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44 PCB007 MAGAZINE I NOVEMBER 2018 To preserve the integrity of samples and maintain a transparent surface to observe what's going on inside the de- vice, Michigan Tech engineers detail how thin hafnium oxide layers act like a cellphone screen protector for micro- devices. Their work was recently pub- lished in the journal Thin Solid Films. Designing a Lab-on-a-chip Jeana Collins, lecturer of chemical engineering at Michigan Tech, is the first author on the paper. She explains how the lab-on-a-chip uses a process called dielectrophoresis. "The dielec- trophoretic response is a movement," she says, "And how can you tell it moved? By watching it move." Collins goes on to explain that a non-uni- form electric field from the electrodes interacts with the charge on the particles or cells in a sample, causing them to migrate. Many biolog- ical lab-on-a-chip devices rely on this kind of electrical response. "As chemical engineers, we deal more with the fluidics side," Collins says, adding that the electronics are also key and a blood glu- cose meter is a prime example, "You have the blood—that's your fluid—and it goes in, you have a test done, then you get a digital read- out. So, it's a combination of fluidics and elec- tronics." Even though a commercialized lab-on-a-chip like a glucose meter is covered, Collins and other engineers need to see what's going on to get a clear picture under a microscope. That's why hafnium oxide, which leaves only a slight hue, is useful in their microdevice design de- velopment. Also, the technology does not apply to a sin- gle device. Because of its simplicity, the haf- nium oxide layer works with many electrode designs, maintains a consistent dielectric con- stant of 20.32, and is hemocompatible, mean- ing it minimizes the Faradaic reactions that can cause cell lysis and fewer red blood cells explode when they come near the electrodes. Collins and her team tested three different thicknesses of hafnium oxide—58, 127, and 239 nanometers. They found that depending on the deposition time—6.5, 13, or 20 minutes— the grain size and structure can be tweaked for the needs of specific devices. The only poten- tial issue would be for fluorescence-based mi- crodevices because the hafnium oxide does in- terfere with certain wavelengths. However, the layer's optical transparency makes it a good so- lution for many biological lab-on-a-chip tests. The project's success is directly tied to the team's complementary skills. By bringing to- gether chemical engineers, electrical engi- neers, and materials scientists in the Michi- gan Tech Microfabrication Shared User Facili- ty, they were able to push the boundaries of all their fields. "Microdevice design has tended toward in- creasing levels of complexity where each lev- el of complexity increases the probability of failure," says Adrienne Minerick, dean of the School of Technology, professor of chemi- cal engineering, and Collins' doctoral adviser. "Simple solutions—while challenging to find— can provide robust, failure-resistant solutions for a wide range of applications. We explored numerous polymeric and inorganic films." The other co-authors include Hector Monca- da Hernandez, Sanaz Habibi, and Zhichao Wang from the Department of Chemical En- gineering; and Chito Kendrick, Nupur Bihari, and Paul Bergstrom from the Department of Electrical and Computer Engineering. PCB007 Hafnium oxide coats the left device and provides both sample protection and optical transparency to help improve the study of microfluidic medical devices.