Nanostructured Titania Coatings for Biological Applications: Fabrication and Characterization - Biological and Biomedical Coatings: Processing and Characterization - page 216

500-nm-long crystalline nanotubes can also induce growth of HA rapidly. Both the Ti-OH
and crystallography influence the bioactivity of titania nanotubes (Tsuchiya et al. 2006).
Kunze et al. have uncovered more details about the precipitation process of HA on tita-
nia nanotube arrays. During the early stages of apatite nucleation, more Ca and P are
found on the nanotubes than on flat TiO 2 . Initial nucleation proceeds better on unannealed
substrates than that on the annealed one on account of a higher degree of hydration. The
crystallography does not appear to play a major role in the initial stage of apatite forma-
tion and it is believed that there are similar nucleation mechanisms for crystalline and
on amorphous nanotubes. At the later stage of precipitation, the crystallography of the
substrate plays an important role in homogeneous apatite formation as stable growth can
only proceed on a crystalline substrate surface. It has also been found that titania nano-
tubes promote formation of very thick apatite films, whereas thin but incompletely closed
precipitated layers form on the compact TiO 2 surface (Julia et al. 2008).
It has been found that titania nanotubes enhance apatite deposition from simulated
body fluids (SBFs) compared to compact TiO 2 layer. A possible reason is the higher nucle-
ation rates on titania nanotubes than on a flat surface. This mainly results from the higher
specific surface area that provides a larger amount of OH − on the surface. The surface area
in a titania nanotube 1.66 μm long, 100 nm in diameter, and 20 nm thick (wall thickness) is
about 45 times larger than that of a flat surface. The TiO 2 /solution interface has positively
polarized titanium and negatively polarized oxygen sites. In body fluids, OH − can adsorb
onto the TiO 2 surface and Ti-OH forms at the solid-liquid interface. The Ti-OH groups are
acidic or basic depending on the pH of the surrounding solution. The isoelectric point at
which the surface shows zero charge corresponds to a pH of about 5.4 for titanium oxide.
In a physiological pH of about 7.4, the surface is slightly negatively charged due to the pres-
ence of deprotonated acidic hydroxides. This negatively charged surface attract cations
such as Ca 2+ followed by the arrival of HPO 4 2− or H 2 PO 4 − , resulting in nucleation of HA.
The HA will grow by consuming Ca and P from the SBFs. In addition, faster and better
nucleation on the nanotubes may stem from the fact that the ions from the SBFs have bet-
ter access to the nanotubular surface because they can diffuse into the channels and form
nuclei uniformly over the walls.
Drug Delivery
Generally, orally supplemented or injection of drugs cannot effectively reach the implant-
tissue interface, particularly necrotic or vascular tissues left after surgery. This limitation
cannot be overcome by increasing the doses because of organ toxicity associated with high
quantities of certain drugs. On the other hand, oral administration leads to increased drug
concentration in the blood plasma immediately after intake but it subsequently decreases
resembling a sinusoidal behavior as a function of time (Popat et al. 2007b). A desirable
method is to effectively load the drug on the surface of implants to achieve sustained and
controlled release of the drug so that the released drug can be effectively absorbed by the
tissues in the vicinity directly.
The high specific area and hollow tubular structure of nanotubes are desirable attributes
in the drug loading and delivery platform. In particular, the TiO 2 surface has abundant
hydroxyl groups that can enable immobilization of many functional substances. Stable
fixation depends largely on the interfacial integrity between the bone tissue and implant.
A promising strategy to elicit specific cellular response is to provide cell-specific signals
to control and improve the osseointegration of implants. RGD peptides, growth factor
including transforming growth factor-β (TGF-β), bone morphogenetic proteins (BMPs),