Fabrication And Performance Of Nanocellulose-based Self-healing/shape Memory Hydrogel

Self-healing and shape memory hydrogels,as prominent representatives of smart hydrogels,have become a frontier in the high-tech field due to their keen stimulus response and excellent bionic properties.However,the preparation of traditional self-healing/shape memory hydrogels usually involves the use of synthetic organic polymers derived from fossil resources,which is not in line with the concept of green and sustainable development,and will also limit its application in biology and medicine due to potential toxicity.Fortunately,it is expected to solve above peroblems thoroughly by using biomass-based natural polymers instead of fossil-based synthetic polymers.In recent years,though varieties of self-healing/shape memory hydrogels based on different biomass have been developed successively,they are generally deficient in mechanical properties and involve complex modification process,which fails to fully demonstrated their superiority.To address the above-mentioned problems,cellulose nanocrystals(CNCs)with excellent mechanical strength,high dispersion stability and chemically active property were selected as raw materials to construct a series of self-healing/shape memory hydrogels.The introduction of self-healing/shape memory mechanisms into CNCs was realized by virtue of gentle physical/chemical modification method,and the advantages of CNCs in self-healing/shape memory hydrogels as strength enhancer,dispersant,template and crosslinking agent were deeply explored as well.Furthermore,the effect of process variables on the performance of self-healing/shape memory hydrogels were explored in detail,and the intrinsic mechanism of CNCs for endowing self-healing/shape memory properties to hydrogel was revealed.In addition,the rapid self-healing/shape memory response behavior and biocompatibility of hydrogels were also emphasized.Periodate oxidation method was first applied to modifying CNCs,and the results showed that aldehyde groups were efficiently introduced into CNCs without sacrificing its rod morphology and colloidal dispersion stability.Then,dialdehyde CNCs(DACNCs),as macromolecular crosslinking agent and strength enhancer,were mixed with acylhydrazine-terminated polyethylene glycol to construct a hydrogel via acylhydrazone crosslinking for the first time.The results were as follows:lower p H and higher gelator concentration were beneficial to accelerating the formation of hydrogel significantly;the tensile and compressive strengths of hydrogel with 20.1%glator concentration were as high as 141 k Pa and 580 k Pa,respectively;excellent self-healing property of hydrogel was guaranteed by dynamic reversible acylhydrazone bond whose healing efficinency was higher than 90%at a wider gelator concentration(11.2%?20.1%);in addition,the hydrogel showed excellent affinity to L-929 cells,and the cell viability was even higher than 100%.Ultra-fine gold nanoparticles(Au NPs)were synthesized successfully for the first time at ambient temperature with DACNCs as the sole reducing agent,template and stabilizer.Mono-dispersed Au NPs with a minimum size of 5.1±1.0 nm was quantitative obtained under the conditions of p H=10.8,0.4%(w/v)DACNCs,0.25 m M HAu Cl₄,53%degree of oxidation(DO).In addition,the as-prepared Au NPs@DACNCs showed excellent catalytic performance for the reduction of p-nitrophenol,in which the reaction rate constant(k)and turnover frequency(TOF)were reached 1.6×10^-3 s^-1 and 279 h^-1,respectively.Furthermore,a double crosslinked hydrogel via helical junction and Schiff base were prepared by compounding Au NPs@DACNCs and gelatin.The results were as follows:glycine could be used to adjust the crosslinking density of Schiff base between DACNCs and gelatin,thereby obtaining a higher shape fixity ratio whose value reached 88.9%under the condition that the molar ratio of glycine to aldehyde group was 0.75;meanwhile,the excellent photothermal effect of Au NPs endowed rapid and efficient shape recovery and self-healing behavior of hydrogel,whose shape recovery ratio and healing efficiency were as high as 100%and 95.6%after illuminating by NIR laser for 30 s and 8 min,respectively.Phenylboronic acid grafted CNCs(CNCs-ABA)were first successfully prepared by a two-step method of periodate oxidation and reductive amination,and the results presented that CNCs-ABA not only retained the chemical framework and nanoscale morphology of CNCs,but also has an excellent stabilizing effect on MWCNTs.Subsequently,the as-prepared CNCs-ABA,as the dispersant of MWCNTs and crosslinking agent,were mixed with MWCNTs and PVA to prepare a double crosslinked hydrogel with both microcrystallization and borate bond.Relevant results were as follows:double crosslinking network endowed excellent mechanical properties to hydrogel,whose tensile strength,elongation at break and elastic modulus were reached 227 k Pa,395%and 9000 Pa,respectively;excellent self-healing ability of hydrogel with healing efficiency of 97%(100 s NIR irradiation)was obtained depending on remarkable photothermal effect of MWCNTs and thermal-induced reversible microcrystallization;meanwhile,dynamic borate bonds between CNCs-ABA and PVA gave the hydrogel excellent shape memory performance,when hydrogels were immersed in dilute acid and alkali for 30 and 10 min,respectively,their shape recovery ratio and fixity ratio could reach 82.1%and 78%,respectively;furthermore,the synergistic effect of MWCNTs and Na OH enhanced the conductivity of hydrogels with a value of 3.8×10^-2 S/m;in addition,the hydrogel was an excellent strain sensor for detecting the human joint motion with superior biocompatibility and fast resistance response to applied strain,which was suitable for human health management.Polyaminophenylboronic acid(PABA)was deposited onto CNCs via in situ chemical oxidation polymerization method for the first time to obtain PABA@CNCs composites,and the result revealed that PABA was wrapped on the surface of CNCs in the form of shell,which not only did not weaken the dispersion stability of CNCs,but also endowed CNCs with excellent photothermal effect.Whereafter,PABA@CNCs were combined with PVA to prepare a double crosslinked hydrogel by virtue of microcrystallization and borate bond,and several representative conclusions were as follows:PABA@CNCs increased the tensile strength of PVA hydrogel by 8.6 times,and the formation of borate bond further increased the tensile strength by 2 times;rapid and efficient self-healing property of hydrogel was guaranteed relying on the excellent photothermal effect of PABA and thermo reversible microcrystallization of PVA,whose healing efficiency reached 96%after exposing to NIR laser for 1.7 min;similarly,superior shape memory performance of hydrogel with shape fixity ratio of 88.9%and shape recovery ratio of 81.9%were also obtained via the p H-induced dissociation and reconstruction of borate bonds;finally,the as-prepared hydrogel showed excellent affinity for L-929 cell.