Green Fabrication Of Xylan-based Hydrogel With Extremely High Compressive Strength And Resilience

Hydrogels are 3D network polymeric materials which are capable of holding large amounts of water,aqueous solutions,or physiological fluids while still maintaining insolubility,and has good hydrophilicity,permeability,biocompatibility and low friction coefficient.Therefore,hydrogels have been widely used in the area of biomaterials and other fields,such as drug delivery,tissue engineering,contact lenses,wound healing,biosensor membranes,microfluidic valves,fluid absorbents,and so on.However,hydrogels are mostly structurally or/and mechanically weak due to their high-water contents and relatively fragile polymer networks in the fully swollen state,which largely limits their applications.Therefore,solving the above-mentioned problem of hydrogel is helpful to broaden its applications in different fields.Polysaccharides,as natural polymers,have inherent characteristics such as biocompatibility,biodegradability,and reproducibility,and the hydrogels prepared from them have natural advantages in the field of biomedicine.Xylan is one of the most important natural polysaccharides in plants.As same as other polysaccharide-based hydrogels,xylan-based hydrogels are inferior in terms of mechanical properties and structural stability,which are insufficient to meet the current requirements for applications in the biomedical field.Some reported covalent crosslinked hydrogels exhibited extremely high compressive strength,however,the fabrication process was usually complicated and the chemicals used for covalent crosslinking were mostly based on polyacrylamide（PAM）and its derivatives such as acrylamide,acrylonitrile,acrylic acid,methylene bis-acrylamide or combination of many other chemicals,which are toxic,possibly explosive,strong corrosive,with pungent smells,and non-biocompatible.This,therefore,limited its application in the biomedicine field.In this work,xylan,cellulose nanofibrils（CNF）and polyvinyl alcohol（PVA）were used as the raw materials to prepare hydrogels with sodium trimetaphosphate（STMP）as the crosslinking agent via two pathways,e.g.reinforcement of CNF to xylan/PVA hydrogels and interpenetrating crosslinking network construction（IPN）.In the hydrogel systems,CNF,xylan and PVA have excellent biocompatibility and degradability.As a crosslinking agent,STMP is a non-toxic chemical derived from dietary sodium dihydrogen phosphate（Na H₂PO₄）,which has been widely used and approved by the Food and Drug Administration.Firstly,CNF with good aspect ratio was prepared from bleached bagasse pulp by microfluidizer after acid hydrolysis with recoverable formic acid.Subsequently,a nanocomposite hydrogel（using macromolecular long-chain Xylan and PVA as the matrix,the hydrogel was formed by crosslinking of PVA with STMP,and CNF was used physically to reinforce the overall mechanical properties of the hydrogel）was prepared.The results showed that the hydrogels had high compressive strength（the maximum compressive stress was 4.36MPa）,good compressive recovery（physical height and compressive stress were restored to99.7%and 88.5%of the initial value,respectively）,and high initial degradation temperature（337-350°C）.However,with the increase of the CNF addition,the compressive stress of the obtained hydrogel gradually decreased,but its compression recovery gradually increased,reflecting the significant influence of CNF on the compressive recovery of the hydrogel.Secondly,in order to further improve the mechanical properties of xylan-based hydrogels,a IPN construction of hydrogel was applied to prepare a Xylan/PVA semi-IPN hydrogel.In the construction of the hydrogel,PVA and STMP were chemically cross-linked to form a network,while linear Xylan macromolecules were interpenetrated in the first cross-linked network and connected with the network via hydrogen bondings.The results showed that the successful construction of the Semi-IPN structure endows Xylan/PVA hydrogel extremely high compressive strength（maximum compressive stress of 28.8 MPa）and good compressive recovery（the highest physical height and compressive stress return to 95.4%and 82.3%of the initial value respectively）,excellent creep recovery ability（physical height returns to 98%of the initial value）as well as higher initial degradation temperature（350-370°C）.In summary,this thesis is mainly based on two hydrogel preparation principles（CNF composite reinforcement and semi-interpenetrating polymer network）,and has developed and designed two methods for preparing xylan-based hydrogels efficiently and environmentally friendly.The obtained products had very high compressive stress,excellent compression recovery and strong heat resistance,which played an important role in promoting the development of mechanical properties of xylan-based and even biomass-based hydrogels.In addition,the raw materials used in the whole system have good biocompatibility,biodegradability and non-toxicity.Therefore,the hydrogel prepared based on the research method in this paper has great application potential in the future in biological tissue engineering and 3D printing materials.