Hierarchical Structures in Biology as a Guide for New Materials Technology (1994) / Chapter Skim
Currently Skimming:
2 NATURAL HIEREARCHICAL MATERIALS
2 NATURAL HIEREARCHICAL MATERIALS
Pages 17-38
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 17... ...
The hierarchical architectures of cellulose aggregates in wood or collagen aggregates in cartilage or tendon provide excellent examples of natural composite materials designed for multifunctional applications. Even the ultrasoft membranes surrounding cells exhibit exceptional properties that emanate from structure on many length scales.
|
From page 18... ...
RECURRENT USE OF MOLECULAR CONSTITUENTS Nature uses collagen in stunningly different ways: in the crimped fibers in tendon, which absorb, store, and transmit forces between muscle and bone; in the junctions between high and low modulus materials in articular cartilage; and as components of hard materials such as bone. At the molecular level, there are relatively minor differences among the collagens in these disparate biomaterials; all are similar in amino acid composition and all occur as collagen "molecules," coils of three interwound helical polypeptides about 300 nm in length (Figure 2-1~.
|
From page 19... ...
These basic fibrils are combined and oriented to form more highly ordered structures with a particular morphology that determines the mechanical properties of the tissue. In the tendon, for example, the parallel alignment of crimped collagen fibers oriented longitudinally between muscle and bone provides nonlinear stress-strain behavior, with a gradual increase in stiffness upon elongation and a limiting elongation of a few percent (Kastelic and Baer, 1980~.
|
From page 20... ...
For example, tendons absorb the shock to the knee joint in landing from a jump. This combination of mechanical properties is accomplished through the unique hierarchical structure of the tendon and the resulting incremental response to mechanical loads that provides initial elasticity, followed by high tensile stiffness and distributed plastic deformation to avoid catastrophic failure modes.
|
From page 21... ...
In the linear region, the fully straightened collagen fibers are further pulled elastically. If the load is released, the tendon will immediately and entirely recover its initial crimped morphology.
|
From page 22... ...
design distributes stresses throughout the levels of structure, thereby minimizing dangerous stress concentrations that could precipitate failure and fracture. The architecture of tendon provides important advantages in dynamic performance.
|
From page 23... ...
embedded in I percent of a "rubbery" matrix of an amorphous polymer with high molecular weight in 91 percent seawater. The matrix surrounding the collagen fibers is probably a protein-polysaccharide complex that forms a dilute gel linked into a permanent network.
|
From page 24... ...
Wood is composed of the high-modulus, highstrength, crystalline polysaccharide, cellulose, in an amorphous matrix of hemicellulose, lignin, and other compounds. The architecture is that of an aggregate of microscopic cylindrical cell walls of the composite, with the cylinders lying parallel to the long axis of the stem, root, or leaf.
|
From page 25... ...
These predictions assume that the creation of new surface area by fiber pu11-out is the major mechanism responsible for fracture toughness. The mechanism accounting for the high fracture toughness for wood is helical column buckling of the cellulose fiberwound cell wall (leronimidis, 1976~.
|
From page 26... ...
In bone, for example, it has been proposed that the interface between collagen and the hundredfold stiffer hydroxyapatite is formed via epitaxial crystallization of the mineral on a phosphorylated collagen template (Glimcher, 1984~. In articular cartilage, collagen orientation changes from parallel to the surface in the outer zone, to a perpendicular orientation at the interface, with fibers extending into the bone (see Case Study-Articular Cartilage)
|
From page 27... ...
Swelling pressures resulting from the presence of water in biological structures help to oppose compressive loads. In articular cartilage, for example, water constitutes 65-80 percent of the tissue and is confined in a swollen network of collagen fibers and proteoglycan aggregates.
|
From page 28... ...
28 Hierarchical Structures in Biology as a Guide for New Materials Technology .
|
From page 29... ...
These properties derive from the intrinsic properties of the collagen molecule and the hydroxypyridinium cross-links that exist between collagen fibers. The proteoglycan aggregates form a labile network that provides the compressive stiffness that results from their bulk compressive stiffness and from the swelling pressure.
|
From page 30... ...
~ I, Deep (30%) _ Articular surface Calcified cartilage / ',~V'~.@c8~7~=,~~cV~ e~t5~_-,~O_ -"v~.v''"~' "v''= ~ A._ ~ Cancellous bone FIGURE 2-6 Ultrastructural organization of collagen fibers throughout the depth of articular cartilage.
|
From page 31... ...
in the collagen network that results from proteoglycan swelling is believed to have an important physiologic function similar to pre-stressed reinforcement bars in concrete beams. The degree of hydration in the cartilage depends on the balance of swelling pressure and the elastic pre-stress developed in the solid matrix and is the most important factor governing cartilage mechanical properties and function.
|
From page 32... ...
Ulkro (10 7m-10~m) FIGURE 2-7 Hierarchical architecture of diarthrodial joints and the constituent articular cartilage.
|
From page 33... ...
Neurotransmitters (to which the isolated ligaments are sensitive) may be produced by the attendant nerve cells to control the ionic environment and thus the mechanical properties of the ligaments.
|
From page 34... ...
The increase in stiffness apparently allows the skin to act in transmission of force from muscle in the anterior parts of the fish to manipulate the tail. It is likely that the study of biological materials capable of this kind of environmental response will reveal a rich variety of mechanisms for coupling of sensory information and the physical and mechanical properties of materials.
|
From page 35... ...
Thus, in the red blood cell, a rigid polymer scaffolding has been interfaced with a fluid membrane to form a compatible composite that is two orders of magnitude softer than existing synthetic elastomers. At the next higher level of the structural hierarchy, a soft tissue, such as skin or liver, can be described as an aggregate of fluid membrane capsules which is supported by internal networks that are connected to form a compatible composite that is resistant to wear and fracture.
|
From page 36... ...
At any time during manufacture, the evolving shape of a biological structure is the product of multiple, successive hen-and-egg events: the cells that are currently making matrix are at specific sites because of earlier events in the development of the organism, and they, in turn, influence the sites where future cells will produce additional matrix. The number of synthetically active cells at a given site depends on earlier replication of cells by cell division; on migration of cells over preexisting structures; and on numerous genetic programs that control cell division, specialization of cells, and synthetic acitivity.
|
From page 37... ...
From the above, it follows that the intimate relationships between the local manufacturer of matrix components, for example, a fibroblast cell, and the matrix to which it adheres, which may be part of a tendon or bone, influence both the local composition and the orientation of new matrix and, eventually, the shape of the completed tendon or bone. The local composition readily acquires a layered, or even interwoven, structure due to repeated cycles of deposition by sets of cells.
|
From page 38... ...
Hierarchical Structures in Biology as a Guide for New Afatenals Technology FIGURE 2-8 Scanning electron micrographe of the rasping tongue of the mollusk Urosalpir~x cinereaSol~yensis. The mineralized structure contains crystalline magnetite in an ordered matrix of organic fibrile.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.