BIOMEDICAL APPLICATIONS V: INFLUENCE OF CARBON NANOTUBES IN NEURONAL LIVING NETWORKS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 158

types. The level of toxicity was found to be dependent on the agglomeration
state of the CNTs. Treatment of mixed neuroglial cultures with up to 30 μg/
mL SWNTs signiicantly decreased the overall DNA content. This effect was
more pronounced when cells were exposed to highly agglomerated SWNTs-a
compared with better-dispersed SWNTs-b. Indeed, higher concentrations of
SWNTs-a were more toxic than the same amount of SWNTs-b. This result is
in agreement with previous studies from Wick et al. in which the aggregation
state of SWNTs might be a crucial factor in terms of toxic effects. 32 Additionally,
SWNTs reduced the amount of glial cells in both derived cultures as measured
by ELISA. The authors observed that neurons were only affected in DRG-
derived cultures with regard to their ionic conductance (diminished inward
conductivity) and resting membrane potential (more positive) according to
whole-cell patch recordings. On the contrary, the neurite outgrowth and the
electrophysiological properties of neurons derived from SPC cultures were
not affected.
6.3 ELECTRICAL STIMULATION OF NEURONAL CELLS GROWN
ON CARBON NANOTUBEBASED SUBSTRATES
Electrical stimulation of neuronal cells is widely employed in basic
neuroscience research, in neural prostheses 33 and in clinical therapy (e.g.,
treatment of Parkinson's disease, dystonia and chronic pain). 34 These
applications require an implanted microelectrode array (MEA) with the
capacity to stimulate neurons. Neuroprosthetic devices currently face various
issues, including (i) long-term inlammatory response of the neuronal
tissues, resulting in neuron depletion around the electrodes and their
replacement with reactive astrocytes that prevent signal transduction; (ii)
delamination and degradation of thin metal electrodes; (iii) miniaturisation
of the electrodes and (iv) mechanical compliance with neuronal tissues for
long-term performance. Currently, semiconductor devices can only partially
solve some of these problems. Nevertheless, CNTs are excellent candidates
for MEA applications because of their unique set of properties which offer the
possibility of constructing small electrodes with high current density.
As will be detailed in this part, microelectrodes coated with CNTs have
low impedance and high charge transfer characteristics and provide a rough
surface that favours excellent cell-electrode coupling, while remaining
chemically inert and biocompatible. Different systems have been utilised to
guide neuronal cell growth and have been tested for their function in cultured
neuronal networks, such as metal electrodes coated with CNTs, patterned
CNT surfaces and CNT-polymer composite thin ilms.