Page 50 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 2
instability time and the solvent drying time. Water vapor di usion into the elongating jet happens faster if the surrounding environment is closer to saturation and the solvent system is hygroscopic, so that phase separation kinetic is also influenced by relative humidity and solvent properties. Moreover, the dependence of surface porosity on the molecular weight of the polymer further confirms the role of the kinetics of phase separation.8,15
2.4.3 Minimum uptake speed for fiber alignment
At constant temperature and relative humidity fiber orientation is gained by increasing the rotation speed of the cylindrical collector, as can be observed from the SEM images of Figure 2.3. Figure 2.3d shows the monolayer-like structure deposited on the collector at high rotation speed. See also Figure 2.5 for more details on the aligned morphology. The degree of alignment was quantified via FFT analysis of the SEM micrographs, and the resulting output images are reported in Figure 2.3e-h. Randomly oriented fibers result in a symmetric distribution (Figure 2.3e), i.e. the intensity (a.u.) is equally distributed in azimuthal direction (along the red line in Figure 2.3e), see Figure 2.3i. Intensity distribution narrows in the equatorial direction (perpendicular to the fiber direction) with increasing alignment, Figure 2.3j-k, and turns slightly broader again when the self-assembled monolayers occur, Figure 2.3k. The identified critical uptake speed leading to the minimum WHH for each value of relative humidity
(at 20°C) are summarized in Figure 2.4. Below values of 50%, relative humidity seems to have no e ect on the critical uptake speed, which remains at 1.75±0.15 m/s. Whereas with critical uptake speeds of 2.1±0.1 m/s and 2.3±0.15 m/s at relative humidity of 70% and 90% respectively, a significant (p<0.05) higher rotation speed is necessary to gain fiber orientation at higher relative humidity levels.
2.4.4 Self-assembling
Increasing the uptake speed above the minimum speed required to align fibers leads to vertical monolayer like structures, as reported in Figure 2.3d. When these self-assembled monolayers occur the pixel intensity in the corresponding FFT output images turns broader again (Figure 2.3h).
Figure 2.5 shows details of these structures. It is evident that fibers are collected during electrospinning as self-assembled vertical monolayer of perfectly aligned fibers. The wavy appearance of the material at low magnifications is an artifact, induced when removing and flatten the electrospun mesh from the target, due to the di erence in inner and outer radius of the construct. When collecting fibers for 5 minutes, these structures could already be obtained when the uptake speed was 0.1 m/s above the minimum uptake speed required for spinning aligned fibers. The first layers on the target were like in Figure 2.3c; hence the transition from highly aligned fibers to these monolayer-like structures is time dependent.
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