BIOMEDICAL APPLICATIONS V: INFLUENCE OF CARBON NANOTUBES IN NEURONAL LIVING NETWORKS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 162

The irst example of CNT-coated electrodes implanted into different
brain areas in rats or monkeys was reported by Keefer et al. 44 These
electrodes showed the ability to enhance the detection of neuronal signals
and to stimulate neurons. Indeed, the results indicated that the CNT-coated
electrodes improved the electrochemical and functional properties of cultured
neurons. CNT coating enhanced both recording and electrical stimulation of
neurons in culture, rats and monkeys by decreasing the electrode impedance
and increasing charge transfer. Electrical stimulation experiments were
performed with cultured neuronal networks grown on CNT-based electrodes
to determine if the CNT coating could alter the capacity to activate neurons.
The CNT coating was found to be permissive for neuronal growth and function.
Moreover, the performances of the CNT-coated electrodes were much higher
than those of gold-coated control electrodes in evoking neuronal responses.
This result highlights the potential of CNTs for the development of electrical
brain interfaces.
The unique physical, mechanical, chemical and electronic properties
of CNTs can be, in part, transferred into CNT composites to combine
high electrical conductivity, chemical stability and physical strength with
structural lexibility. CNT composite constituted of thin ilms were prepared
by layer-by-layer (LBL) assembly by Kotov and coworkers, as will be detailed
later. They demonstrated that CNT composite ilms were suitable substrates
to support growth, proliferation and differentiation, as well as to electrically
stimulate neuronal cells.
Applications of CNT substrates in medical implementations, such as
implanted scaffolding, would require free-standing structure instead of CNTs
attached to supporting glass or plastic. For this purpose, Kotov et al. reported
the fabrication of free-standing SWNT-polymer thin-ilm membranes that
are biologically compatible with neuronal cell cultures. 45 The membranes
constituted of SWNT-polymer composite were prepared by the LBL assembly
process. This technique allows for the construction of multilayered composite
coatings and free-standing ilms of SWNTs with versatile architectures, which
can be engineered at the nanoscale to attain desirable mechanical, structural,
biological and electrical properties. It is based on alternating layers of
CNTs and polymers. Coatings made by LBL are generally very mechanically
robust. 46 SWNTs were dispersed in an aqueous solution of the amphiphilic
poly( N -cetyl-4-vinylpyridinium bromide- co - N -ethyl-4-vinylpyridinium bro-
mide- co -4-vinylpyridine). This polymer bears positively charged groups
that favour cell adhesion. The biocompatibility of the SWNT composite thin
ilms towards NG108-15 neuroblastoma × glioma hybrid culture cells was
assessed using confocal microscopy and calcein. NG108-15 cells are used
as neuronal model system since they exhibit, after differentiation, many