## 5 Additives in Melt Electrospinning
### Salt, Viscosity Modifiers, Stabilizers and Dyes
PCL fibers have shown great advantages, such as biodegradation and non-toxicity, which make them excellent candidates in tissue engineering and wound dressing. However, the superhydrophobicity of PCL makes it insoluble in nontoxic or scarcely toxic solvents, such as water or dimethyl sulfoxide [77]. Therefore, melt electrospinning makes a great solution to manufacture PCL fibers for biomedical applications such as drug delivery, wound healing and tissue engineering [78]. In these applications, the improvement of the electrical conductivity and the decrease of melt viscosity should logically be performed by adding nontoxic additives. Sodium chloride (NaCl) is an excellent option due to lack of toxicity and easy extraction from the obtained fiber [79]. [PERSON] et al. [80] observed that whilst continuous and smooth PCL fibers could be melt electrospun without any additive, their diameter was quite large (\(\sim\)23 \(\upmu\)m). On the other hand, the addition of NaCl up to 8 wt.% decreased the size of the fibers down to \(\sim\)3 \(\upmu\)m, but led to slightly rough surfaces and the disruption of the fiber continuity. A higher content of NaCl (i.e., 25 wt.%) had the opposite effect and gave continuous and smooth fibers and a diameter increase up to values similar to those of pure PCL (Figure 7). The melting point was also altered, giving the lowest value at a concentration of 8 wt.% (i.e., 55 \({}^{\circ}\)C). This change was attributed to the size of crystalline domains. Curiously, some NaCl was found in the glass syringe after the process, thereby indicating a non-complete ejection through the nozzle. Salts have also been used as additives in the melt electrospinning of PLA fibers, the most commercially available biobased and biodegradable polymer. This semi-crystalline thermoplastic has a vast number of applications because of its high tensile strength and Young's modulus. Brittleness and high hydrophobicity are the main drawbacks of PLA that may limit, for example, the number of biological applications. Blending PLA with softer polymers or forming composites would be good solutions to improve properties [81]. [PERSON] et al. [82] studied the effect of NaCl, SS, and a polyester (polyethylene succinate, PES) on PLA properties at both laboratory and pilot scales. PES can be employed as a plasticizer to reduce the viscosity of polyesters (e.g., PLA), PP and polyamides. At laboratory scale, a single-fiber melt-electrospinning device was used, and different temperatures were tested depending on the additive. Melt electrospinning of PLA/NaCl and PLA/PES was performed at 250 \({}^{\circ}\)C, whereas PLA/SS was melt electrospun at 190 \({}^{\circ}\)C to avoid degradation. Fiber diameter were in the order of 16 to 20 \(\upmu\)m for additive contents of 2-4 wt.%. SS was the only additive that provided both thinner fibers and a stable melt-electrospinning process. Therefore, addition of SS was tested at pilot scale considering different concentrations, spin pump speeds, and temperatures. Results pointed out that using a 6 wt.% of SS, an spineret temperature of 195 \({}^{\circ}\)C and a spin pump speed of 0.5 rpm allowed the obtainment of the minimum fiber diameter, which was around 3.8 \(\upmu\)m.  
Addition of three biobased dyes (i.e., alizarin, quercetin, and haematoxylin) to increase PLA conductivity has also been evaluated [83]. These environmentally friendly compounds have as main advantage a good solubility/affinity with PLA. Haematoxylin provided the lowest fiber diameters values (i.e., 16.04 um at 2 wt.%), as consequence of its plasticizing effect and unfortunately for causing a significant polymer degradation. The presence of the indicated additives led to a fivefold decrease of PLA electrical resistance (around 5.0 G\(\Omega\)), being alizarin the more conductive compound [84]. [PERSON] et al. [85] studied the effect of acetyl tributyl citrate (ATBC) on the diameter of PLA melt electrospun fibers.