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Encyclopedia of Physical Science and Technology EN002J-63 May 18, 2001 14:16

196 Biomineralization and Biomimetic Materials

sponge, Monoraphis, that is attached to the seabed by a faces. These work in concert with the supply of calcium
large silica rod, 1 m long. The structure of this material is and carbonate from the mantle tissue. The mantle adds
a series of concentric layers where the weaker interlayer new shell at the existing edge by extruding over the lip to
regions may enhance the toughness of the structure, like extend the outer and inner faces.
a laminated glass windshield. The vaterite form of calcium carbonate often occurs
Siliceous diatoms are single-celled prokaryotes, about in laboratory crystallization and is occasionally found in
100 µm in diameter, enclosed by a porous silica shell, shells. There is increasing evidence for precipitation of
in the form of two overlapping dishes, like a petri dish. amorphous, hydrated calcium carbonate as a metastable
The organism multiplies by dividing into two disc-shaped precursor for the crystal. In some cases, including lobster
cells and then forming two new dishes back-to-back. The cuticle and a sponge, stable amorphous calcium carbonate
cells then separate, each with one new half shell and one is found.
old one. Recent work on the proteins involved in silica
mineralization has started to clarify the details of silica
C. Hydroxyapatite
deposition in diatoms and sponges. Plants often contain
silica as a reinforcing material, and the structure of bam- The human body is supersaturated for both calcium car-
boo has been much studied in this context. Seawater and bonate and for hydroxyapatite, Ca 10 (OH) 2 (PO 4 ) 6 , the
soil water contain low levels of dissolved silica as sili- mineral of bone and tooth. Since the environmental avail-
cic acid, which the cell probably binds and transports as ability of phosphorus is limited, its use as a reinforcement
a complex with catechols (dihydroxybenzene) and then seems peculiar. Possibly bone acts a phosphorus reservoir.
converts this to silica nanoparticles. Boneisessentiallyapolymer-matrixcompositereinforced
with ribbons of hydroxyapatite. The structure and proper-
ties of bone are discussed by Currey et al. (1995). A key
B. Carbonate
current question is the way the lamellar structure gives rise
Calcium carbonate is widespread as a protective shell to a combination of high stiffness, high strength, and high
in marine animals, from single-celled coccoliths through toughness. Similar synthetic composites tend to break at
coral, gastropods, and bivalves. It also occurs as a rein- relatively low strain and so have low toughness.
forcement in the cuticle of crustaceans such as crabs. The Dentine has a structure similar to bone but with signifi-
use of calcium carbonate for shells, rather than silica, may cantdifferencesinthemechanismofmineralization.Tooth
reflect the greater control of structure available through enamel is almost wholly mineral with a fibrous structure
calcium-binding proteins and through control of crystal that forms under the control of a completely different set
morphology with nucleators and growth inhibitors. Silica of mineralization proteins, including amelogenin.
does occur as a component of many mollusk teeth, as do
various iron oxides.
There has been much recent work on the formation, V. BIOLOGICAL AND SYNTHETIC
structure, and properties of mollusk shell. Most shells are PROCESSING
constructed of calcite and aragonite in various arrange-
ments with small amounts of protein, up to about 5% by In practical engineering, where cost is very important,
weight. Higher protein levels would be expected to in- there is little sense in designing a part without a very good
crease the toughness of the shell but at the expense of idea of how it will actually be made. Materials selections
reduced stiffness. It can be assumed that particular shell are then made once the manufacturing route and basic
structures are adapted to the particular lifestyles of the an- design have been decided. The same must be true in biol-
imals and the stresses encountered. For instance, it might ogy; many interesting structural features may be primarily
be expected that a swimming bivalve, such as a scallop, a consequence of the growth process and only secondarily
would have a stiffer, lighter shell structure. One of the most a source of improved properties.
studied shell structures is nacre, or mother-of-pearl, which Small numbers of synthetic parts can be made by sub-
is a layered structure of aragonite plates, 0.5 µm thick by tractive processes such as machining but the process is
several microns wide. Between each layer of plates is a slow and wasteful. Molding is generally cheaper and
thin protein sheet that is responsible for nucleation of the faster. Hot liquid material is injected into a hollow tool and
aragonite layers and acts as a crack stopper, which gives allowed to solidify. Extrusion, forging, rolling, and other
wet nacre a very high fracture energy. hot processes can be viewed as variants on molding. The
While the details of shell formation are not understood, sintering of ceramics is a separate case; compacted powder
it is clear that organic content contains proteins capable of isheateduntilsurfaceenergydrivestheslowshrinkageand
selectively nucleating calcite or aragonite in a specific ori- densification to a solid. Chemical processing is not com-
entation and proteins that inhibit growth of specific crystal mon in the synthetic world. Thermosetting resins such as