Development of a Digital Microfluidic Toolkit: Alternative Fabrication Technologies for Chemical and Biological Assay Platforms

This thesis proposes the development of a digital microfluidics (DMF) device using alternative fabrication methods and materials for application in chemical and biological assays. DMF technology which relies on electrowetting-on-dielectric (EWOD) mechanism, offers several advantages such as reduced sample volume, faster analysis, device flexibility, and portability. It is however not without shortcomings as the fabrication of DMF devices is expensive while the reliability of such devices is reduced due to surface contamination when highly concentrated biomolecular samples (e.g. protein and cells) are used. The first experimental work in this thesis aims to reduce the cost of electrode patterning of DMF devices by investigating the use of inkjet printing method in conjunction with several combinations of conductive ink and substrate. It has been found that EWOD device made of PEDOT:PSS, a type of conductive polymer ink printed on Melinex®, a polyethylene terephthalate substrate presents the most reliable droplet actuation performance with velocity comparable to the standard chrome-on-glass device. Two types of inkjet-printed PEDOT:PSS-on-Melinex® device have been fabricated; one is a 3D 4 × 4 electrode array device and the other is a magnetic micro-immunoassay device establishing the feasibility of the proposed method. The 3D 4 × 4 electrode array device which utilises both sides of the substrate (i.e. top and bottom surfaces) for electrode patterning allows for future construction of multi-level DMF devices with large functional area. Implementation of such electrode design increases throughput as it made multiple parallel assays possible. The second inkjet-printed device demonstrates the possibility of employing the PEDOT:PSS-on-Melinex® device in heterogeneous immunoassay by successfully performing mixing and merging of two droplets and more importantly the magnetic beads separation operation. The second experimental investigation concerns the search for substitute materials for the dielectric and hydrophobic components of EWOD device using off-the-shelf products. For the dielectric component, the best performing material in terms of electrowetting reversibility is Rust-Oleum® Polyurethane Finish while for the hydrophobic surface is Top Coating of NeverWet® superhydrophobic material. Both are low-cost materials which employ a very simple spraying technique as their fabrication method. The NeverWet® superhydrophobic material has been selected for detailed investigation due to its other potential function as an anti-biofouling surface to either eliminate or minimise the biomolecules adsorption problem. The superhydrophobic material has shown great potential by demonstrating droplet contact angle reversibility and low roll-off angle for highly concentrated protein solution indicating low adsorption of protein on its surface. A superhydrophobic EWOD device has been fabricated using the Top Coating of NeverWet® as the actuating surface and the device has reliably transported concentrated protein droplets across its surface. It is hoped that the findings in the thesis will assist towards the future realisation of low-cost and robust DMF devices for a wide range of biological and chemical assays applications outside of conventional laboratory environment

Protein droplet actuation on superhydrophobic surfaces: A new approach toward anti-biofouling electrowetting systems

© 2017 The Royal Society of Chemistry. This is an Open Access article, distributed under the terms of the Creative Commons Attribution 3.0 Unported (CC BY 3.0) licence https://creativecommons.org/licenses/by/3.0/.Among Lab-on-a-chip techniques, Digital microfluidics (DMF), allowing the precise actuation of discrete droplets, is a highly promising, flexible, biochemical assay platform for biomedical and bio-detection applications. However the durability of DMF systems remains a challenge due to biofouling of the droplet-actuating surface when high concentrations of biomolecules are employed. To address this issue, the use of superhydrophobic materials as the actuating surface in DMF devices is examined. The change in contact angle by electrowetting of deionised water and ovalbumin protein samples is characterised on different surfaces (hydrophobic and superhydrophobic). Ovalbumin droplets at 1 mg ml-1 concentration display better electrowetting reversibility on Neverwet®, a commercial superhydrophobic material, than on Cytop®, a typical DMF hydrophobic material. Biofouling rate, characterised by roll-off angle measurement of ovalbumin loaded droplets and further confirmed by measurements of the mean fluorescence intensity of labelled fibrinogen, appears greatly reduced on Neverwet®. Transportation of protein laden droplets (fibrinogen at concentration 0.1 mg ml-1 and ovalbumin at concentration 1 mg ml-1 and 10 mg ml-1) is successfully demonstrated using electrowetting actuation on both single-plate and parallel-plate configurations with performance comparable to that of DI water actuation. In addition, although droplet splitting requires further attention, merging and efficient mixing are demonstrated.Peer reviewe