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**Tissue engineering** is a rapidly developing branch of medicine, consisting in the treatment of various pathologies by the introduction into the site of the lesion of live auto- or allogeneic cells on the surface or inside any carrier, which most often are extracellular matrix proteins and, among them, primarily collagen. **Collagen** is a fibrillar protein, which is the basis of connective tissue and accounts for 30% of all body proteins. Collagen has a unique amino acid composition-33% glycine and about 20% amino acid proline and hydroxyproline; its molecule consists of three polypeptide chains folded into a triple, so-called collagen, helix, which in the solution has a rigid, almost rod-shaped structure. Now we know 27 genetically different types of Collagen. In mammals, 95% is for Type I collagen, which is mainly used in tissue engineering. In the future, collagen will be understood as type I collagen. In vivo or in vitro under physiological conditions (ionic strength, pH and temperature) and at a sufficient concentration, the rodlike molecules aggregate with lateral surfaces with a certain shift, forming fibrils with the correct specific structure, the same for the fibrils formed in vivo and in vitro. At the same time, gel precipitates are formed from the fibrils, into which cells can be incorporated, and almost all cells of the body feel great in these gels, reconstruct them and multiply into them, which is the basis of so-called tissue equivalents and to a large extent all tissue engineering. At the edges of the triple helix of collagen are small non-collagen sequences, the so-called telopeptides containing lysine and arginine, involved in the formation of intermolecular cross-links that provide the strength of fibrils and collagen gels.
       The wide medical and biotechnological use of collagen is promoted by its weak (thanks to a unique and almost identical for all organisms amino acid composition) immunogenicity; when administered subcutaneously, collagen practically does not elicit an immune response.
The immunogenicity of collagen preparations is often explained by the impurity of non-collagen proteins, and so a very thorough cleansing of collagen for tissue engineering is necessary.
Collagen for technological, medical and other purposes is obtained by extracting it from rich tissues, usually skin or tendons. *Three methods of collagen extraction are most common: extraction with acidic solvents after preliminary removal of impurities (non-collagen proteins and acid proteoglycans), enzymatic and alkaline-salt extraction.*