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

198 Biomineralization and Biomimetic Materials

and this will lead to a change in crystal habit. One essence materials, particularly with a view to producing ceramic
of control is that signals can be turned on and off. Prote- and composite materials with improved toughness, analo-
olytic enzymes which degrade specific inhibitors can also gous to shell, tooth, and bone. There was some discomfort
be expected to be part of a mineralization process. in the materials community with the idea that we wanted
Studieshavealsodemonstratedthatamorphouscalcium to mimic biological materials, in the sense of producing
carbonate does occur in some species and is a transient an indistinguishable copy. However, mimesis more gen-
mineral in some others. This structure presumably results erally refers to copying some essential aspects of a thing
from high levels of incorporated protein in the structure. rather than duplicating or faking it. The phrase “bioin-
Other work on solution growth has shown that metastable spired materials” is also used to express the idea in more
complexes of calcium and acidic polypeptide can be familiar terms but is grammatically less desirable as it is
important in growth of carbonate films. It has long been a Greek/Latin hybrid. The field has now spread to include
known that silica species in solution will promote precipi- a group of loosely linked goals in new materials and pro-
tation of hydroxyapatite on many substrates. Again, some cesses, which are surveyed below.
metastable complex is presumably involved. De Guire It should be kept in mind that materials development is
et al. (1998) have studied biomimetic growth of various considerably upstream from the development of new prod-
minerals on substrates treated with self-assembled ucts. It is quite typical for new materials to find their way
monolayers. They have shown that some cases seem to into commercial products about 20 years after their disco-
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correspond to growth by addition of individual ions, while very. Examples include Kevlar , high-Tc ceramic super-
others involve colloidal attraction of preformed clusters conductors, piezoelectric polymers, and gallium arsenide.
to the surface and aggregation. Matijevic has shown that It is also typical for the first applications to be quite dif-
apparently crystalline, faceted particles of many minerals ferent from those originally proposed and for their impact
formed from dilute solution are actually aggregates of to be modest compared to that suggested during the initial
clusters. Thus, many mineralization processes in complex excitement. A personal view of the status and prospects for
solutionsmayinvolveanintermediateclusterorpolymeric biomimeticmaterialswillbegivenattheendofthisarticle.
state.
One striking example of the complexity of biominer-
A. Polymers
alization is the fact that bone is largely mineralized by
hydroxyapatite entrained in collagen fibrils, where it is The core difference between the proteins and synthetic
believed to nucleate at the acidic terminal regions of col- polymers is that protein synthesis provides total con-
lagen triple helices. Many synthetic studies have produced trol of the sequence of units along a chain while the
the mineral on collagen fibrils but none has produced min- best-controlled polymerizations can only provide several
eral in the fibrils. We are missing some key aspect of the blocks of functional units on a chain. One obvious goal is
process. the formation of synthetic polymers with enzyme-like cat-
It should also be kept in mind that mineralization is a alytic activity. Many enzyme-active sites have an array of
process that occurs in space and time. Mineralized tissue active groups held in close proximity so as to interact with
is generally formed by a layer of cells that sequentially the substrate (target molecule) and to reduce the activation
deposit organic matrix and mineral and move back. There energy for reaction by a precise spatial array of ionic or
is a structural gradient away from the cell surface; some hydrogen-bonding interactions. To achieve such a precise
species will act locally while others must diffuse some spacing of active groups on a synthetic polymer would
way to their site of action. In bone formation, matrix vesi- require a rigid structure, which would in turn normally
cles also provide the cells with the possibility of delivering render the material insoluble and so inactive. Recent stud-
species into the mineralizing zone, several microns from ies of dendrimer molecules with highly branched struc-
the cell surface. In mollusk shell, day–night cycles may tures may lead us to the required combination of flexible
also provide the structural sequence. Studies of “flat pearl” and soluble outer structures combined with a highly struc-
have also shown the progression of structure formation on tured core. Many proteins also go through large changes
a glass surface embedded under the mantle of a mollusk. in shape in response to binding of substrates or other en-
Most studies of biomimetic mineralization have used con- ergy inputs. This seems to require a structure where two
stant precipitation conditions. well-defined conformations are closely balanced in en-
ergy so the molecule can flip from one to another. Such a
VII. BIOMIMETIC MATERIALS change is not likely in a wholly flexible system but again
some subtle combination of flexible segments and rigid
The concept of biomimesis has long been used in chem- units is required. We still have much to learn in these as-
istry in the context of compounds with enzyme-like cat- pects of macromolecular design but are acquiring both the
alytic action. Since the mid-1980s it has been applied to synthetic tools and the understanding.