Mechanical Properties of Biological Polymers

Soft connective tissues are designed to serve unique functions in mammals. Each such tissue possesses a complex architecture made from a few components organized into a macrolllOlecular, multicomposite struc­ ture. In general, these complex morphologies consist of collagen fibrils embedded in a gel-like matrix. In this review, the biological role of soft connective tissue is illustrated by five selected examples. The first system we examine, tendon, functions mainly in uniaxial tension and serves as the primary linkage between muscle and bone. Intestine and aorta, our next examples, must work in more complex deformation modes, which impose multiaxial tension. Major differences in structure-property relationships occur between these systems, which serve as tube-like conduits that are anisotropic in the hoop and longitudinal directions. Articular cartilage and intervertebral discs are soft tissues that function primarily under compression conditions. Articular cartilage is important under compressed sliding conditions, while the disc acts as a pad between rigid bone structures during compression. All of these soft connective tissues contain high modulus fibrils composed of Types I, II, or III collagen. Only the aorta has a significant amount of additional elastic fiber, made up of the protein elastin. The collagen fibril, a basic building block in all connective tissues, is an oriented composite structure (Figure 1) ( 1). Collagen is a fibrous protein whose basic chemical constituents are glycine, proline, and hydroxyproline, with some other amino acids present in variable proportions. These chains of amino acids, which are uniformly 290 nm long, form a triple helical structure called tropocollagen. It is generally believed that five tropocollagen molecules are