Biomedical Engineering Reference
In-Depth Information
and roughness, of CNT scaffolds can also affect neuronal morphology. In the
follow-up section, we present the use of water-dispersible CNTs as agents that can be
applied to neurons and affect neurite outgrowth. It is this approach that has been
demonstrated to aid regeneration after spinal cord injury in vivo. We only discuss the
use of neurons in primary culture, rather than reporting on results using various cell
lines established from neural cells, which we have reviewed elsewhere (Lee and
Parpura
2009
). When information is available, we also report on the brain regions
from which primary neurons were isolated and on the animal type/species used, since
some of the reported effects could be species and region specifi c. An emerging pic-
ture is that CNTs are neuron-compatible substrates/scaffolds and injectable agents
which may fi nd future utilization in medicine.
2
CNTs as Retainable Substrates/Scaffolds
for Neuronal Growth
The fi rst evidence for applicability of CNTs as a permissive planar substrate for
neuronal growth came from the study by Mattson et al. (Mattson et al.
2000
) . By
using fi xed materials and scanning electron microscopy (SEM), the authors demon-
strated that hippocampal neurons from primary cultures of prenatal (embryonic day 18)
rat brains survived for at least 8 days, and formed one or two neuronal processes,
neurites, when grown on the surfaces of bare/unmodifi ed MWCNTs. Such MWCNTs
were prepared by a catalytic decomposition of ferrocene-xylene mixture (Andrews
et al.
1999
) resulting in MWCNT sheets containing MWCNTs with diameters of
~20 nm and lengths between 20-200 mm. MWCNT sheets were dispersed in ethanol
by sonication. Dispersed MWCNTs were then deposited onto glass coverslips pre-
coated with polyethyleneimine (PEI), a cationic polymer commonly used to grow
neural cells in culture. After ethanol evaporation, the resulting glass coverslip sur-
face contained patches of MWCNTs on top of the PEI layer as well as areas coated
with PEI alone so that neurons could grow on either/both of these surfaces. The
direction of the neurite outgrowth from somata of cultured hippocampal neurons
was not infl uenced by the orientation of the MWCNTs as neurites grew across or
along CNTs. Additionally, neurons displayed more preponderant and branched neu-
rites when they were grown on top of the PEI surface, rather than on top of MWCNTs.
These observations suggest that although unmodifi ed MWCNTs are a permissive
substrate for neuronal growth and neurite outgrowth, they may restrict some ele-
ments of neuronal growth. Thus, if enhanced neurite outgrowth and multibranching
are of interest, then MWCNTs alone may not be an optimal cell scaffold/substrate
and their modifi cations should be considered. To address whether functionalization
of MWCNTs with biological molecules can be used to control the number of neu-
rites, their outgrowth, and branching, Mattson et al. noncovalently coated MWCNTs
with 4-hydroxynonenal (4-HNE) by physical adsorption. The rationale for the usage
of this lipid peroxidation product came from its effect on intracellular Ca
2+
levels in
cultured hippocampal neurons (Mark et al.
1997
) . Since Ca
2+
infl ux can regulate neurite
