Page 56 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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

Biomineralization and Biomimetic Materials 197

epoxies are solidified by chemical reaction and there is volume as a soluble polymer assembles into fibers or as a
some exotic reactive processing of ceramics. Generally, mineral precipitates will lead to highly porous structures
chemistry has the problem that small changes in starting unless the deposition rate is extremely slow. Any structure
conditions or purity can cause big changes in the reaction forming in a diffusion field will tend to grow towards the
kinetics, so reliability is poor. source rather than filling in gaps in a layer. Very slow
In biology, all processing is essentially chemical. Solu- growth will allow equilibration, which will favor dense
ble reagents are fed to a site where they combine to make structures. One solution to this is to lay down a mesh of
a solid plus dissolved by-products. In the case of epoxies, strong fibers, fill in the pores with a softer matrix, and
mentioned above, there is little volume change between then arrange for this to slowly expel water and harden
the liquid and solid states. In forming solids from solu- chemically. This kind of process is seen in wood and in
tion, there is also a massive shrinkage to be accommo- cuticle and very similar issues occur in the formation of
dated. The biological equivalent of molding can be seen carbon–carbon composites. For such reasons, biological
in the formation of isolated particles within an envelope of materials must be composites.
lipid membrane. Examples include the formation of silica
in sponge spicules and diatoms and calcite in coccolith VI. THE PROCESS OF BIOMINERALIZATION
skeletons. The wall of the “mold” is now permeable to al-
low reagents in and soluble products out. This approach is It has long been recognized that most biological tissues
suitable for isolated particles, such as the magnetic parti- mineralize by precipitation in an existing organic matrix
cles in magnetotactic bacteria and for the wall of a single- that controls crystal form, size, and orientation. Originally,
celled diatom. The particles may also be later assembled it was thought that the key step would be nucleation on a
into a simple framework, as in the sponges. However, this protein where a suitable spacing of charged groups would
method does not lend itself to strong, dense, load-bearing match the crystal lattice to be formed. This picture was
structures suitable for large plants and animals. supported by the observation that acidic proteins extracted
A problem with building a large solid object by chem- frommolluskshellwouldinhibitcrystalgrowthofcalcium
ical precipitation is to avoid surrounding and entombing carbonate from a protein solution but would nucleate crys-
the cellular machinery for providing the reagents and re- tal growth if the protein were immobilized on a surface.
moving the products. The obvious solution is to build the Since then the picture has become much more compli-
solid layer-by-layer, such that a layer of cells provides ma- cated. Studies on nucleation at Langmuir monolayers and
terial to add to the surface of the growing solid and retreat on self-assembled monolayers have shown that surface
ahead of it. The layer approach is apparent in the growth ionic charge is important but there is no strong evidence
rings of trees. Bone, tooth, and shell form in the same way. for lattice matching. While the nucleation effect can be
In wood and skin, it is the cells themselves that become the important in growing surface-attached mineral films, it
structural unit by depositing solid cellulose or keratin in seems to work only in a window of concentration just
or on the cell. Each new layer starts as a new layer of cells. below that at which nucleation occurs readily in solution.
In bone and tooth, the deposition is external to the cells. This window can sometimes be widened by adding growth
In the case of bone growth, the cell layer continuously inhibitor to the solution. The films that form often seem to
forms new collagenous matrix material. This then miner- be limited in thickness, contrary to the reasonable expec-
alizes and converts to hard bone as new matrix is deposited tation that growth would readily continue once a mineral
over it. The control system thus promotes crystal growth layer had covered the substrate. In the case of molluscan
in layers several microns from the cell surface while not nacre it has been shown that some species, at least, nucle-
mineralizing the freshly formed matrix. Many proteins are ate each new layer of aragonite by growing through holes
known to be associated with this process control but it is in a protein layer, rather than via a nucleating protein as
not yet clear how they work together. had been thought.
One natural consequence of this layerwise growth pro- Studies of protein synthesis during biological miner-
cess is that layers of different material can readily be alization have shown that large numbers of proteins are
formed within a single solid. As will be seen below, bi- being produced during the growth of tooth or bone. This
ology makes extensive use of layered structures to add gives us the problem of too much information, as it is not
toughness to strong materials. Recent developments in ce- at all clear why so many are needed and what they all
ramics have also focused on the use of layered structures do. It is clear that much of the control is via inhibition of
to add toughness. New methods of freeform fabrication growth on specific crystal faces, leading to control of crys-
should ultimately allow the production of complex lay- tal shape and orientation. At first it might seem strange that
ered structures resembling those of biological materials. a protein can bind so specifically to one crystal face, but
A result of chemical precipitation is that it is relatively a difference in binding strength may be all that is needed
difficult to make dense structures. The large change in to change the relative growth rates of two crystal faces