Biomedical Engineering Reference
In-Depth Information
that with the help of this technique, liver tumors or metastases as small as
2-3 mm can be detected. Through conjugation of iron oxide NPs with
hydrophilic polymer coatings, such as dextran or PEG, it is possible to ob-
tain a sterically preventing opsonisation of NPs in the serum and a reduction
of their uptake by the RES [143]. Recently antibiofouling polymer-coated
magnetic NPs as nanoprobes for MRI have been characterized. SPION
were coated with the protein- or cell-resistant polymer, poly(TMSMA-t-
PEGMA), to generate stable, protein-resistant MRI probes. The compound
could detect tumors in vivo using MRI, and can be used as a potentially ef-
ficient cancer diagnostic probe [101]. MNPs exhibit acute toxicity in vivo,
which has limited their clinical translation. Oxidative stress and interference
with mitochondrial energy production by MNPs can lead to cytotoxicity.
5.7 Carbon nanotubes
Carbon nanotubes (CNs) are essentially cylindrical molecules made of car-
bon atoms. CNs are synthesized by rolling sheets of graphene into hollow
tubes that are single-walled (SWNTs) (0.4- to 2-nm diameter), double-
walled (1- to 3.5-nm diameter), or multi-walled (MWNTs) (2- to 100-nm
diameter). CNs can be synthesized by heating carbon black and graphite in
a controlled flame environment. One of the main advantages of the CN is
its ability to deliver drugs directly to cancer cells. It has also been suggested
that CNs could be used as nanocarriers for delivering drugs into the body
via injectable routes [144]. Drugs can either attach to the outer surface of
the CNs via functional groups, or be loaded inside the CNs. Attachment
of the anticancer drug to the outer surface of the CNs can be through ei-
ther covalent or noncovalent binding, including hydrophobic, π-π stacking,
and electrostatic interactions [145]. The mechanism by which CNs enter
cells is unclear. The evaluated processes are the passive diffusion of CNs
through the lipid bilayers of the cell membrane, and the attachment of CNs
to the external cell membrane, resulting in its absorption by the cell, using
an energy-dependent process. Generally speaking, small CNs with a length
of up to 400 nm are internalized by a diffusion mechanism, while CNs of
400 nm in length are internalized by endocytosis [146]. Functionalization
and alteration of CNs and other graphite nanoplatfom surface chemistry
can reduce or eliminate complement activation, while making the CNs more
biocompatiable [147]. Functionalized SWNTs were conjugated with pacli-
taxel through branched PEG chains via a cleavable ester bond. The resultant
formulation was more effective in suppressing tumor growth in vivo than
Taxol or paclitaxel-PEG conjugated in a 4T1 breast cancer animal model
[148]. Similar findings have been obtained when paclitaxel was loaded into
PEGylated SWNTs or MWNTs using HeLa cells and MCF-7 cancer cells
lines [149]. Kam et al. [150] have shown the possibility to direct nanotubes
to specifically targeted cancer cells by using coating of nanotube surface
with folic acid. In this way carbon nanotubes bind specifically to cancer
