oxide

Oxides and hydroxides
As discussed in the previous section, it would seem that the layer of hydrated
silica, formed at the surface of implanted bioglasses and vitroceramics, plays a very
important role in the formation of the neo-formed apatitic crystals, hence the idea of
using pure silica or titanium hydrogels as bioactive compounds. Thus, gels of silica,
titanium or zirconia, after being subjected to heat treatment, can induce the
formation of the apatite when these are immersed in a metastable solution analogous
to biological fluids, while alumina gels are not bioactive. The development through
sol-gel processes is an interesting method for the preparation of bioactive materials,
since it helps to obtain a product favoring the heterogenous nucleation of the apatite.
While the silica gel is heated to a temperature greater than 900°C, the formation of
the apatite is delayed [LI 93a]. It therefore appears that the speed of rehydroxilation
and the rate of hydroxyl group on the surface of the silica gel control the formation
of apatite. In the same way, sufficiently hydrated titanium dioxide, that is with Ti-
OH groups on the surface, can join with the bone while it is placed in a bone site.
This property offers the possibility to render titanium bioactive by a treatment before
implantation, in such a way as to form a gel or a very hydrated layer at the surface of
the implant [KOK 99; LI 93b].
12.2.3.5. Composites
The development of mineral-organic composite materials offers the possibility of
combining the favorable properties of bioceramics such as the HAP, alumina or
titanium dioxide with the molding capacity of biocompatible polymers
(polymethylmethacrylate): PMMA [KHO 92], poly(L-lactic) acid: PLLA [ROD 95],
poly(ethylene): PE [DOW 91]). It is also conceivable to attain a value of the
modulus of elasticity near to that of the bone.
We can differentiate composites as bioresorbable or non-bioresorbable. The non-
bioresorbable composites are the result of the combination of a non-bioresorbable
calcium phosphate (HAP) with a non-bioresorbable polymer (PMMA, PE). In this
case, we have to avoid the covering of ceramic grains of the surface, so as to
preserve their biological activity.
Bioceramics 503
Bioresorbable composites combine a bioresorbable polymer (PLLA,
poly(glycolic) acids and poly(butyric) acids) with HAP particles or of resorptive
calcium phosphate. In order to guarantee a successful combination of calcium
phosphates with a bioresorbable polymer, it is important to adapt the resorption of
the two constituents to avoid inflammatory reactions due to the release of ceramic
particles.
These materials should grow in the future on account of the great many
combination possibilities and their aptitude at combining a biological activity with
mechanical properties similar to those of the bone.
Besides, various combinations of calcium phosphates – with chitosan [VIA 98],
with cellulosic composites [OKA 97], with collagen [NIS 95] – have also been well
studied, but these combinations cannot be considered as real composite materials. It
refers most often to sintered ceramic particles, quite coarse (in order to avoid an
inflammatory reaction) and poorly bonded by the macromolecule. Some injectable
formulations have been developed.
Certain ceramic-ceramic composites have also been studied, particularly in the
fluorohydroxyapatite-alumina and hydroxyapatite-alumina systems [DIM 95;
GAU 95].
12.3. Biological properties
Biological properties of ceramics have been studied only recently and essentially
from a practical point of view with respect to the envisaged application. There is no
possibility at present to predict, from a knowledge of the composition and surfaces
properties of ceramic, their biological behavior (hemocompatibility, adhesion
proliferation and cell expression, bonding with the tissues, etc.) despite some
attempts having been made. We will describe in this section the biological properties
of some ceramics and what we can learn from them. Different levels of interaction
can be discerned: with biological fluids, with tissues and with cells. Furthermore, the
biological behavior gets modified with time, often related to modifications in the
surface of the implanted ceramics or their degradation.
12.3.1. Ceramic-tissue interactions
Ceramic-tissue interaction determines the integration of ceramic in its
environment (biointegration). Several parameters play an important role in this
interaction, and we can distinguish the mechanical anchorage, which ensures the
initial stability of the implant, and the chemical anchorage, which actually
determines the integration of the implant with its host organism.