Browsing by Subject "Electrowetting"

The quantitative measurement of inorganic ions in the atmosphere is an important aspect in environmental science. The three most important inorganic ions are sulfate, nitrate and ammonium, which are the most abundant components of atmospheric pollutants and have a significant impact on rainfall, atmospheric visibility and human health. To accurately and quickly measure the distribution of inorganic ions in the vertical and horizontal directions of the atmosphere, a compact and automatic real-time detection system is in need.

The research performed in this study is aimed at developing the science and technology for an aerosol detection system that combines digital microfluidics technology, aerosol impaction and chemical detection on the same chip. The system will be smaller and faster with respect to current aerosol analyzing instruments. The chip in this study performs the integrated functions of aerosol collection, extraction, and quantitative detection in real-time, unlike current benchtop methods that require operator handling and laboratory equipment. All functions are realized in dedicated sections on a digital microfluidic platform.

This thesis will present the design and test of individual components of the aforementioned functions. The digital microfluidics chip design includes transparent top and bottom plates for light absorbance measurement. The droplets are dispensed, transported and mixed on chip with other droplets by activating electrodes individually with a 50V AC sine voltage.

In Chapters 3 and 4, the issues involving droplet transportation are addressed, including droplet movement between the air and silicone oil media and droplet transport across the aerosol impaction area. Next, an aerosol impactor and a chip-to-world chamber are demonstrated and tested with lab generated sulfate aerosol. The collected aerosol showed a clear pattern on the impaction plate, and the collection efficiency inside the chip was 96%.

In Chapter 5, the development of colorimetric methods are described as well as experimental testing for inorganic ion detection. Three well-known tests for detecting sulfate, nitrate and ammonium were first adjusted to adapt to on-chip measurement conditions, the adjustments including the choices of solvent, concentration ranges and mixing ratios. The particle measurement results using a conventional spectrometer were compared with on-chip measurements in terms of absorbance range, limit of detection, sensitivity (based on the coefficient of determination and the slope of the linear regression) and signal-to- noise ratio (presented with standard deviation/average of absorbance measurements).

The thin oil film between the droplet and the top/bottom plate, which is naturally formed, plays an important role in lubrication and reduces contact angle hysteresis. However, these oil films are not always uniform in thickness. During the absorbance measurement tests, varied sizes of oil lenses were observed at the oil/top plate interface, and the size and position of the oil lenses randomly changed when a droplet moved between electrodes. The absorbance measured in the normal direction to the chip’s surface was affected by these oil lenses and, thus, not stable for multiple measurements of the same droplet or for different droplets. To solve this problem, optical fibers were introduced horizontally inside the chip, and measurements taken in this direction proved to produce stable results.

Prototypes of the chip have been fabricated, and the impaction and on-chip colorimetric tests for sulfate and ammonium were successful. Although this study was designed to build the fundamentals of a novel detection system of inorganic ions in aerosol, the potential use of the designed system is not limited to atmospheric studies. Applications can extend to testing the quality of drinking water, detection of nitroaromatic explosives or other experiments based on colorimetry.

Digital microfluidics as implemented in electrowetting-on-dielectric (EWD) technology has been widely used as a platform for miniaturizing the biomedical or biochemical laboratory on a chip in recent years. DNA pyrosequencing, one of the DNA sequencing-by-synthesis methods, has been successfully integrated on EWD devices. However, this platform requires microliters of reagents and 200~300V of applied voltages, which contributes to higher costs and limits the feasibility of a portable system. This dissertation proposes a low voltage EWD device using multi-layer insulators that can manipulate picoliter droplets on chip. A 300pl droplet was dispensed and actuated at voltages as low as 11.4Vrms and 7.2Vrms respectively on a 95um electrode a EWD device with a 20um SU8 gasket. The stacked insulators in the actuator consisted of 135nm tantalum pentoxide (Ta2O5) and 180nm parylene C films deposited and coated with 70 nm of CYTOP. The physical scaling of electrodes was further demonstrated for 33um and 21um electrode devices, resulting in droplets of 12pl and 5pl respectively in conjunction with 3um gaskets. Manipulation of magnetic beads during dispensing, droplet splitting and merging, and droplet transport were also demonstrated on the scaled EWD devices. The chemiluminescent light produced by the on-chip reaction of 100pl ATP-luciferin and luciferase could be detected with an external cooled CCD camera, but detecting this reaction with smaller-scale droplet reactions was limited by the external detector's sensitivity. Based on fundamental theories and experiments, the actuation voltage and dimensional scaling of EWD devices have been demonstrated, but the use of picoliter droplets in biochemical applications will required improved sensing methods.

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.