Biomedical Engineering Reference
In-Depth Information
transition, crystallisation and bioactivity. In other words, it is quite a
challenge to design a glass composition that can be sintered without
crystallising but also remains bioactive. One line of thought is that
the network connectivity should be approximately 2 (like Bioglass)
for a glass to be bioactive (Chapter 2). However, glasses with network
connectivity of 2 have the problem of easy crystallisation. Increasing
the silica content reduces the tendency of a glass to crystallise, but
this reduces the degradation rate and bioactivity. We therefore need
to 'trick' the glass network by incorporating a variety of network
modifiers, substituting for calcium and sodium, to keep network
connectivity constant. The variety of modifiers makes crystallisation
energetically unfavourable, as the structure is more difficult to organise.
New compositions have been designed not to crystallise on sinter-
ing. One is 13-93 (54.6mol% SiO
2
,6mol%Na
2
O, 22.1mol% CaO,
1.7mol% P
2
O
5
, 7.9mol%K
2
O, 7.7mol%MgO), which was developed
in Finland by Brink and co-workers. This glass takes seven days to form
a hydroxycarbonate apatite layer in simulated body fluid tests (Bioglass
particles formed the same layer within 8 hours). This is because the
network connectivity is higher in glass composition 13-93 compared to
45S5 Bioglass owing to the increased silica content.
In order to obtain a similar result without compromising bioactiv-
ity, ICIE16 (49.46mol% SiO
2
, 36.27mol% CaO, 6.6mol% Na
2
O,
1.07mol% P
2
O
5
and 6.6mol% K
2
O) was developed by Elgayar and
co-workers.
12.5 MAKING POROUS GLASSES
Sintering alone cannot make pores large enough to create a pore network
that can encourage vascularised bone growth. Pores that are left behind
(Figure 12.3) are considered defects and sources of weakness (crack
nucleation sites and sites of stress concentration). The aim is to create
large pores with diameters in excess of 500
μ
m with interconnects
greater than 100
m, while having highly sintered struts that provide as
much strength as possible. The aim is to mimic the porous structure of
cancellous bone (Figure 12.2).
μ
12.5.1 Space Holder Method
The most common method for making porous ceramics is to take a
particulate and pack the particles around a sacrificial template of some
