Abstract
Intrinsic self-healing and highly stretchable electro-conductive hydrogels demonstrate wide-ranging utilization in intelligent electronic skin. Herein, we propose a new class of strain sensors prepared by cellulose nanofibers (CNFs) and graphene (GN) co-incorporated poly (vinyl alcohol)-borax (GN-CNF@PVA) hydrogel. The borax can reversibly and dynamically associate with poly (vinyl alcohol) (PVA) and GN-CNF nanocomplexes as a cross-linking agent, providing a tough and flexible network with the hydrogels. CNFs act as a bio-template and dispersant to support GN to create homogeneous GN-CNF aqueous dispersion, endowing the GN-CNF@PVA gels with promoted mechanical flexibility, strength and good conductivity. The resulting composite gels have high stretchability (break-up elongation up to 1000%), excellent viscoelasticity (storage modulus up to 3.7 kPa), rapid self-healing ability (20 s) and high healing efficiency (97.7 ± 1.2%). Due to effective electric pathways provided by GN-CNF nanocomplexes, the strain sensors integrated by GN-CNF@PVA hydrogel with good responsiveness, stability and repeatability can efficiently identify and monitor the various human motions with the gauge factor (GF) of about 3.8, showing promising applications in the field of wearable sensing devices.
Highlights
Flexible, stretchable, self-healing and human-friendly devices have gained widespread attention for multi-functional wearable electronics [1,2]
Most electro-conductive hydrogels (ECHs) currently suffer from weak mechanical strength, insufficient viscoelasticity, lack of self-healing ability and poor electrical conductivity, which can hardly satisfy the demands of practical applications
Biomass-derived cellulose nanofibers (CNFs) acts as templates and nanocarriers to support GN to create uniformly dispersed GN-CNF nanocomplexes in polyvinyl alcohol (PVA) gel matrix, enhancing the viscoelasticity, mechanical strength and electro-conductivity of the GN-CNF@PVA hydrogel
Summary
Flexible, stretchable, self-healing and human-friendly devices have gained widespread attention for multi-functional wearable electronics [1,2]. Hydrogels with a 3D polymer network can preserve large amounts of water During deformation, these soft materials can maintain their structural integrity and exhibit intrinsic flexibility, stretchability and even self-healing ability, which are considered as a suitable candidate for the fabrication of strain sensors [2]. Nanocellulose with cellulose backbone chains inherently forms a hierarchical layered structure This architecture allows for strong interaction between polymer gel matrix and adjacent nanocellulose, which can contribute to the enhancement of mechanical properties [11,12,13]. We successfully fabricated a type of intrinsic self-healing, highly conductive and stretchable hydrogels towards flexible strain sensors. The hydrogen bonding system, entangling of PVA and GN-CNF, and dynamic and reversible multi-complexation that resulted from borax cross-linking contributed to the construction of the combined reinforcing and conductive network within the composite hydrogels. Nanomaterials 2019, 9, 937 could sense and monitor the real-time body motion, demonstrating potential application in wearable sensing devices
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