Collagen is a natural, polymeric protein and the most important structural
material in vertebrates (Martin 1996b). Collagen constitutes 36% of bone by weight, the second largest component after hydroxyapatite. The form of collagen found in bone is termed Type I and is the dominant form throughout the body ,being found in tendons, ligaments, and skin.
to Collagen contains three tropocollagen molecules with left-handed spiral structures in turn wound into a right-handed super helix. The more than a dozen types of collagen are distinguished by variations in the patterns of amino acids in their polymeric chains. Molecular cross-linking is common and can be intermolecular between two of the three polypeptides in a single molecule or intermolecular. Precise fiber
alignment is necessary to facilitate intermolecular cross-linking, leading to a characteristic banding structure with a 6.4 nm repeat dimension. An abundance of cross-linking in tendons and ligaments leads to relatively rigid mechanical behavior. Skin, on the other hand, is highly visco-elastic with rubber-like mechanical behavior. shows the contrasting stress-strain behavior for these various systems. Obviously, the maximum elastic modulus of collagen (as indicated by tendons) is about 1,000 MPa and the maximum strain is 10-20%. It is interesting to note that the aging of skin is the result of continuing cross linking leading to reduced deformability or “stretch.”
4.3
DENTIN
The inner portion of a tooth is composed of dentin (Martin 1996b). It is more highly mineralized than bone, with approximately 80% hydroxyapatite in an organic matrix which accounts for the remaining 20%. Approximately 90% of the matrix is collagen. Because of the high concentration of hydroxyapatite, dentin is a relatively brittle material.
4.4
CHITIN
After collagen, chitin is the second most common component of inactive tissues in animals (Martin 1996b). Chitin is chemically similar to cellulose and has a lamellar structure similar to wood or bone. Its elastic modulus s close to that of hardwoods and bone, and it tends to be mixed with protein for the purpose of adjusting its mechanical properties to the specific structural need. Its mineral content is
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|