Biomedical Engineering Reference
In-Depth Information
However, glass-ceramics always present a lower surface reactivity than
glasses and consequently a lower bioactive response. the reason is the
decrease in the number of Si-OH groups at the surface and the more difficult
release of calcium ions that are usually entrapped at the crystalline phases.
This decrease can be quantified by using the bioactivity index
I
B
:
137
I
B
= 100/
t
0,5bb
(d
-1
)
Where
t
0,5bb
is the time in days necessary for bonding to bone of 50% of the
bioceramic surface after
in vivo
implantation. Using this definition, the higher
I
B
,
the
higher the bioactivity. this index can be determined only for bioceramics that
have been implanted. experimental
values of
I
B
of bioceramics used clinically
are: 0 for alumina and other almost bioinert ceramics, 2.3 for HA, 3.2 for A/W
glass ceramic and 12.5, the maximum value obtained for a synthetic material,
for Bioglass
®
45S5.
When a large series of new bioceramics are evaluated, their bioactivity can be
initially approximated considering the time required for the HCa formation after
soaking in SBF. thus, glass-ceramics with a very quick bioactive response, that
are able to be coated by HCa after 1 day in SBF were synthesized by thermal
treatment of sol-gel glasses.
42
the mechanical properties of glass-ceramics
are clearly better than those of the parent glasses but still lower than cortical
bone.
138
Before describing the third generation ceramics, the clinical applications of
first and second generation bioceramics are compared in Table 7.6. As observed,
for certain applications bioactive ceramics, such as Ha, compete with almost
bioinert ceramics, such as al
2
O
3
.
7.4 Ceramics in bone regeneration: third
generation ceramics
the main purpose of third generation bioceramics is to obtain porous ceramics
that act as scaffolds for cells and inducting molecules and that are able to
drive self regeneration of tissues. With these requirements, second generation
bioceramics with added porosity are being studied, although the design of new
advanced ceramics is also explored, with added porosity. this porosity should
be in agreement with biological requirements. apatites, shaped as pieces with
interconnected and hierarchical porosity, within the micrometre range, would
be a good starting point for fabricating these scaffolds.
the fabrication of ceramic scaffolds for tissue engineering requires a
conformation method that yields a material with interconnected porosity and
pores in the 2-400 mm range.
139
Therefore, the first step would be to find
methods of conformation that yield monoliths with interconnected porosity in
the micrometre range.
140-142
this should be possible with the second generation
bioceramics discussed previously. nowadays, there are several conformation
