Biomedical Engineering Reference
In-Depth Information
(10-24%), P
2
O
5
(0-8%), B
2
O
3
(0-3%), Al
2
O
3
(0-3%). The main
advantage of CaO and P
2
O
5
is obviously calcium and phosphorus,
which are major constituents of the mineral phase of bone. CaO may
be partially replaced by MgO or CaF
2
, and Na
2
O by K
2
O, with little
changes in bioactivity. Al
2
O
3
and B
2
O
3
may substitute SiO
2
to alter the
glass production process or their surface dissolution rates, but they
have to be added at low content to avoid the inhibition of the
bioactivity [DEA 07]. Despite the development of numerous bioactive
glasses, the original Bioglass® 45S5 has been found to have the best
biological properties with a high bioactive index (12.5) (Table 3.3).
The interface that forms between Bioglass® 45S5 and bone is thick
(200 μm) but possesses a low shear strength. Moreover, the material
has low mechanical strength and toughness, and is therefore used as
coatings in particulate form or in low load-bearing applications, such
as alveolar ridge maintenance or to replace the ossicular chain of the
middle ear [HEN 98].
Bioactive glass-ceramics are polycrystalline materials obtained
through appropriate thermal treatment of glass. They possess a fine
and homogeneous grain size with few or no residual porosity, leading
to improved mechanical properties (good mechanical strength and
toughness). Bioactive glass-ceramics are based on the composition of
Bioglass® with low amounts of alkali oxides and therefore have
intermediate bioactivity index. One of the most bioactive glass-
ceramics used in biomedical applications is Cerabone® A-W
(Table 3.3). This material is constituted of two crystalline phases: a
β-wollastonite (CaO.SiO
2
) phase (34%) and a oxyfluorapatite
(Ca
10
(PO
4
)
6
(O,F
2
)) phase (38%), plus a residual glass phase (28%).
Both crystal phases are homogeneously distributed in the glass matrix
with a grain size of 50-100 nm. The β-wollastonite phase acts as a
reinforcing phase which prevents straight propagation of cracks.
Cerabone® A-W has the highest mechanical strength among all
bioactive glasses and glass-ceramics. Its fracture toughness
(2.0 MPa.m
1/2
) is nearly tripled in comparison with its parent glass
(0.8 MPa.m
1/2
) [WAN 04]. Its moderate bioactivity index (3.2) leads
to a thin bonding interface (10-20 μm) which, however, possesses a
high resistance to shear. Indeed, it is reported that a value of
bioactivity index around 4 leads to an optimal interfacial bonding
