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.
Figure 12.2. Events happening at the surface of a bioactive ceramic
leading to the formation of a bone tissue
506 Ceramic Materials
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 nucleator 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 by calcium phosphates with apatitic
structure (hydroxypatite, fluoropatite, carbonated apatite) and certain oxides or
hydroxides (titanium oxide, titanium hydroxides, silica gels) (see Figure 12.3). The
latter are constituted by hydrolyzable 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.
Figure 12.3. Photograph of the formation of immature bone tissue
(after removal of the organic material) at the surface of an HAP layer
on a hip prosthesis implanted in a man for several weeks
Bioceramics 507
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 question
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|