The Emerging Applications of Graphene Oxide and Graphene in Tissue Engineering - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 284

GAP-43 gene expression, and morphological characterization. GAP-43
is a specifi c cytoplasmic protein for the neural tissues and is involved in
the neurite generation and growth [69]. This protein was signifi cantly
up-regulated on the graphene substrates compared to the conventional
tissue culture polystyrene substrates. The complex interactions among
various stimuli (i.e., mechanical, electrical, and chemical cues) imposed
by the graphene substrates made it diffi cult to determine the origins of
the effects of graphene on neural cells. However, the small difference
between the average roughness values of the underlying substrates (i.e.,
graphene and conventional tissue culture substrates) indicated that the
effects of graphene are not due to differences in surface roughness. Some
graphene composites, such as graphene-heparin/poly-L-lysine, were also
used as conductive and biocompatible scaffolds for neural TE. The lat-
ter scaffold was fabricated in 2D and 3D structures using layer-by-layer
deposition and electrospinning techniques, respectively [70]. Both 2D and
3D scaffolds with tunable conductivity and surface chemistry supported
the cellular adhesion and neurite sprouting. Post thermal annealing and
hydrodynamic fl ow procedures were used to obtain uniform coverage
area of graphene-heparin/poly-L-lysine on the substrates, and therefore
a controlled electrical resistance of scaffolds was obtained. Note that the
scaffold capability to transmit applied electrical stimuli is important in
the neural TE. A hydrothermal approach was employed to fabricate 3D
graphene hydrogels as suitable scaffolds for TE applications [71]. GO was
initially used as a building block for the scaffolds, and the lateral size and
concentration of GO sheets had signifi cant effects on the hydrogel struc-
ture. Reduction of GO through hydrothermal treatment at 180 C for 24 h

yielded porous and interconnected graphene hydrogels. MG63 cells were
able to proliferate within these scaffolds. Note that biological cells experi-
ence a 3D environment in vivo . Therefore, there has been a great interest
in fabricating 3D scaffold structures resembling the 3D ECM for the cells
in vivo [72, 73]. Fabrication of 3D scaffolds, consisting of graphene or its
derivatives, has been mainly achieved using the aforementioned hydro-
thermal reduction, fi ltration, and supramolecular interactions [74]. Ruoff
et al. recently demonstrated that paper-like GO structures can be formed
through the vacuum fi ltration of dispersed GO sheets [75, 76]. Fabricated
GO papers exhibited a superior strength and stiffness and a unique struc-
ture where GO sheets were tiled together approximately in a parallel
fashion. Reduced GO was also fabricated by the same procedure [66].
Biocompatibility of later structures was confi rmed as they were exposed
to fi broblast cells. Supramolecular interactions in GO can be controlled to
transform them into 3D hydrogels. Here, various gelators, such as metal
ions, polymers, and acid, can be used [77]. Since GO consists of various
oxygen-contained functional groups, depending on which type of gelator
is used, the bonding force of GO is increased or decreased. For example,

°