Biomedical Engineering Reference
In-Depth Information
mechanism of bone-like apatite on the surface of the hydrogen-implanted silicon wafer, two com-
parative experiments have been conducted. One experiment is to investigate the bioactivity of
hydrogenated silicon wafer with no surface damage, and the other one is to evaluate the bioactivity
of argon-implanted silicon wafer, which possesses an amorphous surface but no hydrogen. After
the hydrogenated silicon wafer and argon-implanted wafer are soaked in SBF for 28 days, no apatite
particles can be found on either surface, indicating poor bioactivity on both the samples. The results
suggest that only the formation of an amorphous hydrogenated silicon (a-Si:H
x
) surface can improve
the bioactivity of silicon wafer and can result in the formation of bone-like apatite on its surface
after treatment in SBF. Experimental evidence has so far suggested that the formation of apatite
requires the surface to be both amorphous and hydrogenated.
Hydrogen is known to interact with silicon in a wide variety of ways, including passivating the
surface, deactivating dopants, and passivating shallow as well as deep levels [50]. In amorphous
and polycrystalline silicon, hydrogen passivates dangling bonds by forming Si
H bonds. In our
hydrogen PIII sample, many dangling bonds are produced [49], and the surface of the silicon wafer
exhibits an amorphous network with disorders and defects [51]. In silicon, it is estimated that each
H ion implanted at 30-100 keV produces approximately 10 Frenkel pairs. These defects provide
many Si dangling bonds. The implantation-induced or preexisting dangling bonds can interact
immediately with the implanted hydrogen to form Si
-
H bonds [52].
When the hydrogen-implanted silicon wafer is soaked in the SBF solution, the following reac-
tions are believed to occur. The
-
≡
Si
-
H structure is fi rst hydrated to form silanol (
≡
Si
-
OH) by
the following reactions:
H
2
O
≡
Si
-
H
+
→
≡
Si
-
OH
+
H
2
(19.1)
OH) reacts with the hydroxyl ion to produce a negatively charged
surface with the functional group (
Afterward, the silanol (
≡
Si
-
O
-
) as follows:
≡
Si
-
OH
-
O
-
+
≡
Si
-
OH
+
→
≡
Si
-
H
2
O
(19.2)
H
bonds or Si dangling bonds in the subsurface of the hydrogen-implanted silicon wafer to form an
amorphous hydrated silicon layer. Therefore, after immersion in SBF, a negatively charged, amor-
phous, and hydrated silicon surface is formed.
The formation of a negatively charged surface on bioceramics and bioglasses is generally
regarded to be important in the precipitation of apatite [53-55]. Due to the formation of the nega-
tively charged surface, the calcium ions in the SBF solution are attracted to the negatively charged
surface site of the silicon wafer. This action is followed by the arrival of HPO
2
-
resulting in the for-
mation of a hydrated precursor cluster consisting of calcium hydrogen phosphate. After the precur-
sor clusters are formed, they spontaneously grow by consuming calcium and phosphate ions from
the surrounding body fl uid. The calcium phosphate phase that accumulates on the surface of the
silicon wafer is initially amorphous. It later crystallizes to a carbonate-containing HA structure by
incorporating carbonate anions from the solution within the amorphous calcium phosphate phase.
At the same time, some water molecules may diffuse through the surface to react with the Si
-
19.3.1.2 Improvement of Bioactivity on Nano-TiO
2
Coatings
Plasma-sprayed TiO
2
as bonding or composite coatings on Ti alloys has recently shown promis-
ing
in vivo
corrosion behavior as it acts as a chemical barrier against release of metal ions from
the implants [56,57] in addition to its excellent biocompatibility [58], but its poor bioactivity has
limited its application as the coating on hard tissue replacements. Nanosized surface topography
may give biomedical implants special and favorable properties in a biological environments. Webster
et al. [59-61] revealed that nanophase ceramics could promote osteointegration that is critical to the
