Abstract
Hydrogel-based strain sensors inspired by nature have attracted tremendous attention for their promising applications in advanced wearable electronics. Nevertheless, achieving a skin-like stretchable conductive hydrogel with synergistic characteristics, such as ideal stretchability, excellent sensing performance and high self-healing efficiency, remains challenging. Herein, a highly stretchable, self-healing and electro-conductive hydrogel with a hierarchically triple-network structure was developed through a facile two-step preparation process. Firstly, 2, 2, 6, 6-tetrametylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils were homogeneously dispersed into polyacrylic acid hydrogel, with the presence of ferric ions as an ionic crosslinker to synthesize TEMPO-oxidized cellulose nanofibrils/polyacrylic acid hydrogel via a one-pot free radical polymerization. A polypyrrole conductive network was then incorporated into the synthetic hydrogel matrix as the third-level gel network by polymerizing pyrrole monomers. The hierarchical 3D network was mutually interlocked through hydrogen bonds, ionic coordination interactions and physical entanglements of polymer chains to achieve the target composite hydrogels with a homogeneous texture, enhanced mechanical stretchability (elongation at break of ~890%), high viscoelasticity (maximum storage modulus of ~27.1 kPa), intrinsic self-healing ability (electrical and mechanical healing efficiencies of ~99.4% and 98.3%) and ideal electro-conductibility (~3.9 S m−1). The strain sensor assembled by the hybrid hydrogel, with a desired gauge factor of ~7.3, exhibits a sensitive, fast and stable current response for monitoring small/large-scale human movements in real-time, demonstrating promising applications in damage-free wearable electronics.
Highlights
The concept of mimicking human skin to develop artificial tissue-like electronic devices has drawn widespread interest due to their broad potential applications in soft robotics, wearable devices, tissue engineering, bioelectronics and artificial intelligence [1,2,3]
The double-network TEMPO-oxidized cellulose nanofibers (TOCNFs)/Polyacrylic acid (PAA) hydrogel matrix was prepared via one-pot in situ free radical polymerization of acrylic acid (AA) monomers using ammonium persulfate (APS) as an initiator, MBA as a chemical cross-linker and Fe3+ as an ionic cross-linker in the presence of TOCNFs as a reinforcing phase
The presence of -COOH and -OH groups in the PAA and TOCNF structure built up plenty of inter- and intramolecular hydrogen bonds, stabilizing the double network of the TOCNF/PAA hydrogel [21]
Summary
The concept of mimicking human skin to develop artificial tissue-like electronic devices has drawn widespread interest due to their broad potential applications in soft robotics, wearable devices, tissue engineering, bioelectronics and artificial intelligence [1,2,3]. As the essential component of skin-inspired electronics, flexible and wearable strain sensors with a sensitivity similar to human skin tactile sensations can transduce mechanical deformations into electrical signals and generate repeatable electrical responses upon external forces, which has inspired remarkable efforts in fabricating innovative soft materials with special functional features [4,5]. To construct high-performance sensing materials, the integration of superior stretchability and self-healability is considered vital for the long-term practical application of personalized electronics [6,7]. One the other hand, integrating self-healing ability into sensing materials can avoid damage to device performance during repeated deformations and considerably improve the durability of the electronics [9]. Developing electro-conductive strain sensing materials with combined high stretchability and intrinsic self-healing capability still remains challenging [7,10,11,12]
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