Biomedical Engineering Reference
In-Depth Information
as opposed to CoCr alloys (1610
ÿ6
/ëC) (Sun et al., 2001). As a result,
titanium alloys are more suitable for the HA coating method.
Nano-HA coatings have received a lot of attention recently due to its
biomimicking nano-size and enhancement in long-term bone growth compared
with conventional micron-size HA coatings by plasma spraying. This has been
achieved by room-temperature deposition techniques such as electrophoretic
deposition (Cornia et al., 2008). It has been claimed that such nano-HA coatings
have higher bond strength (60MPa compared with 30MPa of plasma sprayed
HA coating), higher density and crystallinity (100% crystallinity, thus less
degradation and fewer wear problems) to significantly improve the lifetime of
HA-coated metal hip prostheses (Zhang et al., 2007).
2.4.4 Novel hip implant surface modification techniques
The aforementioned porous metals and bioactive coatings are two major
research directions to improve current titanium and cobalt±chromium alloy hip
prostheses. Besides, novel surface modification techniques such as anodization
provide a great opportunity to take advantages of nanostructured metal surfaces
in order to promote new bone growth (Yao and Webster, 2006).
Anodization to promote tissue integration
Anodization or anodic oxidation is a well-established surface modification
technique for valve metals that produce protective layers. During anodization, a
constant voltage or current is applied between the anode and cathode, electrode
reactions (oxidation and reduction) in combination with field-driven ion
diffusion lead to the formation of an oxide layer on the anode surface.
The anodization technique was discovered in the early 1930s and was widely
studied in the 1960s to enhance titanium implant osseointegration. These pro-
cesses usually adopted high voltage anodization (called ASD) of titanium in
electrolyte solutions whose ions would be embedded into the oxide coating,
resulting in a microporous structure. For electrolytes containing Ca and P, such as
calcium glycerophosphate (Ca-GP) and calcium acetate (CA), both Ca and P
were contained in the oxide layer with a Ca/P ratio closer to HA (1.67). After an
additional hydrothermal treatment (e.g., high pressure steaming), HA crystals
were randomly precipitated on the anodic oxide film. This could be an alternative
way to create HA coatings on titanium implants as opposed to plasma spraying.
Recently, research efforts have focused on creating biologically inspired
nanometer surface structures on titanium implants using specific anodization
parameters. Studies have shown that nanoporous structures can be created by
anodizing titanium in chromic acid at 10±40 V (Baun, 1980). Another unique
surface morphology obtained through titanium anodization is self-ordered nano-
tubular structures (Fig. 2.2a). For these studies, fluorine electrolyte solutions are
