Collagen is a protein that is relatively abundant in mammalian tissues. Studies have found that collagen mainly participates in the composition of the extracellular matrix in mammalian cells. It plays an important role in maintaining the structure and function of mammal tissues and organs. Collagen has become an ideal biomedical and cosmetic skin care material due to its biodegradability, low immunogenicity and ability to promote cell proliferation and differentiation.
The Structure of Collagen
Collagen is a fibrous protein assembled from multiple procollagens. Procollagen is the basic unit of collagen and consists of 3 polypeptide chains. The primary structure of tropocollagen peptide chain has (Gly-X-Y)n repeating units, in which X and Y are often proline and hydroxyproline, and a few are lysine and hydroxylysine. Three procollagen peptide chains can coil around each other through inter-chain hydrogen bonds to form a stable triple helix structure. During the stabilization process of the triple helix conformation, the hydroxyl group of hydroxyproline can form strong hydrogen bonds, which can improve the strength of collagen; proline and hydroxyproline can cause the triple helix structure of collagen to sharply distort, improving the stability of collagen.
Classification, Distribution and Function of Collagen
Currently, collagen genes have been identified from more than 40 species of vertebrates, encoding about 29 types of collagen. Based on the primary structure, the length of the triple helical domain, the molecular weight, the interruption of the triple helical structure, and the size and shape of the terminal domain, collagen can be divided into four major categories: the first type is fibrillar collagen, which includes types I, II, III, V, XI, XXIV and XXVII. Among them, types I, II and III are more distributed in vertebrates, while types V and XI are less distributed, but they play an auxiliary role in the assembly of types I, II and III; types XXIV and XXVII are interrupted when the triple helical domain is shorter. Types III, V and XI collagen retain the C-terminal peptide and part of the N-terminal peptide domain after processing; the second type is triple helical fibrillar collagen (FACIT), which includes types IX, XII, XIV, XVI, XIX and XXII. FACIT collagen does not form collagen, but can interact with fibrillar collagen, regulate the formation and size of collagen fibers, and control collagen synthesis in the extracellular matrix; the third type of reticular collagen, there are types IV, VII and XXVIII. Among them, type IV collagen forms a fibrous reticular structure, while type VII collagen assembles into anchoring fibers that connect the epidermis to the dermis; the fourth type of interrupted helical collagen (MACITs), there are types XIII, XXIII and XXV. Among them, type XIII, XXIII and XXV collagens are type II transmembrane proteins, consisting of a hydrophobic transmembrane domain, a short N-terminal cytoplasmic domain and an extracellular domain composed of three collagens.
Application of Collagen
In recent years, collagen has been widely used in the fields of biological scaffold materials, cosmetics, and medical devices. The development of collagen-based products mainly focuses on bone repair, skin wound repair dressings, tendon repair, drug delivery and beauty.
Figure 1. Medical applications of collagen. (Priyanka Kulkarni, et al.; 2020)
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Wound Healing and Tissue Repair
As one of the important components of the extracellular matrix, collagen can induce epithelial cell proliferation, differentiation and migration. Compared with biosynthetic dressings, collagen composite dressings interact with fibroblasts to form wound contraction force, reducing wound adhesion and contraction strength. It can be seen that collagen is widely involved in the process of tissue repair and wound healing, and collagen-based biomaterials play an important role in tissue repair and wound healing.
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Cosmetics and Medical Beauty
Collagen is the main component of the skin. With the increase of age, collagen is lost, causing the skin to lose elasticity and luster, and even aging phenomena such as spots will appear. Therefore, collagen is widely developed into skin care product formulas. Skin care products containing collagen have strong affinity with the skin and have biological activities such as moisturizing, softening, anti-oxidation and UV protection. The collagen commonly used in cosmetic formulas is hydrolyzed collagen and recombinant collagen. Compared with natural collagen, hydrolyzed collagen and recombinant collagen have a small molecular weight and have excellent solubility, water binding and easy penetration into the dermis under neutral conditions. In medical cosmetology, collagen injections have been widely used to repair skin defects and subcutaneous diseases. Local facial injection of collagen can achieve the effects of facial contour correction, wrinkle and scar repair, etc.
Collagen has become an ideal biomedical material due to its low immunogenicity, biodegradability, repair properties and hemostatic properties. It is widely used in medical dressings, bone repair materials (artificial bones), surgical sutures and drug carriers. Many key factors should be considered when selecting materials for tissue engineering, such as biocompatibility, biodegradability, and mechanical support. The realization of these functions will determine whether the scaffold is suitable for use as a biomaterial that simulates the natural physiological environment of cells. Collagen, as the main structural component of the natural extracellular matrix, has high biocompatibility, biodegradability and ductility; however, the fibrous structure scaffold formed by collagen itself exhibits poor mechanical properties, so the material needs to be modified to achieve the best effect. At present, the modification strategy is mainly to combine collagen with other natural polymers, such as combining collagen with chitosan, which has high biocompatibility, moderate degradation and antibacterial properties, to obtain a sheet structure with stronger mechanical properties and viability. Compared with single collagen materials, blending collagen with natural polymers can develop a new type of material with improved mechanical properties and biocompatibility. In addition, collagen can also be mixed with different polymers for the repair of skin, bones and mucosa. In addition to binary mixtures, mixtures made of collagen and a variety of different polymers are also a new trend. This material is usually based on the integration of cells, bioactive molecules and materials, and can act as an extracellular matrix to provide structural support for tissues, tissue regeneration, allow the diffusion of nutrients and gases, and provide the required microenvironment for cell proliferation. For example, a composite of collagen, hyaluronic acid and chitosan can be used to produce an artificial mixture with unique structural and mechanical properties.
In medical devices, the main uses of collagen include wound healing, skin repair, dermatitis, eczema, hemorrhoids, oral mucositis, oral ulcers, scars and allergic rhinitis.
References
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Priyanka Kulkarni, et al.; Utilization of fish collagen in pharmaceutical and biomedical industries: Waste to wealth creation. Life Science Informatics Publication. 2020, 6(3):1-10.
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Sorushanova A, et al.; The Collagen Suprafamily: From Biosynthesis to Advanced Biomaterial Development. Adv Mater. 2019, 31(1):e1801651.
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León-López A, et al.; Hydrolyzed Collagen-Sources and Applications. Molecules. 2019, 24(22):4031.
