Biomedical Engineering Reference
In-Depth Information
H
2
O
[7]
, NaH
2
PO
4
/HF
[9]
, and Na
2
SO
4
/HF
[9]
. The diameter of the NTs can be regulated to vary
from 15 to 140 nm and the length can range from 200 to 1000 nm by adjusting the electrolyte con-
tent and anodization voltage
[21]
. When using polar organic electrolytes, much longer NTs of hun-
dreds of micrometers can be fabricated
[22]
. Ethylene glycol (EG) is the commonly used organic
solvent to fabricate NTs. The typical NTs formed by anodization of a Ti foil in an EG solution
with 0.5 wt% NH
4
F, 5 vol% CH
3
OH, and 5 vol% H
2
O at 10 V for 1 h and 40 V for 40 min are
shown in
Figure 17.1C and D
, respectively.
17.3
Factors influencing the bioactivity of the NTs
Various factors can influence the bioactivity of the NTs during their preparation and cell culture
process. A better understanding of these factors helps to optimize the NTs for better biological
performance. Here, we summarize the factors such as the sterilization process, phenotype of cells,
protein concentration in the culture medium, and smoothness of the top ends of the tube walls that
affect the bioactivity of the NTs.
17.3.1
Influence of sterilization on the bioactivity of the NTs
Sterilization process is essential for the in vitro bioactivity assay and finally in vivo applications
of dental implants. However, many researchers choose sterilization methods arbitrarily when study-
ing the biocompatibility of NTs. In some experiments, autoclaving is used
[10
13,23]
, whereas in
some others, ultraviolet (UV) irradiation or ethanol immersion is used
[24
26]
. Sterilization meth-
ods can be considered as a posttreatment of the samples, and the various sterilization methods can
change the surface properties of the samples and corresponding bioactivity. We have compared the
effects of three commonly used sterilization methods, namely autoclaving, UV irradiation, and eth-
anol immersion on the bioactivity of NTs.
We have found that the sterilization process can modify the surface of the biomaterials. Strong
discoloration of samples is occasionally observed after autoclaving, but it is usually not found after
UV or ethanol sterilization. The most significantly modified characteristics after sterilization are
surface chemistry and wettability. Autoclaving decreases the surface free energy of biomedical
implants, but contrarily UV treatment dramatically enhances that of Ti (
Figure 17.2A
). The
decrease in hydrophilicity by autoclaving is related to the deposition of hydrophobic contaminants
on the implant surfaces
[27]
. The enhancement in the surface free energy of Ti by UV treatment is
associated with the molecular structure alteration of surface titania with abundant Ti
OH group
formation and removal of surface hydrophobic contaminants especially hydrocarbons
[28,29]
.
The changes in the surface properties subsequently lead to differential cell responses.
UV and ethanol sterilizations induce higher initial adherent cell numbers (
Figure 17.2B
) and cell
proliferation than autoclaving, and UV irradiation leads to the best cell functionalities including
adhesion, proliferation, as well as differentiation represented by related gene expressions. UV steril-
ization appears to be the optimal sterilization method from the viewpoint of eliminating surface
contamination.
Oh et al.
[30]
have investigated the influence of two sterilization methods of wet autoclaving
versus dry autoclaving on the functionalities of osteoblasts cultured on the NTs. Their results
