Research progress in regulating the mechanical properties of silk fibroin films
Silk fibroin shows biocompatibility,biodegradability,and a lack of immunogenicity and allergenicity,possessing great potential in biomedical applications.In various forms of silk fibroin biomaterials,silk fibroin films are attracting increasing attention due to their easy processing.Among the physic-chemical properties of silk fibroin films,mechanical properties have great impact on their subsequent applications in vitro and in vivo.First,this paper briefly discussed the relationship between the hierarchical structures and mechanical properties of silk fibroin.Then,it summarized the influence of preparation parameters on mechanical properties,and the influence of different treatments on micro structure of silk films and the resulting changes in mechanical properties.Moreover,it discussed the relationship between structural changes and mechanical properties of silk fibroin films.Finally,it looked to the future to provide reference for the development of silk fibroin films to meet a variety of needs.The mechanical properties of silk fibroin materials are structure dependent.Silk fibroin materials have five levels of hierarchical structures,in which the content of β-sheet in the secondary structure,the size and content of β-crystallites in the tertiary structure,and the density and orientation of β-crystallites in the quaternary structure are all critical parameters to the mechanical properties.The multi-level structure of silk fibroin can progressively influence the macroscopic mechanical properties step by step.In the aggregated state,it is generally accepted that the strength and stiffness of the silk fibroin material are determined by Silk Ⅱ while ductility and flexibility are controlled by Silk Ⅰ.The rational design and reconstruction of silk fibroin structure can be implemented by controlling the preparation process.Pure silk fibroin films can be prepared in three steps:solution fabrication,film formation and post-treatment.Solution fabrication consists of degumming and silk fibroin dissolution,which will reduce the molecular weight of silk fibroin and therefore weaken the mechanical properties.As for film formation,solution casting or spin coating is often used to produce dense,porous,and lamellar films.The mechanical property of silk fibroin films fabricated via solution casting is controlled by controlling the drying rate,while films fabricated via spin-on method are limited by thickness.The pore size,pore density and pore distribution are important to mechanical properties for porous films,while the mechanical properties of lamellar films are related to the interlayer bond compactness.Post-treatments,such as organic solution immersion,water vapor annealing,external forces,and relative humidity in storage,can improve mechanical properties by changing crystallinity and orientation.Additive modification is an effective way to improve the mechanical properties of silk fibroin films.Blending,plasticizing,and crosslinking are common additive modification methods.Mechanical properties are changed by forming specific intermolecular interactions between silk fibroin and additives.The interactions between blends and silk fibroin can be roughly divided into phase separation,strong interaction,and network formation.Among them,network formation needs an appropriate mixing ratio to form an excellent balance interaction.Crosslinkers form covalent links between molecules and increase the molecular weight.In this way,physical crosslinking and chemical crosslinking need to be balanced.Plasticizing can improve the flexibility and ductility of silk fibroin films.The commonly used plasticizers are glycerin,water,and Ca2+,and the combination of other additives gives silk fibroin films excellent comprehensive mechanical properties.All in all,the hierarchical structures and the mechanic-structure dependence of silk fibroin materials make silk fibroin films adaptive to meet the applications requiring different mechanical properties.In the future,it is necessary to continue to expand the preparation methods of silk fibroin,and have an in-depth knowledge of the relationship between the structure and properties of silk fibroin,so as to accurately regulate the mechanical properties of silk fibroin films and further study the relationship between mechanical properties and other relevant properties.Mechanical property modification,functionalization and biocompatibility need to be considered together in the application studies of silk fibroin materials,so as to further expand the application range of silk fibroin materials.
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