A collagen is a fibrous protein found in all multicellular organisms. The collagen is a main component of skins or bones, and occupies 25% of total proteins in mammals. A typical collagen molecular has a rope-like superhelical structure, which is referred to as a triple helical structure, comprising three collagen polypeptide chains. The collagen is particularly rich in proline (Pro) and glycine (Gly). These two amino acid residues are important to form a stable triple helical structure of the collagen.
As methods for using a collagen as a biomaterial, there may be mentioned, for example, a method of grafting or transplanting an intact or lyophilized skin tissue derived from a pig on a skin area damaged by a burn or scald, a method of removing cellular components from a tissue with enzyme treatment, and a method of using a collagen which is solubilized by a treatment with an acidic solution or an enzyme to reconstitute a desirable form. A common preparation method and a common qualitative method are described in Methods Enzymol., Vol. 82, pp. 33 to 64, 1982.
There are various suggestions to utilize a collagen. For example, Japanese Patent Application Laid-Open No. 08-027192 (JP-08-027192A) discloses a production process of a collagen derivative for imparting moisture and smoothness to skin, which comprises esterifying and modifying an animal tissue containing a collagen with alcohol, and extracting the modified collagen, as well as a cosmetic base material using the collagen derivative.
Japanese Patent Application Laid-Open No. 07-097454 (JP-07-097454A) discloses a production process of a water-soluble crosslinked collagen which shows a high regeneration rate of a triple helical structure after thermal denaturation, and the process comprises subjecting a water-soluble collagen to a crosslinking treatment with a bifunctional alkylene diimidate cross-linker having imide ester groups at both ends of the methylene chain.
Japanese Patent Application Laid-Open No. 08-053548 (JP-08-053548A) discloses a matrix of a collagen and a synthetic polymer (a collagen-synthetic polymer matrix) which has a low immunogenicity and is useful for preparation of biocompatible implants utilized for various medical applications, and a production process of the matrix comprises reacting a collagen with a first synthetic hydrophilic polymer to form a collagen-synthetic polymer matrix, and further reacting the collagen-synthetic polymer matrix with a reactant such as a second synthetic hydrophilic polymer, a biologically active substance, a glycosaminoglycan and a derivative thereof, a chemical crosslinking agent, an esterifying agent, an amidating agent, an acylating agent, an amino acid, a polypeptide, or others.
Japanese Patent Application Laid-Open No. 07-278312 (JP-07-278312A) discloses a unit material containing a hydrophilic synthetic polymer covalently bonded to a chemically modified collagen which is substantially a nonfiberous form at pH 7. The literature discloses that the unit material is particularly useful for ophthalmological devices and optically transparent, and that the unit material has a biocompatibility.
Japanese Patent Application Laid-Open No. 05-000158 (JP-05-000158A) discloses a production process of a collagenic membrane-like substance, which comprises crushing a collagen matrix, centrifuging the crushed matrix under a high centrifugal field, homogenizing the resultant precipitate to obtain a paste, casting the paste, and drying the cast paste at a temperature of not higher than 37° C. The literature also discloses that the collagen membrane-like substance has a biocompatibility and a non-inflammatory property, and is useful for repairing a tissue as an artificial implantation matter.
Japanese Patent Application Laid-Open No. 05-125100 (JP-05-125100A) discloses a soluble fish scale collagen having high-purity and a production process thereof, and the process comprises pepsinating an intact or deashed fish scale to a pepsin treatment.
Japanese Patent Application Laid-Open No. 06-228506 (JP-06-228506A) discloses a production process of a dry particulate or powdery soluble collagen, which comprises injecting a collagen solution through a nozzle into 70 to 90% ethanol medium to form a strand-like or membranous product, drying the product, and chopping or grinding the dried product.
Japanese Patent Application Laid-Open No. 08-276003 (JP-08-276003A) discloses use of an unbaked single-crystal hydroxyapatite as a material for repairing a biological hard tissue (such as a bone), through attaching the single crystal to at least part of a low antigenic collagen fiber.
Japanese Patent Application Laid-Open No. 08-041425 (JP-08-041425A) discloses a method which comprises removing fragments of cells or tissues in a collagen solution and subjecting the residue to an alkali treatment, for removing prion in a collagen derived from an animal or human being, and discloses a collagen obtained by this method.
Moreover, regarding methods for chemical synthesis of collagen analogues, it has been reported that a soluble polyamide having a molecular weight of 16,000 to 21,000 is obtained by dissolving p-nitrophenyl ester of Pro-Ser-Gly or p-nitrophenyl ester of Pro-Ala-Gly in dimethylformamide, and adding triethylamine thereto, and allowing to stand the mixture for 24 hours (J. Mol. Biol., Vol. 63, pp. 85 to 99, 1972). In this literature, the soluble polyamide is estimated to form a triple helical structure based on the circular dichroism spectra. However there are not referred to properties of the obtained polymer.
It also has been reported that a method for obtaining a polyamide, which comprises dissolving a 50-mer peptide containing the sequence Val-Pro-Gly-Val-Gly derived from elastin in dimethylsulfoxide, adding 2 equivalents of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide, 1 equivalent of 1-hydroxybenzotriazole and 1.6 equivalents of N-methylmorpholine thereto, allowing to stand the mixture for 14 days, and dialyzing the resultant mixture with a dialysis membrane (molecular weight cut-off: 50,000) (Int. J. Peptide Protein Res., Vol. 46, pp. 453 to 463, 1995).
Meanwhile, as described in the above-mentioned Japanese Patent Application Laid-Open No. 08-041425 (JP-08-041425A), a causative substance of sheep tremor or bovine spongiform encephalopathy is an infectious protein called as prion, and the infectious protein is considered as one of causes of human Creutzfeldt-Jakob disease infection. Prion is a protein, and it is indicated that prion is hard to deactivate with a conventional pasteurization or sterilization method, further that prion is infectious over species (Nature Review, Vol. 2, pp. 118 to 126, 2001).
In general, a collagen derived from bovine or pig is frequently used as a raw material for medical kits (devices) or pharmaceutical preparations, and cosmetic preparations. Accordingly, there have been always existed the risk of an infection (or a transmission) to pathogenic organisms or a causative factor such as prion which cannot be removed by conventional pasteurization or sterilization.
Moreover, since various cell adhesion sites are found in a naturally occurring collagen, the naturally occurring collagen cannot exert cell selectivity for any applications. For example, in the case using a collagen as a material for inducing a nerval axon, migration or growth rate of surrounding fibroblast is more than elongation rate of the axon resulting in forming scarring tissue, and the axon cannot be elongated. It is therefore necessary to take a step to cover around the collagen with a material for protecting migration of fibroblast, or others.