| Hydrogel,consisting of 3D polymer chains network and large amount of water,is a type of soft material which has many similarities with biological soft tissues.Due to its outstanding characteristics,hydrogel can be widely used in biomediacal applications such as drug delivery,tissue engineering and artificial muscles.In recent years,the research of natural polymers has entered a period of rapid development due to its biologically renewable,biocompatible,and biodegradable properties.Natural polymers,deriving from biomass resources,mainly include cellulose,chitin,starch,protein,and alginate.In particular,cellulose,the most abundant natural crystalline polysaccharide,has become a research hotspot.This thesis aims to construct fuctional cellulose hydrogel materials with excellent mechanical properties and functionality through energy-saving,green and facile approachs.The structure and performance of as-prepared materials were evaluated by using scanning electron microscopy(SEM),transmission electron microscopy(TEM),atomic force microscopy(AFM),solid-state nuclear magnetic resonance spectroscopy(13C NMR),small-angle X-ray scattering(SAXS),wide-angle X-ray diffraction(WAXS)and mechanical test.The properties of the resultant materials make them useful in force sensors,tissue engineering,light management and antibacterial materials.The main innovations of this thesis include:(1)Cellulose hydrogels with high-strength,high-toughness and mechano-responsive behavior were constructed for the first time,which has potential as a new type of force sensor;(2)Combing pre-stretching with hybrid cross-linking strategy,diversified anisotropic cellulose hydrogels(ACHs)were prepared simply and efficiently.Especially,ACHs with microgroove-like and ordered nanofibrous architectures induced the directionally growth of cardiomyocytes;(3)Dual chemically cross-linked cellulose hydrogels with high-strength and high-toughness were fabricated through introducing short-chain and long-chain chemical crosslinker for regulating the crosslinking density of the hydrogel network;(4)Anisotropic and super clear nanocellulose film was simultaneously strengthened and toughened after intergrating orientation and dual-crosslinking strategy in a film at the first time,which was applied in light management;(5)Cellulose/nanosilver composite sponge was fabricated through freeze-drying of the prefabricated cellulose-nanosilver composite hydrogel,which was in situ synthesis of nanosilver in a chemically cross-linked cellulose hydrogel using hydrothermal method.The sponge with 3D interconnected porous structures,high water absorption ability,high water retention capacity,biocompatibilitys as well as antibacterial property,was used in the treatment of infected wounds.The main contents and conclusions of this thesis are divided into the following parts.Cellulose was dissolved in a pre-cooled alkali/urea aqueous system,resulting in a cellulose solution.Then,a novel hybrid cross-linked cellulose hydrogel with high-strength,high-toughness,mechano-responsive behavior and hierarchical structure was fabricated by combining chemical crosslinking strategy and rapid acid treatment induced physical crosslinking strategy on account of the fact that the spontaneous aggregation of cellulose chains into nanofibers in cellulose solution.The loose chemical networks constituted by submicrobundles could maintain the high elasticity of the hydrogel,whereas the densely physical networks comprised of cellulose nanofibers were reversible,which could dissipate mechanical energy through the breaking of"sacrificial bonds" under large deformation,leading to the high strength and toughness of hydrogels.Under deformation,the densely physical networks synchronized easily with chemical network framework to align and orient,leading to a rapidly mechano-responsive birefringence behavior.The switchable birefringence of hydrogel revealed its sensibility to detect small external force,which made the hydrogel have potential application as a force sensor for monitoring small external force.This polysaccharide-based hydrogel greatly expands the application of cellulose in the areas of smart soft materials and biomedical applications.The loosely chemically cross-linked gels(LCGs)were fabricated by adding a small amount of chemical crosslinker in cellulose solution.Then,anisotropic cellulose hydrogels(ACHs)with highly ordered structure were constructed for the first time through combing pre-stretching of LCGs with subsequently fixing the hydrogel orientation using acid treatment.LCGs could be pre-stretched to form a temporarily orientated structure under small external force.Upon rapid acid treatment(<1 min),the alkali/urea solvent shells on the cellulose chains could be destroyed,leading to a new aggregated structure,and permanently locking the hydrogel orientation.The resultant hydrogels displayed a long-range aligned structure,entirely differential mechanical performances along the parallel and perpendicular directions of the hydrogel orientation and optical birefringence,which could be controlled by adjusting the pre-stretching draw ratio.Importantly,ACHs with microgroove-like structure promoted well the adhesion and orientation arrangement of cardiomyocytes,demonstrating its potential application in directionally culturing of cardiomyocytes in vitro.This work provides a simple,green,and efficient method for constructing of cellulose hydrogels with highly oriented structure for biomedical engineering and regenerative medicine.Based on the dual chemically cross-linking strategy,we used low-and high-molecular-weight cross-linkers to crosslink successively the cellulose molecular chains and prepared robust dual chemically cross-linked cellulose hydrogels(DCHs).The 3D Raman microscopic imaging technique could image the distribution of chemical crosslinking points inside the hydrogel and monitored the variation of the cross-linking areas after the first and second crosslinking reaction.DCHs have densely network structure,and excellent mechanical properties.The ruptured stress and fracture strain in tensile and compression test were 1.7 MPa,94.5%,9.4 MPa and 91.9%,respectively.DCHs with short and long chemical crosslinking bonds can dissipate energy through the successive cleavage of crosslinking networks in which the first breaking of a low-molecular-weight chemical crosslinking agent acted as sactrified bonds to dissipate energy to guarantee the elasticity of the hydrogel.DCHs with significantly improved mechanical properties greatly broadened the application prospect of cellulose hydrogels.Highly anisotropic cellulose films,fabricated from cellulose solution through a bottom-up approach(including cellulose solution,chemical gel,anisotropic hydrogel and film),were simultaneously strengthened and toughened using a novel mechano-assisted dual-crosslinking strategy,where a secondary physical crosslinking strategy was introduced to reserve the aligned structure on the firstly highly-stretched chemical cellulose gels.Nanofibers can be tightly stacked,interlocked and oriented towards the imparted pre-stretching direction in films,leading to a highly oriented nanostructure and iridescent birefringence patterns between crossed polarizers.The as-prepared anisotropic dual-cross-linked cellulose films resulted in previously unreached toughness of 41.1 MJ m-3 and also exhibited a maximum fracture stress of 253.2 MPa and a stiffness of 13.9 GPa owing to the alignment of nanofibers along stretching direction.This strategy might be very scalable in fabricating anisotropic cellulose films,combining high-strength,toughness,controllable orientation degrees and morphologies,just through adjusting the pre-stretching draw ratio.To improve the exudates absorptive properties and antibacterial activity,a series of regenerated cellulose/nanosilver sponges were constructed by combing with hydrothermal method and freeze-drying of chemical cellulose hydrogels.The silver nanoparticles(Ag NPs)were uniformly immobilized in cellulose sponges through hydrothermal method,where the interconnecting pores of sponge not only used as micro-reactors to synthesize Ag NPs but also inhibited the aggregation of Ag NPs.The results of in vitro experiments indicated that the sponges exhibited good biocompatibility and excellent antibacterial activity.Further,in vivo tests confirmed that the cellulose/nanosilver composite sponges had an ability to accelerate infected wound healing.This work provides a facial and "green" way of constructing of composite cellulose sponge with excellent antibacterial properties to act as an ideal material for wound dressings.The most abundant and renewable natural resource,cellulose,was used as raw materials for developing various fuctional cellulose materials,including hydrogels,films and sponges,via a bottom-up approach.The structure and properties of hydrogels,films and sponges were in detail evaluated,promoting the utilizations of cellulose in sensing force,inducing cells udirectional growth,managing light and healing infected wounds.Therefore,there were of great scientific values and application potentiality for a sustainably development. |
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