Biomedical Engineering Reference
In-Depth Information
Dry methods . The simplest way to prepare ceramic green bodies is the dry powder method
where powders are directly compressed by pressing (uniaxially or isostatically) into molds, thereby
forming green bodies. Pore diameters decrease and mechanical properties increase as the packing
density of the particles in the green bodies increases. A densifi cation step by sintering at high tem-
perature is required (see Section 1.4.1.1.3). Mechanical properties can be increased further by hot-
isostatic pressing (HIP) [168] or by uniaxial hot pressing. These pressure-assisted methods decrease
the pore diameter as well. The addition of porogens, such as sucrose and camphor, enhances the
formation of pores [159].
Slip casting . Slip is a creamy (relatively thick) slurry. In this method, the slurry is cast into a
porous mold. The liquid of the slurry is absorbed into the porous mold, and as a result the particles
in the slurry are fi ltered, which adhere to the mold surface. After this process, a porous green body
is obtained through further drying [161,169].
Phase separation/freeze-drying. In this method, a ceramic slurry is poured into a container,
which is immersed in a freezing bath. Thus, ice is stimulated to grow and ceramic particles are piled
up between the columns of the growing ice. After the slurry is completely frozen, the container is
dried in a drying vessel, usually under vacuum [170]. The pores are created by the ice crystals that
sublimate at a reduced pressure. Freeze-drying removal of ice crystals creates 3-D interconnected
pore channels with complex structures. The porous structure can be customized by the variation of
the slurry concentration, freezing temperature, and pressure.
Replication technique . This method, which is also called the polymer-sponge method, was
patented for the manufacturing of ceramic foams [171]. In the polymer-replication process, the
green bodies of ceramic foams are prepared by coating a polymer (e.g., polyurethane) foam
with a ceramic slurry. The polymer foam, which already has the desired macrostructure, simply
serves as a sacrifi cial template for the ceramic coating. The polymer template is immersed in
the slurry, which subsequently infi ltrates the structure, and so the ceramic particles adhere to
the surface of the polymer substrate. Excess slurry is squeezed out leaving a ceramic coating on
the foam struts. After it is dried, the polymer is slowly burned out in order to minimize dam-
age to the porous ceramic coating. After the removal of the polymer, the ceramic is sintered to
the desired density. The process replicates the macroporous structure of the polymer foam and
results in a rather distinctive microstructure within the struts. A fl owchart of the process is given
in Figure 1.4 [172]. This method has been applied for the preparation of foam-like scaffolds for
tissue engineering, including porous calcium phosphates [173], Bioglass [80], and other inert
bioceramics [172,174].
Ceramic powder
Add
Prepare slurry from the powder
Binder
Coat a polymer foam with the slurry
Dry, burn out the polymer substrate,
and sinter the green body
Ceramic foam
FIGURE 1.4
Flowchart of the replication process to produce a ceramic foam.
 
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