Biomedical Engineering Reference
In-Depth Information
Production of MWNTs by the laser vaporization method is achieved by fir-
ing a high-power laser toward a graphite target inside a furnace at 1200°C.
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Addition of Ni or Co to the graphite target, or using a CO
2
laser focused on a
graphite-metal target, results in the production of SWNTs.
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Ultrafast laser
pulses were found to be most effective in increasing the produced quantities
of SWNTs.
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The CVD method is used to produce well-aligned MWNTs and SWNTs
with good uniformity in size. Besides these advantages over the arc-
discharge and laser vaporization methods, this production process can also
be scaled up.
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CVD is based on exposure of metal particles to a medium
containing hydrocarbon gases, in which the formation of CNTs is catalyzed.
In the near future, there will hopefully be precise control over the nanotube
orientation, position, density, and joining.
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Finally, the pyrolytic method is based on heating hydrocarbons, or any
organic precursor containing carbon, in the presence of a transition metal cat-
alyst such as nickel, cobalt, or iron. This method is used to produce MWNTs,
as well as SWNTs, and the production process depends strongly on the type
and dimension of the metal particles, the temperature, the amount of hydro-
carbons, and the type of gases involved in the process.
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A well-characterized
pyrolytic production process is the HiPco process. Here, SWNTs are pro-
duced by thermolysis of Fe(CO)
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in the presence of CO at elevated pressures
(<10 atm) and temperatures (800°C-1200°C). This extremely efficient method
is very applicable for producing bulk amounts.
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4.4 Nanomaterial-Cell Interaction: Health Issues
To conceive a better understanding of the interactions between cells and
nanomaterials created for bone tissue engineering, it is important to know
that there are three levels of structures in bone: the first is the nanostruc-
ture, which consists of noncollagenous organic proteins, fibrillar collagen,
and embedded minerals with an average grain size of 20-50 nm in length
and 2-5 nm in diameter; this is followed by the microstructure, consisting
of lamellae, osteons, and the Haversian systems; and finally the macrostruc-
ture, which consists of the cortical and cancellous bone.
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After contact of a tissue-engineered construct with body fluids, proteins
are immediately adsorbed onto the surface. These proteins determine the cell
type that interacts with the construct since the proteins interact with specific
cell membrane receptors. This interaction is dependent on the type, concen-
tration, and conformation of the proteins. The surface chemistry, hydrophi-
licity, hydrophobicity, charge, topography, roughness, and energy determine
the type of protein that absorbs to the implant. Smooth surfaces have been
shown to favor fibrous tissue integration, which ultimately encapsulates
