Biomedical Engineering Reference
In-Depth Information
the form of a core encapsulated by a polymer layer (co-axial electrospinning) (Figure 8.4A).
The bioactive molecules within the nanofiber mesh are subsequently released over time and
absorbed by cells during culture [81].
The release kinetics of these molecules is dependent on both the solubility characteristics
of the bioactive molecules and the degradation characteristics of the nanofiber scaffold.
Molecule diffusibility and solubility are dominant parameters that determine the release
kinetics of bioactive molecules through the nondegradable and slow-degrading scaffolds,
while for fast-degradable scaffolds, the release kinetics is also coupled with scaffold degrada-
tion [81]. Another interesting method (Figure 8.4B) for incorporation of biomolecules into
a nanofibrous scaffold is simultaneous electrospinning of a polymer solution and electro-
spraying of nanoparticles containing biomolecules [82].
Surface Biofunctionalization
Numerous surface modification protocols are available for conjugation of bioactive mole-
cules onto the nanofiber surfaces. Plasma or UV-initiated grafting treatments and chemical
hydrolysis methods like aminolysis were initially employed to functionalize the nanofiber
surfaces with simple functional groups like carboxylic acid, amine, or aldehyde groups.
Peptides, proteins, glycosaminoglycans, and other ligands have also subsequently been
conjugated onto the functionalized surfaces via chemical crosslinkers (e.g., glutaraldehyde,
carbodiimide, etc.) [81]. The easiest way to load biomolecules onto electrospun scaffolds is
to dip the scaffolds into an aqueous phase containing biomolecules (Figure 8.4A). During
physical adsorption, biomolecules which are in the form of pure solution or emulsions
might attach to the scaffolds via electrostatic forces, but such interactions might not be
strong or permanent over time [83].
Architecture Designing
Fiber Diameter
The diameter of electrospun fibers can be controlled through electrospinning parameters.
For example, when the applied voltage is increased, the fiber diameter initially decreases and
subsequently increases. The fiber diameter also increases with increased flow rate of the
polymer solution and with increased polymer concentrations. However, fiber diameter
decreases as the distance between the capillary and collector plate is increased [71]. It has
been found that increasing the solution conductivity or charge density can be used to
produce more-uniform fibers with fewer beads. It was also found that an increased fiber
diameter correlated directly to a decrease in the surface area of the electrospun mats [78].
Fiber Alignment
The most common method for aligning fibers is using a rotating mandrel as the collector. The
mandrel is rotated at a very high speed up to thousands of revolutions per minute, so that the
fibers are taken up along the circumference of the mandrel [9] (Figure 8.5A). Aligned nanofi-
brous scaffolds can also be fabricated using a rotating disk collector (Figure 8.5B) with a
sharp edge [84]. Another method for fabrication of aligned fibers is placing two parallel
conducting electrodes below the needle with a gap between them (Figure 8.5C) to collect
electrospun fibers. Since it is known that the electrospinning jet is influenced by the electrostatic
field profile, the jet would stretch itself across the gap as the field lines are attracted towards
the electrodes. This results in electrospun fibers aligning across the gap between the electrodes.
Due to the presence of charges on the electrospun fibers, mutual repulsion between the depos-
ited fibers will enhance the parallel and relatively even distribution of the fibers [9, 85].
