Page 180 - Biodegradable Polyesters

158 7 Electrospun Scaffolds of Biodegradable Polyesters: Manufacturing and Biomedical Application

(a) (b)
Figure 7.1 Electrospun fibers produced with 8% (a) and 20% (b) PLGA w/v in HFIP.

sufficient viscosity for it to be stretched during electrospinning, which generates
a stable and continuous solution jet until final deposition on the collector [19, 24].
Solution concentration is a major determinant of morphology and fiber diameter.
There was a common consensus in the research groups that the polymer con-
centration and/or its viscosity have direct relationship with the electrospun fiber
diameter [25, 26]. At low polymer concentrations, fibers with smaller diameters,
in the nanometer range, are formed. However, solutions with low concentrations
of polymer can generate an unstable electrospinning jet, resulting in droplet
formation and broken and bead fibers. Meanwhile, when solutions with higher
concentrations of polymers are used, thicker fibers, without beads are obtained.
Thus, it is possible to create different types of fiber scaffolds with the same
polyester and solvent system, but with a concentration variance. One example
of this can be visualized in Figure 7.1, where the fibers were produced with
PLGA (poly (lactic-co-glycolic acid)) in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP),
using the same electrospinning parameters (voltage, flow rate, and distance from
needle to collector plate). The concentration of PLGA was the only parameter
which varied. The molecular weight of the polymer is another factor which
affects the viscosity of the solution. The increase of molecular weight results
in greater entanglement of the polymer chains, increasing the viscosity of the
solution [19].
Different solvents are employed to dissolve polyesters and to prepare the poly-
meric solution for electrospinning. The solvent used in polyester solutions has a
great influence on its spinnability. For successful fiber formation, the solvent needs
to be correctly selected. The solvent should maintain the integrity of the polymer,
show good volatility, and appropriate dielectric constant [21].
Listed in Table 7.1 are the most commonly used biodegradable polyesters to
create electrospun scaffolds for tissue engineering applications. The correspond-
ing solvents and concentrations of polymer solutions reported in the literature are
also described.