3D Printed Double-network Hydrogels With High Toughness,High Elasticity And Osteogenic Properties

Hydrogel with numerous advantages,e.g.,flexibility,high-water content,biocompatibility and smart response,is widely used in biomedical engineering,such as flexible devices,drug delivery,cartilage regeneration,etc.However,due to the weak mechanical properties and poor processability,its further applications were limited,especially in bone tissue regeneration.The modulus of bone is normally in the megapascal level,while the modulus of traditional hydrogel is only in the 1-100 k Pa level.Therefore,hydrogels with high toughness and intelligent processability are meaningful in biomedical engineering,Currently,3D printing is an emerging technology in processability.Contrary to the traditional methods,it shows high efficiency and high accuracy.Although many natural hydrogels（e.g.,gelatin,hyaluronic acid,cellulose）can be 3D printed smoothly,they remain the drawbacks of soft and weak properties.Chitosan is a natural alkaline polysaccharide with good biocompatibility and degradability,which is extensively applied in drug delivery,wound dressing,tissue engineering,etc.Its’3D printability could be improved by increasing ink concentration or adding chemical cross-linking points.But such adjustments might cause a decline in biocompatibility.Therefore,it’s a challenge for chitosan to be 3D printed.Herein,this study aims to create a 3D printed hydrogel from chitosan and poly（ethylene glycol）diacrylate（PEGDA）based on the strengthening double-network mechanism,which would not only show high toughness but also has osteogenic properties.The main researches are as follows:1.To achieve the double-network strengthening mechanism,PEGDA was added to this system with chitosan.In the ionic network,the coordination bonds formed by the N-glucosamine unit on chitosan and Cit^3-act as"sacrificial bonds"to dissipate energy;in the covalent network,the covalent bonds at both ends of PEGDA form covalent cross-linking points by UV triggering.Through a series of mechanical tests,such as tensile tests,compressive tests,and loading-unloading tests,it was shown that this DN hydrogel from chitosan and PEGDA exhibited excellent mechanical properties.The optimal group had the elastic modulus（3.84±0.4 MPa）,the tensile strength（7.23±0.7MPa）,and the fracture energy(1744±200 J m^-2).2.In the CS/PEGDA pre-gel system,we added the citrate solution to build the internal ionic cross-linking network for improving printability.Through rheological tests,it was confirmed that the citrate solution not only promoted the sol-gel transition of the system but also reduced the viscosity of the system,which enabled the ink to be printed at room temperature and low pressure.Also,3D complex models could be obtained,such as bionic graphene structures with hexagonal holes,human noses,human ears,and mesh.Meanwhile,cell culture tests showed that the DN CS/PEGDA hydrogel had good biocompatibility and ionic conductivity.3.Based on the CS/PEGDA double network hydrogel,the calcium phosphate（Ca P）composite layer can be synthesized in situ on the surface of the DN hydrogel by the"cyclic soaking method".Through scanning electron microscopy and X-Ray diffraction characterization methods,it was proved that calcium phosphate mineralized layer was formed on the surface of DN hydrogel.Besides,the soaking times could affect the morphology of the mineralized layer.In a series of cell cultures,this DN-Ca P hydrogel exhibited good cell adhesion,cell spreading,and cell proliferation.Moreover,in alkaline phosphatase（ALP）tests,DN-Ca P exhibited osteogenic properties.