Construction And Properties Of Moldable Marine Polysaccharide/Laponite Nanocomposites

Injectable and 3D-printable materials,as moldable materials,have a wider range of applications in biomedical fields than block or granular materials,which can be filled or customized to irregular shapes.Injectable materials are easy to handle and suitable for small-scale filling.For large scale filling,3D printable materials are more advantageous for customizing the shape and pore structure of scaffolds,but they are more difficult to construct.Marine polysaccharides,such as chitosan and alginate,have good biocompatibility,degradability,viscoelasticity,ease of chemical modification,and structural properties similar to extracellular matrix,which are highly advantageous in constructing biocompatible moldable materials.However,moldable materials prepared from single marine polysaccharide suffer from insufficient injectability/3D printing performance,insufficient mechanical strength,and complicated preparation process.Laponite,has nanosheet structure and charge heterogeneity,is gradually used to modulate the moldable and mechanical properties of marine polysaccharide-based materials.In the existing studies,laponite requires high content to significantly improve the printability and mechanical properties of marine polysaccharides,which is prone to safety risks.In this thesis,the molecular structure of marine polysaccharide and its interaction with laponite were regulated to construct injectable hydrogels,3D printable hydrogels,and mineral-based materials with secondary deformation after 3D printing,respectively,in low laponite content（≤2%）.The main findings and conclusions are as follow:1.Preparation of injectable catechol chitosan/Laponite nanocomposite hydrogelCatechol chitosan（CC）was prepared from chitosan and compounded with Laponite（LAP）at the molecular level,which was further oxidized by sodium periodate to construct injectable,adhesive,antibacterial and antioxidant CC/LAP（OCCL）nanocomposite hydrogels.The antioxidant properties of CC were investigated and were found to be positively correlated with the degree of substitution.By examining the effect of LAP incorporation on the oxidation process and mechanical properties of the nanocomposite hydrogels,it was found that the confinement effect and the alkaline environment provided by LAP resulted in limited oxidation of the catechol groups on CC.When the LAP content was 0.5%,the composite hydrogel exhibited higher adhesive strength（49.8 k Pa）,faster gelling time（449 s）and stronger hydroxyl radical scavenging ability（70.1%）,allowing it to be injected into and adhere to a wide range of materials.The in vitro degradability of the OCCL nanocomposite hydrogel was investigated and proved to be biodegradable（28 d degradation rate of 31.4%）.Further,the antimicrobial properties of the nanocomposite hydrogels were investigated and the kill rates of E.coli and S.aureus were found to be 99.7%and 99.6%.In addition,in vitro cellular experiments initially demonstrated that the multifunctional,injectable nanocomposite hydrogel has good biocompatibility.This study provides a new idea for the development of multifunctional moldable materials.2.Preparation and properties of injectable methylacrylyl hydroxypropyl chitosan/Laponite nanocomposite hydrogelsCompared with the above-mentioned hydrogels in which the gelation time cannot be precisely controlled,photocrosslinked hydrogels are more practical due to their spatio-temporal precision and controllability.To this end,in this section,methylacrylyl hydroxypropyl chitosan（HM）,which can be cross-linked under UV irradiation,was prepared from positively charged hydroxypropyl chitosan and compounded with LAP to construct a photocrosslinkable,injectable,soft and tough HM/LAP（HML）nanocomposite hydrogel.The mechanism of self-assembly of HM and LAP into nanoparticles was investigated by experiments and simulations,and it was hypothesized that electrostatic and hydrogen bonding interactions were the main driving forces of self-assembly.Using cryo-scanning electron microscopy,the HML nanocomposite hydrogels were found to have a collagen fibril-like network formed by the tandem association between nanoparticles.When the content of LAP was 0.5%,the hydrogels had low hardness（G’<2 k Pa）,high compressive strength（709 k Pa）and excellent anti-swelling properties（only 1.07 swelling rate in phosphate buffered solution for 24 h）.In addition,to realize 3D printing of hydrogels,the conditions of processing nanocomposite hydrogels into microgels were explored.The 3D printing performance and rheological properties of microgels,as well as mechanical properties of gel scaffolds after secondary cross-linking were investigated.These results showed that this processing method could obtain 3D printed hydrogels with stable structure（compressive strength>150 k Pa）.Meanwhile,cytotoxicity tests demonstrated that theses hydrogels have good biocompatibility.This study provides a new strategy for the development of anti-swelling,soft and tough injectable materials as well as their 3D printing.3.Preparation and properties of 3D printed methylacrylyl carboxymethyl chitosan/Laponite nanocomposite hydrogelsAlthough the above studies can process hydrogels into microgels that can be 3D printed,the preparation steps are complicated and the printability needs to be further improved.To this end,in this section,methacrylate-carboxymethyl chitosan（CM）,which can be cross-linked under UV irradiation,was prepared from negatively charged carboxymethyl chitosan and compounded with LAP to construct a new hydrogel ink that can be directly 3D printed,and CM/LAP（CML）nanocomposite hydrogels were obtained after photo-cross-linking.The effects of LAP addition on the rheological properties,maximum injection length and 3D printing performance of the hydrogel ink were investigated,and the results showed that the increase of LAP content could improve the shear thinning performance,elasticity and shape fidelity of CML ink.It was further demonstrated through experiments and simulations that this 3D printing ink with excellent printing performance is formed by the physical interaction of amino and carboxyl groups on CM and methacryloyl groups with LAP through electrostatic and hydrogen bonding.The best printing performance of CML ink is achieved when the LAP content is 1.5%,with high elasticity（Tanδ<0.1）and high shape fidelity（～1.0）,allowing direct printing of microtubules with small diameters（inner diameter<500μm）as well as fine and complex structures with large dimensions（height>2 cm）without other auxiliary means.Further,the mechanical properties,swelling resistance,in vitro degradability,and biocompatibility of the photo-crosslinked CML nanocomposite hydrogels were investigated,and the results showed that the nanocomposite hydrogels have good compressive strength（186.0 k Pa）,swelling resistance（<1.1 in PBS）,degradability（24.5%at 28 d）,and cytocompatibility.This study provides a new approach for the development of 3D printing hydrogel inks with easy preparation,excellent printing performance and wide printing range.4.Preparation and properties of 3D printed alginate/Laponite/calcium phosphate bone cement composite scaffoldsCompared with hydrogels,mineral-based scaffolds have more advantages in terms of mechanical properties and are more suitable for use in hard tissues.Herein,this chapter developed a bone cement ink that can be secondarily deformed after 3D printing,using alginate/LAP composite as the liquid phase andα-tricalcium phosphate（α-TCP）as the solid phase of bone cement,and taking advantage of the property that alginate can cross-link with Ca²⁺.The effects of LAP addition on the rheological properties and printing performance of the bone cement ink were investigated.The experimental results showed that the bone cement ink had the best 3D printing performance,high shape fidelity（～1.0）and the potential to print large-sized complex structures（height>8 mm）when the LAP content were 2%.Meanwhile,solidification time,XRD,and SEM were used to test the effects of LAP and Ca²⁺treatment on the hydration ofα-TCP,and the results showed that this preparation method delayed the early hydration process ofα-TCP for 3D printing,but did not affect the final hydration products.In addition,the Ca²⁺-treated flexible scaffolds were investigated for their flexibility in secondary processing（bending,folding,twisting,hole-piercing,and mold setting）.In addition,the mechanical properties of the composite scaffold were examined and it was found that the compressive strength of the bone cement increased from 11.4 MPa to 17.9 MPa with the addition of LAP.These results indicated that this study achieves 3D printing and post-printing secondary deformation of bone cement,which provides a new direction for processing and manufacturing of bone cement scaffolds.5.Preparation and properties of bioactive ceramic 3D printed scaffolds with complex structures assisted by alginate/Laponite compositesIn order to investigate the applicability of this processing strategy in bioactive ceramics,which developed a processing strategy using Ca²⁺curing to obtain flexibility for post-printing secondary deformation,this study combined alginate/LAP composites with akermanite（AKT）to prepare stronger bioactive ceramic composite scaffolds.By examining the rheological properties of the ink,its 3D printing performance,and its flexibility analysis after Ca²⁺treatment,it was found that the alginate/LAP composite allows AKT to achieve high fidelity3D printing while gaining the ability to be secondarily deformed for the construction of complex structures.After sintering,it was found that the temperature,the addition of LAP,and Ca²⁺treatment all affected the mechanical properties and microstructure of the bioactive ceramic composite scaffolds.Ca²⁺inhibited grain growth and limited the densification of the ceramic,while LAP limited the internal penetration of Ca²⁺to a certain extent,obtaining a porous surface and dense internal microstructure and restoring mechanical properties.In addition,scaffolds containing 3D interconnected channels were successfully constructed using its flexibility,and the scaffolds sintered at 1250°C exhibited good mechanical properties（5.37 MPa）,abundant microporous structures（66.5%）,degradable properties（34.88%degradation rate at 40 d）,and excellent in vitro mineralization ability.The results indicate that mineral-based materials such as bone cements and bioactive ceramics with complex shapes and structures can be easily constructed using alginate/LAP composites,providing a new preparation strategy for personalized and customized scaffold materials.