Biomedical Engineering Reference
In-Depth Information
charges and surface adsorption of various molecules, and their impact on the one-
dimensional electron conduction in individual CNTs, have been so far neglected but
are expected to play important roles. Indeed, environmental electrostatic infl uences
closely resemble the voltage-dependent gating of semiconductor conductance (i.e.,
the fi eld effect), exploited in metal-oxide-semiconductor (MOS) transistors (i.e.,
MOS-FETs). This has been almost exclusively studied under the perspectives of
engineering new generations of CNT-based transistors (Avouris et al. 2005 ) . There,
the semiconducting properties of SWNTs and their modulation by external electric
fi elds are crucially placed and represent the basis of the transistor effect.
Given the nanoscale of the CNT meshwork details, do time-varying ion currents
infl uence CNT lateral conduction, when occurring at the neuron-CNT interface?
A more in-depth description of semiconductive phenomena and CNT-CNT resis-
tive junctions will soon become imperative. In particular, local extracellular electric
fi elds, induced by the neuronal membrane during an action potential (Rall 1962 ) ,
might directly affect the instantaneous lateral conduction in the nanotube substrate
(Kim and Kim 2007 ). If this is the case, then one could envision the activation of
spike-triggered percolation networks on a mesoscopic scale, with electrical shunt-
ing of proximal neurons that fi red synchronously. Only additional experiments and
the advance in our understanding of the CNT/neuron biophysics in culture clarify
whether these scenarios are relevant and benefi cial for future neuroprosthetics.
Another central issue that has been explored in the context of biosensing applica-
tions, but never related so far to the neurophysiology of neurons in contacts with
CNTs, is represented by the nonspecifi c binding of proteins to the nanotube surface
(Chen et al. 2003 ). This phenomenon can modulate the electrical transport proper-
ties of isolated CNTs, similarly to the gating electric fi eld discussed above. We
certainly expect that future developments and investigations will ultimately focus
on the understanding of nanotubes in close proximity with a living biological tissue
matrix. Cells are known to synthesize their own extracellular adhesion proteins
(Lutolf and Hubbell 2005 ), which may interact directly with the CNT substrate and
affect its electrical properties. In addition, excitable cells like neurons might even
(endogenously) induce selective protein adsorption or release from the substrate or
from previously engineered substrates (e.g., loaded with neuroactive amino acid
compounds) (Mattson et al. 2000 ). These phenomena might certainly have a sub-
stantial impact on the substrate lateral conductivity. The manipulation of this class
of phenomena is exploited for nano-neuroengineering applications (Patolsky et al.
2006 ), opening exciting perspectives for future applied clinical research applica-
tions (Silva 2006 ; Benabid et al. 2005 ; LlinĂ¡s et al. 2005 ) .
Characterizing the electrical properties of the interface between CNTs and neurons
help in elucidating the potential of CNTs as novel materials for (neuroprosthetics)
electrodes. More specifi cally, innovative aspects in CNT/neuron interactions, such
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