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and cell-matrix interactions [ 245 – 247 ]. In general, dimensionality plays a major

role in neurite extension, retraction, branching, and maturation into axons and den-

drites. Further, neurons cultured in 3D versus 2D environments display distinctly

different morphologies, as 3D cultures give rise to neuritic geometries that are more

morphologically reminiscent of those that occur in vivo [ 244 ].

Substrate charge, stiffness, and dimensionality are also important regulators of

stem cell phenotypic fate. Environmental stiffness is such a potent controller of cell

fate that MSCs will differentiate into neuronal, muscle, and bone cells as gel stiff-

ness is increased [ 248 , 249 ]. Interestingly, however, when MSCs migrate from soft

to stiff substrates, some cells preserve neural markers, suggesting that not just stiff-

ness, but variation in stiffness may also impact cell fate [ 250 ]. Saha et al. observed

neural progenitor/stem cells (NPSCs) to preferentially differentiate into neurons on

2D substrates of soft to intermediate stiffness and into astrocytes on stiffer sub-

strates [ 251 ]. Likewise, 3D hydrogel systems with mechanical moduli similar to

that of the brain were found to induce neuronal differentiation of NPSCs [ 252 ]. In

addition to stiffness and dimensionality, there is some evidence to suggest that sub-

strate charge plays a role in cell fate. For example, mouse embryoid bodies cultured

on negatively and neutrally charged hydrogel substrates were found to differentiate

into all three germ layers and just mesoderm, respectively [ 236 ]. Further, Hynes

et al. present data that may indicate that neuronal differentiation of NPSCs is due,

in part, to the charge of PLL hydrogels [ 227 ].

Another promising methodology for hydrogel nanotechnology is self- assembling

peptides (SAPs). Such self-assembling systems facilitate non-invasive delivery

directly into an irregular shaped lesion. SAPs aggregate in situ via van der Waals

forces, hydrogen bonds, and electrostatic forces to form a stable network with mini-

mal secondary damage [ 253 , 254 ]. Many studies have demonstrated that a wide

variety of peptides and proteins can be utilized to produce very stable and well-

ordered nanofi ber structures with exceptional regularity [ 255 – 258 ]. Additionally,

SAPs will collapse into non-toxic L -amino acids, which can potentially be used by

local cells for growth and repair [ 256 ]. The diameter of self-assembled nanofi bers

ranges from 10 to 100 times smaller than typical electrospun fi bers (discussed in the

next section), which is of particular relevance to tissue engineering. This property

suggests that SAPs can provide cells with a more realistic 3D microenvironment

[ 259 ]. Self-assembling nanofi ber scaffolds have also been observed as a possible

treatment to induce axonal growth as well as prevent signifi cant lesioning of the

brain in experimental brain injury [ 260 , 261 ]. As such, a number of in vitro and

in vivo studies have been performed to investigate the effi cacy of SAPs in the con-

text of neuroregenerative medicine.

SAP scaffolds can be chemically designed to incorporate specifi c functional

ligands, such as integrin-binding epitopes, to enhance endogenous repair mechanisms.

Of particular interest are the laminin epitope, IKVAV; the ionic self- complementary

RADA epitope; and modifi cations thereof. SAPs containing IKVAV sequences have

been found to suppress astrocytic differentiation from NPSCs and to promote neurite

outgrowth from cultured neurons [ 262 ]. In vivo injections of IKVAV functionalized

SAPs into a spinal cord compression model reduced astrogliosis and cell death at the

A. Roussas et al.