At the Schaefer School of Engineering and Science, Ph.D. student Robert Bauer has been conducting research in the applications of micro- and nanotechnologies for the last three years. His work is specifically focused on the design and fabrication of a non-invasive scalable sensor for the purpose of recording the action potentials of thousands of nerve cells within the human body. Bauer’s experiments and inquiries are handled under the supervision of Professor Yong Shi from the Department of Mechanical Engineering.
Prior to his research, Bauer received his bachelor’s degree in Mechanical Engineering at Cal Poly Pomona University in 2013 and later worked at Northrop Grumman as a major aerospace defense contractor. After four years at Northrop Grumman, he felt the need for a career change. In 2019, Bauer completed his master’s degree at Stevens and applied for the school’s Ph.D. program. When asked why he decided to study at Stevens, Bauer explained that he was attracted to the “collaborative set of courses” offered by different disciplines, most notably within the nanotechnology program.
The focus of Bauer’s current work is to fabricate a sensor capable of capturing “the signals generated by nerve cells.” These are known as action potential signals. Neural cells produce them when there is an unequal distribution of charged ions on opposite sides of a cell membrane. “The result is that there’s a very large voltage across,” he explains, “about 70 mV, which is large in terms of the scale we’re talking about.”
There is an “inherent limitation” to the measurement of these signals with modern-day technology. Most people take measurements directly, using a probe and a wire to carry the voltage. This is damaging to the cell membrane. “Down at the cellular level, your fabrication technologies are primarily microfabrication, and those are planar, meaning 2D fabrication techniques,” said Bauer. At this level, a probe cannot be positioned where a wire is crossed. As you increase the number of probes being used to capture cell signals, probe density lowers, thus restricting the recording process.
His solution to this issue is to develop a sensor that can respond to the electric fields being emitted by a cell. “We’re going to put a material near a cell, and that material is sensitive to the electric field, so when the field changes, then we expect to see a material property change,” he said. Bauer had to search for a material with a strong enough relationship between applied electric field, the dielectric constant (the way light bends and changes medium,) and the extinction coefficient (how much light is absorbed.) The material must be thin, with no complex structure, so it will be fairly easy to fabricate a large piece. With the use of light, he explains how there would be no need for wires. “You can use a laser and scan the material surface very quickly at high resolution, and we expect to be able to map out what cells are doing over the whole area.”
The material that Bauer is using for fabrication is called PZT, which stands for Lead Zirconium Titanium. PZT has very high piezoelectric properties and is commonly used as actuators and sensors. Piezoelectricity refers to a property where there is a relation between the strain placed on a material and a generated electric field. Bauer wants to utilize this connection for his research.
Before deciding on PZT, Bauer had spent the first year or so trying to identify what would be a good material choice. “Originally, we weren’t sure what kind of material we were going to use,” he acknowledged. “We were actually looking at a material called electrochromic.” Electrochromic can change opacity with applied electric fields and is used in everyday objects such as car rearview mirrors. Nevertheless, Bauer had to reject the material because of its slow response time, which is about two seconds. As action potentials are generated in one to two milliseconds, an electrochromic sensor would not have the ability to document any signal. Additionally, fabricating electrochromic has proven to be a long and exhausting process.
Once he narrowed down his focus to PZT, Bauer has spent the last year and a half trying to find a way to properly produce it. Manufacturing PZT requires identifying how to create a “high-quality film” which should have “uniform thickness, no defects, no cracks, and no pinholes.” Recently, he managed to produce his very first successfully fabricated sample. “In order to make this work, [Bauer] had to put down a layer of platinum underneath the PZT layer,” to create a clean and uniform crystal phase for the PZT. This is due to the fact that platinum has a very similar lattice constant to PZT, thus lowering the nucleation energy required to form the material sheet. The discovery of the need for a supplementary layer of platinum was a big breakthrough for Bauer.
When asked about the assistance he has received from Professor Shi as he moved through new stages of his research, Bauer explained that his advisor has been there to ask him questions and help him think hard about the problems with which he is confronted. “A big part of research is that any assumptions that you make, you have to validate. I have to justify why it’s more complicated to fabricate electrochromic than PZT, and why the time constant of the domain is slower, and why that is a critical factor to the research.” Professor Shi has been there to ensure that Bauer presents his experimentation and conclusions with clarity by urging him to take all information into consideration and asking questions such as “where is your explanation for that? How are you evaluating this? How do you know? How many references do you have for that?” Along with being a point of contact for any data that is generated from the research, Professor Shi assists Bauer in writing grant applications that will ultimately fund his work.
“My goal is to graduate in the Spring of 2022, which would be a five-year program, normal for a Ph.D.,” Bauer said. “The reality is, with a Ph.D., you must generate new scientific knowledge, something that was not previously known or understood, and you must present that to the scientific community. In some senses you are not following a map, you’re making a map. It is sort of challenging, but the reality is that I will complete my degree when the research is done. With hard work and a good idea of what the scope is and what you’re trying to accomplish, you can do things in a reasonable amount of time.”
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