Biomedical Engineering Reference
In-Depth Information
are stable up to 2800
C in vacuum, possess a thermal conductivity whose value is twice that of
diamond, and have an electric-current carrying capacity that is 1000 times that of copper
[1]
.
Studies
[2,3]
have shown that nanotubes display extraordinary mechanical properties—tensile
modulus of 1 TPa, tensile strength in the range of 50
150 GPa, and a failure strain in excess of
5%. The elastic modulus and strengths are one to two orders of magnitude higher than that of the
strongest steel. They also display outstanding electrical and thermal properties. These extraordinary
properties of nanotubes have sparked an interest in using them as reinforcing materials in compo-
sites or as additives to impart novel functionalities. Given their unique properties, CNT-based mate-
rials have attracted attention in the field of biomaterials with potential applications in load-bearing
application, radiotracers, Magnetic Resonance Imaging (MRI) contrast agents, drug delivery, tissue
engineering, and sensors. In addition, CNT-based scaffolds that are electrically conducting is also
an attractive potential. However, before its wide spread usage, the safety of CNT-based materials
must be established.
3.2
Preparation of CNT composites
There have been different techniques used to prepare composites using CNTs
[4]
. These include
melt blending
[5]
, solution blending
[6]
, in situ polymerization
[7]
, electrospinning
[8,9]
, and layer-
by-layer (LbL) assembly
[10,11]
. In melt blending, the polymer in a molten state and the nanotubes
are mixed in a shear environment in a mixing device (typically in a screw extruder). The objective
is to uniformly disperse the nanotubes in the polymer matrix for reinforcement. In solution blend-
ing, the polymer is dissolved in solution and the nanotubes are added. Since the tubes are held
together by van der Waals forces, they are separated and dispersed in solution using sonication.
Once adequate dispersion and homogeneity are obtained, the solvent is evaporated to yield the
nanotube-filled polymer. This process is mainly used where the polymer is soluble in common
organic solvents. CNT composites using in situ polymerization involve polymerizing vinyl mono-
mers and CNT. This process is very attractive for polymers that are thermally unstable or are insol-
uble in solvents. It is possible to have a high nanotube loading
[12]
, including grafting the polymer
to nanotube surface which promotes interfacial adhesion between the polymer and the nanotube
increasing its bulk properties. Electrospinning is a technique for the production of fibers with
diameters ranging from microns to few nanometers. It was originally applied to polymers, but more
recently the process has been applied to the production of glass, metal, and ceramic
[13]
. In the
electrospinning process, static electric charges are induced on the polymeric solution which is
extruded through a syringe. Initially, the polymer solution is held by its surface tension in the form
of a droplet at the end of a capillary. If the charge density is high enough, the repulsive force over-
comes the surface tension. Within a few centimeters of travel from the tip, the discharged jet
undergoes bending instability and begins to whip and split into bundles of smaller fibers. In addi-
tion to bending instability, the jet undergoes elongation that causes it to become thinner. The sol-
vent evaporates leading to the solidification of the fluid jet. The fibers are collected on a collector,
usually in the form of nonwoven fabric. The LbL technique exploits the electrostatic attraction
between oppositely charged species to induce the growth of one-dimensional (1D) structure. An
important characteristic of the LbL assembly is to precisely control the thickness of individual
layers. It is also possible to incorporate a number of different materials, particularly biomolecules,
