Controlled dielectric breakdown in microfluidic devices

Controlled dielectric breakdown is a fabrication technique based on the use of electric fields in an electrolyte solution. It is one of the easiest methods for seamless integration of nanofractures into a mic Controlled dielectric breakdown is a fabrication technique based on the use of electric fields in an electrolyte solution. It is one of the easiest methods for seamless integration of nanofractures into a microfluidic device. This method relies on applying a voltage across a dielectric insulating material by setting a threshold current limit. Based on the literature, this is a very convenient nanofabrication technique for achieving dimensional control at the nanometre scale by changing the breakdown current threshold. When a dielectric material is placed in the electric field, it gets polarised. In a polarised material, each atom of the material gets stretched to the corresponding opposite charge accumulated in the conductive capacitor plates. With an increase in the electric field, the atoms in the dielectric material become more stretched. However, when the strength of the electric field surpasses the material's dielectric strength, a partial electric discharge starts to occur continuously, resulting in a breakdown. This thesis presents some factors which significantly affect the dielectric breakdown method, namely the electrolyte concentration and design of the microfluidic device. In this thesis, a nanometre-sized hole formed by dielectric breakdown defines the nanostructure. The hole was created by a localised process in the dielectric material due to the intense electric field. It hasn't been etched, molded, or ablated. The dielectric breakdown resulted in the formation of the nanometre-sized hole, which may involve vaporization of the material at the site of the breakdown. The first aim of this thesis was to understand the impact of a variation in the electrolyte concentration on nanofracture fabricated by dielectric breakdown. A three‐dimensional‐ printed microfluidic device made of a thermoplastic material was used to study the creation of nanofractures by controlled dielectric breakdown to function as a molecular filter. The device was made from acrylonitrile butadiene styrene by a fused deposition modeling three‐ dimensional printer and consisted of two V‐shaped sample compartments separated by 750 µm of extruded plastic. Nanofractures were formed in the thin piece of acrylonitrile butadiene styrene separating the two V channels by controlled dielectric breakdown through application of voltage (15–20 kV) with the voltage terminated when reaching a defined current threshold. Variation of the size of the nanofractures was achieved by both variation of the current threshold and by variation of the ionic strength of the electrolyte used during the breakdown step. Electrophoretic transport of two proteins, R‐phycoerythrin (RPE; <10 nm in size) and fluorescamine‐labeled BSA (f‐BSA; 2–4 nm), was used to monitor the size and transport properties of the nanofractures. Using 1 mM phosphate buffer, both RPE and f‐BSA passed through the nanofractures when the current threshold was set to 25 μA. However, when the threshold was lowered to 10 μA or lower, RPE was restricted from moving through the nanofractures. When the electrolyte concentration was increased during breakdown from 1 to 10 mM phosphate buffer, BSA passed but RPE was blocked when the threshold was equal to, or lower than, 25 μA. This demonstrates that nanofracture size (pore area) is directly related to the breakdown current threshold but inversely related to the concentration of the electrolyte used for the breakdown process. The second aim of the thesis was to examine the internal geometry of the device to understand the impact on dielectric breakdown. Controlled dielectric breakdown was performed in microfluidic devices fabricated by using two thermoplastic materials cyclic olefin copolymer (COC) and Poly(methyl methacrylate) (PMMA). The COC device was made by injection moulding, and the PMMA device was created by Hot-embossing. Both devices contained two V-shaped sample compartments separated by a defined plastic gap. Channels were formed in both COC, and PMMA devices by controlled dielectric breakdown by application voltage of 4 kV with voltage terminated when reaching a defined current threshold. The influence of alterations in the current threshold and microchannel dimensions was examined using imaging technologies, including light microscope and scanning electron microscope (SEM) to visualise the channels created by controlled dielectric breakdown. As established in the first aim, the size of the channels depends on the breakdown current threshold. Additionally, the cross-sectional area of the conductive capacitor plates, which are the opposite parts of the plastic gap between the two V-shaped compartments, was found to significantly impact the channel size. The charge accumulation on the capacitor plates and the electrostatic force also substantially affect the channels created by controlled dielectric breakdown. The larger area of the capacitor plates resulted in larger channels, a crucial finding for the fabrication of micro/nano-channels in fluidic devices by controlled dielectric breakdown. The third aim examined of the thesis, dielectric breakdown method in a microfluidic chip printed with a Polyjet 3D-printer in different orientations. The Objet Eden260VS 3D printer was used to print microfluidic devices in the VeroClear™ material. The previous double V-shaped design was used again with different thickness gap (16 μm, 32 μm, 48 μm, 96 μm, 150 μm and 200 μm) between the two V-shaped microchannels. The devices were printed in six different orientations: Orientation 1 (flat, with the insulating gap in the X-axis), Orientation 2 (flat, with the insulating gap in the Y-axis), Orientation 3 (laying on the edge, with the insulating gap in the X-axis), Orientation 4 (laying on the edge, with the insulating gap in the Y-axis), Orientation 5 (standing upright, with the insulating gap in the Y-axis) and Orientation 6 (standing upright, with the insulating gap in the X-axis). Of the six orientations, breakdown was only observed in the chips in two orientations (Orientation 5 and Orientation 6) with gap thickness of 16 μm, 32 μm and 48 μm. It was observed from the dielectric breakdown experiment that the breakdown mechanism of solid insulating material under the influence of a high electric field is similar to the function of a capacitor shorting. Therefore, the experimental setup can be considered as a physical resistor-capacitor (RC) circuit system. The plastic region between the two microchannels (breakdown area) will act as a capacitor while the electrolyte in the channels will act as resistors. Experiments conducted with a gap thickness of 48 μm and varying the shape of the 'V' microchannels in the third aim reinforced the findings of the second aim, as changes in the area of the capacitor plates influenced the size of the fractures produced. Different orientations led to variations in the distribution of the electric field and charge accumulation, ultimately affecting the efficiency of the breakdown process. This finding emphasizes the need to optimize the 3D-printing process by selecting the appropriate device orientation to ensure successful application of controlled dielectric breakdown in microfluidic devices. This optimization should be carried out in conjunction with the previously established parameters from the first and second aims, such as breakdown current threshold, electrolyte concentration, and capacitor plate area, to achieve precise control over the size and properties of the nanofractures formed. The third aim, therefore, complements and builds upon the findings of the first two aims, emphasizing the importance of considering all aspects of the fabrication process to achieve reliable and consistent results in fabrication of nanofracture in a microfluidic device using controlled dielectric breakdown.