NANOMATERIALS FOR IMPROVED ORTHOPEDIC AND BONE TISSUE ENGINEERING APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 216

technology, a nanostructured diamond fi lm with high hardness, enhanced tough-
ness and good adhesion to alloys can be obtained. Catledge et al. have demon-
strated that nanostructured diamond fi lms can be tailored on metallic surfaces
with hardness values ranging from 10 GPa to 100 GPa by changing the feed
gas (N 2 /CH 4 ) ratio [73]. The above excellent properties of nanostructured
diamond make it promising as a coating for load-bearing articulating surfaces
[54] in orthopedic implants.
In summary, nanoceramics can offer signifi cantly improved mechanical prop-
erties, good wear properties, as well as excellent biocompatibility compared to
conventional ceramics. Although further investigations of nanoceramics are
needed to justify and apply them into real orthopedic applications, nanoceramics
are clearly promising future orthopedic materials.
7.3.2 Nanostructured Metals
Artifi cial joints or prostheses have employed a variety of metallic components
because of their durability, physical strength and physiological inertness. For ex-
ample, titanium, Ti alloys (such as Ti6Al4V), metal alloys (such as CoCrMo) and
stainless steel are commonly used in orthopedics. Most metallic components such
as hip ball and sockets are made of stainless steels or titanium, which allows bone
ingrowth. Commercially pure titanium and Ti6Al4V alloys are the two of the
most common titanium-based implant biomaterials [77].
Because of the fact that titanium is not ferromagnetic, titanium implants can
be safely examined with magnetic resonance imaging, which makes them espe-
cially useful for long-term implants. Moreover, titanium develops an oxide layer
in contact with water or air, resulting in high biocompatibility. Thus, titanium and
titanium alloys are considered the best choice for manufacturing permanent non-
biodegradable implants [77,78]. Importantly, the modulus of elasticity of titanium
can be exploited to closely match the modulus of the bone [79]. However, the
stiffness of titanium alloys is still more than twice that of bone.
The micro or nanoscale surface properties of metals, just like ceramics, such as
surface composition and topography, can affect bone formation. Coatings and/or
surface modifi cations are interrelated with topography and surface energy. For this
reason, it is very diffi cult to determine how these affect—as an independent factor—
the fi nal bone formation result [77]. The topographical features obtained on the
metal implant surface can range from nanometers to millimeters, which are far
below the size of osteoblasts but have been shown to promote bone formation [80].
Numerous treatment processes, including machining or micromachining, par-
ticle blasting, Ti plasma spraying, HA plasma spraying, chemical or electrochemi-
cal etching, and anodization are available to modify Ti surface topography. In
addition, anodizing titanium under different conditions can change the nature of
the oxide layer (thickness, porosity and crystallinity) on the nanoscale to strongly
affect in vivo bone formation [77].
The topography of an implant surface can be defi ned in terms of form, wavi-
ness and roughness (Figure 7.5), with the waviness and roughness often presented