Introduction

Silk fibroin (SF), a protein derived from the silk of various insects, has garnered significant attention for its potential applications in the field of tissue engineering. Leveraging its unique mechanical properties, biocompatibility, and biodegradability, SF offers a promising platform for developing various biomaterials.

Sources of Silk Fibroin

Silk proteins are synthesized by a range of insects such as silkworms, spiders, lacewings, and mites. Among these, silks from silkworms, particularly Bombyx mori, and spiders are most commonly employed in biological applications. Bombyx mori, a mulberry-feeding silkworm, produces high-quality fibers superior to those from Antheraea species, which feed on non-mulberry plants.

Interestingly, the structural and biochemical properties of SF vary depending on the source. For instance, non-mulberry silks like Antheraea mylitta and Antheraea assama contain an abundant Arg-Gly-Asp (RGD) motif, enhancing cell adhesion and proliferation compared to the mulberry variant. Additionally, the poly(-Ala-) β-sheets in non-mulberry silk confer greater hydrophobicity and mechanical stability.

Properties of Silk Fibroin

Silk fibroin is a natural protein-based polymer composed primarily of two chains: a heavy chain (~390 kDa) and a light chain (~26 kDa). The heavy chain consists of repetitive sequences of glycine, alanine, and serine, leading to the formation of stable β-sheet structures, while the light chain contributes elasticity due to its non-repetitive sequences. These chains are linked by disulfide bonds, giving silk fibroin exceptional mechanical properties, including high tensile strength (300-700 MPa), extensibility (4-26%), and toughness (70-78 MJ/m³).

Silk fibroin's structural and chemical composition imparts excellent biocompatibility and biodegradability. Its degradation products-primarily amino acids and peptides-are non-toxic and can even support tissue regeneration. Silk fibroin has been widely used in medical fields for surgical sutures and wound dressings.

Types of SF-Based Biomaterials

SF's diverse fabrication methods enable the creation of various material types, each with unique properties and applications:

Fig. 1 Types of scaffolds based on silk fibroin nanocomposites used for tissue engineering (Zuluaga-Velez A.; et al. 2021).

Hydrogels

SF hydrogels are hydrophilic, 3D network polymers ideal for mimicking the natural extracellular matrix (ECM). Hydrogels offer high water content, biocompatibility, and the ability to encapsulate cells and bioactive molecules. These properties make them suitable for cartilage and osteochondral repair. Injectable hydrogels allow for in situ curing and better integration with native tissues.

Scaffolds

3D porous scaffolds are designed to provide structural support and a conducive microenvironment for cell growth and differentiation. They are particularly valuable in cartilage and bone tissue engineering. Gao et al. developed DCM/SF scaffolds that effectively supported cartilage regeneration and BMSC differentiation, while Li et al. created SF/graphene oxide meniscus scaffolds with functional coatings that enhanced inflammatory response regulation and osteochondral repair.

Films

SF films are two-dimensional structures with applications ranging from drug delivery to implantable devices. Techniques such as spin coating and vertical deposition are used to fabricate SF films, which can then be modified to enhance specific properties like roughness, hydrophilicity, and rigidity.

Mats and Textiles

Electrospinning is often used to create fibrous mats and textiles, which mimic the natural fibers of the ECM and support cell adhesion and growth. These materials can be combined with bioactive ceramics, like hydroxyapatite, to enhance osteoconductivity and overall cell differentiation.

Mixed Conformations

These scaffolds combine multiple structures, such as hydrogels embedded within sponge-like structures or covered with electrospun mats. Mixed conformations are particularly useful for emulating the multilayered nature of native tissues and facilitating the slow release of growth factors for enhanced tissue regeneration. Felfel et al. developed a hybrid scaffold combining SELR hydrogel with polylactide structures, showing enhanced biological and physicochemical properties.

Preparation Methods of SF-Based Biomaterials

3D Bioprinting

3D bioprinting is a rapid prototyping technology that allows for the precise layer-by-layer construction of biomaterials. This technique is used to fabricate SF-based materials with complex structures and personalized sizes, making them suitable for cartilage and osteochondral repair. The key components of 3D bioprinting are the biological printer and ink. Extrusion printing, in particular, is favored for its ability to process higher-resolution patterns cost-effectively.

Electrospinning

Electrospinning is a highly controllable technique that produces long fibers with small diameters and high surface area-to-volume ratios. SF scaffolds prepared by electrospinning have interconnected pore structures similar to the extracellular matrix (ECM), promoting cell migration and proliferation. Electrospinning can be combined with other preparation methods to enhance the properties of SF-based biomaterials for cartilage and osteochondral repair.

Freeze-Drying

Freeze-drying is a flexible and controllable technique that adjusts the pore size of biomaterials by controlling the growth of ice nuclei. This method is environmentally friendly and suitable for preparing SF-based biomaterials with specific pore structures. Freeze-dried SF scaffolds can carry drugs and growth factors, facilitating targeted tissue regeneration.

Salt Leaching

Salt leaching is a cost-effective method for preparing porous SF biomaterials. This technique involves adding salt particles to the SF solution, followed by the removal of undissolved salt particles to create pores. Salt-leaching is often combined with other methods, such as freeze-drying, to produce SF-based materials with optimal porosity and mechanical properties.

Cross-Linking

SF can be cross-linked using enzymatic methods, crosslinkers, photo cross-linking, or ultrasonication. Enzymatic cross-linking maintains the mechanical properties and biocompatibility of SF, while photo cross-linking offers rapid cross-linking rates. Ultrasonication accelerates cross-linking by increasing local temperature and shear force, avoiding cytotoxicity issues associated with other methods.

Silk Fibroin in Dental Applications

Silk fibroin is an emerging biomaterial in dental applications, showing potential for periodontal and maxillofacial therapies. Its non-toxic nature and ability to promote cell proliferation make it ideal for regenerative dentistry. Studies have demonstrated that silk fibroin scaffolds support the formation of mineralized dental tissues like osteodentin and can effectively cultivate human dental stem cells, maintaining important mesenchymal markers. Combining fibroin with graphene oxide enhances its therapeutic potential. Additionally, a novel silk fibroin structure has been developed for ligament regeneration, improving the secretion of key proteins by periodontal ligament fibroblasts. Silk fibroin is also beneficial in implant therapy, improving cell adhesion and bone formation, particularly when used with platelet-rich fibrin or 4-hexylresorcinol. It aids in wound repair and tissue regeneration, showcasing its broad potential in dental medicine.

Conclusion

Silk fibroin is a versatile biomaterial with excellent mechanical properties, biocompatibility, and biodegradability. Its ability to be processed into various morphologies and functionalized with bioactive components makes it a promising candidate for tissue engineering and regenerative medicine. Future research should focus on optimizing the processing techniques and exploring new applications for SF-based biomaterials to realize their full potential in clinical settings.