Polymer Microresonator Sensors Embedded in Digital Electrowetting on Dielectric Microfluidics Systems
Integrated sensing systems are designed to address a variety of problems, including clinical diagnosis, water quality testing, and air quality testing. The growing prevalence of tropical diseases in the developing world, such as malaria, trypanosomiasis (sleeping sickness), and tuberculosis, provides a clear and present impetus for portable, low cost diagnostics both to improve treatment outcomes and to prevent the development of drug resistance in a population. The increasing scarcity of available clean, fresh water, especially noticeable in the developing world, also presents a motivation for low-cost water quality diagnostic tools to prevent exposure of people to contaminated water supplies and to monitor those water supplies to effectively mitigate their contamination. In the developed world, the impact of second-hand cigarette smoke is receiving increased attention, and measuring its effects on public health have become a priority. The `point-of-need' technologies required to address these sensing problems cannot achieve a widespread and effective level of use unless low-cost, small form-factor, portable sensing devices can be realized. Optical sensors based on low cost polymer materials have the potential to address the aforementioned `point-of-need' sensing problems by leveraging low-cost materials and fabrication processes. For portable clinical diagnostics and water quality testing in particular, on-chip sample preparation is a necessity. Electrowetting-on-dielectric (EWD) technology is an enabling technology for chip-scale sample preparation, due to its very low power consumption compared to other microfluidics technologies and the ability to move fluids without bulky external pumps. Potentially, these technologies could be combined into a cell phone sized portable sensing device.
Towards the goal of developing a portable diagnostic device using EWD microfluidics with an embedded polymer microresonator sensor, this thesis describes a viable fabrication process for the system and explores the design trade-offs of such a system. The main design challenges for this system are optimization of the sensor's limit-of-detection, minimization of the insertion loss of the optical system, and maintaining the ability to actuate droplets onto and off of the sensor embedded in the microfluidic system. The polymer microresonator sensor was designed to optimize the limit-of-detection (LOD) using SU-8 polymer as the bus waveguide and microresonator material and SiO2 as the substrate cladding material. The fabrication process and methodology were explored with test devices using a tunable laser system working around a wavelength of 1550 nm using glucose solutions as a refractive index standard. This sensor design was then utilized to embed the sensor and bus waveguides into an EWD top plate in order to minimize the impact of the sensor integration on microfluidic operations. Finally, the performance of the embedded sensor embedded was evaluated in the same manner and compared to the performance of the sensor without the microfluidic system.
The primary result of this research was the successful demonstration of a high performance polymer microresonator sensor embedded in the top plate of an electrowetting microfluidic device. The embedded sensor had the highest reported figure-of-merit for any microresonator integrated with electrowetting microfluidics. The embedded microresonator sensor was also the first fully-embedded microresonator in an EWD system. Because the sensor was embedded in the top plate, full functionality of the EWD system was maintained, including the ability to move droplets onto and off of the sensor and to address the sensor with single droplets. Furthermore, the highest figure-of-merit for an SU-8 microresonator sensor yet reported at a probe wavelength of 1550 nm was measured on a test device fabricated with the embedded sensor structure described herein. Optimization of the sensor sensitivity utilized recently developed waveguide sensor design theory, which accurately predicted the measured sensitivity of the sensors. Altogether, the results show that embedding of a microresonator sensor in an EWD microfluidics system is a viable approach to develop a portable diagnostic system with the high efficiency sample preparation capability provided by EWD microfluidics and the versatile sensing capability of the microresonator sensor.
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