Cross-Linking Methods of the Silk Protein Hydrogel in Oral and Craniomaxillofacial Tissue Regeneration

Abstract

Background: Craniomaxillofacial tissue defects are clinical defects involving craniomaxillofacial and oral soft and hard tissues. They are characterized by defect-shaped irregularities, bacterial and inflammatory environments, and the need for functional recovery. Conventional clinical treatments are currently unable to achieve regeneration of high-quality oral craniomaxillofacial tissue. As a natural biomaterial, silk fibroin (SF) has been widely studied in biomedicine and has broad prospects for use in tissue regeneration. Hydrogels made of SF showed excellent water retention, biocompatibility, safety and the ability to combine with other materials.

Methods: To gain an in-depth understanding of the current development of SF, this article reviews the structure, preparation and application prospects in oral and craniomaxillofacial tissue regenerative medicine. It first briefly introduces the structure of SF and then summarizes the principles, advantages and disadvantages of the different cross-linking methods (physical cross-linking, chemical cross-linking and double network structure) of SF. Finally, the existing research on the use of SF in tissue engineering and the prospects of using SF with different cross-linking methods in oral and craniomaxillofacial tissue regeneration are also discussed.

Conclusions: This review is intended to show the advantages of SF hydrogels in tissue engineering and provides theoretical support for establishing novel and viable silk protein hydrogels for regeneration.

Keywords: Crosslinking reagents; Guided tissue regeneration; Pulp tissue; Silk fibroin.

Fig. 1

Summary of the applications associated with SF hydrogels in oral and craniomaxillofacial tissue regeneration

Fig. 2

SF hydrogel crosslinking methods include physical and chemical crosslinking. Methods used for physical crosslinking include temperature and pH-mediated self-assembly, ultrasonication, electric fields, etc. Methods used for chemical crosslinking include chemical cross-linking agents, photopolymerization, γ-ray irradiation, enzyme cross-linking, etc. “Application of Silk-Fibroin-Based Hydrogels in Tissue Engineering” by Yihan Lyu et al. is licensed and used under CC BY. Excerpted from the original

Fig. 3

Schematic representation of nanofiber network formation at different gelation stages during four-stage physical gelation of silk fibroin. SF molecules first gather to form nuclei; the crystal nuclei grow into β-crystallites and cause the β-crystallite networks (nanofibrils) to form a primary network, and these nanofibrils further grow and branch into single-domain fibril networks. Silk fibrils interpenetrate to form a multidomain fibril network and finally the final network. “Crystal Networks in Silk Fibrous Materials: From Hierarchical Structure to Ultra Performance” by Anh Tuan Nguyen et al. is licensed and used under CC BY. Excerpted from the original

Fig. 4

Schematic showing enzymatic (HRP/H₂O₂) crosslinking and visible light photocrosslinking (Ru/SPS) mechanisms to fabricate silk fibroin hydrogels through di-tyrosine bond formation. “Rapid Photocrosslinking of SF hydrogel with High Cell Density and Enhanced Shape Fidelity” by Xiaolin Cui et al. is licensed and used under CC BY. Excerpted from the original

Fig. 5

Modified silk fibroin and hyaluronic acid are linked with highly reactive vinyl groups. Covalent cross-linking of silk fibroin and hyaluronic acid to form hydrogels under the action of I2959 and ultraviolet light. “Photocross-linked silk fibroin/hyaluronic acid hydrogel loaded with hDPSC for pulp regeneration” by Lu Wang et al. is licensed and used under CC BY. Excerpted from the original

Fig. 6

Schematic illustration showing the preparation of the SF/GA/Zn hybrid hydrogel. “Immunoregulation in Diabetic Wound Repair with a Photoenhanced Glycyrrhizic Acid Hydrogel Scaffold” by Yuna Qian et al. is licensed and used under CC BY. Excerpted from the original

Fig. 7

Methacrylic anhydride (red)-modified hyaluronic acid (green)-induced weak self-assembly of fibroin (blue) after homogeneous mixing (1st cross-link). Methacrylated HA was photocrosslinked, forming a rigid but brittle hydrogel network (2nd cross-link). Ethanol treatment led to strong self-assembly of the fibroin (3rd cross-link). ‘‘Stepwise Cross-Linking of Fibroin and Hyaluronic for 3D Printing Flexible Scaffolds with Tunable Mechanical Properties’’ by Muyang Sun et al. is licensed and used under CC BY. Excerpted from the original

Fig. 8

Schematic diagram showing enzymatic preparation of a SF hydrogel and its responses to acids, alkalis, and alcohols. A SF solution preparation; B HA modification using TA and EDC/NHS; C enzymatic coupling of mHA onto SF; and D responses to soaking conditions. “Efficient Regulation of the Behaviors of Silk Fibroin Hydrogel via Enzyme-Catalyzed Coupling of Hyaluronic Acid” by Lin Wang et al. is licensed and used under CC BY. Excerpted from the original

Fig. 9

Fabrication of a physically and chemically double-crosslinked SF/HPMC double-network hydrogel. A Bioprinting the SF/HPMC scaffold, B the actual SF/HPMC hydrogel, and C the double network mechanism for the SF/HPMC hydrogel. ‘‘3D Bioprinting of Bone Marrow Mesenchymal Stem Cell-Laden Silk Fibroin Double Network Scaffolds for Cartilage Tissue Repair’’ by Tianyu Ni et al. is licensed and used under CC BY. Excerpted from the original

Fig. 10

Schematic showing the criteria for the ideal hydrogel scaffold. ‘‘Hydrogels and Dentin–Pulp Complex Regeneration: from the Benchtop to Clinical Translation’’ by Marwa M. S. Abbass et al. is licensed and used under CC BY. Excerpted from the original