BIOMEDICAL APPLICATIONS I: DELIVERY OF DRUGS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 23

Several DDS have been investigated, such as liposomes, dendrimers and
smart polymers, iron and gold nanoparticles, fullerenes and nanohorns.
Nevertheless, none of the above-mentioned systems can be considered “ideal”
for drug delivery. For example, liposomes (the most studied DDS) show
excellent biocompatibility and low toxicity but show also big dimensions and
signiicant instability in solution. 3-5 Dendrimers and smart polymers possess
a high controllable size and surface functionalization but display also a slow
release rate and cytotoxicity (up to 200 nM). 6-12 A quite signiicant toxicity
was also observed for gold and iron nanoparticles, 13-25 while fullerenes
showed accumulation in the liver 26-30 and nanohorns were found to be poorly
soluble in aqueous media and resulted in self-assembly (potential toxicity) as
a consequence. 31-33
Taking into account all these experimental observations, the use of carbon
nanotubes (CNTs) as potential carrier for DDS appeared to be a promising
option. 34
In fact, they show several interesting properties, such as high aspect ratio,
ultra-light weight, great strength, high thermal conductivity and remarkable
electronic properties similar to those of metallic to semiconductors. 35-39
Produced carbon nanotubes (pCNTs) are made of a series of condensed
benzene rings and wrapped in a tubular form and can contain either one
(SWCNTs) or multiple (MWCNTs) graphene sheets.
At present it is not possible to demonstrate which of the two systems is more
advantageous. In fact, while SWCNTs offer the additional photoluminescence
property that can be considered promising in diagnostics, MWCNTs present
a wider surface and internal volume that can facilitate encapsulation and
external functionalisation with active molecules. 40-44
One of the major problems related to the use of CNTs as DDS is the
lack of solubility (both in organic solvents and aqueous solutions), which
signiicantly compromises their biocompatibility and immunogenicity. 45
However, these observations can be considered appropriate for pCNTs
only, indicating that further modiications can lead to functionalised CNTs
( f -CNTs) with such desirable properties that may enable them to be
considered as good biomaterials for drug delivery. In fact, introduction of
multiple functions on their surface can render them dispersible in aqueous
media, thereby overcoming the major disadvantage of pCNTs. 46,47
In particular, the application of f -CNTs as new nanovectors for drug
delivery became feasible soon after the demonstration of cellular uptake of
this new material. In fact, it has been clearly demonstrated that f -CNTs can
be internalised into the cells regardless the type of functionalisation on the