Biomedical Engineering Reference
In-Depth Information
is dispersed through a print head on each layer of powder. Similar to previous methods, this printer
works on a layer-by-layer basis, starting from the bottom and going up. The printer has two different
powder zones: one for building the structure and the other for feeding the powder, so that when the
feeding area goes up, the building area lowers down, moving a distance equivalent to the required layer
thickness. A counter-rotating roller spreads the powder from the feeding bed over to the building bed,
followed by the injection of the binder based on an image corresponding with the slice layer data of the
3D structure. This technique is very fast in the building process and is significantly cheaper than other
techniques (
Woesz, 2008b
). This machine was used indirectly to produce a porous metal scaffold by
printing a sacrificial mold from alumina powder to cast the Co-Cr alloy which is then removed through
several thermal and chemical steps (
Curodeau et al., 2000
). Similarly, the same principle has been
used to create a Ti scaffold and wax template as an alternative sacrificial mold (
Ryan et al., 2008
). This
method of manufacturing provides a precise surface texture. However, the time- consuming process
through the multiple stages of manufacturing is considered to be the main disadvantage of the indirect
3DP technique. This attempt has fascinated and motivated many researchers to replicate this success
through direct printing of the desired metal scaffold, followed by sintering of the green part under a
high temperature vacuum furnace (
Hutmacher et al., 2004; Basalah et al., 2012
). This technique has
fewer stages than indirect printing. With 3DP, several parameters are able to form the microstructure
of the desired part such as powder size, sintering temperature, and duration (
Basalah et al., 2012
). The
layer thickness is primarily chosen based on the particle size and cannot be thinner than the largest
particle in the powder (
Hutmacher et al., 2004
). However, the resolution is lower than the laser-based
techniques because the binder penetrates the powders adjacent to the targeted area (
Woesz, 2008b
).
Difficulties in removing the trapped powder from the green part are the main disadvantage of this tech-
nique (
Hutmacher et al., 2004; Woesz, 2008b
;
Basalah et al., 2012
).
Using the 3DP method and CP Ti as a building material, the authors' group has been able to develop
a set of optimized processing conditions to assure control over a microstructure and the associated
mechanical and physical properties of the structure (
Basalah et al., 2012
). Material powder size is
one of the most influential parameters on changing the strength and density of the structure, as shown
in
Figure 11.3
. Also, the physical appearance of the final product is crucial in the fabrication of the
orthopedic implant, since a high level of shrinkage in the implant is undesirable, and the particle size
influences the shrinkage, as shown in
Figure 11.4
.
As a whole, AM techniques offer a good control over the manufacturing of the external and internal
details of metallic bone substitute scaffolds, with limitations in the creation of complex internal inter-
connected macro pores (
Yarlagadda et al., 2005
).
11.5
BIOCERAMIC BONE SUBSTITUTES
The appropriate materials for constructing bone substitutes for tissue regeneration or augmentation
should adhere to a basic set of criteria stating that: (i) the material must be biocompatible to avoid the
expression of unwanted immune responses, and must be osteoconductive to encourage rapid integration
with surrounding bone, and ideally osteoinductive to promote formation of new bone at the site (
Porter
et al., 2009; Yang et al., 2001
); (ii) the material should be bioresorbable such that it is capable of bulk
degradation and resorption through natural pathways (
Hutmacher, 2000; Porter et al., 2009; Bohner
et al., 2012
); and (iii) the material should provide the physical support, starting from the seeding process
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