Page 111 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

allowing the creation of sca olds that closely mimic the mechanical anisotropy of a heart valve leaflet.55
Most of the tissue engineering studies depend on the reproducibility and control of the fiber geometry that electrospun architectures promise to provide. In the past, this has led to significant e orts in the field of electrospinning focused on elucidating the influence of processing parameters on both the fiber diameter as well as morphology. The properties of the polymer solution have a large influence in the electrospinning process and on the resulting fiber diameter and morphology. So will the viscosity of the solution and its electrical properties define the elongation of the jet, thereby influencing the diameter of the resulting electrospun fiber, whereas surface tension is crucial in the process for spinning either beaded or straight fibers. With increasing the viscosity the diameter of the electrospun fiber increases.77–79 This may be attributed by the increased resistance of the solution to be stretched in the jet.80 Increased fiber diameter with higher concentration of the polymer solution is attributed by the jet instability that occurs at increasing distance from the needle. As a result, the jet path is reduced, resulting in less stretching of the polymer solution in the jet. With increased conductivity of the solution, more charges can be carried by the jet. The conductivity can be increased by adding salt or polyelectrolyte or selecting solvents with a higher conductivity. Since the stretching of the jet is caused by repulsion
of the charges at the surface, a higher conductivity
will increase the stretching rate of the solution in the
jet. As a result, beading in fibers is suppressed. In
addition, the fiber diameter is reduced as the higher
conductivity causes the jet instability to shi towards
the needle.81 This results in the opposite e ect of
the higher viscosity; the jet path is increased, more
stretching occurs and the fibers are spread over a
bigger area on the target.82 The same occurs with a
higher dielectric constant of the solution, resulting
in a reduced fiber diameter and suppression of
bead formation83–85 The fiber diameter can further
be adjusted by external factors. Increasing the
polymer feeding rate directly increases the diameter.
Increasing the field strength, by either varying the
needle to target distance or the applied voltage, 5 creates greater columbic forces in the jet, resulting
in increased stretching of the solution in the jet and thus, smaller fiber diameters.86,87 This process occurs up to a point when the higher voltage increases the acceleration of the electrospinning jet, resulting in decreased flight time and hence, thicker fibers.88
By using the various processing parameters described here, fiber diameters from ~4 nm up to about 20 μm can be electrospun. This gives ways to mimic the heart valve ECM in a similar level of complexity. Micron sized fibers give structure and mechanical stability and can mimic the collagen structure in the heart valve leaflet. Nanometer fibers, on the other hand, have a much bigger surface
ELECTROSPINNING FOR HEART VALVE TISSUE REGENERATION
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