Biomedical Engineering Reference
In-Depth Information
the HA crystals but is made up of a continuous phase, evidenced by
verygoodstrengthoftheboneaftercompleteremovaloftheorganic
phase.
12
Due to this unique hierarchical structure of human bones with
a macroporous structure and nano-scale organization, ideal bone
implant materials must possess the hierarchical structure. This
impliesthatthescaffoldsshouldhavenotonlysu
cient3Dinternal
space and interconnective open channels that allow for mass trans-
port, cell migration, attachment, and proliferation, as well as tissue
ingrowth, but also 1D nanophase materials on the surface in order
to match the lowestorganization of human bones.
Becauseofexcellentmechanicalpropertiesandbiocompatibility,
Ti-based biometals, such as Ti, Ti6Al4V, and nickel-titanium (NiTi),
are used widely in various orthopedic applications. We have pro-
duced 3D porous NiTi and Ti scaffolds using capsule-free hot iso-
staticpressing(CF-HIP)powdermetallurgy(PM),andtheprocessis
described here.
3.2 Fabrication and Characteristics of
Macroporous Ti-Based Alloys
CF-HIP is a proven PM process to fabricate porous materials. With
the use of a space holder, such as ammonia hydrogen carbonate
(NH
4
HCO
3
),porousTi-basedmetalscaffoldswithadjustableporous
structures,suchasTiandNiTi,havebeenproducedsuccessfully.
15
,
16
Fig. 3.4 shows the general fabrication process of Ti-based metal
scaffolds using CF-HIP. In this process, metallic elemental powders
including titanium are the starting materials with the ratios fol-
lowing the chemical compositions of the given Ti-based metals.
They are mixed with a certain proportion of space holder powders
(NH
4
HCO
3
) and pressed hydraulically under cold pressure to pro-
ducegreencompacts.Thesegreencompactsarepreheatedat200
◦
C
to remove the space holders to acquire more initial internal pores,
andtheseporesarefilledwithargongaswhentheHIPunitisevacu-
atedandbackfilledwithargongasathighpressure.Afterward,these
compacts are sintered at high temperatures in two steps, an initial
shrinkage stage and afinal expansionstage.
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