(Invited) Atmospheric Pressure Plasma-Assisted Electrospinning of Bead-Free Honeycomb-Patterned Nanofibers Via a Controlled Self-Assembly-Driven Phenomenon

Abstract

The end of the 20th century has marked an enormous surge of interest in electrospinning after discovering its capacity of generating fibers from various polymers. This technique has actually shown several attractive strengths such as its versatility, affordability, simplicity and ability to adjust the diameter of the electrospun fibers from hundreds of micrometers down to tens of nanometers. The large surface area to volume ratio and high porosity of electrospun structures have rendered the technology appealing in numerous applications e.g. optical devices, catalyst supports, batteries, textiles and sensors. Moreover, electrospun fibers are very promising candidates in tissue engineering applications given their biomimicry of the natural extracellular matrix. The massive research advances in electrospinning have even led to the generation of fascinating bioinspired-patterned fibrous structures such as the lotus leaf, feather, silver ragwort leaf, spider web, plant tendril, bear hair and honeycomb. In particular, honeycomb-patterned nanofibrous structures are potentially very promising as tissue engineering scaffolds as they exhibit, in addition to the large surface area, a high structural stability and an architecture integrating interconnected microporous voids with nanofibers. Such structures were as such shown to synergistically promote the uniform infiltration of cells and their adhesion, proliferation and differentiation thus creating a stimulating microenvironment for the regeneration of different tissues such as blood vessel and bones. In fact, the geometry of the honeycombs also mimics the natural microvascular network and the spongy structure of the bone tissue. Interestingly, some research groups have introduced the possibility of electrospinning honeycomb-patterned nanofibers in a simple and rapid way by carefully varying some process parameters such as the polymer solution concentration and applied voltage without even the use of a sophisticated patterned collector. Actually, at a sufficiently low concentration, wet and still charged nanofibers can be deposited on the collector and can subsequently self-assemble into honeycomb structures via competitive actions between: 1) the surface tension that drives the fibers to merge together at a cross point and 2) the electrostatic repulsion that, in contrast, pushes adjacent sections away to form polygonal channels with three-branched walls. Polyacrylonitrile, polyvinyl alcohol, polyethylene oxide, polyurethane and polycaprolactone (PCL) were reported to be successfully electrospun into honeycomb structures via a self-assembly-driven phenomenon. Nonetheless, the low solution concentrations used to trigger this self-assembly have always led, in all the reported results so far, to the formation of beaded poor-quality nanofibers. In order to overcome this issue and obtain for the first time in literature bead-free fibrous honeycomb structures, the present work aimed at performing a pre-electrospinning treatment of a PCL solution by a non-thermal plasma generated directly inside the solution itself. To do so, a distinctive atmospheric pressure plasma reactor engendering a plasma jet with an afterglow in contact with the solution was designed in-house and was used with argon as working gas. This treatment is expected to trigger a plasma-induced degradation of the solvent molecules generating new species such as HNO3 and HCL thus enhancing the solution conductivity and the polymer electrospinnability. An extensive parametric study of the electrospinning process was first conducted with different solvents systems (chloroform/dimethylformamide (DMF) 4/1, dichloromethane (DCM)/DMF 4/1, DCM/DMF 3/2 and formic acid/acetic acid 9/1), PCL molecular weights (80000, 45000 and 37000 g/mol), polymer concentrations (ranging from 6 to 20% w/v), applied voltages and tip-to-collector distances. Concurrently, the operational plasma parameters (gas flow rate, applied voltage and plasma exposure time) were delicately tailored to treat the different solutions prior to the electrospinning. Results revealed that the solvent system chloroform/DMF 4/1, a PCL weight of 37000 g/mol, a PCL concentration of 17.5% w/v, an applied voltage of 22.5 kV and a tip-to-collector distance of 15 cm led to an optimal self-assembly of the PCL fibers into a clear honeycomb pattern as visualized by scanning electron microscopy images. Moreover, for all the used electrospinning parameters, a pre-plasma treatment of the solution triggered a significant enhancement in the fibers quality with less to no beads. In particular, a plasma treatment time of 15 s, argon flow rate of 0.5 slm and applied voltage of 3 kV combined with the pre-mentioned optimal electrospinning parameters resulted in well-balanced morphology combining a honeycomb pattern with bead-free fibers. Overall, this work introduces a novel interesting application of atmospheric pressure plasma treatment illustrated by the generation of high-quality bead-free honeycomb-patterned nanofibrous structures. Figure 1