Highly stretchable and sensitive strain sensor based on polypyrrole coated bacterial cellulose fibrous network for human motion detection

https://doi.org/10.1016/j.compositesb.2021.108665Get rights and content

Highlights

  • A NR strain sensor was prepared based on polypyrrole coated bacterial cellulose nanofiber network.

  • The cPPy/BCNF@NR exhibited excellent elongation at break of 388.0 % at only 6% PPy loading.

  • A gauge factor of 355.3 was achieved in the strain range over 300%.

  • The cPPy/BCNF@NR sensor satisfied the detecting requirements of various range of human body motions.

Abstract

Recently, wearable strain sensors based on elastomeric conductive composites have attracted tremendous attention in human motions and physical signals detection. However, it is still challenging to achieve good conjunction between broad strain range and high sensitivity performance for a wide range of applications. Here, we construct a highly stretchable and sensitive natural rubber (NR) based strain sensor from polypyrrole modified bacterial cellulose nanofiber (cPPy/BCNF) network. Due to the large aspect ratio and strong interfacial interaction of BCNF templates, cPPy/BCNF could form continuous conductive pathways in NR matrix with an extremely low loading, thus greatly reducing the permeability threshold of the elastomer. Consequently, our strain sensor performs remarkable sensitivity over a broad strain range (0–388%) and long-term reliability (3000 cycles at 60% and 180% strain). Particularly, a high gauge factor of 355.3 recorded in the strain range of 279–388% has been achieved, outperforming most reported NR-based stretchable strain sensor. Meanwhile, the sensor also exhibits stable strain sensing behavior under different amplitude of human motions such as finger, wrist, elbow, and knee joint bending. Our work provides more opportunities for the design of effective elastomeric conductors in the application of wearable electronic devices.

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).

References (68)

  • S.T. Han et al.

    An overview of the development of flexible sensors

    Adv Mater

    (2017)
  • Q. Zhang et al.

    Electromagnetic shielding hybrid nanogenerator for health monitoring and protection

    Adv Funct Mater

    (2018)
  • X. Fang et al.

    High-performance wearable strain sensors based on fragmented carbonized melamine sponges for human motion detection

    Nanoscale

    (2017)
  • J. Luo et al.

    Superhydrophobic and breathable smart MXene-based textile for multifunctional wearable sensing electronics

    Chem Eng J

    (2021)
  • S. Choi et al.

    High-performance stretchable conductive nanocomposites: materials, processes, and device applications

    Chem Soc Rev

    (2019)
  • C.P. Feng et al.

    A facile route to fabricate highly anisotropic thermally conductive elastomeric POE/NG composites for thermal management

    Adv Mater Interfaces

    (2018)
  • T. Xiao et al.

    3D printing of flexible strain sensor array based on UV-curable multiwalled carbon nanotube/elastomer composite

    Adv. Mater. Technol.

    (2020)
  • J.E.Q. Quinsaat et al.

    Stretchable piezoelectric elastic composites for sensors and energy generators

    Compos B Eng

    (2020)
  • B. Li et al.

    A highly stretchable, super-hydrophobic strain sensor based on polydopamine and graphene reinforced nanofiber composite for human motion monitoring

    Compos B Eng

    (2020)
  • T. Yamada et al.

    A stretchable carbon nanotube strain sensor for human-motion detection

    Nat Nanotechnol

    (2011)
  • F. Xu et al.

    Highly conductive and stretchable silver nanowire conductors

    Adv Mater

    (2012)
  • D. Drotlef et al.

    Bioinspired composite microfibers for skin adhesion and signal amplification of wearable sensors

    Adv Mater

    (2017)
  • Y.Q. Li et al.

    Highly flexible strain sensor from tissue paper for wearable electronics

    ACS Sustainable Chem Eng

    (2016)
  • S.Y. Wu et al.

    Ultrasensitive and stretchable strain sensors based on mazelike vertical graphene network

    ACS Appl Mater Interfaces

    (2018)
  • Y.C. Cai et al.

    Stretchable Ti3C2Tx MXene/carbon nanotube composite based strain sensor with ultrahigh sensitivity and tunable sensing range

    ACS Nano

    (2017)
  • M. Abshirini et al.

    3D printing of highly stretchable strain sensors based on carbon nanotube nanocomposites

    Adv Eng Mater

    (2018)
  • Q. Li et al.

    Engineering of carbon nanotube/polydimethylsiloxane nanocomposites with enhanced sensitivity for wearable motion sensors

    J Mater Chem C

    (2017)
  • Y.R. Jeong et al.

    A skin-attachable, stretchable integrated system based on liquid GaInSn for wireless human motion monitoring with multi-site sensing capabilities

    NPG Asia Mater

    (2017)
  • D.Y. Choi et al.

    Highly stretchable, hysteresis-free ionic liquid-based strain sensor for precise human motion monitoring

    ACS Appl Mater Interfaces

    (2017)
  • B. Guo et al.

    Properties of conductive polymer hydrogels and their application in sensors

    J Polym Sci Part B

    (2019)
  • C. Cochrane et al.

    A flexible strain sensor based on a Conductive Polymer Composite for in situ measurement of parachute canopy deformation

    Sensors

    (2010)
  • H.G. Wu et al.

    Fibrous strain sensor with ultra-sensitivity, wide sensing range, and large linearity for full-range detection of human motion

    Nanoscale

    (2018)
  • C.M. Boutry et al.

    A stretchable and biodegradable strain and pressure sensor for orthopaedic application

    Nat Electron

    (2018)
  • M. Amjadi et al.

    Stretchable, skin-mountable, and wearable strain sensors and their potential applications: a review

    Adv Funct Mater

    (2016)
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