Biomedical Engineering Reference
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found to have comparable performances to conventional Ag/AgCl electrodes.
The CNT probes, with the tip pressed against an axon, were able to detect
an action potential extracellularly and some small spikes (i.e., potential
waveforms) associated with other neurons in the nerve cord. The results
obtained from these extracellular experiments were in agreement with those
demonstrated with microelectrodes coated with CNTs.
41,42,54
The intracellular recording and stimulation with CNT probes has been
investigated for the irst time. The conduction mechanism was investigated
on the basis of impedance measurements and cyclic voltammetry. CNT
probes were shown to transmit electrical signals through not only capacitive
coupling but also resistive conduction to a comparable extent, contrary to
the suggestion that capacitive impedance is the main mechanism to record
and stimulate neural activity.
41,42,54
The resistive conduction helps record
postsynaptic potentials and equilibrium membrane potentials intracellularly.
It also facilitates the delivery of direct-current stimulation. The authors
pointed out that the recording capability of the CNTs did not degrade and
even improved after delivering direct-current stimuli for a long period of
time. This highlights that the CNT probes are suitable for long-term use with
a longer endurance compared with conventional Ag/AgCl electrodes, as
the latter are ineficient once the silver chloride is reduced to silver. Thus,
this work supports the potential use of CNT probes as a promising new
neurophysiological tool.
Ballerini and coworkers reported that CNT substrates can boost neuronal
electrical signalling under chronic growth conditions.
54
They demonstrated
that the growth of primary hippocampal neurons on MWNTs increased
the frequency of spontaneous postsynaptic currents, but did not affect the
general electrophysiological characteristics of the neurons.
The substrates coated with CNTs were prepared by deposition of a
homogeneous dispersion of pure MWNTs obtained by functionalisation using
the 1,3-dipolar cycloaddition of azomethine ylides. Defunctionalisation of
the resulting pyrrolidine-MWNTs was then induced at 350 °C under nitrogen
atmosphere to eliminate the organic functional groups introduced on the
nanotube surface. This treatment provided puriied non-functionalised
MWNTs layered on the glass substrate. Previous attempts to prepare glass
coverslips coated with as-produced MWNTs were not satisfactory because of
poor reproducibility in terms of neurite growth and elongation. Hippocampal
neurons were then seeded on glass coverslips. Attachment and growth of
neurons were observed on the substrates covered with puriied MWNTs, along
with neurite extension. Hence, puriied MWNTs layered on glass substrate
were found to be permissive substrates to support neuron adhesion, survival
and dendrite elongation.
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