Nanostructured Titania Coatings for Biological Applications: Fabrication and Characterization - Biological and Biomedical Coatings: Processing and Characterization - page 204

etc.) of natural tissues and may provide the preferred environment for tissue growth (Zhang
and Webster 2007). There is increasing evidence that nanoscaled topography can enhance
the activity of alkaline phosphate (ALP), accelerate hydroxyapatite (HA) nucleation and
growth, and promote osteoblast adhesion and proliferation. Nanoporous structures have
been observed to not only accelerate HA growth and improve adhesion and proliferation of
osteoblasts, but also enhance biological fixation of the implants to the natural tissues (Sun
et al. 2007; Popat et al. 2007a; Yao et al. 2006, 2008; Karlsson et al. 2003; Khang et al. 2008a;
Khang et al. 2008b; Ruckh et al. 2009). These results suggest that if the surface architecture
can be further refined, optimization of the various cell/matrix/substrate interactions and
gene expression on the nanoscale can be accomplished (Nanci et al. 2006).
Biocompatible titania possesses excellent tribological performance and good corrosion
resistance. The abundant Ti-OH groups on the surface can immobilize various functional
substances. Owing to the nanosize effect, a nanostructured titania coating is a desirable
candidate on biological devices. In this chapter, the strategies pertaining to the fabrication
of various TiO 2 nanostructured coatings including nanotube arrays, nanoporous struc-
tures, and nanocrystalline films are reviewed. Structure characterization of the derived
nanopatterns is described and the biological performance and applications of these nano-
sized titania coatings are succinctly discussed. Finally, the future trend of titania coating
is discussed.
Titania
Titania is an n-type semiconductor and the conductivity decreases with O 2 partial pres-
sure at temperature above 600°C. The activation energy for electronic conductivity is
found to be 1.75 eV and the band gap energy is reported to be 3.05 eV. Rutile, anatase, and
brookite constitute the three main phases of TiO 2 . There are three main crystal faces in
the rutile structure in which two of them, (110) and (100), possess quite low energy. The
most thermally stable one is (110) in which an O atom is connected to two Ti atoms. The
Ti atoms are 6-coordinated. (001) is less thermally stable and restructures above 475°C.
There are 5-coordinate Ti atoms parallel to the rows of bridging O (Ramamoorthy et al.
1994). The two low-energy faces in the anatase structure are (101) and (001). The (101) face
is the most prevalent one in anatase nanoscrystals and is corrugated with 5-coordinate Ti
atoms (Burnside et al. 1998). In the brookite phase, the order of stability of the crystal faces
is (010) < (110) < (100) (Beltran et al. 2006; Fujishima et al. 2008). The stability of the various
nanoscaled titania phases is believed to be size-dependent. Rutile is the most stable phase
when the particle size is above 35 nm. Below 11 nm, anatase is more stable. Brookite is
found to be the most stable for particle size ranging from 11 to 35 nm (Zhang and Banfield
2000; Shklover et al. 1997; Fujishima et al. 2008).
Good biocompatibility of titania has been demonstrated in many in vivo and in vitro
experiments (Erli et al. 2006; Li et al. 2004; Khang et al. 2008b). Titania coatings also pos-
sess ultrahigh hardness, excellent tribology behavior, as well as good corrosion resistance
(Liu and Ding 2002; Zhao et al. 2007). TiO 2 is well known for its photocatalytical proper-
ties. The photogenerated charge carriers from reactive oxygen species include O 2 − and OH •
radicals. Matasunaga et al. demonstrate that UV-irradiated platinized TiO 2 powders can
kill bacteria in an aqueous environment (Matsunaga et al. 1985). Titania is now becoming
a desirable candidate as functional coatings and materials on biomedical devices.