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

194 Biomineralization and Biomimetic Materials

A key observation concerning biological materials is II. STRUCTURAL PROTEINS
that they are all composites. At any scale above 10 microns
there are no uniform structures in biology. This makes Silks have been reviewed Kaplan et al. (1997). Many in-
the concept of a “biological material” rather uncertain be- sects and arachnids make extensive use of these protein
cause structures and properties change with position in fibers for a range of purposes. Spiders typically produce
the body, with the individual plant or animal, and with five different silks for the radial and spiral parts of the
time. Thus, we can consider “bone” to be a material, but web, for a sticky web coating, for wrapping prey, and for
then distinguish the structures of woven, Haversian, and the dragline. The properties variations come from differ-
lamellar bone. We then subdivide the bone into dense and ences in the amino acid sequence of the polymer. Silks
cancellous. Finally, the details of the structure will vary are stored in a gland as an aqueous solution, at least some
with site. Also, in contrast to assembled machines, sharp of the time in a lyotropic liquid crystalline state. For the
boundaries between structural materials are rare so it is strong silks, shear at the spinneret leads to a change in
quite difficult to define where bone stops and mineral- conformation to a very highly oriented and stiff fiber. The
ized cartilage or mineralized tendon starts. For these rea- combination of strength, stiffness, and toughness shown
sons, it is also relatively difficult to characterize tissues by spider dragline silks has led to efforts to characterize,
and compare their properties with those of plastics, ce- clone, and produce a synthetic version of this material .
ramics, or metals. This article will summarize biological Bulk crystalline polymers, such as nylon or polyethy-
polymer matrix materials, discuss mineralized tissues, and lene, have an amorphous fraction of 30 to 50% that arises
then discuss biomimetic composites and ceramics. from the inability of entangled polymer chains to become
completely ordered. This crystallinity can be controlled to
I. STRUCTURAL BIOLOGICAL some extent by processing and can be reduced by copoly-
MATERIALS merization to introduce random irregularity into the chain.
Fiber structures are less easy to resolve but behave sim-
In their survey of biological materials, Wainwright et al. ilarly. In many silks, the structure seems to be blocky,
(1986) make a division into tensile materials, rigid mate- with irregular sections spaced along the chain to define
rials, and pliant materials. Following a more conventional noncrystallizable sections. We are not yet able to resolve
materials division, we will discuss polymers, ceramics and the role of these irregular sections in stabilizing the liq-
mineralized polymers, and gels; however, such boundaries uid crystalline state and in defining the final structure and
are even less clear in biology than in the synthetic world. properties of the fiber. In all biological polymers, there is
Two processing-induced limitations should be recog- a degree of control over the molecular structure that may
nized for structural polymers in biology. First, they are all be very important for properties and cannot be duplicated
formed from aqueous solution and so are all very sensitive synthetically.
to plasticization by moisture. In most cases, the properties Collagen is the structural material of skeletal animals
of the dry material have little relevance because they will and has properties that are quite inferior to those of cellu-
only occur in a dead organism. The plasticizing effect of lose, chitin, or silk. The key to its use seems to lie in the
water on biological polymers can be regarded as parallel versatile processability. Soluble procollagen is formed in
to the softening effect of increased temperature on syn- the cell and exported into the growing tissue. An enzyme
thetic amorphous polymers. As temperature will take a cleaves a bulky end section from the molecule to form
hard polymer through the glass transition into a rubbery tropocollagen that organizes into a triple helical structure.
state, so will increased water content convert amorphous These triple helices self-assemble into collagen fibrils that
proteins from glass to rubbery. There is little sense in mea- make up the bulk of tensile structures such as tendon and
suring the mechanical properties of biological materials ligament. As it ages, the collagen becomes cross-linked,
without defining the water content. which increases the stiffness and strength but reduces the
Second, the growth process allows a variety of routes to toughness. Baer and co-workers (1991) have discussed
the formation of fibers but there are no simple ways to form the structure and properties of collagen in tendon and
a dense isotropic plastic. Thus, even essentially isotropic ligament.
materials will have a fibrous composite microstructure. Keratin can be seen as a biological answer to the need
Weiner et al. (2000) have argued that many biological for a tough plastic equivalent to nylon. As hair and fur, it
structures can be viewed as a search for isotropic prop- is a fiber. As epidermis, it is a film, and as hoof or horn,
erties, or at least orthotropic properties (strong in two di- it is tough solid. The structure contains fibrils, which are
mensions), from fibrous materials. This would reflect the built from three-stranded ropes of alpha-helical chains in
unpredictability of stresses encountered by a structure in a coiled-coil arrangement. The fibrils are embedded in a
a dynamic environment. cross-linked matrix of amorphous protein that is heavily