Overview Biological Materials Mechanics Structural Biological Composites: An Overview Marc A. Meyers, Albert Y.M. Lin, Yasuaki Seki, Po-Yu Chen, Bimal K. Kad, and Sara Bodde Biological materials are complex com- biology is a mature science, the study of strength and ductility leads to high posites that are hierarchically structured of biological materials and systems energy absorption prior to failure. The and multifunctional. Their mechanical by materials scientists and engineers most common mineral components are properties are often outstanding, consid- is recent. It is intended, ultimately, to calcium carbonate, calcium phosphate ering the weak constituents from which accomplish two purposes. First, this (hydroxyapatite), and amorphous silica, they are assembled. They are for the most study provides the tools for the develop- although no more than 20 minerals have part composed of brittle (often, mineral) ment of biologically inspired materials. been identifi ed, with principal elements eld, also called biomimetics, 3 is and ductile (organic) components. These This fi being Ca, Mg, Si, Fe, Mn, P, S, C, and the complex structures, which have risen attracting increasing attention and is light elements H and O. These minerals from millions of years of evolution, are one of the new frontiers in materials are embedded in a complex assemblage of organic macromolecules 4 that are inspiring materials scientists in the research. Second, the study of biological design of novel materials. This paper materials enhances the understanding hierarchically organized. The best known discusses the overall design principles of the interaction of synthetic materials are keratin, collagen, and chitin. in biological structural composites and and biological structures with the goal The extent and complexity of the illustrates them for fi ve examples: sea of enabling the introduction of new and subject are daunting and will require spicules, the abalone shell, the conch complex systems in the human body, many years of global research effort to be shell, the toucan and hornbill beaks, leading eventually to organ supplemen- elucidated. Thus, the focus here is on fi ve and the sheep crab exoskeleton. tation and substitution. These are the systems that have attracted the interest so-called biomaterials. of the authors. The silica spicules have INTRODUCTION One of the defi ning features of the been studied and extensively described by Mayer and coworkers. 5,6 The four Many biological systems have rigid biological systems that comprise mechanical properties that are far beyond a signifi cant fraction of the structural other systems have been investigated by the authors: abalone, 7–9 conch, 9,10 those that can be achieved using the biological materials is the existence of toucan, 11,12 and crab exoskeleton. 13 same synthetic materials with present two components: mineral and organic. technologies. 1 This is because biological The intercalation of these components HIERARCHICAL organisms produce composites that are can occur at the nano-, micro-, or meso- ORGANIZATION OF organized in terms of composition and scale and often takes place at more than STRUCTURE structure, containing both inorganic and one dimensional scale. Table I exempli- organic components in complex struc- fi es this for a number of systems. The It could be argued that all materials tures. They are hierarchically organized mineral component provides the strength are hierarchically structured, since the at the nano-, micro-, and meso-levels. whereas the organic component contrib- changes in dimensional scale bring about Additionally, most biological materials utes to the ductility. This combination different mechanisms of deformation are multifunctional 2 (i.e., they accu- mulate functions). For example, bone Table I. Principal Components of Common Structural Biological Composites provides structural support for the body Mineral Organic plus blood cell formation; the chitin- based exoskeleton in arthropods offers Biological Calcium Hydroxy Composite Carbonate Ca Silica Apatite Other Keratin Collagen Chitin Cellulose Other an attachment for muscles, environmen- Shells X X tal protection, and a water barrier; sea Horns X X spicules offer light transmission plus Bones X X structural support; and roots anchor trees Teeth X X Bird Beaks X X plus provide nutrient transport. A third Crustacean X X X defi ning characteristic of biological sys- Exoskeleton tems, in contrast with current synthetic Insect Cuticle X X Woods X systems, is their self-healing ability. This Spicules X X is nearly universal in nature. Although 2006 July • JOM 35
and damage. However, in biological materials this hierarchical organization is inherent to the design. The design of the material and structure are intimately connected in biological systems, whereas in synthetic materials there is a disciplin- ary separation, based largely on tradition, between materials (materials engineers) and structures (mechanical engineers). This is illustrated by three examples in Figure 1 (bone), Figure 2 (abalone shell), and Figure 3 (crab exoskeleton). In bone (Figure 1), the building block of the organic component is the collagen, Figure 1. The hierarchical organization of bone. which is a triple helix with a diameter of approximately 1.5 nm. These tropocol- lagen molecules are intercalated with the mineral phase (hydoxyapatite, a calcium phosphate) forming fi brils that, on their turn, curl into helicoids of alternating directions. These osteons are the basic building blocks of bones. The weight fraction distribution between the organic and mineral phase is approximately 40/60, making bone unquestionably a complex hierarchically structured bio- logical composite. Similarly, the abalone shell (Figure 2) owes its extraordinary mechanical properties (much superior to monolithic CaCO 3 ) to a hierarchically organized structure, starting at the nano-level, with an organic layer having a thickness of 20– 30 nm, proceeding with single crystals of the aragonite polymorph of CaCO 3 , consisting of “bricks” with dimensions Figure 2. The hierarchy of abalone structure. Clockwise from top left: entire shell; of 0.5 µ m vs. 10 µ m (microstructure), mesostructure with mesolayers; microstructure with aragonite tiles; nanostructure showing organic interlayer comprising 5 wt.% of overall shell. and fi nishing with layers approximately 0.3 mm (mesostructure). Crabs are arthropods whose carapace comprises a mineralized hard compo- nent, which exhibits brittle fracture, and a softer, organic component, which is primarily chitin. These two components are shown in the scanning-electron micrograph (SEM) of Figure 3. The brittle component is arranged in a heli- cal pattern called a Bouligand structure. There are canals linking the inside to the outside of the shell; they are bound by tubules shown in the micrograph and in a schematic fashion. At higher magni- fi cation, this consists of a chitin-protein mixture. SPONGE SPICULES Sea sponges have often long rods that protrude out. Their outstanding fl exural Figure 3. The hierarchy of spider crab structure. toughness was fi rst discovered by Levi et JOM • July 2006 36
Recommend
More recommend
