Investigation of polymers as functional layers of a light actuated digital microfluidic device

Van Eetveldt, Henri (2021-03)

Thesis (MEng)--Stellenbosch University, 2021.


ENGLISH ABSTRACT: Microfluidic devices, or lab-on-a-chip technologies, are designed to simulate certain labo-ratory processes where functionality such as transporting, merging, splitting and mixingof fluids are necessary. The devices are also designed for functional autonomy and easeof use to allow an operator with no laboratory experience to perform experiments underthe supervision of a professional who is familiar with the device. The size, weight andpotential autonomy of these devices, combined with their ability to be programmed toperform a particular task of analysis, could bring medical diagnostics to rural areas, wherethe infrastructure development of a laboratory is not possible.Microfluidic devices are already used in various industries. However, the light actuateddigital microfluidic (LADM) model has yet to be developed for commercial use. The devicediffers from the industry standard in terms of how the fluid droplets are actuated. Thenew design model holds potential advantages, such as reducing manufacturing costs andsignificantly increasing processing speed, as well as expanding the array of possible devicefunctions. This project investigates the possibility of using polymers as functional layers of the LADMdevice. During the project, the material properties of two polymers are tested for theirsuitability as a photoconductive layer and a hydrophobic layer in LADM devices. Theproject also identifies the mathematical correlation between the varying process parametersin the deposition process of a hydrophobic fluoropolymer to the contact angle and thedeposition rate of the polymer. These results directly contribute to the growing body ofresearch on polymer based microfluidics.A literature study contextualises the history of microfluidics and the various microfluidicplatforms. A more detailed view of LADM devices and the increasing role of polymers inmicrofluidics lays the platform for the experimental chapters. A series of experiments areconducted to identify critical material properties and performance characteristics of thepolymer materials. The observations and conclusions are discussed and recommendationsfor future work are proposed.The experimental research on the polymer materials was conducted at the state-of-the-artFraunhofer ENAS institute under supervision of the divisional head, Dr. J ̈org Nestler.The instrumentation and optical setup used for the photoresistance measurements werecalibrated by the laboratory technicians. The hardware and software used for the mea-surement of the droplet contact angle is manufactured by the industry leading supplier.The methods and results of the studies were verified by the Fraunhofer ENAS technicians. The polymer P(VdF- HFP) is found to be a suitable dual layer (dielectric and hydrophobic)of a LADM device. Contact angles of water droplets ranged between 98◦and 104◦indicatingits hydrophobicity, complementing its dielectric properties shown in the literature.The polymer P3HT:PCBM is exposed to light (580nm and white light) for short periodsof time. The resistance measured through the layer at the illuminated area respondedwithin 0.5s of light exposure, as well as to the removal of the light source. The minimumresistance measured was 64MΩ. Continuous and prolonged light exposure resulted in areduced resistance long after the light source was removed.19 Si-wafers coated with a fluoropolymer, in a process varying input parameters, weremeasured for their thickness and contact angle. Two equations are obtained from aregression analysis which yields the contact angle and deposition rate of the fluoropolymerbased on the variable input parameters of the deposition process.

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