Biomedical Engineering Reference
In-Depth Information
shape of pores was found to transform from strongly oblate to
round at higher sintering temperatures [460]. The total porosity
(macroporosity + microporosity) of calcium orthophosphate
bioceramics was reported to be ~70% [461] or ~85% [450, 451]
of the bioceramic volume. In the case of coralline HA or bovine-
derived apatites, the porosity of the original biologic material
(coral or bovine bone) is usually preserved during processing [94].
To conclude this topic, creation of the desired porosity in calcium
orthophosphate bioceramics is a rather complicated engineering
task and the interested readers are referred to other publications on
this subject [112, 377, 402, 416, 462-489].
Figure 4.8
SEM pictures of HA bioceramics sintered at (A) 1050°C and (B)
1200°C. Note the presence of microporosity in A and not in B.
Reprinted from Ref. [459] with permission.
Studies revealed that increasing of both the specific surface
area and pore volume of bioceramics might greatly accelerate the
in vivo
process of apatite deposition and, therefore, enhance the
bone-forming bioactivity. More importantly, a precise control over
the porosity, pore dimensions and internal pore architecture of
bioceramics on different length scales is essential for understanding
of the structure-bioactivity relationship and the rational design
of better bone-forming biomaterials [483, 490, 491]. Namely,
in antibiotic charging experiments, a calcium orthophosphate
bioceramics with nanodimensional (<100 nm) pores showed a much
higher charging capacity (1621 μg/g) than that of commercially
available calcium orthophosphate (100 μg/g), which did not contain
nanodimensional porosity [478]. In other experiments, porous
blocks of HA were found to be viable carriers with sustained release
profiles for drugs [492] and antibiotics over 12 days [493] and 12
weeks [494], respectively. Unfortunately, porosity significantly
decreases the strength of implants [233, 307, 325]. Thus, porous
calcium orthophosphate implants cannot be loaded and are used
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