Biomedical Engineering Reference
In-Depth Information
Primary particle
Pore among agglomerates
Pore among primary particles
Agglomerate of
primary particles
FIGURE 1.3
Schematic illustration of pores among agglomerates and particles [159].
TABLE 1.9
Methods for Obtaining Ceramic Bodies for 3-D Porous Ceramics [159]
Dry processes
Loose packing
Compaction
Uniaxial pressing
Cold isostatic pressing (CIPing)
Wet processes
Slip casting
Injection molding
Phase separation/freeze-drying
Polymer replication
Gelcasting
slurry, such as sucrose, gelatine, and PMMA microbeads, and a wetting agent (i.e., a surfactant)
can increase porosity. These chemicals, which are called porogens, are evaporated or burned out
during sintering, and as a result pores are formed [2,159]. One successful formulation has been the
use of hydroxyapatite powder slurries (dispersed with vegetable oil) added with gelatine solution
[162], which has led to porous scaffolds with interconnected pore structure with pore diameters of
∼
100 µm. A similar process has been used to prepare melt-derived Bioglass scaffolds using cam-
phor (C
10
H
16
O) as the porogen [163].
Binders are also added to slurries. The most important function of a binder is to improve the
strength of the green body in order to provide structural integrity for handling (green strength)
before the product is sintered [164]. Polysaccharides [165], polyvinyl alcohol (PVA) [166], and poly-
vinyl butyl (PVB) [167] are the frequently added binders in bioceramic slurries.
1.4.1.1.2
Formation of Green Bodies
In ceramic production, a green body is always porous, and its structure largely determines that of
the sintered product. Table 1.9 lists different methods of obtaining green bodies for 3-D porous
ceramics. These methods can be classifi ed into two categories: dry and wet processes [159].
They lead to different porous structures and pore volume fractions. Certain techniques, such as
tape casting, extrusion, slurry dipping, and spraying, are not included here; because they aim at
achieving a predetermined geometric shape of ceramic parts (such as rods, tubes, sheets, and
coating on fi lms), instead of a given porous structure. Except injection molding, all conventional
processes listed in Table 1.9 have been applied to synthesize ceramic scaffolds for tissue engi-
neering as discussed below.
