29-05-2013, 02:27 PM
Carbon Nanotubes for Biomedical Applications
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Abstract
Carbon nanotubes (CNTs) have many unique physical,
mechanical, and electronic properties. These distinct properties
may be exploited such that they can be used for numerous
applications ranging from sensors and actuators to composites. As
a result, in a very short duration, CNTs appear to have drawn the
attention of both the industry and the academia. However, there
are certain challenges that need proper attention before the CNTbased
devices can be realized on a large scale in the commercial
market. In this paper, we report the use of CNTs for biomedical
applications. The paper describes the distinct physical, electronic,
and mechanical properties of nanotubes. The basics of synthesis
and purification of CNTs are also reviewed. The challenges associated
with CNTs, which remain to be fully addressed for their maximum
utilization for biomedical applications, are discussed.
INTRODUCTION
IN RECENT years, miniaturized products have become increasingly
dominant in every aspect of life. The benefits
of having smaller components, and hence a device with enhanced
capabilities and functionalities, are obvious from the following
engineering perspectives: smaller systems tend to move
more quickly than larger systems because of lower inertia of
mass, the minute sizes of small devices encounter fewer problems
in thermal distortion and vibration, and they consume less
power [1]. Because of these advantages, miniaturization of systems
and devices has become an active area of research. In the
past decade, enormous progress has been made in developing
new fabrication techniques and materials for developing small
devices. One of the most promising applications of miniaturization
technology is in the biomedical industry. The biomedical
industry, today, is characterized by irreconcilable demands:
1) patients are seeking better care; while 2) healthcare providers
and insurance companies are calling for increasingly cost-effective
diagnoses and treatments. The biomedical industry thus
faces the challenge of developing devices and materials that
offer benefits to both constituencies [2].
SYNTHESIS AND PURIFICATION TECHNIQUES
Generally, three techniques are being used for producing
CNTs: 1) the carbon arc-discharge technique [3], [8]–[16];
2) the laser-ablation technique [17]–[20]; and 3) the chemical
vapor deposition (CVD) technique [21]–[31]. Among the
CNTs, MWNTs were first discovered by Ijima in 1991 by the
arc-discharge method [3]. After two years, Ijima and Ichihashi
[10] and Bethune et al. [11] produced SWNTs. The SWNTs
were produced using metal catalyst in the arc-discharge method.
Thess et al. [17] synthesized bundles of aligned SWNTs by the
laser-ablation technique. For the first time, catalytic growth of
MWNTs by CVD was proposed by Yacaman et al. [21]. The
three techniques are discussed in detail below.
Laser-Ablation Technique
In the laser-ablation technique used by Thess et al. [17] for
producing CNTs, intense laser pulses were utilized to ablate a
carbon target. The pulsed laser-ablation of graphite in the presence
of an inert gas and catalyst formed SWNTs at 1200 C.
Fig. 4 shows the schematic diagram of the laser-ablation technique.
The X-ray diffraction (XRD) and TEM revealed that the
SWNTs produced by laser-ablation were ropes (or bundles)
of 5–20 nm diameter and tens to hundreds of micrometers
of length. Braidy et al. [18] synthesized SWNTs and other
nanotubular structures (graphite nanocages and low aspect ratio
nanotubules) by pulsed KrF laser-ablation of a graphite pellet
at an argon pressure of 500 torr, a temperature of 1150 C,
and a laser intensity of 8 10 W/cm .
CVD Technique
In the CVD method, CNTs are synthesized by taking hydrocarbons
(the commonly used sources are methane, ethylene,
and acetylene) and using an energy source, such as electron
beam or resistive heating, to impart energy to them. The energy
source breaks the molecule into reactive radical species in
the temperature range of 550–750 C. These reactive species
then diffuse down to the substrate, which is heated and coated
in a catalyst (usually a first-row transition metal such as Ni,
Fe, or, Co), where it remains bonded. As a result, the CNTs
are formed. Fig. 5 shows the schematic diagram of the CVD
technique. Yacman et al. [21] synthesized microtubules of up to
50 m length of CNTs by catalytic decomposition of acetylene
over iron particles at 700 C. Li et al. [22] used iron nanoparticles
(embedded in mesoporous silica) as catalyst for large-scale
synthesis of aligned CNTs.
Purification
In all the above-mentioned preparation methods, the nanotubes
come with a number of impurities whose type and
amount depend on the technique used. The most common
impurities are carbonaceous materials, whereas metals are
the other types of impurities generally observed [32]. In the
carbon arc-discharge method, the impurities can be purified by
oxidation as the carbonaceous impurities have high oxidation
rates. However, in this case, 95% of the starting materials are
destroyed and the remaining samples require annealing at high
temperature 2800 [37].
CONCLUSION
The past 13 years have seen phenomenal growth in the research
activity in the area of CNTs. In this paper, we have made
an effort to provide the most contemporary overview possible of
synthesis, properties, and potential biomedical applications of
CNTs through recent examples. The exceptional physical, mechanical,
and electronic properties of CNTs allow them to be
used in sensors, probes, actuators, nanoelectronic devices, and
drug delivery systems within biomedical applications. With the
increasing interest shown by the nanotechnology research community
in this field, it is expected that plenty of applications of
CNTs will be explored in future. At the same time, it is believed
that the continued development and application of CNTs can
enhance the practice of biomedical industries. However, amidst
all the hope and hype, CNTs have yet to cross many technological
hurdles in order to fulfill their potential as the most preferred
material for biomedical applications. It is hoped that the descriptions
provided and references to the literature therein will allow
researchers to develop new applications besides proposing improvement
in the current application areas.