Designing sensors and antennas of the future
Shopping one day at Home Depot, Craig Grimes noticed a man who had just pulled a set of drill bits off the rack by the exit door. As the man took a couple of steps toward the door, with the unpaid-for merchandise, Grimes thought about the alarm that would sound if the shopper decided to dash out of the store just then. For Grimes at that moment, a new research interest was born.
"I started thinking about the little white tags attached to store merchandise and how, if they're not deactivated, these markers interact electromagnetically with the 'interrogation coils,' as I call them. It's this interaction that sets off an alarm," says Grimes, an associate professor of electrical engineering at the University of Kentucky. These coils, a few meters apart and situated in front of an exit door, are now so commonplace, Grimes adds, that we hardly give them a thought. "I began to consider how these anti-theft markers work and whether or not this technology could be extended to chemical and environmental sensing."
Developing new uses for sensors is only one of Grimes's wide-ranging research interests. He is also breaking new ground in the field of antenna technology, he's working to characterize and control thin-film materials and devices, and he's recently ventured into the young science of nanotechnologyworking to understand the properties of nanotubes.
Grimes admits he's always had a healthy curiosity, but says his path into science wasn't a clear and direct one.
"As an undergraduate, I was really unfocused. I started out as an accounting major, switched to econ, then industrial engineering, then mechanical engineering," Grimes says. "Then I dropped out and kind of spent a couple of years skiing. I was kicked out of the University of Texas as an undergrad because my GPA was so low."
Grimes ultimately ended up driving a schoolbus in Austin, and the reality of the job drove him back to the university classroom. "I got serious about my career while driving a bus for $4.85 an hour," he says. "That job put things into crystal clarity."
Grimes left Austin in 1983 and headed to Penn State, where his father was a professor of electrical engineering and chairman of the department. "I still couldn't make up my mind what I wanted to focus on, so my dad, in exasperation, told me that if I was incapable of picking something, I might as well go into electrical engineering. And here I am."
Grimes got serious at Penn State, earning both a B.S. in electrical engineering and a B.S. in physics in 1984. By then, his undergraduate experience at the University of Texas had begun to fade into the past like an image in the rearview mirror of a schoolbus. But Grimes wasn't finished with Austin yet. "I went back to the University of Texas in '84 to do my graduate work," he says. He left Austin again in 1990, this time with both a master's degree and a Ph.D. in electrical engineering.
Grimes came to UK in the fall of 1994 as an assistant professor of electrical engineering and was promoted with tenure to associate professor last summer. His work is currently funded by four grants from NSF, two from NIH, two from the U.S. Air Force Office of Scientific Research, one from the Department of Education, and one from NASA for a combined $3 million. Grimes supervises a staff of five postdocs and seven graduate students who work with him on his sensor and antenna research.
Sensors: simple and sophisticated
When Grimes talks about the sensors he and his research associates are developing, he does so with unrestrained enthusiasm. "We're just talking about a small piece of mostly iron and nickel," he says, "but it supports a really sophisticated technology. And one of the beauties of this technology is that it's inexpensiveone of these sensors that identifies your package as you leave a store costs about a penny." A kilogram of the basic material, enough to make several thousand sensors, comes in a roll (imagine a reel of film that would snap into a movie projector) and costs about $100. Triggered by a standard electromagnetic coil, sensors work without batteries or attachments of any kind.
As any of us who took Physics 101 may recall, electrical current can produce magnetic effects, and vice-versa. If an electrical current runs through a wire, a magnetic compass near that wire will move. A moving compass will generate electricity in a nearby wire. This relationship between electricity and magnetism, discovered by the Danish experimenter Christian Oersted, is generally seen as one of the highlights of 19th-century physics. "It is by this effect that we detect and monitor our sensors," Grimes says. "The sensors we use in our researchwe commonly refer to them as 'ribbons'are composed of iron and nickel, with a little phosphorous, silicon and boron thrown in," says Grimes.
How do the sensors work? "Our sensors are the magnetic equivalent of a church bell," Grimes explains. "If you hit the bell with a hammer, you've transformed kinetic energy into an elastic wave that deforms the bell, which rings at its resonant frequency." This frequency is inherent in the size of the bellfrom the boom of a cathedral bell to the tinkle of a hand-held dinner bell.
"Our sensors 'ring' magnetically," Grimes explains. "Instead of hitting a bell with a hammer, we magnetically pulse a sensor by passing a current through a wire-wound coil to generate a magnetic field." The energy that is in the magnetic field is absorbed by the sensor, which causes it to mechanically deform, like the bell. But instead of sending out acoustic waves, the ribbon generates magnetic waves.
"Using a pick-up coil, we listen to the sensor and determine its resonant frequency. By looking at changes in its resonant frequency, we can gather environmental information," Grimes explains.
"Just as a church bell will change its resonant frequency in response to temperature, pressure, or mass loadconsider the effect of a coat of paint on a bellso does the resonant frequency of our magnetic sensor," he explains.
Sensors in your car and in your Cheerios
For the past four years, Grimes has been developing sensors with an eye toward various practical applications of this technology. He and his team have fine-tuned sensors to measure several different environmental parameters, including pressure, temperature, the viscosity of liquids, the surface tension of liquids, and mass load. "One immediate application that I'd think would interest the motor industry would be the use of a reliable, inexpensive sensor to monitor oil viscosity in cars," says Grimes. "The sensor, which would cost only pennies, would tell drivers when they needed to change the oil in their car."
"In one application of the technology, we coat the sensors with a thin layer of ceramic aluminum-oxide," Grimes says. "Aluminum-oxide changes mass in response to humidity levels, thereby changing the resonant frequency of the adjoining sensor." The result is a disposable, inexpensive humidity sensor that can be monitored from inside sealed packages.
The cereal industry could be one of the first customers for Grimes's new technology. If a sensor were placed, for example, in each box of Cheerios as it was hustled down a conveyor belt, temperature, humidity and moisturethe natural enemies of crispness and freshnesscould be monitored from inside each sealed box. The sensors could be monitored in the warehouse or in the grocery store. "In this way, product freshness could be assured," Grimes says. "The spoiled food products could be weeded out easily."
As this technology advances, a grocery employee might one day take a hand-held tester down the cereal aisle to "pulse" the boxes on the shelves. Depending upon the response of the sensors, the employee would know whether or not a box of cereal was fresh enough to sell.
Another application of the sensor technology Grimes has developed could be in the medical field. Grimes and research associates Chaohui Tong and Qingyun Cai developed a glucose-responding polymer to detect sugar levels. A polymer is a plastic-like material designed to capture certain organic molecules, in this case the glucose-sugar molecule. Thin layers of these polymers are applied to the magnetic sensors. Once the sensor is prepared, Grimes tests it by alternately immersing the sensor in high or low solutions of glucose to see how it responds.
"What is very exciting about this technology is that a small magnetic sensor could be placed under the skin of someone with diabetes, under the skin of his hand, for example, and glucose concentration could then be measured by the patient simply passing his hand in front of a pick-up coil," says Grimes. "You would get an immediate reading, thereby providing a quick bloodless and painless method for diabetics to take sugar-level readings." Currently, someone with diabetes has to prick the end of a finger and draw a drop of blood to analyze sugar concentration.
Craig Grimes followed in his father's footsteps to become an electrical engineering professor. The two have collaborated on antenna research since 1983 when Craig was a student at Penn State. Dale Grimes is currently a visiting professor in the UK Department of Physics.
Another medical application would benefit patients diagnosed with an ulcer. For a doctor to measure how acidic a patient's stomach contents are, current practice involves having the patient swallow a tube that remains attached to an external machine. This procedure, to say the least, isn't very user-friendly.
"Instead of having patients swallow this long tube, they could swallow a tiny sensor encased in a pill," Grimes explains. "The sensor inside the pill could immediately and continuously be monitored as it passes through the digestive tract. The pill would cost around a nickel and as it went down it would measure pH and pressure." To develop and commercialize this sensor technology, Grimes formed a company in 1998 called SenTech Corporation. The corporation recently received an NIH Small Business Innovative Research award for $100,000 to support Grimes and Nick Nickls, a professor of internal medicine at the UK medical center, in this sensor research.
A separate research program of Grimes's is focused on developing efficient, electrically small antennas. Antennas are used to transmit and receive electromagnetic energy, such as sunlight and radio waves. "Current antenna technology is stymied," says Grimes, "because for efficient operation antennas have to have a physical dimension roughly equivalent to the wavelength of energy being radiated. This size-wavelength relationship is a tremendous problem if you want to operate at low frequencies or use miniature antennas."
Low-wavelength electromagnetic waves would be of tremendous value for radar mapping of the sea floor, prospecting, or simply locating buried pipes. However, generation of low-frequency, long-wavelength electromagnetic waves that can effectively penetrate earth or sea water would require an antenna several miles long.
A project proposed in the 1980s by the U.S. Navy illustrates the scope of this problem. Some navy engineers contemplated building an antenna across the length of the Upper Peninsula of Michigan to enable communication with submerged submarines. This gargantuan antenna was never built, but the fact that this project (Project Sanguine) was seriously proposed is an acknowledgment of the antenna size problem.
And the limitation of known antenna technology is not so esoteric as deep-sea communications. Today, the largest single item in a cellular telephone is the antenna. Furthermore, some types of tumors have been found to self-destruct when directly exposed to low-frequency radiation. However, because extremely large antennas are needed to generate these waves, destruction of tumors using low-frequency energy has not become a practical reality.
"Interestingly enough, while antenna technology is faced with this severe operational constraint, mother nature is not," Grimes says. "Atoms, for example, efficiently radiate energy at wavelengths several hundred times greater than the physical size of the atom. What does mother nature know that antenna engineers do not?"
This was the question that Dale Grimes, Craig's father, set out to answer in the mid-1960s as a professor at the University of Michigan. (He is now retired and a visiting professor in the Department of Physics here at UK). Craig began working with his father on this problem in 1983 when still a student at Penn State. After several decades of work, the two came to understand just how an atom emits energy.
"The trick then became transferring our understanding of how an atom operates to a working model on a macroscopic scale of dimensions, something, for example, the size of a breadbox," Grimes says. Many mathematical questions first had to be answered. Then, idealized, spherical mathematical functions had to be transferred into a working antenna.
"The other key obstacle is how you convince a funding agency to give you some of their research dollars to enable the actual doing," Grimes says. "Keep in mind, this is a very controversial path of research. Efficient, electrically small antennas were declared impossible several times over the past 50 years by several very famous scientists. The fact that mother nature was doing it every day, all around us and in us, has continually been dismissed as 'quantum effects.'"
Grimes was pleased that his funding proposal to the U.S. Air Force Office of Scientific Research was met with a lot of interest. Arje Nachman, an Air Force physicist with a strong medical background, thought Grimes was on to something. "It was Dr. Nachman's opinion that we were on the right path, and with his support AFOSR provided us the resources to do the work," Grimes says.
Keat Ghee Ong, an electrical engineering doctoral student, tests an antenna inside an electromagnetic anechoic chamber. The chamber walls are designed to absorb all energy, which allows the antenna to behave as if it were in outer space. The pyramid shape of the chamber walls helps to prevent unwanted reflections of the energy back out into the chamber.
The further good news, Grimes happily reports, is that he and his research team have built a working antenna 100 times more efficient than what anyone thought possible. It consists of three cross-pieces of copper wire attached at 90-degree angles, and looks, frankly, like something a child might accidentally build.
Grimes agrees. "Yes, it looks rudimentary, but it supports very precise field symmetries that enable the antenna to behave as if it were more than 100 times larger. Needless to say, we've applied for a patent." He adds that if all goes well, commercial fabrication and production could happen relatively soon. "The next few years will be an interesting ride."
The funding challenge
Grimes admits that his research, especially his antenna work, is controversial, and he says such leading-edge work can often fail to attract funding support, especially among more "conservative" funding agencies. "For half a century now, very respected scientists in the electromagnetics community have said that building functional, electrically small antennas is impossible, and this has become something of a mantra to the antenna community," Grimes says. "When we would argue theory with some people, things would get a little touchy at times."
But as Grimes and his antenna team continued to develop and refine a convincing mathematical framework for the development of smaller antennas, funding agencies began to pay more and more attention.
"By our best count, Craig has written and submitted 46 grant proposals since coming to UK," says Fitzgerald Bramwell, vice president of Research and Graduate Studies. "Fifteen of these have been funded and eight are pending. That's an extremely impressive success rate, and it reflects the interest his work continues to generate at various funding agencies around the country."
Grimes concedes that grant writing takes a lot of his time but says it's simply part of the "professorial landscape." He says that one reason for his funding success is his writing skills. "You need to have good ideas and be able to convey those ideas effectively to the reviewers of your proposal," Grimes says. "If you don't you don't get far. It's a very competitive environment."
"Craig's willingness to invest an enormous amount of time working with funding agencies is remarkable," says Joe Fink, UK assistant vice president for Research and Graduate Studies. Fink cites Grimes's Air Force funding as one example of this resolve.
"Craig went to Washington and spent a day sitting down with Air Force officials to go through the theory with them, just sat in one room all day and talked with physicists and mathematicians," says Fink. "At the end of the day, they said, 'Well, you got the math to work and nobody else has been able to do that, so we'll give you some money to see if you can build such an antenna.' This is a feather in the cap of the Air Force scientists, too, I think, who were willing to listen and support a unique project like this."
As a result of both his sensor and antenna work, Grimes has six patents pending. "I have a great deal of time, emotional and mental energy involved with all of them, and hope to see the ideas captured in these patents translated to commercial products."
Strong recognition for Grimes's work came recently in the form of a prestigious 1999 NSF CAREER Award. Formerly called the Presidential Young Investigator Award, this support is given to outstanding scientists at or near the beginning of their careers. Grimes will receive from NSF $210,000 for four years.
"Craig Grimes is in the vanguard of relatively young and extremely talented faculty who are assuming the leadership of the research enterprise for the College of Engineering and the university," says Thomas Lester, dean of the college. Lester says that in addition to Grimes's innovative research, his approaches to funding have also been novel. "It's unusual for an electrical engineer to attract funding from, for example, the National Institutes of Health, but Craig has been successful in doing this." The engineering dean adds that since medicine is relying more and more upon technical devices to assist in quality of life and in treating diseases, such "crossover" funding will likely become more prevalent.
All in the family
Anyone who takes a backward glance at Grimes's life might conclude that he should have known all along that he was destined to become an engineering professor. "My father was a professor of engineering, and all of our family and friends were professors and scientists. My sister is a professor, my brother-in-law is a professor, my cousin is a professor, and my uncle is a professor."
Grimes adds that although he grew up around "a lot of science," he wasn't indoctrinated in any formal sense. "It wasn't like my father and I would get together after dinner and he'd teach me quantum physics, but there might well have been an osmosis effect," he says.
And Grimes is happy to mention yet another "very bright scientist" now in his immediate familyhis wife Beth Dickey, an assistant professor of materials engineering at UK. "Of course, I chose my wife wisely," Grimes says, laughing. "She certainly is one of the best scientists I have ever met. She is also an excellent writer, so it is a pleasure to collaborate with her on grants." Grimes says the two recently had a DoE grant funded for which they were co-PIs, and they have two grant applications currently under review.
"One of my six patents pending is a joint invention between Beth and me," Grimes says. "She is able to ask very thought-provoking questions. For example, while we were stuck in traffic a few months ago, she asked a couple questions about how one could align carbon nanotubes into useful electronic structures. Once the question emerged, we considered different solutions. By the time we got out of traffic, we had the complete invention worked out."