Natural composites Bone: Hydroxyapatite (calcium phosphate) mineral ribbons reinforcing collagen fibers Horn and hoof: Keratin fibers stuck together with resin Wood: Cellulose fibers stuck together by lignin and hemicelluloses Insect Cuticle: Chitin fibers stuck together with resin. Mineralized in crustaceans BONE The composition of bone is collagen 1/3 of dry weight, 50% of volume. The reinforcing mineral is hydroxyapatite Ca10(PO4)6(OH)2, in plates 2nm x 20-50nm (aspect ratio 10-20). This (density about 2 g/cc) is 65 wt%, 50vol% of the bone. Bone also contains water which keeps it tougher. Bone is a short fiber composite. The crystals are ribbons with an l/t ratio here is of about 25. For maximum strength we need an interfacial strength about 1/50-1/10 of crystal strength (=s c.t/2l), but we don't know what is in bone. The obvious advantages of the bone structure are the high volume fraction of mineral and its parallel orientation. This is hard to achieve in moldable synthetic composites where the maximum packing density is low. Antler (totally different from horn, which is keratin) is selected for toughness, while enamel has chemical resistance and hardness. The mineral (HAP) modulus is 130GPa, the strength about 100MPa in an unnatural sintered form. Collagen has a modulus of 1 GPa, and strength of 50 MPa. At 50% parallel fibers the modulus would be 65.5GPa according to the parallel model, the serial model (for spherical filler or transverse fibers) gives 2 GPa. Hence bone is much better than a simple filled composite. The strength is better than any model as we can't get good HAP for comparison, it is not even clear that a "reference" can exist except as a very long, 4nm thick, HAP ribbon. Hydroxyapatite naturally contains a high level of carbonate. Fluoride can be picked up to replace the OH, but fluorapatite is much less soluble. The thin ribbon-like morphology is not formed synthetically, there is some kind of control by the system. Sharks have calcified cartilage, not bone. There is acellular bone in some fish. Shark teeth are fluorapatite, with F replacing the OH. Bone remodels, grows on compressive, not tensile faces to strengthen heavily loaded regions. The rules are not clear but may be driven by a tendency to eliminate surface stress gradients. Bone is piezoelectric and there has been discussion of whether control is due to the piezoelectric response of HAP or collagen. It isn’t. Compact Bone Parietal bone of the Cancellous bone skull: cross-section cut. Compare to metal: same right through Bone does not act as a simple solid material but is arranged in a complex hierarchy. a) Woven bone: Random collagen fibrils plus mineral b) Lamellar bone: planes few microns thick, with a preferred direction of collagen. These two are arranged as Woven (W), Lamellar (L), Haversian cylinders round a blood vessel (L) or Laminar (mix of planes of W&L). These in turn are arranged at the higher length scale into Compact or Cancellous. For cancellous bone the modulus is about 1/60 of compact and the strength 1/30. It provides a light support structure to the interior of the bones. Bone shows a 10% extension to break, 10x any equivalent synthetic material. This occurs through microcracking, small holes forming, probably between the lamellae. The tearing of collagen across these cracks may be the source of the energy absorption and so toughness. Exactly how this works is not yet known. John Currey is the expert in this field and has written many reviews. In compact bone the osteons run in various directions but with an overall orientation. Hence the structure is not highly aligned and modulus does not fall rapidly with change of angle. This agrees with the intermediate modulus between the parallel and series estimates. Joints between osteons may be a source of creep and splintering. Fracture starts from osteocytes and blood vessels. These are the price to be paid for adaptability. High rate gives smoother surfaces. There is also calcified tendon, which works differently. In our tendons, local calcification smooths the stress transfer to bone. Long turkey leg tendons are all calcified. Synthetic hydroxyapatite-polymer composites are much inferior to bone How do bones grow? Around the outside and from the middle Bone is a composite Bone is Anisotropic Bone is hierarchical Bone repairs itself Bone remodels Growth factors control the deposition of bone and conversion of mesenchymal cells to chondrocytes, osteoblasts, osteoclasts Keratin Properties Feather keratin: beta-sheet with an 8 GPa modulus. It is a block copolymer with crystalline and non-crystalline regions, like silk. The choice for feathers is probably based on stiffness/weight ratio. Skin strength at low humidity is about 2 MPa, relatively low. The main function of keratin in skin is to be a flexible barrier layer, so it must fail by intercellular separation. It cracks when dry and softens when wet but is maintained moist but not wet by a continuous flux of water vapor from the body. Amphibian skin is a beta-sheet keratin because of the need to tolerate continuous water immersion. Hair styling is a combination of stretching to beta sheet and recoiling in steam and breaking and remaking the crosslinked amorphous material by mercaptans (-SH) : R1-S-S-R2 + R3-SH <--> R1-S-S-H + R3-S-S-R2 Wool and hair are not uniform in cross-section but have a skin layer and a core. Why is not clear. Keratin seems like a real plastic: available as moldings, films and fiber with properties tuned to the application. The modulus of wool is 1.2 GPa and that of hoof and hair 4GPa. Yields at 150 MPa, modulus drops to 0.4 GPa after yield as the chains unravel to beta-sheets. Keratin is very tough (excellent for hooves), and has a high hysteresis, due to the alpha- beta transition. Chitin This insoluble polysaccharide is expressed and embedded in a swollen protein. Mechanism of formation unknown? Oxidative cross-linking hardens the protein to form hard cuticle.
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