Biomedical Engineering Reference
In-Depth Information
densities from ~0.35 to ~1.44 g cm
-3
, and the natural apatite precursor is not
converted to pure HAp, but retains many of the ionic substituents found
in bone mineral, notably carbonate, sodium, and magnesium ions. Such a
material, porous, inorganic, and bioactive (containing some biologically
essential trace minerals), should be highly compatible with body tissue, and
when used as bone implants may become essentially integral with the bone.
Moreover, the struts of the HAp material is not fully dense but have retained
some traces of the network of osteocyte lacunae. Macrostructural analysis
can demonstrate the complex interrelationship between the structural fea-
tures of an open pore structure. It is also noted that the pore connectivity
and mechanical strength are sensitive to macrostructural anisotropy and
apparent density (Hing et al. 1999). Tancred et al. reported another nega-
tive-negative replica method that can also be used to fabricate the bonelike
HAp porous bioceramics. Cancellous bone is used to create a negative rep-
lica; afterward acid is used to remove the bone and a HAp negative replica of
wax mold is formed (Tancred et al. 1998).
5.2.2 Volatile Additives to Create Macropore Bioceramics
Macroporosity is usually formed due to the release of volatile materials and,
therefore, the incorporation of pore creating additives is the most popular
technique to create macropore bioceramics. The conventional porogens such
as paraffin spheres, naphthalene, hydrogen peroxide, and polyvinyl butyral
have been widely used. These additives are mixed to CaP powders or slur-
ries. After molding, the organics burn away from the molding body dur-
ing sintering. This approach allows direct control of the pore characteristics,
which are a function of the amount and properties of the volatile phase.
However, porous CaP ceramics processed by high-temperature treatment
present a significant reduction of bioreactivity and growth kinetics of new
bone due to the lack of resorbability. The low surface reactivity influence the
osteogenic cell activation and osteoconduction (Dorozhkin 2010).
The conventional CaP bioceramics are strong but brittle, and thereafter
are not suitable for most load-bearing applications. Bioceramics fail by sud-
den catastrophic fracture, which is not a trait that engineers find endearing.
These materials behave in this way because they absorb very little energy
during fracture. These difficulties could be alleviated if an energy-absorb-
ing mechanism was built into the microstructure of materials to increase
toughness. Previous reports of the extraordinary toughness and strength of
mollusk shells have inspired much research because the shells, consisting
of 99% CC, are hundreds of times tougher than simple polycrystalline lime-
stone. The tough structure of shellfish nacre has been replicated in a ceramic
by a simple water-freezing method (Deville, Saiz, Nalla, et al. 2006). Before
that, the potential of freeze-casting as a means to create ceramics with a con-
trolled and complex cellular architecture has started to attract research atten-
tion (Fukasawa et al. 2001), and the first reports on freeze-cast biomedical
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