rapidly established their status as the preferred equipment for human movement detection [[1], [2], [3], [4], [5]]. Among various strain sensing materials, elastomeric conductive polymer composite (ECPC), functioned on the electrical resistance changes upon external tensile strain, are widely used for detecting motion signals including human voice, finger motion and joint motion, etc [[6], [7], [8], [9], [10]]. Generally, the key factor affecting the electrical sensitivity of elastomers depends on the contact efficiency between conductive fillers [[11], [12], [13], [14]]. Nowadays, with the development of the high conductivity nanofillers such as graphene [7,15,16], carbon nanotube [[17], [18], [19]], liquid metal [20,21] and conductive polymer [22,23], the electrical sensitivity of the ECPC have been significantly improved. However, due to the intrinsic low strain tolerance or elastic of these carbon or metal-based fillers, irreversible cracks or fractures will inevitably occur under high strains with the increase of filler contents [24,25]. Consequently, it is still challenging to effectively construct a compact but flexible conductive network with a low loading of nanofillers to achieve good conjunction between broad detection range and high sensitivity performance [[26], [27], [28], [29]].
For this purpose, supporting or template materials that can establish strong interfacial interaction with conductive fillers should favor the construction of conductive network with lower electrical conductivity percolation threshold, thus improving the flexibility and sensitivity [[30], [31], [32]]. Biomass bacterial cellulose nanofiber (BCNF), biologically synthesized by Acetobacter xylinum, has been widely used as environmentally friendly reinforcing materials in various applications [[33], [34], [35]]. The BC product obtained through fermentation process exhibits ultra-thin nanofibers (30–50 nm) and unique three-dimensional (3D) interconnected network structure [36]. Compared with natural plant cellulose nanofiber (CNF), the BCNF also processes higher crystallinity, higher aspect ratio and higher mechanical strength, which ensure its excellent mechanical enhancement even at a low loading in polymer matrix [37,38]. For instance, the Blaker's group fabricated the self-reinforced polylactide composites with only 2 wt % of modified BCNF, resulting in the improvements in viscoelastic properties of up to 175% in terms of storage moduli at bending [39]. More attractively, the abundant hydroxyl groups exposed on the surface of the BCNF enable the polymeric monomer to be evenly dispersed and coated along the fiber for the construction of conductive network with high electrical properties [40,41]. For example, the Wang's group prepared the BCNF/polyaniline (PANI) composite paper via chemical grafting of PANI onto epoxy modified BCNF, the conductivity of the composite paper could be up to 1.08 S m−1 [42]. Therefore, we expect the design of using BCNF as substrate to construct continuous conductive pathways in elastomeric matrix could overcome the trade-off relationship between mechanical properties and sensing performance, thus meeting the needs for a wide range of applications [[43], [44], [45]].
In this study, we constructed a PPy coated BCNF conductive network for the preparation of NR-based ECPCs sensor. The cPPy/BCNF are found to form a continuous conductive network in the NR matrix because of the strong inter-fiber connections and high aspect ratio of the modified BCNF. Consequently, the prepared cPPy/BCNF@NR strain sensor exhibit both broad strain range to 388% strain and high gauge factor (GF) of 355.3 with the filler loading of only 6 wt%. Moreover, long-term durability over 3000 cycles at 60% and 180% strain stretching-releasing deformation is also achieved. Under different amplitude of human motions such as such as swallowing, finger, wrist, elbow, and knee joint bending, the sensor shows stable strain sensing behavior, indicating its tremendous potential applications in future intelligent electronics.
Section snippets
Materials
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 98%), sodium hypochlorite (NaClO, 99%), sodium chlorite (NaClO₂, 98%), ethanol (99.5%), hydrochloric acid (HCl, 36%), pyrrole (Py, 99%), polyvinylpyrrolidine (PVP, 99%) were obtained from Aladdin Reagent Co., Ltd. Sulfuric acid (H2SO4, 98%), ammonium hydroxide (NH4OH, 28%), ferric trichloride (FeCl3, 99.99%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Aqueous natural rubber (NR) latex was obtained from Thailand Sankeshu natural latex
Results and discussion
The structural design of the elastomer is illustrated in Fig. 1a. The most intriguing design of our work is using modified BCNF to construct continuous cPPy/BCNF conductive network for the preparation of NR latex-based sensing material. In our work, BCNF is firstly treated by a gentle TEMPO-oxidation treatment accompanied with high-pressure homogenization for improving their dispersibility [46]. Hence, from the TEM micrographs in Fig. 1b, one can see interconnections between fibers could be
Conclusion
In summary, using BCNF as substrate, we reported the construction of a NR-based flexible strain sensor based on highly dispersed cPPy/BCNF conductive nanofillers. The cPPy/BCNF with strong interfacial interactions could form a continuous network throughout the NR matrix. Consequently, at a PPy loading of only 6%, the conductivity of cPPy/BCNF@NR was significantly improved and excellent mechanical properties were retained (with the elongation at break over 388%). Notably, a gauge factor of 355.3
CRediT authorship contribution statement
Xuran Xu: Methodology, Formal analysis, Investigation, Data curation, Writing - original draft. Shuaining Wu: Investigation. Jian Cui: Investigation. Luyu Yang: Validation. Kai Wu: Visualization. Xiao Chen: Writing - original draft, Supervision. Dongping Sun: Resources.
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.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (52003121), the Natural Science Foundation of Jiangsu Province (BK20200501), China Postdoctoral Science Foundation (2020M671497, 2020T130300), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, China).
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