Abstract:

Significance The application of bone tissue engineering (BTE) techniques to repair bone injuries and defects arising due to trauma, infection, tumors, or pathological fractures remains a major challenge. Silk fibroin (SF) is a natural biomaterial with excellent biocompatibility and controllable biodegradability, and can be a mineralization template to induce the growth of hydroxyapatite (HAp). Recently, SF has received more attention for the application in bone regeneration. Although the development of SF-based bone repair materials has achieved surprising results, many SF-based bone scaffolds with excellent functionality are still in the laboratory stage. Therefore, in the present article, the latest research progress of SF-based materials in bone repair was reviewed, in particular, the strategies and methods to improve the mechanical properties and osteogenic performance were highlighted, in order to promote the innovative development of SF-based bone regeneration scaffolds.

Progress SF is usually prepared into films and hydrogels due to the good film forming and sol-gel transformation properties, and it can also be constructed into porous scaffolds by biomimetic design. Generally, SF films are prepared by casting or electrospinning, then the post-treatments make films insoluble. The softness and flexibility of SF films induced by physical treatment using alcohol reagents are poor, but can be significantly improved by chemical crosslinking or increasing the molecular weight of SF. In addition, combination with sericin can not only improve the softness of the films, but also promote the deposition of HAp. SF hydrogels are typically formed by self-assembly using physical methods like concentration, shearing, ultrasound and electric fields, and also can be prepared by chemical crosslinking. The SF hydrogel yielded by the horseradish peroxidase (HRP)/H₂O₂ reaction system shows impressive viscoelasticity and biocompatibility. Nevertheless, the stability of SF hydrogel is poor in vivo, its mechanical properties and stability can be further improved through double-crosslinking. Freeze-drying, salting-out and 3D printing are commonly used to prepare SF porous scaffolds. Regarding the freeze-drying method, the pore characteristics and mechanical properties of the SF scaffolds can be adjusted through freezing temperature and solution concentration. It is also possible to design a temperature gradient to induce the directivity of the pores. However, it cannot accurately pre-design the internal structure. Salting-outing method can stably control the pore structure, but it is easy to produce more salt residues if the pores connect incompletely. 3D printing allows for pre-designing the internal structure of the scaffolds, but the technology still faces great challenges due to the fluidity of the SF ink. Throughout decades of researches, the mechanical properties of SF materials have been far from satisfactory for the application in BTE. Inspired by the composition and structure of natural bone, inorganic materials such as HAp have usually been considered to modify SF to improve the mechanical properties of the SF scaffolds. Moreover, growth factors and cells are usually incorporated into the SF/HAp composite scaffolds to further enhance osteogenic capacity.

Conclusion and Prospect SF is a natural biomaterial with remarkable biomedical properties. No matter which form of SF material is used for bone defect repair, it shows promising application prospects. However, current researches indicate that the mechanical properties of pure SF-based materials are insufficient, and still have a significant gap compared to natural bone. These deficiencies and osteogenic potential of SF scaffolds can be significantly improved by changing preparation strategies or incorporating inorganic reinforcement materials such as HAp. Furthermore, adding growth factors or cells to SF/HAp composite materials can achieve the repair of critical-size bone defects. In the future research of SF-based bone repair scaffolds, studies focusing on mechanical conduction, neovascularization and matched material degradation need to be considered. Maintaining the activity of growth factors or cells during bone repair is also an urgent problem. These are pivotal for the precise design of SF scaffolds or composite scaffolds that align with guiding bone regeneration and functional recovery. With the development of regenerative medicine and tissue engineering, SF materials are promising candidates to create osteogenic niches with multiple cues and develop different medical devices used in clinical bone regeneration.

Key words: silk fibroin, scaffolds, mechanical property, osteogenic capacity, bone repair, bone regeneration material