Identifier

etd-08192008-160519

Degree

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Document Type

Thesis

Abstract

Cryopreservation is storage of biological systems at ultra low temperatures for a prolonged duration; such that they can be thawed and restored to the same living state. It is important to understand the behavior of cells when they are subjected to subzero temperatures. Research in this area has shown the occurrence of two main biophysical events; cellular dehydration and intracellular ice formation. Current techniques for characterizing the dehydration in cells as part of a tissue are not adequate for studying intracellular ice formation in tissues. An integrated device consisting of an array of thermal sensors (microthermocouples) and actuators (microthermoelectric coolers) would help to detect intracellular ice formation by measuring and modulating the heat release of individual cells during freezing. This requires a dense wiring layer below the devices which can act as a heat sink in turn affecting the performance of the device. To alleviate this problem fabrication of bump structures was proposed to isolate the dense wiring layer from the array. Modeling was used to assess the effect of the bumps on the performance of the thermoelectric cooler. Bismuth telluride posts of 10 µm diameter and 20 µm height yielded optimal cooling with a bump radius of 5 µm. A maximum effective change in temperature of 3.47 K was achieved for an applied current of 23.7 mA and Joule’s breakdown was found to occur at 47.7 mA. To avoid the complexities in the measurements due to the presence of second junction, copper and constantan were chosen as bump material. Electrodeposition along with UV-LIGA microfabrication technique was used to fabricate the bumps. Copper and constantan micro bumps, with mean diameters of 6.5 & 27.76 µm and heights of 7.81 and 12.04 µm were fabricated with dimensional variation of ±0.5 µm with a 95% confidence interval. A custom printed circuit board was fabricated on FR4 laminate using lithography and liftoff technique. The mean length and width of the structures were found to be 4151.98 ±1.86 µm and 1003.21 ±0.55 µm, respectively with 95% confidence interval. There is a need for future work to precisely fabricate metal features on FR4 laminate.

Date

2008

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Ram V. Devireddy

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