NANOTECHNOLOGY AND 3D BIOPRINTING FOR NEURAL TISSUE REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 307

excellent candidate for neural tissue engineering. In addition to 2D graphene films, a study by Li et al.
investigated neural stem cell growth on 3D graphene foams ( Li et al., 2013 ). It was found that neural
stem cells can actively proliferate on 3D graphene when compared to 2D graphene films. In addition,
3D graphene promoted neural stem cell differentiation to astrocytes and neurons, as well as efficiently
mediated electrical stimulation for differentiated neural stem cells.
Carbon nanotubes are secondary structures composed of single- or multilayered graphene sheets
that have been rolled into a tubular structure, resulting in the formation of single-walled (SWCNTs)
or multiwalled (MWCNTs) carbon nanotubes ( Dai, 2002 ). The incorporation of CNTs within nano-
composite scaffolds can create neural interfaces with increased interfacial area, conductivity, and elec-
trochemical stability ( Keefer et al., 2008 ). Both in vitro and in vivo experiments have investigated the
cytotoxicity and biocompatibility of CNTs for potential use in neural regenerative applications. For
instance, Aldinucci et al. studied the immune-modulatory action of human dendritic cells on MW-
CNTs in vitro ( Aldinucci et al., 2013 ). Based on their findings, differentiated and activated dendritic
cells presented a lower immunogenic profile when interfaced with MWCNTs and the immune reaction
modulation was related to topographical and physical features of the growth surface. It was also found
that neuronal viability of postnatal mouse dorsal root ganglia was reduced when exposed in higher
concentration of MWCNTs containing culture media ( Gladwin et al., 2013 ). In that study, 250 m g/ml of
MWCNT-containing media exhibited neuronal death and abnormal neurite morphology while 5 m g/ml
of MWCNT-containing media presented no cytotoxicity over 14 day culture. Although there have been
cytotoxicity concerns raised about CNTs, thus far the exact mechanisms of CNT's effects on cells are
still not fully known. But the results presented show that nanotubes are not cytocompatible only under
certain conditions (e.g. certain tube lengths, high concentration, hydrophobicity of bare nanomaterial,
and dispersion of nanotubes). It was suggested that CNTs can serve as an excellent nanobiomaterial for
neural regeneration through surface and structural modifications for enhanced biocompatibility and cell
growth. In the following, we will briefly overview current research progress in this field.
The overall success of tissue regeneration depends heavily on initial cell-scaffold interaction, which
is largely determined by the surface properties of the scaffold ( Vasita et al., 2008 ). Modification of sur-
face charge/chemistry as well as integration of signaling complexes on the CNT surfaces can regulate
cell adhesion, proliferation, differentiation, matrix remodeling, and tissue organization ( Yang et al.,
2007 ). To date, multiple strategies including topographical alterations and chemical functionalization
have been explored to modify CNT-based scaffold surface. With regards to topographical alteration,
Fan et al. designed super-aligned CNT yarn scaffolds and demonstrated neurite outgrowth extended
along the CNT yarns with decreased branching ( Fan et al., 2012 ). Similarly, Béduer et al. investigated
neuron growth on SiO 2 substrates patterned with double-walled CNTs ( Béduer et al., 2012 ). It was
found that neurons were able to sense the physical and chemical properties of the scaffold surface in
a contact-dependent manner. Furthermore, neurons prefer to grow on substrates with patterned CNTs
and directed neurite outgrowth can also be modulated by micrometric CNT patterning. It is postulated
that directed neuronal outgrowth is attributed to preferential adsorption of specific culture medium
proteins. In addition, the topography of CNTs impacts neural differentiation of stem cells when ex-
posed to a chemical inducer. In a study by Park et al., MSCs cultured in neural differentiation media
showed enhanced neural gene expression when grown on linear CNT networks when compared to bulk
randomly-oriented CNT-based films ( Park et al., 2013b ). In another study, Park et al. developed a CNT
network pattern for selective growth and controlled neuronal differentiation of human neural stem
cells into neurons ( Park et al., 2011 ). They illustrated that the patterned CNT network could provide