Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramics
Dr. Shaker J. Edrees
12.3.2. Cell-ceramic interactions
The presence of materials may modify cell activity and affect tissue
reconstruction. We have known for many years how to grow eukaryotic
cells on different materials and studies have been made on a wide variety
of ceramics. Concerning ceramics used in orthopedics, there is a
succession of events which ends in the integration of the implant. The
sketch in Figure 12.2 details these events in the case of a bioactive
ceramic.
1. Equilibrium of the ceramic surface and the solution. Eventual release
of mineral ions, adsorption of
ions and/or of proteins.
2. Nucleation and growth of a layer of carbonated apatite analogous to
bone mineral from
supersaturated biologic fluids. This layer contains a number of bioactive
proteins.
3. Cells go towards the modified surface on which these will settle and
become differentiated in order to
give rise to osteoblasts.
4. The osteoblasts multiply and colonize the surface of the biomaterial.
5. The cell layer synthesizes a collagenic organic matrix.
6. The organic matrix mineralizes and the new bone is deposited.
12.3.2.1. Surface phenomena
Biological fluids are generally supersaturated with respect to the apatite
of bone tissues. The presence of nucleation and crystal growth inhibitors
of calcium phosphates, however, generally prevent the phenomena of
uncontrolled mineralization. In this environment, the nucleation and the
crystalline growth of apatitic calcium phosphate analogous to bone
mineral are relatively easy. It has been shown that all bioactive
compounds which facilitate the formation of bone tissues also favor
nucleation of calcium phosphates. Several authors in fact now consider
this property as a direct measurement of the biological activity of
ceramics. We can
distinguish ceramics which simply play the role of a nucleate or without
bringing mineral ions from those which modify in biological environment
with release of mineral ions and accelerate the formation of a layer of
neo-formed apatite analogous to bone mineral. The first are constituted
Babylon University / College of Materials Engineering
Non-Metallic Materials Engineering Department- Ceramic Branch
Subject: Bioceramics
Dr. Shaker J. Edrees
by calcium phosphates with apatitic structure (hydroxyapatite,
fluoropatite, carbonated apatite) and certain oxides or hydroxides
(titanium oxide, titanium hydroxides, silica gels) (see Figure 12.3). The
latter are constituted by hydrolysable phases with release of calcium ions,
phosphate
and/or hydroxide or carbonate (non-apatitic calcium phosphates,
bioglasses, alkaline surfaces, or calcium carbonates). The latter are
capable of generating a neo-form crystalline layer faster than the former
and can also create a denser crystalline layer. In certain cases, we can
combine composites of the first family with those of the second.
Let us note that the formation of an apatitic layer in vivo is not desirable
for ceramic applications other than for bone substitutes (cardiac valves,
friction surfaces of joints). While choosing ceramics for such
applications, we have to take care to choose ceramics which do not favor
nucleation of these crystals. It has been precisely shown that alpha
alumina is a very bad substrate for the nucleation of calcium phosphates.
On the contrary, titanium nitrides and carbides also proposed as friction
surfaces are quite good nucleators [ROY 93] and yet these can be
questioned with respect to their use in friction couples for joint repair