NANOTECHNOLOGY AND 3D BIOPRINTING FOR NEURAL TISSUE REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 308

synergistic cues by selective laminin adsorption and optimal nanotopography guidance. The synergistic
cues successfully controlled the neuronal differentiation at the level of individual axon or neurite.
In addition to topographical modification, CNTs can be readily functionalized through various
chemical methods to sites at the end, side, or induced defect sites ( Peng and Wong, 2009 ). Treatment
with strong acids (such as HNO 3 or H 2 SO 4 ) or other oxidative agents will generate oxygenated groups
(such as carboxylic acid groups) at CNT ends or defect sites which can undergo further modifica-
tion by linking specific functional groups ( Bekyarova et al., 2005; Banerjee et al., 2005 ). Chemical
functionalization can alter the physical and chemical properties of CNTs including solubility, and
electrical and mechanical properties ( Bekyarova et al., 2005; Balasubramanian and Burghard, 2005;
Hirsch, 2002; Matsumoto et al., 2010; Ni et al., 2005 ). Several studies have shown the potential of
functionalized CNTs to improve biocompatibility and cellular responses. Hu et al. demonstrated, for
the first time, the influence of functionalized CNTs with various surface charge on the outgrowth and
branching pattern of neurons ( Hu et al., 2004 ). Their studies focused on three kinds of functionalized
MWCNTs fabricated by covalently conjugating -COOH, -PABS (poly-m-aminobenzene sulfonic
acid), and -EN (ethylenediamine) with negative, neutral, and positive charge at physiological pH,
respectively. Neurons grown on MWCNTs with positive surface charge exhibited outstanding cellular
response, much more growth cones, extended neurite length, and elaborate neurite branching when
compared to neurons grown upon neutral or negatively charged MWCNTs ( Figure 14.6 ). Based on
these findings, numerous functionalized CNTs have now been reported. A recent study by Gottipati
et al. chemically functionalized SWCNT films to modulate the morphology and proliferation of as-
trocytes ( Gottipati et al., 2013 ). In this work, SWCNTs were covalently linked to polyethylene glycol
(PEG) followed by spraying into films of various thicknesses. When compared to astrocytes grown
on unmodified samples, cells grown on SWCNT-PEG films were bigger and rounder in morphology,
exhibited decreased immunoreactivity of glial fibrillary acidic protein, and promoted cell proliferation
which is associated with the dedifferentiation of astrocytes. In a report by Jan et al., SWCNTs were
functionalized with polyelectrolyte (PEI) and assembled layer-by-layer to form films ( Jan and Ko-
tov, 2007 ). The SWCNT-PEI induced mouse embryonic neural stem cell differentiation into neurons,
FIGURE 14.6
Schematic of neuron growth on positive, neutral, and negatively charged MWCNTs. Images are adopted from
Hu et al. (2004) .