Effect of atmospheric pressure non-equilibrium plasma treatment on poly-L-lactic acid electrospinnability: Investigating the roles of plasma source and voltage waveform

Summary form only given. Poly-L-lactic acid (PLLA) is a biocompatible and biodegradable polymer, which is soluble in organic solvents, such as dichloromethane (DCM) and dimethylformamide (DMF). The latter is usually added to DCM in appropriate amount in order to increase the dielectric constant of the solution, thus ensuring a better electrospinnability. However, the boiling temperature of DMF is around 153°C (much higher than that of DCM which is about 40°C), and it is therefore difficult to completely avoid traces of residual DMF in the scaffold after the electrospinning process. Electrospun nonwoven mats, composed by nanofibers of high surface area, are suitable substrates for cell growth, therefore the possibility to produce solvent-free scaffolds for biomedical applications is a great chance to increase material compatibility. The aim of this work is to investigate the effect of different plasma sources and voltage waveforms on a PLLA solution in DCM before the electrospinning process (preelectrospinning solution). The focus of the work is the use of non-thermal process to improve the electrospinnability of PLLA in a 100% DCM solution without the need to add the high-boiling point DMF as second solvent to increase conductivity, thus avoiding the use of toxic solvents. Results will be presented concerning the process of exposure of PLLA dissolved at a concentration of 13% w/V in 100% DCM to the plasma generated by (i) a multi-gas plasma jet developed by the authors, (ii) a direct liquid phase discharge reactor and (iii) a gas phase discharge reactor with liquid electrode. Moreover, sinusoidal, triangular and square waveforms with nanosecond or microsecond rise times are here used to drive the plasma sources; the effect of bias and frequency are considered as well: since active species generation and plasma temperature are influenced by these parameters, voltage and frequency have an indirect, though not negligible, influence on electrospinnability improvement. Fiber morphology (fiber diameter distribution, presence of defect such as beads along fiber axis, etc.) and solid state properties of mats produced from a plasma treated solution are compared with those of mats fabricated from an untreated PLLA solution.