Biomedical Engineering Reference
In-Depth Information
tumor growth
[49]
. This possibly suggests that CNTs themselves could possibly be surface-engi-
neered to have anticancer activity by inducing an immune response against tumor.
20.3.3
CNTs as Carriers for Antimicrobial Molecules
Surface-engineered CNTs may be able to capture pathogenic bacteria in liquid medium
[50-52]
.
Thus, CNTs themselves might have antimicrobial activity since microorganisms may be adsorbed
onto the engineered surfaces of CNTs. Moreover, using
Escherichia coli
as a model microorganism,
it has been reported that the electronic properties of SWNTs may regulate their antibacterial activity.
The antibacterial effect was attributed to CNT-induced oxidation of the intracellular antioxidant glu-
tathione, resulting in increased oxidative stress on the bacterial cells and eventual death
[53]
.
Functionalized CNTs have been previously demonstrated to be able to act as carriers for antimi-
crobial agents such as the antifungal amphotericin B. CNTs can attach covalently to amphotericin B
and transport it into mammalian cells. This reduced the antifungal toxicity as compared to the toxicity
of the free drug since 40% of the cells were killed by the CNTs-free formulation compared to no cell
death by the CNTs formulation. It has also been reported that the antifungal activity was increased
using the CNTs
[54]
.
20.3.4
Photothermal Therapy of Cancer Using CNTs
CNTs are capable to absorb light in the near infrared (NIR) region, resulting in heating of the nano-
tubes
[55]
. This unique property of CNTs has been exploited to kill cancer cells by thermal effect
[34,56-66]
.
Optical coupling of light with CNTs is predicted to be at highest when the length of the nanotubes is
more than half the wavelength of the incident light beam, which has been previously determined by the
antenna theory
[67]
. Engineering the structure of MWNTs by creating intentional surface defects might
enhance the antenna properties of the nanotubes. Such engineered “defects” is known as dopants and
will cause scattering in the travelling currents and also increase the heating of the nanotube. This physi-
coelectronic characteristic of the engineered MWNTs can be employed to thermally destruct the tumor
cells by using MWNTs that have good heat conducting properties. The process of involving dopants in
the structure of the nanotubes is called doping, and examples of that are boron doping
[68]
and nitrogen
doping (N-doping)
[57]
. N-doped MWNTs have been shown to produce photo-ablative kill of model
kidney cancer cells when NIR light was used. Moreover, the length of nanotubes has been found to be a
major determinant of nanotube ability to transfer heat and kill the tumor with lengths between 700 and
1,100 nm being most desirable to kill the tumor
[57]
(
Figure 20.9
).
In a study conducted by Gannon and coworkers
[56]
, SWNTs were functionalized using Kentera
(a polyphenylene ethynylene-based polymer). The incubation of the nanotubes with hepatic tumor
cells followed by application of radiofrequency field caused a concentration dependent thermal
destruction of the tumor cells which was demonstrated by development of apoptotic cells that caused
complete necrosis of the tumor cells. By contrast, tumor cells that were injected with the Kentera
alone (without CNTs) were viable after the application of the radiofrequency field. In the same study,
it has been reported that
in vivo
injection of the Kentera-functionalized SWNTs was tolerated by rab-
bits
[56]
. Unfortunately, the resultant thermal destruction is not selective toward cancer cells and the
access to deep tumor areas is generally poor, necessitating the inclusion of targeting moieties such as
