# Poly(lactic acid)-Based Electrospun Fibrous Structures for Biomedical Applications
## 4 Electrospinning of PLA Fibers
### Solvent Systems  
Different research works performed on basic concepts of electrospinningn have been approved so that the solvent system has a great influence on the ultimate properties of the electrospun fibers and structures, including morphology and mechanical properties. The vapor pressure of the solvent characterizes its evaporation velocity and so the solidification speed of the polymer jet. Generally, a very high evaporation rate is not proper for the electrospinningn, because the jet may solidify immediately after exiting from the nozzle. Since the volatility is too low, the fibers may still be wet when deposited on the collector. In addition, the dielectric constant of the solvent is also important; this feature controls the value of the electrostatic repulsion. Depending on the dielectric constant of the solvent, the required applied voltage during the electrospinningn process will differ [35]. Dichloromethane (DCM), chloroform (CHCl\({}_{3}\)), dimethylformamide (DMF), hexafluoroiso-propanol (HFIP), and 2,2,2-trifluoroethanol (TFE), are the commonly used solvents for electrospinningn of PLA-based solutions [35]. Several attempts have been made to consider the effect of the solvent system on the electrospinningnability of the PLA and the ultimate properties of the fibers. In research works performed by [PERSON] et al., the effect of solvent type on the electrospinningn of PLA has been studied [36; 37]. For this purpose, different solvents of CHCl\({}_{3}\), DCM, TFE, or HFIP were considered for electrospinningn of the PLA-based structures applicable for medical devices. The scanning electron microscopy (SEM) images of electrospun PLA structures from different solvents are illustrated in Figures 4 and 5. Figure 4: SEM images of fibers electrospun from PLA solutions using different solvents of (**a**) HFIP (unpublished original picture by the authors), (**b**) TFE (unpublished original picture by the authors), (**c**) CHCl\({}_{3}\) (unpublished original picture by the authors), (**d**) DCM (unpublished original picture by the authors). The use of solvents with different features led to notable variations in the shape, surface morphology, and diameter of the electrospun fibers. HFIP (Figure 4a) and TFE (Figures 4b and 5a) based electrospun fibers had smaller diameters with a smooth surface morphology and were more uniform compared to those obtained with other solvents. In contrast, using CHCl\({}_{3}\) and DCM as solvents, broad fiber diameter distributions were obtained with higher diameters (Figures 3 and 4) [36; 37]; moreover, the electrospinning PLA solutions using solvents with high vapor pressure and lower boiling points, such as DCM (Figures 4d and 5b) and CHCl\({}_{3}\) (Figures 4c and 5c), may produce nanofibers with porous surface morphology. These pores form due to the solvent evaporation and successive phase separation during the procedure [36; 37]. In addition to the morphology and diameter, the studies have revealed that the characteristics of the solvent affect the crystallinity and consequently the mechanical properties of the electrospun structure. In a study performed by [PERSON] et al. [36; 37], it was found that in the case of the solvents with high vapor pressure, such as DCM, it is not enough time for crystal growth, thus the electrospun fibers had lower crystallinity. When using solvents with lower volatility, such as TFE, however, the crystals have more time to grow and this caused to development of a structure with relatively high crystallinity; in addition, the dielectric constant of the solvent is also effective. The solution conductivity influences the electrostatic forces which are required to apply during the electrospinning. For example, the lower conductivity of PLA/DCM solution caused a decrease in repulsive coulombic forces on the charged jet and resulted in less stretching on the way between the spinneret and the collector, whereas the higher conductivity of the TFE solution boosted the molecular orientation in the electrospun PLA fibers [36; 37].