STEM CELLS AND NANOTECHNOLOGY IN TISSUE ENGINEERING AND REGENERATIVE MEDICINE - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 14

FIGURE 1.4 General structure of self-assembled peptide amphiphile (PA). (a) Molecular
model of the PA showing the overall shape of the molecule. The narrow gray area represents
the hydrophobic alkyl tail and the thicker head region is composed of hydrophilic amino acids
containing functional groups that can provide signals to the cells to influence their behavior.
(b) The PAs self-assemble into nanofibrous structures upon exposure to physiological
conditions with the hydrophobic tail in the core and the head region facing the outside to
interact with cells. (c) Vitreous ice cryotransmission electron microscopy image of hydrated
PA fibers (scale bar ¼ 200 nm). Modified with permission from Ref. [70].
incorporated into PAs with adhesion-promoting RGD head sequences. These
phosphoserine residues serve as a template for nucleation of hydroxyapatite crystals
that align along the long axis of the nanofibers, similar to the native bone structure. 71
The RGD peptides on the outer region of the PA promote cell adhesion. Other studies
have shown that the use of self-assembled PAs can promote neural regeneration 72 as
well as angiogenesis. 73 This technique not only provides biological cues to induce
tissue formation but also mimics the basic steps of ECM biosynthesis. However, this
method of nanofiber synthesis is quite complex and of relatively low yield and may
not be suitable for large-scale tissue engineering applications. 74
1.4.4.3 Electrospinning Electrospinning has recently become the most commonly
used method for the fabrication of nanofibrous biomaterials. This method involves
the application of a high electric field to a polymer solution delivered at a constant
rate through a needle. At a high enough voltage, the charge on the polymer
overcomes the surface tension of the solution and causes emission of a fine polymer
jet. This jet undergoes a whipping process, and the fibers are further elongated as the
solvent evaporates and fibers are deposited on a grounded collector (Fig. 1.5). Both
natural and synthetic polymer scaffolds have been successfully created using the
electrospinning method. The ability to generate three-dimensional scaffolds with
tailored architecture, mechanical properties, and degradation characteristics has
made electrospinning a popular method in tissue engineering applications. Altering
parameters during the electrospinning process, such as polymer concentration, flow