Heat Transfer Sets the Noise Floor for Ultrasensitive Electronics

A team of engineers and scientists has identified a source of electronic noise that could affect the functioning of instruments operating at very low temperatures, such as devices used in radio telescopes and advanced physics experiments.

The findings, detailed in the November 10 issue of the journal Nature Materials, could have implications for the future design of transistors and other electronic components.

The electronic noise the team identified is related to the temperature of the electrons in a given device, which in turn is governed by heat transfer due to packets of vibrational energy, called phonons, that are present in all crystals. "A phonon is similar to a photon, which is a discrete packet of light," says Austin Minnich, an assistant professor of mechanical engineering and applied physics in Caltech's Division of Engineering and Applied Science and corresponding author of the new paper. "In many crystals, from ordinary table salt to the indium phosphide crystals used to make transistors, heat is carried mostly by phonons."

Phonons are important for electronics because they help carry away the thermal energy that is injected into devices in the form of electrons. How swiftly and efficiently phonons ferry away heat is partly dependent on the temperature at which the device is operated: at high temperatures, phonons collide with one another and with imperfections in the crystal in a phenomenon called scattering, and this creates phonon traffic jams that result in a temperature rise.

One way that engineers have traditionally reduced phonon scattering is to use high-quality materials that contain as few defects as possible. "The fewer defects you have, the fewer 'road blocks' there are for the moving phonons," Minnich says.

A more common solution, however, is to operate electronics in extremely cold conditions because scattering drops off dramatically when the temperature dips below about 50 kelvins, or about –370 degrees Fahrenheit. "As a result, the main strategy for reducing noise is to operate the devices at colder and colder temperatures," Minnich says.

But the new findings by Minnich's team suggest that while this strategy is effective, another phonon transfer mechanism comes into play at extremely low temperatures and severely restricts the heat transfer away from a device.

Using a combination of computer simulations and real-world experiments, Minnich and his team showed that at around 20 kelvins, or –424 degrees Fahrenheit, the high-energy phonons that are most efficient at transporting heat away quickly are unlikely to be present in a crystal. "At 20 kelvins, many phonon modes become deactivated, and the crystal has only low-energy phonons that don't have enough energy to carry away the heat," Minnich says. "As a result, the transistor heats up until the temperature has increased enough that high-energy phonons become available again."

As an analogy, Minnich says to imagine an object that is heated until it is white hot. "When something is white hot, the full spectrum of photons, from red to blue, contribute to the heat transfer, and we know from everyday experience that something white hot is extremely hot," he says. "When something is not as hot it glows red, and in this case heat is only carried by red photons with low energy. The physics for phonons is exactly the same—even the equations are the same."

The electronic noise that the team identified has been known about for many years, but until now it was not thought to play an important role at low temperatures. That discovery happened because of a chance encounter between Minnich and Joel Schleeh, a postdoctoral scholar from Chalmers University of Technology in Sweden and first author of the new study, who was at Caltech visiting the lab of Sander Weinreb, a senior faculty associate in electrical engineering.

Schleeh had noticed that the noise he was measuring in an amplifier was higher than what theory predicted. Schleeh mentioned the problem to Weinreb, and Weinreb recommended he connect with Minnich, whose lab studies heat transfer by phonons. "At another university, I don't think I would have had this chance," Minnich says. "Neither of us would have had the chance to interact like we did here. Caltech is a small campus, so when you talk to someone, almost by definition they're outside of your field."

The pair's findings could have implications for numerous fields of science that rely on superchilled instruments to make sensitive measurements. "In radio astronomy, you're trying to detect very weak electromagnetic waves from space, so you need the lowest noise possible," Minnich says.

Electronic noise poses a similar problem for quantum-physics experiments. "Here at Caltech, we have physicists trying to observe certain quantum-physics effects. The signal that they're looking for is very tiny, and it's essential to use the lowest-noise electronics possible," Minnich says.

The news is not all gloomy, however, because the team's findings also suggest that it may be possible to develop engineering strategies to make phonon heat transfer more efficient at low temperatures. For example, one possibility might be to change the design of transistors so that phonon generation takes place over a broader volume. "If you can make the phonon generation more spread out, then in principle you could reduce the temperature rise that occurs," Minnich says.

"We don't know what the precise strategy will be yet, but now we know the direction we should be going. That's an improvement."

In addition to Minnich and Schleeh, the other coauthors of the paper, "Phonon blackbody radiation limit for heat dissipation in electronics," are Javier Mateos and Ignacio Iñiguez-de-la-Torre of the Universidad de Salamanca in Salamanca, Spain; Niklas Wadefalk of the Low Noise Factory AB in Mölndal, Sweden; and Per A. Nilsson and Jan Grahn of Chalmers University of Technology. Minnich's work on the project at Caltech was funded by a Caltech start-up fund and by the National Science Foundation.

Written by Ker Than

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Making Hotter Engines and Lasting Artwork: An Interview with Katherine Faber

Ceramics are extremely versatile materials. Because they can be formed into a variety of shapes and serve as effective insulators from heat, they're used in thousands of applications ranging from dainty porcelain teacups to hardy barrier coatings in engines. However, the characteristic brittle nature of ceramics can often be the material's Achilles' heel. New faculty member Katherine Faber, Simon Ramo Professor of Materials Science in Caltech's Division of Engineering and Applied Science, studies the reasons why brittle ceramics fracture—and how these materials can be made stronger and tougher in the future.

Faber, who comes to Caltech from Northwestern University, received her bachelor's degree in ceramic engineering from Alfred University, her master's degree in ceramic science from Penn State, and a doctorate in materials science from UC Berkeley.

Recently, she spoke about her work, her background, and how her research interests have been applied to sustainability and the arts.

 

What will you be working on in your laboratory at Caltech?

My training is in ceramic materials, studying the fracture of brittle solids. If we understand enough about how brittle materials fail, we can then design new materials that are more robust. In particular, I am interested in high-temperature materials—those desirable for energy-related applications. This includes, for example, the ceramic coatings that are used in power-generation applications, as thermal-barrier coatings in engines. These very-high-temperature materials provide insulation, protecting underlying metallic materials, so that an engine can run hotter and hence more efficiently. Right now, we are characterizing new coating systems for next-generation engines.

 

Does your laboratory also make brand-new materials?

Yes we do. One thing that I have found through the years is that often times the materials I want to study are ones that don't exist. That has moved my research into ceramic processing. Currently we are looking at strategies to make ceramic materials that are filled with pores. Such ceramics might be used as filters, in fuel cells, or as biological scaffolds into which cells can grow. Beyond processing, it is essential to characterize the pores. It's not just the pore size, or what fraction of the material is porous, but how tortuous the pathway through the porous network is. This will determine the ease of flow or filtration capabilities Each application may require a different level of connectivity and tortuosity. Here, we rely on three-dimensional imaging to illustrate these features.

 

Your work also touches on art conservation. How did that come about?

Yes, that's the other part of my work. It all happened rather serendipitously more than a decade ago. When I was chair of my department at Northwestern, the Art Institute of Chicago received funding to hire its very first PhD-level conservation scientist. Museum staff approached me to see if our department of materials science and engineering might be interested in collaborating with their scientist in order to provide a scholarly community and possible opportunities for research. We gladly forged the partnership. My personal involvement included projects on jades and porcelains. But I also became the matchmaker, finding the right people within the university for the right problems at the Art Institute.

Art conservation is not what I was trained to do, but involvement in museum work has become an important part of my career. It's still materials science and engineering, it's just that the "materials" in the museum projects happen to be valuable works of art.

We've been able to involve students through the years, and they, too, see this as a thrilling opportunity to take their training in materials science and engineering to one of the great museums of the world.

 

Do you also enjoy the arts in your spare time?

I've always loved going to art museums, and I'm really looking forward to exploring the museums here in Los Angeles. To be within walking distance of the Huntington Library is just extraordinary. I also love the theater, so I'm ready to discover the Pasadena Playhouse and beyond. We already have tickets for a couple of shows.

 

Are there any other reasons that you're excited to have made the move from Chicago to Southern California?

Well, given the winter that we had in Chicago last year, I am very excited that I don't have to shovel snow this winter. To wake up in the morning and go outside your door to grab a fresh grapefruit off of a tree—that's pretty cool.

 

What else are you looking forward to about being at Caltech?

One of the best things about moving to a new institution later in one's career is that it provides an opportunity to make new connections and work on new problems. That's what I'm most excited about. I suspect that as I meet people across the campus and at JPL I'll learn about a host of new research problems that will intrigue me.

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Wednesday, October 29, 2014
Center for Student Services 360 (Workshop Space) – Center for Student Services

Meet the Outreach Guys: James & Julius

Caltech Awards International Aerospace Honor to Industry Leader

John Tracy, chief technology officer and senior vice president of engineering, operations, and technology for the Boeing Company, is the 2014 recipient of the International von Kármán Wings Award. The honor—bestowed annually by the Aerospace Historical Society, which is part of the Graduate Aerospace Laboratories of the California Institute of Technology (GALCIT)—acknowledges outstanding contributions by international innovators, leaders, and pioneers in aerospace.

The award recognizes Tracy for his technology and leadership contributions to technical and engineering excellence in the aerospace industry and for leadership in advanced structural and material technologies for commercial aviation, defense programs, and space systems.

"It is an honor for GALCIT and the Aerospace Historical Society to give the International von Kármán Wings Award to John Tracy, whose pioneering work with Boeing continues to transform the aerospace industry. His technical and functional excellence has resulted in transformative structural and material technologies for commercial and defense applications, which have significantly enhanced the reliability of the aerospace systems," says Guruswami (Ravi) Ravichandran, chair of the Aerospace Historical Society, director of GALCIT and the John E. Goode, Jr., Professor of Aerospace and professor of mechanical engineering at Caltech.

In his current position, Tracy oversees the development and implementation of Boeing's technology investment strategy and provides strategic direction to functions, business organizations, and initiatives involving more than 100,000 employees. He is a member of the National Academy of Engineering and a fellow of the American Institute of Aeronautics and Astronautics, the Royal Aeronautical Society, and the American Society of Mechanical Engineers.

"John is passionate about everything aerospace; he loves the intellectual challenge of making it all work," says Boeing chairman and chief executive officer Jim McNerney. "The aerospace industry is stronger because John has been in it."

Ravichandran presented the Wings Award to Tracy at a gala banquet and awards ceremony on October 20 at the Caltech Athenaeum. Also recognized during the banquet was Morgane Grivel, a graduate student in Caltech's aeronautics program, who received the Shirley Thomas Academic Scholarship. The scholarship, which is named in honor of the Aerospace Historical Society's founder, has been awarded annually since 2010.

The Wings Award has annually honored aerospace pioneers for their achievements and role in shaping the aerospace industry for the last 30 years.

Recent recipients of the award include Neal Blue, chairman and CEO of General Atomics; Sir Martin Sweeting, executive chairman of Surrey Satellite Technology Ltd.; and David Thompson, chairman, president, and CEO of Orbital Sciences Corporation.

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How to Grip an Asteroid

On Saturday, October 18, hundreds of undergraduate students shared the results of their projects during SURF Seminar Day. The event provides students with the opportunity to discuss and explain their research to individuals with a wide-range of expertise and interests.

For someone like Edward Fouad, a junior at Caltech who has always been interested in robotics and mechanical engineering, it was an ideal project: help develop robotic technology that could one day fly on a NASA mission to visit and sample an asteroid.

Fouad spent 10 weeks this summer as part of the Summer Undergraduate Research Fellowship (SURF) program working in the lab of Aaron Parness, a group leader at JPL, where researchers are designing, prototyping, and refining technology for a device called a microspine gripper. Looking something like a robotic circular foot with many toes extending radially outward, such a gripper has the ability to grab onto a rocky surface and cling to it even when hanging upside down.

That makes it a good candidate to be included in the robotic capture phase of NASA's Asteroid Redirect Mission, which aims to capture an asteroid and haul it into lunar orbit where robotic and manned missions could study it more easily. One of two concepts that NASA is currently considering for that mission involves using robotic arms to grab a boulder for return from a much larger asteroid. Microspine gripper technology is being evaluated for use on these robotic arms.

Researchers at JPL have been working on this technology for almost five years. The latest version of the gripper is made entirely of metal and consists of two concentric rings of carriages—the toe-like appendages that stick out from the gripper. Each of those carriages is in turn made up of a number of "microspines" with steel hooks at their tips. When the gripper makes contact with a rocky surface, the carriages extend downward onto the rock and then pull inward toward the gripper's center. Because the carriages and microspines all move independently, the gripper is able to conform well to the rock's nooks and crannies.

For his SURF project, Fouad helped with the construction of the latest gripper prototype and worked on improving the design of the microspines for the next generation. In particular, his goal was to design a metal microspine that could conform to a rocky surface and stretch as needed toward the center of the gripper. One of the key elements in such a design is a compliant flexure, a material that can bend and flex, allowing each hook to move independently of its neighbors, to grab onto the crags of an uneven surface. In the past, elastic polymers and metal extension springs have been used for this purpose, but elastic polymers cannot stand up to the extreme temperatures of space, and the springs greatly increase the complexity of the gripper's design and complicate the manufacturing and assembly processes. A different metal option was needed.

"I started by brainstorming many different flexure designs, modeling them on the computer with CAD software, and laser cutting them out of acrylic to test their compliant properties," Fouad says. After repeating that process and improving the designs over several weeks, Fouad and Parness settled on two designs to prototype in metal and test on different rock types. In the end, one of Fouad's designs worked so well in bench-top tests that Parness's group is now incorporating it into their new gripper design.

"Edward did a great job this summer," says Parness. "The SURF program provides a great balance; it ensures an educational experience for the student but also provides a lot of value to the projects and mentors. I always try to work with the students before the summer so that the SURF projects provide some autonomy but give the students a chance to work toward something that could make a long-term contribution to the main project. Edward's project was a good example."

Fouad says he went into the SURF project with a lot of relevant experience. A statics and material mechanics course (ME 35—now ME 12) had provided him with the background he needed to understand how the microspine toes of a particular geometry would deform under different loading conditions. A mechanical design and fabrication class (ME 14) taught him important design skills. And, he says, "The experience I have gained leading the mechanical subgroup of the Caltech Robotics Team was invaluable for my work this summer. Through designing and constructing an autonomous underwater vehicle over the past year, I have acquired a great deal of design and machining techniques as well as the skills necessary to collaborate with others on a large group project."

Fouad says he loved working in Parness's lab and enjoyed having the freedom to pursue the design paths that he found most interesting and promising. And he says that he will now strongly consider pursuing a future career at JPL. "It is an incredible environment for someone looking for exciting robotics opportunities."

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Sweeping Air Devices For Greener Planes

The large amount of jet fuel required to fly an airplane from point A to point B can have negative impacts on the environment and—as higher fuel costs contribute to rising ticket prices—a traveler's wallet. With funding from NASA and the Boeing Company, engineers from the Division of Engineering and Applied Science at Caltech and their collaborators from the University of Arizona have developed a device that lets planes fly with much smaller tails, reducing the planes' overall size and weight, thus increasing fuel efficiency.

On October 8, the researchers—including Emilio Graff, research project manager in aerospace at Caltech and a leader on the project—were presented with a NASA Group Achievement Award "for exceptional achievement executing a full-scale wind-tunnel test, proving the flight feasibility of active flow control."

An airplane's tail forms a critical part of the control system that helps steer the plane during flying. During flight, air rushes around the vertical tail with great force and is deflected by the tail's rudder—a moveable flap at the rear of the tail that can steer the plane by angling air to the left or right. By moving the rudder left or right, a pilot can move the air in one direction or the other, helping to keep the plane flying straight during a strong crosswind.

During the high speeds of flight, the air flow around the tail is so strong that the rudder can control the plane's path with minimal movement. However, during the lower speeds of takeoff and landing, larger rudder deflections are required to maneuver the plane. And in the case of engine failure in a multiengine airplane, the vertical tail must generate enough force to keep the plane going straight by turning "against" the working engine. Airplane manufacturers deal with this challenge by fitting planes with very large vertical tails that can deflect enough air and generate enough force to control the plane—even at low speeds.

"But this means that the planes have a tail that's too big 99 percent of the time," says Emilio Graff, research project manager in aerospace at Caltech and a leader on the project, "because you only need a tail that big if you lose an engine during takeoff or landing. Imagine if the only way you could have airbags in your car was to tow them in a big trailer behind your car, just in case there was an accident. It ends up sucking up a lot of fuel."

The system—designed by Graff and his colleagues in the laboratory of Mory Gharib, Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering—would allow airplanes to be designed with smaller tails by helping to increase the tail's steering effect at low speeds. The work was done in collaboration with Israel Wygnanski, a professor at the University of Arizona.

In their new approach, the researchers installed air-blowing devices called sweeping jet actuators under the outer skin of the tail along the tail's vertical length. The sweeping jet actuators deliver a strong, steady burst of sweeping air just along the rudder, equivalent to the amount of airflow that would normally be encountered by the tail and rudder at higher speeds. The engineers hypothesized that with the sweeping jets turned on, a smaller tail and rudder could straighten the path of the airplane, even at low speeds.

Graff says that, using these devices, airplane manufacturers could reduce the size of airplane tails by 20 percent, only needing to activate the sweeping jet actuators during the low speeds of takeoff and landing. "That means that most of the time when you're flying around normally, you're saving gas because you have a smaller, lighter tail. So even if this system itself uses a lot of energy, it's only on in emergencies," he says. "When you take off or land, the air jets will be on—just in case an engine fails. But on a 12-hour flight, if you're only using the system for 30 minutes, you're still saving gas during 11 hours and 30 minutes."

The fuel savings come not only from reduced drag due to the smaller size, but also from weight savings and structural advantages from having a shorter tail, Graff adds.

The researchers first tested this hypothesis in the approximately five-by-six-foot Lucas Wind Tunnel at Caltech, recording the effect of sweeping jet actuators on a small model—only 15 percent of the size of an actual airplane tail. Because the jets of air created by the device move back and forth, "sweeping" the air over the length of the tail rather than blasting a single, linear burst of air, the researchers discovered that they could increase air flow over the entire tail with just six of the sweeping jets. On the small-scale model, these six jets boosted the effectiveness of the rudder by over 20 percent.

Upon seeing the favorable results from this preliminary experiment, and as part of NASA's Environmentally Responsible Aviation program, Graff and his colleagues designed the system to test the effects of sweeping jet actuators on a full-sized airliner tail. However, since such tails are nearly 27 feet tall, the engineers had to move this stage of their experiment off campus, to the National Full-Scale Aerodynamics Complex at Moffett Field, California—home of the world's two largest wind tunnels.

After machining sized-up sweeping jet actuators at Caltech, the multi-institutional team, which also included engineers from Boeing Research and Technology and NASA's Langley Research Center, installed the devices on a refurbished Boeing 757 tail, found at an airplane parts salvage yard. The large wind tunnel allowed the researchers to simulate wind conditions that realistically would be experienced during takeoff and landing. Data from the full-scale test confirmed that sweeping jet actuators could sufficiently increase the air flow around the rudder to steer the plane in the event of an engine failure.

The technique used by sweeping jet actuators—called flow control—is not new; it has previously been used for quick takeoffs and landings in military applications, Graff says. But those existing systems are not energy-efficient, he adds, "and if you need a third engine to power the system, then you may as well use it to fly the plane." The system designed by Graff and his colleagues is small and efficient enough to be powered by an airliner's auxiliary power unit—the engine that powers the cabin's air conditioning and lights at the gate. "We were able to prove that a system like this can work at the scale of a commercial airliner, without having to add an extra engine," Graff says.

For the next phase of the project, collaborators at Boeing will test the sweeping jet actuators on their Boeing ecoDemonstrator 757, a plane used for testing innovations that could improve the environmental performance of their aircraft.

These findings could one day help Boeing and other manufacturers produce "greener" planes. However, Graff notes, there are still kinks to work out—for example, as currently designed, the sweeping jets could be noisy for passengers—and the adoption of any new features on an aircraft can be a lengthy process. But once adopted, the payoffs could be huge—and improving the tail is not the only goal, Graff says.

"This is only the beginning. The tail is a 'low risk' surface; modifying it puts engineers at ease compared to, for example, modifying wings," he says. "But the data shows that similar systems could be applied to wings to increase the cruise speed of airplanes and allow some maneuvers to be achieved without moving parts.

"I would be surprised if this ends up in the next line of airplanes—since the new planes are already probably years into the design stage—but some version of this device could be adopted in the near future," he says. And the researchers estimate that if all commercial airplanes were fitted with this device and used it for one year, the fuel savings would be the equivalent of taking a year's worth of traffic off of Southern California's notoriously crowded 405 freeway—a worthy goal.

The sweeping jet actuator was developed as part of NASA's Environmentally Responsible Aviation (ERA) project, which aims to reduce the impact of aviation on the environment.

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Wednesday, October 29, 2014
Avery Courtyard – Avery House

Fall Family Festival

Friday, October 17, 2014
Center for Student Services 360 (Workshop Space) – Center for Student Services

TA Training: fall make-up session

Seismology and Resilient Infrastructure: An Interview with Domniki Asimaki

Building homes and other solid structures on a dynamic, changing earth can be a very big challenge. Since we can't prevent an earthquake or a tsunami from happening, scientists strive to understand the impacts of these forces, and structural engineers try to build infrastructure that can survive them. And that intersection is where the work of Domniki Asimaki comes in.

Asimaki, professor of mechanical and civil engineering in the Division of Engineering and Applied Science, is interested in the behavior of geotechnical systems under the influence of forces such as wind, waves, and seismological activity. Using this information, she hopes to make predictive computer models that can lead to the design of an infrastructure that is resilient to natural and man-made hazards. The effects of natural forces on man-made structures can also help in the cost-effective design of infrastructure for sustainable energy harvesting such as offshore wind farms—a promising green energy solution.

Born in Greece, Asimaki earned her bachelor's degree from the National Technical University of Athens before heading to MIT for both her master's and doctoral degrees.

Although Asimaki only joined the Caltech faculty in August, she has been thinking about moving to Pasadena since her first trip to campus a decade ago. Recently, she spoke about her work, her hobbies, and what it's like to finally be at Caltech.

 

What will you be working on at Caltech?

I am interested in the response of soils and foundations to dynamic loading, with emphasis on earthquakes. The work exists at the interface between civil engineering and earth and atmospheric sciences. Specifically for seismic loading, my research is trying to translate the output from simulations done by seismologists into input that engineers can use to design stronger structures.

In general, geotechnical engineering is an old field. Now we know a lot more about how soils behave, and that extends from the foundations of a house to the foundations of a bridge to nuclear reactors to dams. But that knowledge has been disconnected from advancements in earth sciences, and this gap has, in turn, hindered the integration of these advancements into structural design practices. I think it's an area of opportunity.

 

How does this work provide a link between the scientists and structural engineers?

Traditionally, structural engineers designed buildings using empirical data—like actual data from a previous earthquake. Today, with more than half of the global population concentrated in areas prone not only to major earthquakes but also to severe droughts and more extreme climatic events such as sea-level rise, there is an ever-increasing need to improve these empirical models, incorporate new, sustainable construction materials, and to build stronger, more resilient urban environments. I think the big promise of seismological modeling is that rather than using empirical data to make decisions about which ground motions buildings should be designed against in the future, we can actually run real earthquake scenarios in a simulation.

This can help provide a real prediction of the shaking against which the structural engineers can design buildings—provided, among other things, that seismologists have information about the soils on which their structures are built. And that's the gap that I'm hoping to fill.

 

How does this work translate to the harvesting of wind energy?

There is growing interest in offshore wind farms to be used as a source of sustainable energy, but since it's still pretty new, we don't have domestic experience about the best way to build these wind farms. We want to understand how the foundations of offshore wind turbines behave under the mix of forces from the rotor, from the waves, from currents and tide, from wind—regular wind or hurricane wind—and how all of these different types of dynamic loading affect the behavior of the foundation. We also want to understand how the behavior of the foundation, in turn, affects the stability of the wind turbine's performance and capability to harvest energy.

This specific application of my work is a fascinating direction for me. It is an opportunity to ask why design models work and how can we maximize performance capabilities and minimize cost. People like myself with an engineering background, but also with scientific curiosity, can work in areas like this and set the performance and design standards from scratch. But because the energy-harvesting industry is just starting out, we need to make it innovative while still financially feasible.

 

We have a lot of seismology expertise at Caltech. Was that a factor in your decision to come here?

It's a big part of my research interest, and so Caltech has always been the place that I felt I should be. It is a unique place in the sense that it's small enough so that different disciplines are closely connected. And there's a role that I can play, bringing research programs together. It has all the key players that I need in the same space, and it provides a great opportunity for us all to work together and build a seamless research continuum, from seismology to resilient infrastructure monitoring and design.

 

Are there any other reasons you're looking forward to living in Southern California?

Because it's gorgeous! I've never had the opportunity to have such nice weather, which is good because I love to swim, and the pool here is beautiful. I actually went to the pool on campus on the second day that we moved here. I hadn't even started yet, and I said, "I'm new faculty. I promise. I can prove it." And the guy who runs the show there, John Carter, was nice enough to give me a visitor pass so I could swim.


Do you have any other outside interests?

I love to cook. Elaborate cooking, from traditional Greek to exotic Asian cuisine and lots of other things. I am adventurous in my cooking but very traditional at the same time because I make everything from scratch. To graduate from MIT was a little easier than to graduate from a Greek mother.

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Sensors to Simplify Diabetes Management

For many patients diagnosed with diabetes, treating the disease can mean a burdensome and uncomfortable lifelong routine of monitoring blood sugar levels and injecting the insulin that their bodies don't naturally produce. But, as part of their Summer Undergraduate Research Fellowship (SURF) projects at Caltech, several engineering students have contributed to the development of tiny biosensors that could one day eliminate the need for these manual blood sugar tests.

Because certain patients with diabetes are unable to make their own insulin—a hormone that helps transfer glucose, or sugar, from the blood into muscle and other tissues—they need to monitor frequently their blood glucose, manually injecting insulin when sugar levels surge after a meal. Most glucose monitors require that patients prick their fingertips to collect a drop of blood, sometimes up to 10 times a day for the rest of their lives.

In their SURF projects, the students, all from Caltech's Division of Engineering and Applied Science, looked for different ways to do these same tests but painlessly and automatically.

Mehmet SencanSenior applied physics major Mehmet Sencan has approached the problem with a tiny chip that can be implanted under the skin. The sensor, a square just 1.4 millimeters on each side, is designed to detect glucose levels from the interstitial fluid (fluid found in the spaces between cells) that is just under the skin. The glucose levels in this fluid directly relate to the blood glucose concentration.

Sencan has been involved in optimizing the electrochemical method that the chip will use to detect glucose levels. Much like a traditional finger-stick glucose meter, the chip uses glucose oxidase, an enzyme that reacts in the presence of glucose, to create an electrical current. Higher levels of glucose result in a stronger current, allowing the device to measure glucose levels based on the charge that passes through the fluid.

Once the glucose level is detected, the information is wirelessly transmitted via a radio wave frequency to a reader that uses the same frequency to power the device itself. Ultimately an external display will let the patient know if their levels are within range.

Sencan, who works in the laboratory of Axel Scherer, the Bernard Neches Professor of Electrical Engineering, Applied Physics, and Physics, and who is co-mentored by postdoctoral researcher Muhammad Mujeeb-U-Rahman, started this project three years ago during his very first SURF.

"When I started, we were just thinking about what kind of chemistry the sensor would use, and now we have a sensor that is actually designed to do that," he says. Over the summer, he implanted the sensors in rat models, and he will continue the study over the fall and spring terms using both rat and mouse models—a first step in determining if the design is a clinically viable option.

Sith DomrongkitchaipornJunior electrical engineering major Sith Domrongkitchaiporn from the Scherer laboratory, also co-mentored by Mujeeb-U-Rahman, took a different approach to glucose detection, making tiny biosensors that are inconspicuously wearable on the surface of a contact lens. "It's an interesting concept because instead of having to do a procedure to place something under the skin, you can use a less invasive method, placing a sensor on the eye to get the same information," he says.

He used the method optimized by Mehmet to determine blood glucose levels from interstitial fluid and adapted the chemistry to measure glucose in the eyes' tears. This summer, he will be attempting to fabricate the lens itself and improve upon the process whereby radio waves are used to power the sensor and then transmit data from the sensor to an external computer.

Jennifer Chih-Wen LinSURF student and sophomore electrical engineering major Jennifer Chih-Wen Lin wanted to incorporate a different kind of glucose sensor into a contact lens. "The concept—determining glucose readings from tears—is very similar to Sith's, but the method is very different," she says.

Instead of determining the glucose level based on the amount of electrical current that passes through a sample, Lin, who works in the laboratory of Hyuck Choo, assistant professor of electrical engineering, worked on a sensor that detects glucose levels from the interaction between light and molecules.

In her SURF project, she began optimizing the characterization of glucose molecules in a sample of glucose solution using a technique called Raman spectroscopy. When molecules encounter light, they vibrate differently based on their symmetry and the types of bonds that hold their atoms together. This vibrational information provides a unique fingerprint for each type of molecule, which is represented as peaks on the Raman spectrum—and the intensity of these peaks correlates to the concentration of that molecule within the sample.

"This step is important because once I can determine the relationship between peak intensities and glucose concentrations, our sensor can just compare that known spectrum to the reading from a sample of tears to determine the amount of glucose in the sample," she says.

Lin's project is in the very beginning stages, but if it is successful, it could provide a more accurate glucose measurement, and from a smaller volume of liquid, than is possible with the finger-stick method. Perhaps more importantly for patients, it can provide that measurement painlessly.

Sophia ChenAlso in Choo's laboratory, sophomore electrical engineering major Sophia Chen's SURF project involves a new way to power devices like these tiny sensors and other medical implants, using the vibrations from a patient's vocal cords. These vibrations produce the sound of our voice, and also create vibrations in the skull.

"We're using these devices called energy harvesters that can extract energy from vibrations at specific frequencies. When the vibrations go from the vocal folds to the skull, a structure in the energy harvester vibrates at the same frequency, generating energy—energy that can be used to power batteries or charge things," Chen says.

Chen's goal is to determine the frequency of these vibrations—and if the energy that they produce is actually enough to power a tiny device. The hope is that one day these vibrations could power, or at least supplement the power of, medical devices that need to be implanted near the head and that presently run on batteries with finite lifetimes.

Chen and the other students acknowledge that health-monitoring sensors powered by the human body might be years away from entering the clinic. However, this opportunity to apply classroom knowledge to a real-life challenge—such as diabetes treatment—is an important part of their training as tomorrow's scientists and engineers.

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