Biomedical Engineering Reference
In-Depth Information
Several other techniques, such as replication of polymer foams
by impregnation (Fig. 4.6), dual-phase mixing, particulate leaching,
freeze casting, slip casting, stereo lithography, direct foaming of
suspensions, as well as surfactant washing have been applied to
fabricate porous calcium orthophosphate bioceramics [91, 195, 212,
216, 219, 222-224, 267, 268, 376-378, 397-451]. Some of them have
been summarized in Table 4.1. [408]. Furthermore, natural porous
materials, like coral skeletons made of CaCO
, can be converted into
porous HA under the hydrothermal conditions (250°C, 24-48 h) with
the microstructure undamaged [91-93]. Porous HA bioceramics can
also be obtained by hydrothermal hot pressing. This technique allows
solidification of the HA powder at 100-300°C (30 MPa, 2 h) [415]. In
another approach, bi-continuous water-filled microemulsions have
been used as pre-organized systems for the fabrication of needle-
like frameworks of crystalline HA (2°C, 3 weeks) [416, 417]. Porous
HA bioceramics might be prepared by a combination of gel casting
and foam burn out methods [219]. Lithography was used to print a
polymeric material, followed by packing with HA and sintering [418].
Both hot pressing [267, 268] and ice templating [419] techniques
might be applied as well. Besides, an HA suspension can be cast into
a porous CaCO
3
skeleton, which is then dissolved, leaving a porous
network [410]. Three-dimensional periodic macroporous frame of
HA has been fabricated via a template-assisted colloidal processing
technique [420, 421]. A superporous (~85% porosity) HA ceramics
was developed as well [450, 451]. More to the point, porous HA
bioceramics might be prepared by using different starting HA
powders and sintering at various temperatures by a pressureless-
sintering approach [427].
Porous bioceramics with an improved strength might be
fabricated from calcium orthophosphate fibers or whiskers. In
general, fibrous porous materials are known to exhibit an improved
strength due to fiber interlocking, crack deflection and/or pullout
[422]. Namely, porous bioceramics with well-controlled open pores
was processed by sintering of fibrous HA particles [423]. In another
approach, porosity was achieved by firing apatite-fiber compacts
mixed with carbon beads and agar. By varying the compaction
pressure, firing temperature and carbon/HA ratio, the total porosity
was controlled in the ranges from ~40% to ~85% [412]. Additional
examples are available in literature [397, 401, 408-451].
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