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

202 Biomineralization and Biomimetic Materials

of a large extension breaking after yield, such as occurs the gap after the crack has passed; however, the extent
in folded beta-sheet proteins, where the unfolding of the of deformation occurring during the drawing is limited
chains leads to a large energy absorption, as has been dis- by the thinness of the metal layers. As a result, substan-
cussed by Morse (1999). However, polymer layers will tial increases in toughness were only seen when the vol-
also lead to a significant loss of elastic modulus when ume fraction of metal was quite large. In contrast, mol-
compared to a wholly inorganic material. lusk shells show significant toughness with only a small
Much effort has gone into increasing the strength of ce- percent of polymer. It is likely that some combination of
ramic materials by improving their toughness. There was embedded metal, porosity, and interfacial debonding can
initially much effort devoted to fibrous materials, particu- give much enhanced toughness or impact resistance to ce-
larly silicon nitride, but the need for reliable, inexpensive ramics, but current methods for modeling fracture do not
processing methods was not met. Attention then switched provide enough guidance for these designs and the bio-
to ceramic composites with fibrous reinforcements that logical models are only just being interpreted.
would add toughness. Mostly these proved to be unsta- Studies on the influence of polymers on the crystal-
ble or reactive at the temperatures needed for turbine en- lization of calcium carbonates in vitro have shown that
gines,whilestillbeingdifficulttoprocess.Therehavebeen metastable liquid complexes with anionic polymers can
many studies of ceramic/metal layered structures, where be formed that subsequently transform to calcite. Stud-
the metal layers take the role of the polymers in mollusk ies of carbonate biomineralization have shown that amor-
shell. Such materials are of interest both for use in engines phous calcium carbonate forms either transiently or as a
and for armor. There is also interest in incorporating struc- stable phase. It has also been shown that absorbed proteins
tured porosity in materials to improve fracture properties modify the fracture properties of carbonates. In particu-
or to modify elastic properties such as Poisson’s ratio or lar, fracture surfaces show smooth conchoidal fracture,
piezoelectric response. like glass, rather than the faceted fracture characteristic
In the case of metal/ceramic layered materials, the key of normal crystals. This suggests that we have much to
problem is to absorb as much energy as possible within learn about the modification of the properties of crys-
the metal layers. Hwu and Derby have shown that the tals and amorphous solids by entrained polymer (see
major energy absorption is due to metal drawing across Fig. 3). There are parallels with the Lanxide process for

FIGURE 3 Porous interior of a sea urchin spine; calcite single crystal. Polymer coating may serve to prevent fracture
from surface damage; entrained polymer may enhance the toughness.