3D Electrospinning: the combination of electrospinning and 3D-printing for the fast fabrication of designed 3D polymeric macrostructures made of nanofibres
Nanofibrous structures, due to their unique morphology boasting a high surface areato-volume ratio, make them interesting in several research fields. Added to that, recent studies have highlighted the important benefits of assembling 3D structures with nanofibrous features. For example, 3D scaffolds made of nanofibres have been shown to have better cell attachment and growth, because of their close resemblance to the natural extracellular matrix. 3D nanofibrous structures have also been used in high filtration processes. Electrospinning is a good candidate for building these 3D nanofibrous structures. The high versatility of electrospinning allows it to change the morphology of the electrospun fibres easily (porosity, diameter, surface roughness, fibres alignment, etc…). The nature of the polymer used in electrospinning is flexible as well, and with the existing possibility for further functionalizing electrospun fibres, electrospinning is applicable to a wide range of research fields. Furthermore, several methods of inducing 3D build-up via electrospinning have been investigated, however, these methods have several disadvantages such as being time-consuming, made of several steps, requiring an extra support material, or having no control over the shape of the final 3D structures. In this thesis, a device combining the versatility of electrospinning with the manoeuvrability of 3D printing is studied. By inserting specific additives to a polymer solution, self-assembly of 3D structures via electrospinning is possible. The precise control of the movement of the nozzle head during electrospinning, as well as the setting of the collector height allow to direct the position of the deposition area during the whole electrospinning process. By combining these two features together, it is possible to fabricate a designed 3D polymeric macrostructure made of nanofibres, from a simple computer-aided design (CAD) file. Thus, this technology is named “3D electrospinning”. The first aspect of this thesis is to have an in-depth look at the formation mechanism of 3D electrospun structures. The process parameters of 3D electrospinning have been identified and investigated to better understand the formation mechanism of the 3D build-up for polystyrene (PS), the model polymer. It is shown that the crystal phase of the polymer itself, the viscosity and the conductivity of the polymer solution have no influence on the 3D build-up of the electrospun structures. It was instead shown that the rapid solidification of the fibres as well as the in-situ charge induction and polarization of the fibres are inducing the 3D build-up. Overall, it is possible to build a 3~4 cm high macrostructure in a single step in 10 minutes of electrospinning. A thorough study of the experimental parameters of 3D electrospinning allows to optimise the shaping of the 3D electrospun structures, in terms of wall resolution and fibres’ morphology. It is shown that the improper adjustment of any parameters such as polymer concentration, applied voltage, working distance, flow rate or nozzle moving speed can have detrimental effects on the 3D build-up and instead leads to an electrospun flat 2D mat. After identifying the optimal experimental parameters, several shapes such as triangle, square, star or smiley face, are electrospun to showcase the versatility of the 3D electrospinning process. Other polymers such as polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) are then 3D electrospun to extend the range of usable polymers, and thus potential applications, for 3D electrospinning. The differences in shape induced by each polymer are identified. Different types of electrodes are then used to change the electric field profile and alter the path of the electrospun jet. Base electrodes and steering electrodes are shown to have detrimental effects on the 3D build-up leading to either poor drying of the fibres or poor shaping of the 3D structures. Guiding electrodes (electrode at the collector level) were able to further enhance the shaping of the 3D electrospun structures, with an increased wall resolution and no noticeable drawback. However, this beneficial effect was only shown for polystyrene. A pillar support was used as a guiding electrode to force the fabrication of an electrospun 3D structure with radial alignment. The effect of the pillar height, pillar thickness, applied voltage and working distance on the 3D structure and fibres alignment is shown. Finally, the long-term stability and the mechanical stability of the 3D electrospun structures have been investigated. While both PS and PAN structures show high shelflife in ambient conditions, only 3D PS structures demonstrate some shape recovery after compression. Upscaling of the 3D structures was then achieved with both the 3D electrospinning device and a nozzle-free electrospinning setup. Similar to extrusion-based 3D printing, it is possible to raise the working distance during 3D electrospinning to increase the final height of the 3D structure. 3D electrospinning with a nozzle-free electrospinning setup is possible via precise control of the rotation speed during 3D build-up. This opens up the possibility to fabricate electrospun 3D structures on a commercial scale. Carbonization of 3D PAN structures is also demonstrated, to fabricate carbon fibrous structures with 3D features, which can have applications in energy-related fields. The work conducted in this thesis has successfully expanded upon the field of 3D material fabrication. 3D electrospinning is a simple, cost-effective and fast process to build designed 3D structures. It is a versatile process not limited to a single type of polymer. As such, 3D electrospinning is a viable technique for several applications. 3D PS and 3D PVP could both be used as a cell culture material. 3D PAN, which is a typical precursor for carbon fibres, could be used as an electrode material for Lithiumion batteries.