Biomedical Engineering Reference
In-Depth Information
Nanotubes are categorized as single-walled carbon nanotubes (SWCNTs) and multiwalled
carbon nanotubes (MWCNTs). Individual nanotubes naturally align themselves into “ropes” held
together by van der Waals forces, more specifically,
pi
-stacking. Applied quantum chemistry, spe-
cifically orbital hybridization, best describes chemical bonding in nanotubes. The chemical bonding
of nanotubes is composed entirely of
sp
2
bonds, similar to those of graphite. These bonds, which are
stronger than the
sp
3
bonds found in alkanes and diamonds, provide nanotubes with their unique
strength (Gullapalli and Wong, 2011).
SWCNTs are tubes of graphite that are normally capped at the ends. They have a single cylindri-
cal wall. The structure of a SWCNT can be visualized as a layer of graphite a single atom thick,
called graphene, which is rolled into a seamless cylinder. Most SWCNTs typically have a diameter
of close to 1 nm. The tube length, however, can be many thousands of times longer. SWCNTs are
more pliable, yet harder to make, than MWCNTs. They can be twisted, flattened, and bent into
small circles or around sharp bends without breaking. The diameters of MWCNTs are typically in
the range of 5 to 50 nm. The interlayer distance in MWCNTs is close to the distance between gra-
phene layers in graphite. MWCNTs are easier to produce in high-volume quantities than SWCNTs.
However, the structure of MWCNTs is less well understood because of its greater complexity and
variety (Mintmire et al., 1992; Dekker, 1999; Martel et al., 2001).
Double-walled carbon nanotubes (DWCNTs) form a special class of nanotubes because their
morphology and properties are similar to those of SWCNT but their resistance to chemicals is
significantly improved. This is especially important when functionalization is required (this means
grafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT.
In the case of SWCNTs, covalent functionalizations will break some C=C double bonds, leaving
“holes” in the structure on the nanotube and, thus, modifying both its mechanical and electrical
properties. In the case of DWCNTs, only the outer wall is modified. DWCNT synthesis on the gram
scale was first proposed in 2003 (Flahaut et al., 2003) by the CCVD technique from the selective
reduction of oxide solutions in methane and hydrogen.
6.2 POTENTIAL APPLICATIONS
The strength and flexibility of CNTs potentiate their use in controlling other nanoscale structures,
which suggests they will have an important role in nanotechnology engineering. The highest tensile
strength of an individual MWCNT has been tested to be 63 GPa (Yu et al., 2000). Over the years,
new applications have taken advantage of their unique electrical properties, extraordinary strength,
and efficiency in heat conduction. CNTs exhibit unique structural, electromagnetic, chemical,
mechanical, and electrical properties, and have been considered for use in numerous technological
applications (Table 6.1).
The current use and application of nanotubes has mostly been limited to the use of bulk nano-
tubes, which is a mass of rather unorganized fragments of nanotubes. Bulk nanotube materials
may never achieve a tensile strength similar to that of individual tubes, but such composites may,
nevertheless, yield strengths sufficient for many applications. Bulk CNTs have already been used as
composite fibers in polymers to improve the mechanical, thermal, and electrical properties of the
bulk product tips for atomic force microscope (AFM) probes (www.nanoscience.com); and scaf-
folding for bone growth in tissue engineering (Haddon et al., 2006). MWCNT AFM tips offer a
much greater scanning life than standard probes, and combine abilities to achieve high resolution
when measuring high aspect ratio features.
6.3 CNT-MEDIATED TOXICITY
People could be exposed to CNTs through accidental exposures by coming in contact with the
aerosol form of CNTs during the production or exposure as a result of biomedical use. CNTs are
in the nanometer size range and, hence, easily enter into the lungs via the respiratory tract through
