Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramic
12.3.2.2. Recruitment, development and cell expression
The formation of the neo-formed layer is a determining element of
biological activity. The neo-formed nanocrystals of apatite are extremely
reactive and have the property to bond specific bone proteins
(osteopontin, osteocalcin, etc.) which later promote adhesion,
multiplication and cell expression. The formation processes of this layer
are, however, quite different from those that take place during primary
ossification, which can be described as the mineralization of a pre-formed
collagenic matrix. Furthermore, this layer does not have the characteristic
organization of a bone tissue with apatite crystals, well oriented with
respect to the collagen fibers, and should rather be considered as an
uncontrolled mineralization. The events which occur at the surface of
bioactive minerals, however, have a certain analogy with those which
come into play during the bone remodeling in the cementing zone which
limits the bone osteons . The same phenomena and the same proteins
seem to be involved. The neo-formed layer is favorable to the adhesion of
osteoblastic cells, which then multiply and form the collagenic matrix at
first and mineralize it later . We must mention that the osteoblasts can
also attach and develop and form bone on materials which are not
necessarily bioactive, for example, cell culture dishes in polymer
materials, normally more effective than apatite substrate . However, the
chemical bond between the substratum and the tissue is missing in these
systems. To deserve the adjective bioactive, a material should, at the
same time, be able to establish a bond with the tissue and help
in its renewal. The cells generally settle on a substratum by means of
adhesive proteins characterized by the presence of amino acids sequences
(particularly the tripe tide sequence arginine-glycine-aspartic acid
(RGD)). On the other hand, the relationship between secretion and/or
adsorption of these proteins and the surface characteristics remain
unknown. The energy and the surface load appear to play a preponderant
role; thus, the proteins fixed on the substratum with different loads
exhibit different characteristics and can influence cell recruitment and
development [SHE 88].
Generally, the positively charged substrate are more favorable for
recruitment and adhesion of negatively charged cells. Similarly, the
surfaces which have a high bipolar energy encourage cell development .
However, when over a period of time the protein film develops on the
Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramic
surface of the material, these initial factors no longer have an effect on
cell activity. It appears therefore that the kinetic factors of the protein
film formation and the competitive adsorption of different proteins,
probably determined by the characteristics of the surface, play a decisive
role on cell
activity .
12.3.2.3. Cell rehabitation
Some porous materials may be preoccupied by the cells and invaded by a
newly formed bone tissue. These materials should have at the beginning a
definite pore size. Very large pores do not encourage bone growth, while
the very small ones do not allow an adequate vascularization. The pore
diameter should be a minimum of 100 μm, but could go up to 500 μm
(see below). Moreover, tissue rehabitation requires interconnected pores