Biomedical Engineering Reference
In-Depth Information
first tested the suitability of the NTs to serve as a potential drug loading and delivering platform
[43,44]
. They used bovine serum albumin and lysozyme as model proteins to investigate the load-
ing and release efficiencies from the NT platforms. They demonstrated the efficacy of using NTs
as drug eluting coatings for implantable devices. Various amounts of drugs can be incorporated
into the NTs and their release can be adjusted by varying the tube length, diameter, and wall
thickness
[43]
. Another report demonstrated that the NTs can control small molecule delivery
within weeks and larger molecules in months
[44]
. Various agents have been experimentally loaded
into the NTs to attain better bioactivity and extra properties such as antibacterial ability. There are
many reports on the incorporation of growth factors or antibiotics to the NTs and certain bioactivity
and antibacterial ability. However, we believe that the NTs are ideal for loading and delivering
targeted inorganic agents such as silver (Ag), strontium (Sr), and zinc (Zn). First of all, these are
much smaller molecules than growth factors and antibiotics and function at very low doses. Long-
lasting activity can be achieved by increasing the loaded amounts and controlling the release rate
appropriately. Secondly, these agents are stable due to their inorganic nature, thereby facilitating
the use of loading processes and loading methods that tend to have harsh conditions. Thirdly, the
stable properties of the agents may also permit relatively long storage after fabrication of the
implants and it is important to commercial adoption.
As mentioned in the introduction section, postoperation infection remains one of the most com-
mon and serious complications for a dental implant, and so a surface boasting long-term antibacte-
rial ability is highly desirable in order to prevent implant-associated infection. We have fabricated
Ag nanoparticles incorporated NTs (NT
Ag) on Ti implants to achieve this purpose
[5]
. The
Ag nanoparticles adhere tightly to the wall of the NTs prepared by immersion in a AgNO
3
solution
followed by UV light irradiation (
Figure 17.11
). The amount of Ag introduced to the NTs can be
controlled by changing the processing parameters such as the AgNO
3
concentration and immersion
time. The NT
Ag can kill all the planktonic bacteria in the suspension during the first several
days, and the ability of the NT
Ag to prevent bacterial adhesion is maintained without obvious
decline for 30 days, which are normally long enough to prevent postoperation infection in the early
and intermediate stages and perhaps even late infection around the implant. The ability of the
NT
Ag to prevent viable bacterial colonization is vividly displayed by fluorescence staining
(
Figure 17.12
). After 7 days of repeated bacterial invasion every 24 h, there are large amounts of
viable bacteria on the flat Ti and smaller amounts on the TiO
2
NTs. In comparison, the amounts
of viable bacteria are obviously smaller on the NT
Ag samples due to the Ag loading amount.
Although the NT
Ag structure shows some cytotoxicity, it can be reduced by properly controlling
the Ag release rate. This NT
Ag structure with relatively long-term antibacterial ability has prom-
ising applications in bone implants after eradicating the cytotoxicity by properly controlling the
Ag release.
Sr shows the effect to modulate bone turnover toward osteogenesis by enhancing osteoblast func-
tions and inhibiting osteoclast functions, and so Sr-loaded nanotubular structures (NT
Sr) that allow
controlled and long-term Sr release are expected to yield favorable osteogenic effects. Well-ordered
SrTiO
3
NT arrays capable of Sr release at a small rate and for a long time have been successfully fab-
ricated on titanium by simple hydrothermal treatment of anodized titania NTs (
Figure 17.13
)
[20]
.
This surface architecture combines the functions of nanoscale topography and Sr release to enhance
osseointegration while at the same time leaving space for loading of other functional substances. In
vitro experiments reveal that the SrTiO
3
NT arrays possess good biocompatibility (
Figure 17.13
)and
