Recently Carbon nanotubes have been designed to be used as a drug delivery
vector. It has shown potential in tissue engineering, nuclear targeting, and
drug, protein and peptide drug delivery also. The small nanoscale dimension
and astonishing properties make them a distinctive carrier with a wide range
of promising applications. This review briefly outlines some of the important
biomedical application of carbon nanotubes.
Introduction

In the last few decades
various micro and nanoscale drug carrier systems have been developed in order to
find well-organized and proficient carrier systems for drugs, genes, and antigen
which will assist the targeting and delivering of bioactives into specific and
precise organ, tissues and cells. These diverse drug delivery systems include
microemulsion, multiple emulsions, liposomes, niosomes, nanoemulsion,
microspheres, nanoparticle, resealed erythrocyte and dendrimers.

In recent times carbon nanotubes (CNTs) have been designed to be used as a drug delivery carrier.
Carbon nanotubes were discovered by Bacon in the late 1950s. But they were not
fully appreciated at that time. In 1991 Iijima discovered CNTs and proposed it
as an interesting material due to their structural properties1. CNTs
consist of graphite sheets rolled up in to tubular form. These new nanomaterials
belonging to the family of fullerene are the third allotropes of
carbon2. Recently, scientists have also accounted that CNTs hold
potential of a drug delivery systems. The studies have shown that CNTs loaded
with peptides3, proteins4, nucleic acids5 and
drugs6 comprise effective targeting into the cells. Depending upon
the number of graphene sheets, CNTs can be classified as single-wall carbon
nanotubes and multi-wall carbon nanotubes.
1. Single-wall carbon nanotubes

Single-wall carbon
nanotubes (SWNTs) are made of a single graphene sheet. These are seamless
cylinders, were first reported in 19931. Their diameters range from
about 1 to 2 nm, and their length is usually in order of the micrometers. SWNTs
typically team up to form bundles. These bundles consists hexagonally arranged
SWNTs to form a crystal-like structure (figure 1 A).
2. Multi-wall carbon nanotubes

The multi-wall carbon nanotubes (MWNTs) are made up of collection of several
graphene cylinders. MWNTs have a diameter of about 1-100nm and length of about
1-50 micrometers. The distance between each layer of MWNTs is about 0.36nm1
(figure 1 B)

Single-wall carbon nanotubes

Figure 1: (A) Single-wall carbon nanotubes (B) Multi-wall carbon nanotubes

Carbon nano horns and
fullerenes are some structurally related compound to carbon nanotubes. Carbon
nano horns are composed of graphite carbon atom structurally similar to CNTs.
The difference between CNTs and carbon nano horns is that, the latter have an
irregular horn like shape. Fullerene molecules are almost round cages of 60
carbon atoms arranged in interlocking hexagons and pentagons, like the patches
on a soccer ball.

Physicochemical properties
of CNTs include ultra light weight, ordered structure with high aspect ratio,
high mechanical strength and metallic or semi-metallic behavior with high
surface area. There are some limitations of CNTs also, which includes lack of
solubility in most solvents and aggregation. Both these limitations can be
overcome by functionalization or modification of their surface1.
Biomedical application of carbon nanotubes

Owing to the large inner
volume, CNTs proffer attractive advantages for biomedical applications. These
large inner volumes can be filled with desired bioactives of small size as well
as of large size such as proteins and peptides. The targeting and
biocompatibility aspects of bioactive loaded CNTs can also be enhanced by
effective surface functionalization.
Carbon nanotubes mediated drug delivery

In general drug delivery
system is designed to improve the pharmacological and therapeutic profile of a
drug molecule. The large inner volume of CNTs allows encapsulation of both low
as well as high molecular weight drugs. It also permits encapsulation of both
hydrophilic and lipophilic drugs. More than one drug can also be loaded in CNTs
in the case of multi-drug therapy. Ligands and diagnostic moieties can also be
conjugated to surface of CNTs by functionalization to target the drugs to
specific site of action. The CNTs can act as controlled release system for drug
by releasing the loaded drugs for a long period of time. In this way CNTs can be
used multifunctionally for drug delivery and targeting.
Cellular and nuclear targeting

The endeavor behind targeted drug delivery is to enhance
the efficiency and diminishing the noxious effects. The CNTs can be chemically
surface modified such that ligands can be attached to their surface functional
groups. These ligands which are specific to certain receptors can carry the CNTs
directly to the specific site without affecting on non-target site. On the other
hand diagnostic moieties like fluoroisothiocyanate (FITC) can also be attached
to the CNTs for probing their way to the nucleus.
Carbon nanotubes in peptide delivery

The use CNTs in peptide
delivery has also been done by scientist. Application of CNT as a template for
presenting bioactive peptides to the immune system has been done. For this
purpose, by using a bifunctional linker epitope of virus and amine group of CNT
can be covalently link and immunization can be done. Subsequently the
immunogenic features of peptide–CNT conjugates can be assessed in vivo. In this
way CNTs can achieve high value in peptide delivery also7.
Carbon nanotubes in tissue engineering

The main objective of
tissue engineering is to restore unhealthy or damaged tissue with biologic
alternative which can reinstate and preserve regular tasks. The carbon nanotubes
can be used for tissue engineering by visualizing and enhancing cellular
performance and by tracking and labeling of cells8.
Conclusion

It can be concluded that
functionalization of CNTs will open new era in the potential of CNTs in
biomedical field. Some CNT are highly toxic, mostly due to their insolubility,
which is of great concerned in using CNTs. This problem can also be overcome by
functionalization. This offers the possibility of introducing more than one
function on the same CNT molecule to target bioactives, imaging agents, drugs
and ligand moieties at once. Further investigations must be done by the
scientists in the field of CNTs to establish them for their biomedical
applications.