Biomedical Engineering Reference
In-Depth Information
7.3.1 Nanostructured Ceramics
Ceramics are non-metallic inorganic materials which have excellent cytocompat-
ibility and possibly biodegradability properties in the physiological environment
that makes them attractive for orthopedic applications. For these reasons, they
have been widely adopted as orthopedic implants (known as bioceramics for
several decades). According to the tissue response in an osseous environment,
bioceramics can be classifi ed in three groups: bioactive ceramics (such as
hydroxyapatite (HA), tricalcium phosphate (TCP), bioglasses, HA/TCP bi-phase
ceramics, and glass-ceramics); biopassive ceramics (such as alumina, titania
and zirconia); and biodegradable ceramics (such as TCP).
Although traditional bioceramics have long served as bone substitutes and
fi ller materials, structural forms, and surface coatings in orthopedic applications
[43], there often exists a variety of implant failures due to insuffi cient osseointe-
gration, osteolysis, as well as implant wear (see section 7.2.2). There are many
reasons to use nanophase ceramics to overcome these traditional implant failures.
As we know, 70% of the human bone matrix is composed of inorganic crystalline
hydroxyapatite which is typically 20 - 80 nm long and 2 - 5 nm thick [18] . Addition-
ally, other components in the bone matrix (such as collagen and noncollagenous
proteins (laminin, fi bronectin, vitronectin)) are nanometer-scale in dimension [9].
Therefore, novel nanophase ceramics (grain sizes less than 100 nm in diameter)
which biomimic the nanostructure of natural bone have become quite popular in
orthopedics. Present researchers have shown that nanostructured alumina, tita-
nia, ZnO and HA can greatly enhance osteoblast adhesion and promote calcium/
phosphate mineral deposition [14-16,44-48]. It is believed that the special surface
topography, increased wettability and better mechanical properties of nanoc-
eramics may contribute to enhanced osteoblast functions. Obviously nanostruc-
tured ceramics may become a new generation of more promising and effi cient
orthopedic material. In the following section, the advantages of various nano-
structured ceramics compared to conventional ceramics will be discussed.
7.3.1.1 Special Surface Properties of Nanophase Ceramics for Im-
proved Orthopedic Implants.
With decreased grain size as well as decreased
pore diameter, nanophase ceramics have increased surface area, surface rough-
ness (shown in Figure 7.3) and number of grain boundaries at the surface.
For example, from the results of extensive characterization studies (Table
7.1), increased surface roughness and improved surface wettability (decreased
contact angles) are evident in nanophase compared to conventional alumina, HA
and titania [16]. In the case of ZnO, atomic force microscopy (AFM) root-mean-
square roughness values of nanophase and microphase ZnO were 32 and 10 nm,
respectively [57]. Additionally, a 23 nm grain size alumina had approximately
50% more surface area for cell adhesion than that of a 177 nm grain size alumina;
similarly, a 32 nm grain size titania had nearly 35% more surface area than that of
a 2.12
m grain size titania [14] while nanophase ZnO had 25% more surface area
than that of microphase ZnO [57].
μ
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