Biomedical Engineering Reference
In-Depth Information
LbL composites containing CNTs have been assembled using electrostatic interactions
[139,140]
or hydrogen bonding
[141
143]
. Covalent cross-linking can increase the modulus of the films
[144,145]
and is achieved by using polymers that have functional groups that are capable of
reacting with one another or can react with a bifunctional agent (using diamines, diimides, or
dialdehydes). It is generally believed that cell processes are affected by their surrounding micro-
environment which include their mechanical and biochemical stimuli
[146
148]
.
polymer composites depend on the fabrication
and processing technique used. A number of materials have served as a matrix for CNT-based
composite including synthetic polymer, natural polymer, and ceramics. Irrespective of the proces-
sing technique used, two important criteria must be satisfied to effectively improve the material
properties—interfacial adhesion between the CNT and the polymeric matrix, and a homogeneous
dispersion of CNT in the matrix. CNTs have been chemically modified to incorporate functional
groups in the end or sidewalls to enhance interactions with the matrix. Several dispersion techni-
ques (e.g., screw extrusion, agitation, and ultrasonication) have been used to achieve efficient
dispersion. One of the potential drawbacks of CNT composites made using traditional polymer-
processing techniques is its inherent inability to include functional components. Biomolecules such
as proteins are unable to withstand the harsh processing conditions encountered in traditional
polymer techniques. In both the LbL assembly and electrospinning, it is possible to include bio-
molecules into the composite. Hence, the latter two techniques are more appealing for use in the
field of tissue engineering.
In summary, the mechanical properties of CNT
3.3
Conductivity
Composites based on conductive polymer and conductive filler are of interest as biomaterials
[12,149]
. Polyaniline, polypyrrole, and polythiopene are conductive polymers that are available.
However, conductive polymers have limited thermal and electrical stability, poor solubility in
solvents, and poor mechanical properties. CNTs, in addition to improving the mechanical properties
of composite, also make the composite more conductive. CNT
polymer composites become elec-
trically conductive when a critical CNT concentration, referred to as percolation threshold, is
attained. At percolation concentration, CNTs form an interconnected network of conductive path-
ways. The critical concentration is affected by various factors and includes polymer, nanotube type,
processing technique, and processing conditions. For a given nanotube concentration, processing
parameters during molding such as holding pressure and melt and mold temperature have a signifi-
cant effect (
10 orders of magnitude) on the resistivity
[54,150]
. Type of nanotube (single wall
versus multiple wall) affects percolation threshold
[151]
with SWNT requiring significantly lower
amount to reach percolation threshold than MWNT
[152]
. While functionalization aids in disper-
sion and enhance matrix interaction, it decreased the electrical conductivity
[153]
. The mechanism
for charge transport and modulus reinforcement of CNT-based composite materials are different
[154]
. For increased electrical conductivity, nanotube agglomeration is preferred. Pristine CNTs
exhibited lower electrical percolation threshold than amino-functionalized ones, probably due to the
lower aspect ratio of functionalized nanotubes
[55]
. In addition to electrical conductivity, the ther-
mal conductivity of polymers can also be enhanced by the addition of CNTs
[155]
.
5
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