MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 380

11.4.2 Bioactive Glasses
Glasses, as a subclass of ceramics, constitute the most important group of non-
crystalline solids. Glasses are amorphous materials that maintain a disordered
atomic structure, characteristic of liquids of a very high viscosity, which behave
like solids. The most common and technologically the most important group of
glasses are the silicate glasses. Silicate glasses are made up of a network of tetra-
hedra of four large oxygen ions with a silicon ion at the centre. Each oxygen is
shared by two tetrahedra, giving the bulk composition of SiO 2 . However, the basic
silicate network can incorporate virtually all atoms of the periodic table of ele-
ments, and thus silicate glasses of numerous different compositions and proper-
ties can be obtained.
Special compositions of glasses containing the same compounds present
in bones and tissue fl uids (Na 2 O, P 2 O 5 , CaO and SiO 2 ) are bioactive, which
means that bioactive glasses have the ability to react chemically with living
tissues, forming with them mechanically strong and lasting bonds [Hench, 1993;
Yamamuro, 1990; Jones, 2001; Ducheyne, 1998]. Bioactive silica-based glasses are
structurally based on tetrahedral units SiO 4− , where the central silicon atom with
the external electronic confi guration 3 s 2 3 p 2 assumes a tetrahedral hybrid state sp 3
and contributes one electron to each bond. Consequently, two cases can occur;
either:

1. each oxygen atom with electronic confi guration 122 2 2 ssp p p

x

22 2 1 1

z uses y

covalent bonds with two neighbour sili-
con atoms (“bridging oxygen,” BO), or
2. each oxygen uses one unpaired electron in a

their two unpaired electrons in σ

covalent bond with the
neighbour silicon atom, the other unpaired electron being available to
ionic interaction with alkaline or alkaline-earth metals, the so-called net-
work modifi ers ( Na + , K + , Ca 2+ , Mg 2+ , etc.), forming “ non - bridging ” oxygen
(NBO) bonds [Galeener, 1979 ; Gaskell, 1991 ].

σ

The presence of these cations results in a disruption of the continuity of the
glass network. For this reason, NBOs play a key role in the bioactive response of
these glasses [Serra, 2002]. After implantation, the surfaces of these materials in
contact with body fl uids give rise to the formation of a silica hydrogel layer that
leads to subsequent crystallisation of the apatite-like phase. Hench et al. [Hench,
1972, 1993], proposed a theoretical model to explain the interfacial bonding
mechanism of bioactive glasses in inorganic fl uid environment.
The bioactivity process can be summarized in fi ve steps:

1. rapid exchange of alkali or alkali - earth ions with H + or H 3 O + from the
solution;
2. loss of soluble silica in the form of Si(OH) 4 to the solution;
3. condensation and repolymerization of SiO 2 -rich layer on the surface de-
pleted in alkalis and alkaline-earth cations;