Research On Mechanically Robust And Smart Hydrogels Regulated By Cellulose Nanocrystals-Fe³⁺Networks

Hydrogels consist of a large amount of water and three-dimensional(3D)polymer networks,which are similar to the bio-tissues and have attracted a huge research interest because of their potential applications in tissue engineering,drug delivery systems,E-skin,and soft-bodied robots.The integration of decent mechanical properties and versatility for fabricating robust and smart hydrogels is still a great challenge.Inspired by the multifunctional biological soft tissues with multilayer network structures,herein,we designed a synergistic soft and hard network structure using covalent cross-linking and CNCs-Fe3+coordination as binding units to form the robust and smart hydrogels with decent mechanical properties,self-healing capability,and high sensing sensitivity.Firstly,the homogeneous network hydrogels were synthesized by covalent cross-linking of poly(vinyl alcohol)(PVA)and poly(vinyl pyrrolidone)(PVP)triggered by microwave-assisted treatment.Under stress,the homogeneous polymer network leads to a smooth stress-transfer with small dissipate energy,which enables the hydrogels to achieve high mechanical strength(1.16 MPa),stretchability(-700%),and good self-recovery property(96.2%).Such highly mechanical and recoverable hydrogels will stimulate the development of synthetic equivalents that have potential applications in load-bearing artificial soft tissues.Due to the homogeneous network for lack of efficient energy dissipation,the hydrogels are brittle and notch-sensitive.The mechanical properties decrease markedly when hydrogels contain notches.Secondly,in order to make the homogeneous network hydrogels with notch-insensitivity,we introduced cellulose nanocrystals(CNCs)into the homogeneous network,and formed a synergistic nanocomposite network with a smooth stress-transfer and efficient energy dissipation.The synergistic nanocomposite network makes the nanocomposite hydrogels achieving high mechanical strength and stretchability(?1200%).Moreover,the nanocomposite hydrogels are notch-insensitive,and maintain the intact notch.However,the disorder distribution of CNCs inside hydrogels and the irreversible slippage of CNCs under mechanical deformation of the hydrogels will cause a large amount of null dissipate energy,resulting in a marked decrease of self-recovery property.Thirdly,in order to make the CNCs with an order distribution inside polymer network of the hydrogels and realize the controllable and reversible slippage of CNCs to form an efficient and recoverable energy dissipation network,herein,we designed a synergistic soft and hard network structure using covalent cross-linking and CNCs-Fe3+coordination as binding units to fabricate the mechanically robust and smart hydrogels.The synergistic soft and hard structures result in a smooth stress-transfer and an efficient and recoverable energy dissipation,which enable the hydrogels to achieve high mechanical strength(2.1 MPa),toughness(8.9 ± 0.78 MJ m-3),self-healing capability(<5 min)and good self-recovery property.Furthermore,the hierarchically porous networks inside the hydrogels exhibit fast,stable and repeatable deformation toward strain to control the ionic transport,thus resulting in the high strain sensitivity,fast response,and excellent sensing stability.A wearable soft strain sensor was assembled based on the mechanically robust and smart hydrogels,which can quickly and accurately monitor human motion signal and important physiological signals.This study provides theoretical and experimental evidence for the development of multifunctional composite "soft and hard"materials based on the stiff nanofibers and flexibility polymers.