Biomedical Engineering Reference
In-Depth Information
long tubes with a diameter of about 90 nm. The grafted OPDA monolayer after the first
step of anodization generates a hydrophobic top on the tube. After the second anodization
step, a new hydrophilic surface is created. Four loading methods have been developed.
Unmodified nanotubes are loaded by simple immersion in the drug system to allow free
and physisorbed drug molecules inside the tube. In the second approach, capped tubes
are exposed to the drug solution with DMSO, which acts as a surfactant. This approach
leads to free HRP molecules in the lower part of the tubes. After evaporation of DMSO,
the drugs are trapped by the hydrophobic cap. The drugs are grafted onto the hydrophilic
tube walls by an APTES/vitamin C monolayer linker. The fourth approach combines
approaches 2 and 3 resulting in loading of the drugs to the lower part of the nanotubes.
Different release curves in the phosphate buffering solution (PBS) are obtained. Quick and
uncontrolled release is observed in nanotubes loading by approach 1. Almost 90% of the
drug is released during the first 1 min. With regard to the OPDA-capped nanotube and
surface-linked drug nanotube, the release rate can be adjusted by UV irradiation. Strong
UV irradiation yields a faster release rate. The amphiphilic nanotube layer with OPDA and
covalently linked drugs allows controllable release.
Mg, Zn, and Sr are important biological elements. By a simple hydrothermal treatment,
titania nanotubes can be transformed into crystalline SrTiO 3 /MgTiO 3 /ZnTiO 3 nanotube
arrays. More detailed information about the SrTiO 3 nanotubes can be found in (Xin et al.
2009). This SrTiO 3 or (MgTiO 3 and Zn TiO 3 ) can leak Sr (Mg, Zn) slowly for a prolonged
period by slow dissolution and ion exchange. After the hydrothermal treatment, the nano-
tube architecture is retained. A representative morphology of the SrTiO 3 nanotubes arrays
prepared by the hydrothermal treatment in 0.02 mol/L Sr(OH) 2 is shown in Figure 5.15.
As Mg(OH) 2 and Zn(OH) 2 are nearly insoluble, transformation from titania nanotube to
MgTiO 3 nanotubes can be realized in two steps—hydrothermal treatment in a diluted
NaOH solution forming the Na 2 TiO 3 nanotube arrays and subsequent ion exchange reac-
tion of the derived Na 2 TiO 3 in the MgCl 2 or ZnCl 2 solution. It should be kept in mind
that the NaOH solution should not be too concentrated, as a high content of OH destroys
the tubular structure. Generally, the NaOH concentration does not exceed 0.05 mol/L. The
nanotube arrays are capable of leaching biological elements but also leave room for loading
CITY U
SCI
5.0 kV ×40,000 100 nm WD 14.7 mm
FIGURE 5.15
SEM micrograph of SrTiO 3 nanotube arrays fabricated by hydrothermal treatment titania nanotube arrays in
0.02 mol/L Sr(OH) 2 solution.
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