Controlling the spatial deposition of electrospun fibre
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Electrospinning process is a simple and widely used method for producing polymeric nanofibres. However, despite its popularity, significant challenges remain in controlling the fibre deposition due to the complex nature of electrospinning process. The process is renowned for its chaotic motion of fibre deposition, also known as the whipping instability. This instability is caused by electrostatic and fluid dynamics interactions of the charged jet and it is partly responsible for the thinning of the fibres into nanoscale diameters. Due to the instability, an electrospinning process typically deposits random orientated fibres in a circular deposition area. Furthermore, there is no control over the location where the fibres land on the collector electrode except that the fibres always travel through the shortest trajectory between the source and the collector electrodes. In this study, an alternative controlled deposition technique was proposed based on electric field manipulation (EFM). The main hypothesis of this study is that a consistent and repeatable method of controlled deposition can be achieved by using EFM. EFM was achieved by introducing a pair of charged auxiliary electrodes positioned adjacent and perpendicular to the fibre deposition direction. The applied voltage of either direct current (dc) or time-varying (ac) voltage at the auxiliary electrodes act as control to influence the spatial location and size of the deposition area. Samples were produced on black paper substrates and scanned into greyscale images. An image analysis technique was developed to measure the shift and size of the deposition area. A computer simulation was used to calculate the electric field strength and to simulate the behaviour of fibre response based on the trajectory of a charged particle. An image analysis based on greyscale intensity measurement was also developed to examine the uniformity of the deposition area. Finally, fibre characterisation was carried out to examine the fibre morphology, diameter, and orientation based on scanning electron micrographs. The results from this study showed that EFM can provide a consistent and repeatable control of the deposition area. When the auxiliary electrodes were independently charged with two dc voltages, it was observed that the deposition area moved away from the most positive electrode. The magnitude of shift of the deposition area was found to increase linearly with voltage difference between the auxiliary electrodes. Furthermore, the aspect ratio of the deposition area (ratio of width over height) decreased linearly with base voltage i.e. lower of the two auxiliary electrode voltages. These two controls were found to act independently from each other and can be described as two separate controls i.e. voltage difference for spatial location and base voltage for aspect ratio of the deposition area. A similar response was observed in simulation i.e. the particle moved away from the most positive electrode. Simulation results also showed that the x-axis component of the electric field (Ex) was responsible for the shift in location and the reduction of aspect ratio of the deposition area. When the auxiliary electrodes were charged with two antiphase time-varying voltages, continuous scanning of the electrospinning jet was observed producing a wide electrospun fibre mat. It was first thought the smooth oscillation of a sine wave would produce a more uniform deposition pattern compared to a triangle wave, but the results showed otherwise. The inferior uniformity of the sine wave sample was found due to the variability of the jet scanning speed when compared to the constant speed achieved when using a triangle wave. It was also observed that the deposition pattern can be further improved by using two clipped triangle wave voltages. The results open up the possibility for further exploiting the control voltage to achieve the desired deposition pattern. Two case studies were presented to demonstrate the applicability of the technique in real electrospinning applications. In the first case study, it was demonstrated that the continuous scanning of electrospinning jet was capable of eliminating the stripe deposition pattern which is commonly associated to a multi-spinneret electrospinning system. In the second case study, it was found that the alignment and distribution of aligned fibres in a gap electrospinning system can be improved by using the EFM technique. A new technique was also introduced to produce a multi-layer orientated fibre construct. These application examples showed that the EFM technique is ready for the production of engineered electrospun fibre constructs. This would extend the use of electrospun fibres to applications which is currently limited by geometrical constraints of the fibre constructs.