Biomedical Engineering Reference
In-Depth Information
are important parameters that influence the fiber diameter during the spinning, in addition to para-
meters such as the jet length, solution viscosity, surrounding gas, flow rate, and the geometry of
the collector assembly
[59,60]
. Then the polymeric nanofibers are subjected to thermal treatment
(oxidative stabilization and carbonization) to obtain CNFs
[61]
.
Among the various precursors for producing CNFs, polyacrylonitrile (PAN) is the most com-
monly used and important polymer, mainly due to its high carbon yield (up to 56%), flexibility for
tailoring the structure of the final CNF products, and the ease of obtaining stabilized products due
to the formation of a ladder structure via nitrile polymerization
[55]
. In the oxidative stabilization
treatment (200
o
C
300
o
C), cyclization of nitrile groups and cross-linking of the chain molecules
occurs upon heating to prevent PAN fibers melting during subsequent carbonization. In the follow-
ing carbonization procedure, usually 1000
o
C
1500
o
C, noncarbonized components would be
removed in the form of H
2
O, NH
3
, CO, HCN, CO
2
, and N
2
gases. It is noteworthy that applying
tension during stabilization and carbonization is crucial for preparation of CNFs with high mechani-
cal strength
[62]
. Unlike CNTs or conventional CNFs, which are produced by bottom-up methods,
the CNFs produced by electrospinning are through a top-down nanomanufacturing process, which
results in low-cost, continuous nanofibers that do not require further expensive purification and that
are also easy to align, assemble, and process for many applications.
18.5
Cytotoxicity of carbon nanomaterials
However promising a new technology or material might be for biomedical applications, it must be
safe. It is crucial to understand its response to a foreign substance when it is introduced into the
body at any time. There are debates in numerous literatures regarding the cytotoxicity of CNTs and
CNFs
[63]
. There are two types of in vitro studies done on evaluating the cell response to CNTs/
CNFs: one with CNTs/CNFs dispersed in the cell culture and the second with CNTs/CNFs held
in a substrate in contact with the cell culture. The studies with CNTs/CNFs dispersed in the cell
culture showed low biocompatibility
[6,64]
, whereas the other studies showed an obvious prefer-
ence of osteoblasts or neurons cell growth on a CNT/CNF surface
[9,65,66]
.
With such high specific surface areas, CNTs/CNFs have high interfacial chemical and physical
reactivity that translate to biological reactivity. Can a CNT/CNF pierce through a phospholipid
bilayers of living cell? Do the catalytic metal impurities in CNTs/CNFs and poor CNT dispersion
in aqueous media cause toxicity? Great efforts have been made to clarify their cytotoxicity; how-
ever, it is still a controversial topic in the literature. The results should be considered carefully
because the CNTs/CNFs applied for cytotoxicity study were in different size, shape, surface area,
surface chemistry, etc. The first source of toxicity in CNTs/CNFs comes from the catalyst metal
residuals, such as Co, Ni
[16]
. These metals are known to be toxic to biological systems if the pris-
tine products were used. It has been shown that SWCNTs decreased keratinocyte
[67]
and HEK293
cell (human embryonic kidney cells) survival significantly
[64]
, thus raising important concerns
about their biocompatibility. This disadvantage has been eliminated by now for most CNTs/CNFs
used for biomedical application have been chemically modified to improve solubility and biological
properties, a process during which metal ions have been removed with oxidative treatment
[14,18]
.
The chemically functionalized CNTs have been shown to be biocompatible on different types of
