NANOTECHNOLOGY AND 3D BIOPRINTING FOR NEURAL TISSUE REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 304

In designing neural scaffolds, controlling the cellular microenvironment is critical for successful tis-
sue regeneration. Self-assembling scaffolds can also be functionalized with various bioactive molecules
and binding peptides such as the laminin-derived IKVAV (isoluceine-lysine-valine-alanine-valine)
and cell-adherent RGD (arginine-glycine-aspartic acid) moieties for enhanced cellular attachment and
neurite outgrowth ( Abdul Kafi et al., 2012; Cheng et al., 2013 ). In work developed by Tysseling et al., a
peptide amphiphile was incorporated on neuroactive epitope IKVAV and injected into a mouse spinal cord
injury ( Tysseling et al., 2008; Tysseling et al., 2010 ). After 11 weeks, IKVAV PA nanofibers regenerated
both descending motor fibers and ascending sensory fibers. They concluded that 3D self-assembling nano-
fibers with neuroactive epitopes are able to accelerate and improve regeneration of spinal cord injuries.
Moreover, SAPNS can be used as a slow and sustained bioactive factor release system to further enhance
tissue regeneration. For instance, Gelain et al. investigated the diffusive mechanisms of active cytokines
within RADA16-I (neutral charge), RADA16-DGE (Ac-RADARADARADARADAGGDGEA-CONH2)
(negative charge), and RADA16-PFS (Ac-RADARADARADARADAGGPFSSTKT-CONH2) (positive
charge) ( Gelain et al., 2010 ). Results demonstrated that protein mobility is directly related to both, physi-
cal hindrances and the charge between proteins and peptide nanofibers. Cell studies showed the capability
of sustained active cytokine ( b FGF) up to 3 weeks when culturing adult neural stem cells. These experi-
ments have opened new avenues for SAPNS-related research with focus on functional molecular release
strategies for neural regeneration.
In addition to self-assembling peptide amphiphile nanofibers, several other types of self-assem-
bling nanomaterials have been investigated for neural regeneration. For example, Merzlyak et al.
genetically engineered M13 bacteriophages which naturally form nanofiber-like viral structures dis-
playing signaling motifs on their protein coats for directed self-assembly of nanofibrous scaffolds
( Merzlyak et al., 2009 ) ( Figure 14.3 ). After self-assembling into nanofibrous matrices, hippocampal
neural progenitor cells were seeded to verify the biological capacity of the novel phage nanobio-
material. Cell studies exhibited the effectiveness of M13 bacteriophage nanofibrous matrices for
enhanced cell viability, proliferation, and differentiation as well as directed 3D cell growth. An-
other type of self-assembling nanobiomaterial is rosette nanotube (RNT), which has shown substan-
tial promise as a new type of nanobiomaterial for tissue regenerative applications ( Sun et al., 2012;
Zhang et al., 2010, 2009a, 2008, 2009b; Fine et al., 2009 ). RNTs are biologically inspired supramo-
lecular nanomaterials formed via the self-assembly of low molecular weight DNA base pair motifs
(Guanine^Cytosine, G^C, Figure 14.4 ). Nanotubular RNTs composed of a hydrophobic core and
hydrophilic outer surface remain stable via electrostatic interactions, base stacking, and hydrophobic
effects. Morphologically, RNTs exhibit a 3-4 nm outer diameter and several hundred nanometers
in length. One critical feature of RNTs is their flexibility in designing their length, diameter, and
surface chemistry. Through functionalization of the G^C motif, predefined chemical and physical
properties for specific tissue regeneration can be achieved. In our recent work, we explored human
bone marrow mesenchymal stem cell (MSC) adhesion, proliferation, and 4 weeks chondrogenic dif-
ferentiation on twin-based RNTs with cell adherent RGDSK peptide embedded within poly-L-lactic
acid scaffolds ( Childs et al., 2013 ). Our results demonstrated that these biomimetic twin-based nano-
tubes can significantly enhance MSC growth and chondrogenic differentiation when compared to
controls without nanotubes. Both the biomimetic nanostructure and high density of peptides with
well-organized architecture contributed the greatly enhanced stem cell functions in vitro . Theoreti-
cally, any cell-favorable short peptides can be conjugated onto the G^C motifs to modulate surface
chemistry, rendering RNTs as a biomimetic nanotemplate for a variety of tissue/organ regeneration.