Development of a resonant mass sensor for MEMS based cell detection applications

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2012
Eroğlu, Deniz
This thesis reports design and implementation of a MEMS based resonant mass sensor for cell detection applications. The main objective of the thesis is the real-time detection of captured cells inside liquid medium and obtaining the detection results by electronic means, without the aid of any external optical instruments. A new resonant mass sensor architecture is presented that has various advantages over its conventional counterparts. The device oscillates in the lateral direction, eliminating squeeze film damping. A thin parylene layer coated on the device prevents liquids from entering the narrow gaps of the device, further improving the quality factor. The resonator is embedded on the floor of a microchannel. A gold film on the proof mass facilitates antibody based cell capture on the device. Theoretical background regarding resonator operation is investigated. Various resonator designs are presented, taking into account design trade-offs, application v considerations, and fabrication limitations. The design procedure is verified with MATLAB Simulink modeling results and finite element simulations. A new process flow has been developed for resonator fabrication, combining SOI, glass, and polymer micromachining. Modifications have been done on the flow for the solution of problems encountered during device fabrication. Each device has a foot print area of 1.5 x 0.5 cm2. The majority of this area is occupied by fluidic connections and reservoirs. Resonance characterization results in air and water have shown that there is significant quality factor enhancement with the parylene coating method. The quality factor decreases to only 170 in water from 610 in air, when the resonator is coated with a thin layer of parylene. Uniformity and linearity tests revealed that the devices have a standard deviation of only 1.9% for different analyte capture sites and an R2 of 0.997 for mass loads as high as 2.7 ng. Detection of Saccharomyces cerevisiae type yeast cells has been done using the resonators. Mass measurement of single yeast cell (13 pg) and yeast clusters (102 pg) have been performed. Antibody and thiol-gold chemistry based Candida Albicans type bacteria capture and detection has also been made in both air and water environments. The mass of several captured bacterial cells in air has been measured as 95pg. Two bacterial cells have been captured on one device inside water and their mass has been measured as 85 pg. It is worthy to note that all mass measurements are consistent with theoretical expectations.

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Citation Formats
D. Eroğlu, “Development of a resonant mass sensor for MEMS based cell detection applications,” M.S. - Master of Science, Middle East Technical University, 2012.