MICRO-AND NANOTECHNOLOGY FOR VASCULAR TISSUE ENGINEERING - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 252

that formed vascular structures. Injected neonatal cardiomyocytes were able to
survive and proliferate while recruiting endogenous cells.
Physical matrix induction cues can be enhanced with the addition of soluble
factors. The Stupp group sought to bind and release proangiogenic molecules within
the nanofiber gels to provide physical and chemical signals for the encapsulated cells.
Rajangam et al. found that the Cardin-Weintraub heparin-binding domain could be
incorporated into the PAs. 109 These PAs self-assembled into nanofiber gels in the
presence of heparin and demonstrated prolonged release of bound protein for more
than 10 days. The gels promoted significant neovascularization in a rat corneal
angiogenesis assay when loaded with small amounts of VEGF or FGF-2. A dorsal
skin fold chamber and a subcutaneous implant model were used to further analyze
the performance of the gels in vivo. 110 In both models, neovascular formation was
promoted, and the inflammatory response was minimal. Furthermore, the gels were
shown to persist for at least 30 days in the subcutaneous environment.
Self-assembly nanofibers provide a technique to form small ECM-mimicking
fibers with tailorable mechanical properties to promote vascularization. However, a
challenge for the field has been the difficulty to form complex structures with this
approach. Very recently, PA chemistry was also translated to form a peptide-based
membrane construct to bind and release growth factors and promote cell adhesion. 111
HA and a cationic PA were combined to form a self-assembled PA-HA hybrid
membrane. By incorporating the heparin-binding domain again, the group was able
to bind heparin-binding growth factors within the membrane and release them over
time. Additionally, mesenchymal stem cells (MSCs) could adhere and proliferate on
the membranes. In a chick allantoic membrane model, the membranes induced rapid
and robust angiogenesis when loaded with small amount of growth factors compared
with unloaded membranes or soluble growth factors alone. Formation of complex
structures is an important step for self-assembly nanofibers, and future work to create
other structures using self-assembly holds much potential.
Natural materials can also be used to form nanofibrous gels with the ability to
direct cell behavior. These gels can be modified to enhance their mechanical
properties, load and release growth factors, or guide differentiation. Our laboratory
has developed a chemically modified (PEGylated) fibrin gel (Fig. 11.4a) with
tailorable properties such as fiber diameter and storage modulus based on the
type of PEG used. 112,113 Notably, the initial fiber diameter and storage modulus
of unmodified fibrin can be either increased or decreased (Fig. 11.4b-d). We have
demonstrated differentiation of MSCs to an endothelial phenotype and the formation
of tubes in vitro (Fig. 11.4e-g). The cellular response is dependent on the specific
PEG used and is therefore tailorable. We have also demonstrated loading and release
of multiple growth factors within the gel. 68 The growth factors can be loaded via
physical affinity for the fibrin matrix or by covalent conjugation to the PEG chains.
11.6.7 Conclusion
Micro- and nanotechnology have provided techniques to make great strides in the
formation of microvascular networks for tissue engineering scaffolds. Microfluidics