Biomedical Engineering Reference
In-Depth Information
The conductivity of CNT can be advantageous in many biological applications. For example,
small current can stimulate osteogenesis of fractures
[156]
. Electrically induced osteogenesis has
been extensively studied in vitro
[157
164]
. Blends of CNT and polylactic
acid were used as conductive composite and exposed osteoblast cells to electrical stimulation
[165]
.
Results indicate that alternating-current electrical stimulation promotes cell proliferation, gene
expressions for collagenous and noncollagenous proteins, and enhanced calcium deposition in the
extracellular matrix.
The conductive properties of CNT composites can be exploited to offer neural stimulation by
prosthesis used to heal a damaged or diseased portion of the nervous system by delivering electrical
pulses. Electrical conductivity can affect neural signal transmission
[166]
. Traditional neural
electrodes are made from stable metals such as platinum, gold, titanium, and stainless steel. These
metals suffer from poor contact with tissue or scar formation. Several coating materials such as
iridium oxide
[167]
and conducting polymers such as polypyrrole
[168]
and polythiopene
[169]
have emerged as materials for neural interfacing. However, they suffer from long-term instability
[170]
. Recently poly (3,4-ethylenedioxythipene) doped with CNT and deposited on platinum micro-
electrodes gave promising results in terms of stability, toxicity, and in supporting the growth of
neurons
[171,172]
. It has been shown
[173
159]
and in vivo
[160
177]
that CNT microelectrodes have superior electro-
chemical properties, which are further enhanced by surface coating
[178
180]
. The CNT-based
microelectrodes allow the growth and differentiation of neurons.
3.4
CNT cytotoxicity
Given the remarkable properties of CNTs, there is considerable interest in its applications in the
fields of biomaterials, biosensors, drug delivery, and tissue engineering. Hence, it is paramount that
the safety of CNTs to human health and the environment be assessed. It is known that sub-micron
size particles (such as asbestos fibers) influence cell behavior
[181]
. The higher surface area for
small particles makes them more reactive than larger particles
[182,183]
.
Most of the as-produced CNTs contain substantial amounts of impurities such as metal catalyst
(Co, Fe, Ni, Mo, and Pt). Some of the metals (Co, Ni, Co
2
1
) cause cytotoxic or genotoxic effects
as well as lung diseases including fibrosis and asthma
[184]
. Nickel is known to be cytotoxic and
carcinogenic to the human body
[185]
. Hence, the toxic effects of unpurified CNT were evoked by
the heavy metal residues rather than the nanotubes themselves
[186]
. Other factors that induce
cytotoxicity include length and size distribution, surface area, dispersion and aggregation status,
coating or functionalization, immobilization, internalization, or cellular uptake and cell type
[149]
.
Furthermore, these factors could interact with each other. This has led to contradictory results in
the literature. While some studies have reported that CNTs are toxic to mammalian cells
[187
195]
. In general, the
negative results have come from research groups concerned with environmental aspects
[187,190,196]
. The discrepancy lies in the source from where the nanotubes were obtained, dose,
and time of exposure.
Several techniques have been used to purify the as-processed nanotubes. These include acid
treatment
[197,198]
, thermal oxidation
[199]
, acid treatment with thermal oxidation
[200]
, and a
191]
, other reports have suggested that CNTs are biocompatible
[192
