Biomedical Engineering Reference
In-Depth Information
2.4 Ti surface analysis by X-ray photoelectron spectroscopy (XPS)
Traditionally, Ti and its alloys have been reported to be bioinert. When embedded in the
human body, a fibrous tissue encapsulates the implant isolating it from the surrounding
bone forms. Since conventional metals as biomaterials are usually covered with metal
oxides, surface oxide films on metals play an important role not only against corrosion but
also regarding tissue compatibility. The composition of surface oxide film varies according
to environmental changes, although the film is macroscopically stable. Passive surfaces co-
exist in close contact with electrolytes, undergoing a continuous process of partial
dissolution and re-precipitation from the microscopic viewpoint [Kelly, 1982]. In this sense,
the surface composition constantly changes according to the environment. The film on
titanium consists of amorphous or low-crystalline and non-stoichiometric TiO
2
[Kelly, 1982].
The surface oxide film of titanium just after polishing in water contains not only Ti
4+
but
also Ti
3+
and Ti
2+
[Beck, 1973; Kelly, 1982]. Hydrated phosphate ions are adsorbed by a
hydrated titanium oxide surface during the release of protons [Hanawa, 1992]. Calcium ions
are adsorbed by phosphate ions adsorbing on a titanium surface, and, eventually, calcium
phosphate is formed. The above mentioned phenomena are characteristic in titanium and
titanium alloys [Hanawa, 1992]. In this regard, an anatase-like structure is effective for
apatite nucleation [Wei et al., 2002ab], whereas the naturally formed oxide film on titanium
surface is mainly amorphous. The ability of titanium in order to form calcium phosphate on
itself is one of the reasons for its better hard-tissue compatibility than those of other metals.
This property is applied to the surface modification of titanium and its alloys to improve
hard-tissue compatibility. In the case of alkali-heat-treated titanium, calcium and phosphate
are orderly deposited, and calcium deposition is the pre-requisite for phosphate deposition
[Yang et al., 1999].
It is easily expected that the bonding properties of the HA/Ti interface prepared by the
HHP method can be depended on the Ti surface conditions. As preliminary experimental
results, Ti surface finished in wet environment can be achieved bondong to HA ceramics
through the above mentioned HHP method. However, Ti surfaces before the HHP
processing have not been investigated precisely. The present study aims to investigate Ti
surface properties through X-ray photoelectron spectroscopy (XPS) analysis. Particular
attention was been paid to chemical composition and oxidation states of Ti in surface films
in order to explain the bonding mechanism of Ti and HA ceramics by the HHP.
A commercially available pure Ti rod was used. The Ti surfaces were finished using 1500#
emery paper in
air
and
water
conditions. After the emery paper finish, the Ti disks were
cleaned in deionized water and ethanol by using an ultrasonic cleaner, and then dried in air.
XPS was performed with an electron spectrometer (ULVAC-PHI, ESCA1600). All binding
energies given in this paper are relative to the Fermi level, and all spectra were excited with
the monochromatized Al K line (1486.61 eV). The spectrometer was calibrated against Au
4f
7/2
(binding energy, 84.07 eV) and Au 4f
5/2
(87.74 eV) of pure gold and Cu 2p
3/2
(932.53
eV), Cu 2p
1/2
(952.35 eV), and Cu Auger L
3
M
4,5
M
4,5
line (kinetic energy, 918.65 eV) of pure
copper. The energy values were based on published data [Asami & Hashimoto, 1977]. The
reproducibility of the results was confirmed several times under the same conditions.
In order to clarify the surface related chemical characteristics of the Ti, XPS analysis was
performed for the specimens as-polished mechanically in
air
or
water
environments. The
XPS spectra of the specimens over a wide binding energy region exhibited peaks of Ti 2p, O
