# Poly(lactic acid)-Based Electrospun Fibrous Structures for Biomedical Applications
## 4 Electrospinning of PLA Fibers
### Concentration  
In addition to the type of solvent, the electrospinningnability of PLA-based solutions mainly depends on their concentration. The concentration can also affect the viscosity and surface tension of the solution, and both factors may change the morphology and diameter of the electrospun fibers. Aiming to form fibers, a minimum concentration is required to attain the chain entanglement, which is critical to shift from electrospray (i.e., droplet forming) to electrospinning (i.e., continuous jet forming). At a concentration lower than this limit, the jet will break into droplets and thus polymeric beads will form, instead of continuous fibers. By gradually increasing the concentration, the jet may not break up and the shape of the beads will change from spherical to spindle-like (Figure 6a). At high enough concentrations, the adequate viscosity and surface tension allow fabricating uniform and beadles fibers, while at very high concentrations, it is extremely hard to overcome the viscoelastic forces and no jet will be formed [35; 36; 38].  
Figure 6: Different morphologies of PLA fibers are obtained by tuning the electrospinningn parameters: (a) bead-on-string morphology (unpublished original picture by the authors), (**b**) cylindrical shape with a smooth surface (reproduced from an open access paper [4] distributed under the terms of the Creative Commons CC BY license, (**c**) flat ribbon-like morphology, reproduced from an open access paper [4] distributed under the terms of the Creative Commons CC BY license, (**d**) porous reproduced from an open access paper [4] distributed under the terms of the Creative Commons CC BY license, (**e**) bead with the porous surface (unpublished original picture by the authors), (**f**) core–shell structure reproduced from an open access paper [9] distributed under the terms of the Creative Commons CC BY license.  
In an appropriate range, decreasing the concentration leads to thin fibers. The research revealed that at high solution concentration, the average diameter of fibers increased [36]. The low concentrations with reduced viscosity facilitate the elongation of the jet under the electrical field. At high concentrations, under the same electrical forces, the viscous nature of the solution hinders the jet to be completely stretched and results in fibers with increased diameters [36; 38].  
In addition, the solution concentration may affect the crystallinity of the PLA nanofibers. In a research work performed by [PERSON] et al., it was reported that the increase of the solution concentration led to a significant reduction in crystallinity of electrospun PLA fibers. With increasing solution concentrations from 5 wt% to 9 wt%, PLA fibers showed a decrease of around 47%. The lower viscosity of solutions with low concentrations boosts the mobility of molecular chains to arrange to the fiber axis during electrospinning; this allows a higher degree of molecular orientation. In addition, a lower solution concentration has a slower solidification procedure, so affords more time for jet stretching which also leads to molecular orientation in the fiber structure [36].  
The solution concentration also appeared to have a considerable influence on the mechanical performance of the electrospun PLA structures. The experimental results obtained by [PERSON] et al. [38] showed an increase in tensile strength and modulus of electrospun PLA varns by increasing the solution concentration from 5 wt% up to around 7.5 wt%. Beaded fibers were formed at a lower concentration of 5 wt %, which almost influenced the mechanical features of the electrospun PLA structures. By further increasing the concentration from \(\sim\)8 wt % up to 9 wt %, the fiber diameter enhanced and both tensile strength and modulus were reduced [38].