Advances in Microfluidics: A Lab-on-a-Disk Approach to Solve Dairy Industry Problems

Abstract

This interdisciplinary thesis reports a combination of engineering, physics, biology and chemistry to create a centrifugal microfluidic system (CMS), methods of fast prototyping of microfluidic disks and advancements in microfabrication to solve problems in the dairy industry. The case study used was the quantification of fat concentration and progesterone in milk, two of the most valuable measurements needed in production optimisation. This thesis describes in detail the assembly, rationale behind the software user interface (UI) and hardware necessary to make a robust and flexible CMS. The detection method was optical imaging. Therefore, careful studies related to different light emitting sources and the use of a complementary metal oxide semiconductor (CMOS) camera as a detector were carried out. Camera parameters that determined detection sensitivity, such as the gamma coefficient, were optimised. Different imaging analysis software tools were developed and are explained through diagrams and code. Furthermore, this research project identified the advantages and disadvantages of using different materials and background colours in the microfluidic disks. Rapid, simple microchannel prototyping is critical for the development of modern microfluidic devices and platforms. Laser cutting (ablation) using a commercially available continuous wave (CW) CO2 laser followed by thermal bonding is one of the most common approaches for prototyping in thermoplastics such as polymethyl methacrylate (PMMA). However, this technique suffers from poorly controlled channel quality, inconsistent results from solvent-based post-processing, and inconsistency of thermal bonding. This research project has overcome these challenges and developed a method to produce rapid, high-quality microchannels in PMMA that are well suited for rapid prototyping of microfluidic devices and platforms. A systematic study was used to demonstrate the channel ablation in PMMA using a CW CO2 laser, and detailed the effects of experimental parameters on channel dimensions and quality. A new solvent treatment approach using isopropyl alcohol at boiling temperature resulted in clearly improved microchannel quality and processing consistency, with negligible residual solvent verified by Fourier-transform infrared spectroscopy (FTIR). Thermal bonding of the processed material showed four-fold increase in bonding strength with full retention of PMMA’s favourable optical clarity. As proof of concept, a high-quality microfluidic prototype in a 3-layered microfluidic design was fabricated and its performance demonstrated with this new method. The fat concentration in milk was measured by centrifugation on a disc, both by measuring the change in scattered light intensity at a fixed position as a function of time, and by measuring the width of the band of fat formed as a consequence of the centrifugation. Both measurements could be completed in less than 180 seconds. The coefficient of determination (R2) for the first method was 0.942 with coefficient of variation (CV) of 4.1%, and that for the second was 0.923 and 4.9%. Furthermore, the same experiments were done using multiple sample analysis (two samples concurrently) and resulted in R2 up to 0.936 and CV of 6.6% using the light scattering as function of time, whereas the use of the fat band development resulted in 0.887 and 7.0%. Two bioassay methodologies were developed for the detection of low levels of progesterone in undiluted milk. The first assay was based on the use of a single bead, filter tips and centrifuge tubes. This bioassay protocol took approximately 28 minutes and lead to a R2 of 0.959 within <0.625 to 20 ng/ml with CV of 3.9%. The second bioassay involved the transfer of the bead assay to a microfluidic disk. The use of the lab-on-a-disk technique decreased the assay time to approximately 22 minutes and allowed for the measurement of progesterone concentration in the range of 0.625ng/ml to 5ng/ml with R2 of 0.943 and CV of 7.2%. Gelation of the milk samples was investigated, showing disadvantages in using homogenised commercial milk as samples. The shelf-life of the beads was improved using sucrose – tween solutions to coat the beads when they were dried and allowed for bead storage in the disk. Finally, a method has been demonstrated of improving the fluid control in microfluidic channels through the creation of superhydrophobic and superhydrophilic microfluidic valves in polycarbonate by laser modification. Surface wettability was dramatically changed by laser treatment and was dramatically different with different pulse-length and wavelength. Use of a femtosecond (fs) laser at 800nm increased contact angle from ~80o up to ~160o, whilst use of a nanosecond (ns) laser at 248nm generated superhydrophilic surfaces. The surface roughness of the hydrophobic surface was substantially increased compared to the control and hydrophilic surface. Further investigation through FTIR and x-ray photoelectric spectroscopy (XPS) demonstrated that chemical modifications occurred in the ablated samples, where it was found an increase in the hydrophilic chemical groups in both surfaces, but with higher levels in the nanosecond laser modified surface. The coverage of hydrophilic groups was sufficient to cause the surface to become superhydrophilic. The chemical and morphology modification led to a Wenzel state for the superhydrophobic surfaces. The superhydrophilic surface was characteristic of a Cassie state. The laser generated valves comprised a section of altered wettability laser-fabricated in the base and top of a channel. Change in interfacial tension at the boundary between two sections of different wettability resulted in a difference in the pressure, provided by the centrifugal force, required to force the fluid past the modification. Tests using the CMS showed an increase of 29% in the pressure needed to pass through pass a droplet through a microchannel modified with the hydrophobic valves. The use of superhydrophilic valves resulted in high wettability and decrease of 39% in the pressure necessary for the fluid to pass through the channel. However, the high difference in wettability between modified and unmodified area also worked as means to increase the pressure by holding the water droplet in the superhydrophilic region.