Page 38 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

178 Biomaterials, Synthetic Synthesis, Fabrication, and Applications

mineralization systems. Bone Gla-containing proteins are the difference in porosity of cortical and cancellous bone
unique to bone and dentin and as such are expected to have with the former being found where load bearing is impor-
a specific functional role to fulfill in these tissues. tant. Bone is constantly in a state of dynamic equilibrium
Stages in the formation of bone are: (1) synthesis and with its environment and changes with age. Changes with
extracellular assembly of the organic matrix framework, time will also be expected for diseased states and when for-
(2) mineralization of the framework, and (3) secondary eign bodies (e.g., implants) are in close proximity to these
mineralization as the bone constantly forms and reforms. phases although much less is known for such situations.
All of the salts and biomolecules associated with bone An understanding of the structure and dynamics of natu-
described above will play their own role(s) in the develop- ral materials should enable to design of materials for their
ment of bone, the structure of which can vary considerably replacement which will be chemically more compatible
according to the location and use to which the resulting with those they are seeking to replace.
natural composite is to be used. Figure 2 shows pictorially Mineralized cartilage contains much thinner fibers of
collagen than are found in bone, high levels of water of
hydration, and hydroxyapatite crystals, although there is
no regular organization of the crystallites with respect to
the collagen matrix.
Enamel is formed via the assembly of a matrix com-
prising both collagenous and noncollagenous proteins into
which large oriented hydroxyapatite crystals are formed.
The crystals may be of the order of 100 microns in
length, 0.05 microns in diameter, and with an hexagonal
cross-section. At maturity water and protein (including
collagen) are removed from the tooth leaving a collagen-
free composite.

III. GENERAL REPAIR MECHANISMS
AND BIOCOMPATIBILITY

Under normal circumstances most tissues in the body
are able to repair themselves although the process and
the presence or absence of scarring is tissue dependent.
Bone repair occurs either through formation of membra-
nous bone or through mineralization of cartilage. In fa-
cial bones, clavicle, mandible, and subperiosteal bones,
membranous bone growth involves calcification of osteoid
tissue (endochondral bone formation). In long bones the
stages of repair include induction, inflammation, soft cal-
lus formation, callus calcification and remodeling.
Cartilage and skin can also repair themselves although
scarring does occur. For skin, repair involves inflamma-
tion, immunity, blood clotting, platelet aggregation, fibri-
nolysis, and activation of complement and kinin systems.
In the absence of a chronic inflammatory response, dermal
wounds are repaired through deposition and remodeling
of collagen to form scar tissue.
Enamel is not repaired by the body.
Early studies on biomaterials were based upon the idea
FIGURE 2 Schematic drawings of (a) human corticalbone and (b) that implants would not degrade in the human body or be
humand cancellous bone. Note the difference in packing density involved in biological reactions. Hence the search was for
and porosity between the two idealized structures. (Reprinted with
permission from Perry, C. C. (1998). In “Chemistry of Advanced bioinert materials, whatever their chemical composition.
Materials.” (L. V. Interrante and M. J. Hampden-Smith, eds.), pp. However, no material implanted in living tissue is inert
499–562, Wiley VCH, New York. and all materials elicit a response from the host tissue. For