Tech Talk

Clocks low in the gravity well run slower than those higher up. That’s just a relativistic fact of life—one that we take advantage of every time we use the Global Positioning System. (I remember how cool I thought it was when I first learned that clocks in orbiting GPS satellites have to be programmed to correct for relativistic time dilation.) Within the past year, experimentalists comparing the speeds of widely separated atomic clocks (connected via fiber optic cable) have shown that current chronometers, with uncertainties around 10^-16, are sensitive enough to detect the change in time’s flow that accompanies a shift of a meter or less in relative elevation.

The geoid is the Earth-swaddling surface of equal gravitational potential that more or less coincides with global mean sea level. Or, from the chronometric viewpoint, the surface on which all clocks tick at the same rate. The geoid isn’t smooth. As we have known for years (and continue to prove with increasing detail), the geoid undulates. Its rise and fall betray details of objects and events far below the surface.

Writing in Geophysical Journal International (the paper is also available on arXiv.org), a team of researchers from Switzerland, the United States, and Romania shows that clocks accurate to within 10^-18 could map the geoid to within about a centimeter and show the sizes and locations of mass variations far below the surface. Since the accuracy of standard reference clocks has increased at a steady order-of-magnitude-per-decade since the 1950s—and since newly built optical atomic clocks and proposed designs for highly charged ion and nuclear clocks promise to increase that rate substantially—it is clearly time prepare for an age in which we can map the crust and mantle by watching time slip by.

Imagine a bubble of magma 20 percent denser than the surrounding rock. University of Zurich physicist Ruxandra Bondarescu and her co-workers show that a clock with 10^-18 accuracy can easily detect and locate a 1.5-kilometer-radius, high-density sphere centered 2 km below the surface. It could also unmask a 4 km-radius sphere 45 km below our feet. Time changes can reveal subtler differences, too—such as a 1 percent density anomaly 10 km in radius centered 37 km underground.

Established methods (like those used in a recently published report linking variations in the Earth’s magnetic field to mass flows deep under the Atlantic and Indian Oceans) use satellites to detect variations in gravitational force. Bondarescu, et al maintain that chronometric mapping of the geoid presents some distinct advantages. Clock readings measure the potential surface directly. The researchers point out that force (the first derivative of the potential) must be integrated (over a path) to yield figures for potential, reducing resolution. Though acceleration is a vector, moreover, many instruments record only its magnitude, and not its precise direction. This also, say the chronometrists, compromises accuracy of the geoid measurement, and potentially degrades resolution of subterranean features.

Image: European Space Agency ESA, D. Ducros

The interesting start-up Misfit Wearables has unveiled the prototype for its first product, a small and sleek activity tracker called the Misfit Shine that users can clip onto their clothes or wear around their wrists. This elegant and unobtrusive gadget could be a big hit with "quantified selfers," the growing tribe of analytically minded people who are using new gizmos to track their every action and meet their wellness goals. 

The San Francisco-based company has launched a fundraising campaign on Indiegogo, hoping to raise $100 000 by mid-December, and promises to ship the product to its early supporters in March. Misfit CEO Sonny Vu says the company has raised plenty of money via venture capital, and sees the Indiegogo campaign primarily as a way to raise the company's profile and generate some buzz. "We need consumer validation more so than the money," he told me in an email. "We wanted to see if people will actually pay for this thing." 

The Shine will compete with fitness trackers already on the market, like the clip-on Fitbit and the BodyMedia arm band. (For more on these gadgets, check out the IEEE Spectrum feature story "How I Quantified Myself," in which intrepid reporter Emily Waltz tried out a plethora of gadgets for several months.) 

But Vu told me in a phone call that the Shine is distinguished by its simple design and its durability: It's made of "aircraft-grade aluminum" and is waterproof. "It’s a very material-driven project," says Vu. "Other activity trackers, they have decent looking design, but they're made of rubber and plastic. They're not in the same class of product as your Tiffany earrings or your watch." 

To make wearable technology that's appealing to mainstream consumers (i.e. not just data geeks), Vu says companies need to prioritize comfort and fashion. "We want to achieve high wearability," he says. "Once we have that, we can insert more functionality. But we don’t compromise on user experience." To keep that user experience simple, Misfit Wearables designed a gadget that never needs to be plugged in. The Shine's battery lasts about six months, says Vu, and it syncs wirelessly with your smartphone, where you can review your stats. 

For more details, check out the video below.  

Photo and video: Misfit Wearables

It’s been about two weeks now since “Superstorm” Sandy whipped through the eastern United States, flooding beachfronts and low-lying communities and blowing hurricane-force winds from Virginia to Massachusetts. As clean-up crews continue to clear debris and power is restored to the last of the 8.5 million people who lost it, there will no doubt emerge many lessons on how engineers can better protect critical infrastructure if and when the next storm strikes. In Sandy's early aftermath, I went hunting for some of those lessons.

On the first Saturday after the storm, I met up with Alexis Kwasinski in Manhattan to do a bit of what he calls “disaster forensics.” Kwasinksi, who is an assistant professor of electrical engineering at the University of Texas, had come to the area to do an on-the-ground assessment of damage to communications and power infrastructure, which he has done for most major disasters since Hurricane Katrina in 2005. His surveys, though far from comprehensive, often reveal weaknesses or design flaws that official outage reports might miss.

I met Kwasinski around mid-day at 14^th Street near the East River bank, where on Monday night, a massive explosion at an electrical substation had knocked out power to most of the island’s lower half. He wore jeans, a baseball cap, and an orange utility vest. He carried a point-and-shoot camera, which he rarely set down, even while driving.

“You need to document everything,” he said. “If it’s not in the photos, it’s not there.”

[Utility crews address damage after a massive explosion at the 14th Street substation in lower Manhattan. Credit: Alexis Kwasinski]

By the time I caught up with Kwasinski, he had already visited two other sites in lower Manhattan—both central telecommunications offices owned by Verizon, which had flooded during the surge. The offices still had no grid power, and the flooding had very likely also taken their back-up generators out of commission. Kwasinski suspected that the surge had also damaged any copper telecom cables passing through the buildings. Normally, these cables are kept pressurized to prevent water from seeping in and corroding the wires. “But if you don’t have power, you can’t pressurize,” Kwasinski explained. Workers at one office, he said, had wheeled in a couple emergency mobile generators, but by the time they arrived, the damage would have already been done. If the corroded cables aren’t properly replaced, he added, “they’re going have a high failure rate in the future.”

[Seawater is pumped from a Verizon central telecom office in lower Manhattan. Credit: Alexis Kwasinski]

On 14^th Street, Kwasinski snapped a few more photos of the lifeless substation. Then we hopped into his black rental SUV, which he’d loaded with water bottles, snacks, and full gasoline canisters. The storm had disrupted the gasoline supply chain, and the shortage meant that refueling might be a challenge.

In the drivers’ seat, Kwasinski riffled through loose-leaf printouts from Google Maps pinpointing the locations of other central telecom offices he suspected might have suffered damage. The next stop, he decided, would be the Rockaway Peninsula—a tail-like strip of day parks and beach homes that had been all but washed away by the storm surge.

He wove through the streets of Manhattan and the neighboring boroughs of Brooklyn and Queens, snapping photos of dead streetlights, a few downed cables, and many seemingly unscarred but powerless curbside cabinets housing telephone and cable TV equipment. He also photographed dozens of cell towers perched on the tops of buildings. Many were obviously without grid power, and their backup batteries had probably run out days ago.

A few of the roof-top cell sites we passed were connected to mobile generators humming on the sidewalks. But the vast majority remained without back-up power. “The problem is how you get power up there if the building doesn’t have power,” Kwasinski said. Simply getting access to the building, he explained, can be a big hurdle.

Besides the widespread power outages, though, there seemed to be little damage.

Until we arrived at the Rockaways.

The first thing that struck me was the sand. Whipped up off the beaches by the storm, it now coated the streets. Bulldozers were scraping it onto the sidewalk in piles three meters high.

There were felled street lamps and overturned cars. A submerged powerboat had come to rest on the median of the main throughway. Residents were depositing the waterlogged contents of their homes—clothes, toys, furniture—out on the sidewalks.

Power was out across the peninsula and cell service was spotty. Yet despite the massive outages, Kwasinski pointed out, strangely no utility poles seemed to be down or damaged.

As Kwaskinski had suspected, the central telecom office on the peninsula had flooded. When we got there, hard-hatted workers were busy pumping out seawater from the red-bricked former telephone switching station. Kwasinski photographed the building from all sides, noting the lack of windows on the first few floors. The only place the water—which rose only three or four feet—could have gotten in was through the front door.

“This building shouldn’t be vulnerable,” he said, shaking his head. “But it's got a regular door. Why would you have a regular door? In Japan, all the central offices have watertight doors, like on submarines. How much does it cost to build a watertight door? Not much.”

A watertight door may be a simple fix, Kwasinski said, but it would have made a huge difference. In fact, he added, probably none of the telecom offices would have flooded if they’d had better doors.

That evening, after I’d left him, Kwasinski made his way to Staten Island. And on Sunday, he toured the New Jersey coast, although he couldn’t get to many of the most devastated areas, he told me later, because roads were still closed. He didn’t have much more network damage to report other than power outages. He thought we’d probably seen the worst of it in Manhattan and in the Rockaways.

[An explosion at a power substation on Rockaway Peninsula charred the cabinets housing switching equipment. Credit: Alexis Kwasinski]

Now back in Texas, Kwasinski still has to go through the hundreds of photos he took on the trip, synthesize his data, compare it to public reports, and follow up with engineers he knows at Verizon and other telecom operators. But I wondered if he had walked away from the weekend with any obvious lessons.

The lack of watertight doors, he reminded me, was an obvious one. Also, it seemed to him that unlike in most other disaster areas he’s visited, underground infrastructure in this case seemed to fair worse than lines and equipment kept above ground—probably, in New York City at least, because the surges from Sandy were more damaging than its winds.

As with most disasters, he concluded, the biggest problem was power outages, which have ripple effects in other utilities, particularly telecommunications. He believes that if critical infrastructure such as street lights, cell sites, and curbside telecom cabinets could be made less dependent on the grid, it would be much more robust in the face of disaster. One solution may be to encourage power companies to deploy standards that allow cell sites, homes and other independent structures to safely disconnect from the grid and still use local sources of energy, such as microgrids, solar panels, or wind turbines.

[Current standards prevent utility poles from safely using solar panels when disconnected from the grid during a blackout—hence the unlit street lights. Credit: Alexis Kwasinski]

Finally, Kwasinski said there was one last thing he had expected to see in Sandy’s wake but never did:

“In every disaster area there is always a damaged McDonald’s sign."

   The mesmerizing movements of jellyfish have inspired researchers to design all sorts of things, from mechatronic jellyfish that function as autonomous robots to artificial jellyfish built from rat cells and silicone. Now scientists have built a jellyfish-inspired microchip that can capture cancer and other rare cells in human blood.

A jellyfish captures floating food particles with its long tentacles, which are equipped with repeating patterns of sticky structures. Researchers at Brigham and Women's Hospital in Boston used that design concept to build a microfluidic chip coated with long strands of repeating DNA sequences that bind to specific proteins on cancer cells as they float by in the blood.

Capturing cancer cells in the blood stream can provide key information about how a tumor is responding to treatment, and a device like the jellyfish chip could be used not only in diagnosing and monitoring cancer, but also for capturing other rare cells in the blood, such as fetal cells, viruses and bacteria, the researchers reported yesterday in the journal Proceedings of the National Academy Sciences.

Other microfluidic devices that rely on antibodies or engineered nucleic acids have been developed in the past with a similar intent, but have failed to capture large entities in the blood, such as whole cells. The new jellyfish-like device can grab those cells, and more of them. The key was making the three-dimensional DNA strands long, like tentacles, and arranging them in a herringbone pattern inspired by the repeating patterns of sticky structures on the jellyfish. And unlike previous methods, the device can also easily release the cells so that they can be studied in the lab.

In addition to diagnostic applications, the device could also be used therapeutically. "What most people don't realize is that it is the metastasis that kills, not the primary tumor," says Jeffrey Karp, an author of the paper and a bioengineer at Brigham. "Our device has the potential to catch these cells in the act with its 'tentacles' before they may seed a new tumor in a distant organ."

Go jellyfish. Maybe researchers should spend more time at the aquarium staring at these hypnotic marine animals.

Image: Brigham and Women's Hospital

My son rarely listens to “real” radio; he’s more likely to be listening Pandora on his iTouch. When we’re in a hotel, he scrambles for the iPod dock, not a favorite radio station. But I think he—and I—got a new appreciation for radio in the wake of Hurricane Sandy, when we traveled for a family wedding to a storm-struck, electric-powerless region of New Jersey two weeks ago.

The Internet didn’t work, cell phone coverage was spotty, and TVs had gone dark. We stayed at my mother’s house, where her land line telephone worked just fine. But I couldn’t call to check on my elderly aunt; Verizon had upgraded her to its fiber network, and without power in her house, that was useless. The entire region, it seemed, was dark, cold, and silent—except for radios.

My mother has an ancient boombox in her kitchen—with a slot for (way too many) D batteries. She was ahead of the game. As the storm threatened, FEMA Administrator Craig Fugate, on CBS This Morning urged the up to 50 million people living in areas meteorologists predicted would be impacted by the storm to stay informed by tuning into local broadcasting, radio in particular. "Probably one of the things you don't really think about anymore is having a battery powered radio or a hand-cranked radio to get news from your local broadcasters…" Fugate said. "Cellphones may be congested. Radio is oftentimes the way to get those important messages about what's going on in the local community."

My aunt hadn’t heard Fugate’s advice; she figured out for herself that she’d need a battery operated radio. She hunted through stores, ending up with a jogging radio on an armband (and her very first pair of earbuds). It wasn’t quite what she had in mind, but it kept her informed throughout an entire week without power.

People who failed to get a battery operated radio before the storm hunted in vain afterwards. Hunkered down to watch election results in a local bar where power had been restored, I talked to one gentleman who was taking a break from his ongoing search for a battery operated radio. He was getting particularly desperate, figuring he’d be wanting to listen to election news long after the bar closed. (Fortunately for him, the presidential race was called well before closing time.)

And indeed, if you did have a battery-operated or hand-cranked radio (I’m never making fun of those NPR pledge gifts again), you did have something to listen to, because, by and large, radio stations stayed on the air. Radio towers are designed to be hurricane-proof, with backup power for eight to 10 days. (That’s something the cell networks could learn from: post Katrina, the Federal Communications Commission (FCC) had proposed that cell towers be required to have backup power, but the cellular industry resisted, citing the high cost. Post Sandy, one in four cell sites in the affected region failed. So folks used to turning to their smartphones to find out what’s going on were pretty much out of luck.)

Though radio stations, for the most part, were prepared with generators and backup generators, a few did go down.  New Jersey Broadcasters Association President and CEO Paul Rotella told Radio World that “If you have 10 feet of water, a station will go down. But if a station does go down, it doesn’t matter so much because one station alone can reach millions of people. So if you have hundreds of stations and one goes down, people are going to hear it, they are going to get their information. That’s what the ubiquitous nature of radio is all about.”

Rotella called for cell phone manufacturers to include FM chips in cell phones, or to enable chips already installed in the case of emergencies. He’s not the only one arguing for FM chips in phones; some are looking to Congress to mandate the chips' inclusion as a safety issue. Jeff Smulyan, CEO of Emmis Communications, an owner and operator of radio and television stations, has long been lobbying for such a requirement, and the FCC is starting to see things his way. 

Again, the cellular companies are resisting—people listening to Internet radio through cell phones pay for that data stream.

I’m rooting for Smulyan, Rotella, and their compatriots. Put those FM chips in a cell phones; we can charge them from our car batteries if the power is out; we won’t have to grope around on closet shelves to find them in a blackout since they’re likely to be in our pockets; and, the next time a hurricane is bearing down people like my aunt and the guy in the bar, they won’t have to scramble to find battery operated radios.

If you have power and Internet, follow me on Twitter @TeklaPerry. (And thank you to the National Association of Broadcasters for pointing me to some of the information used in this post.)

Mars: the planet that keeps on giving.

The successful summer landing of NASA’s Mars Science Laboratory (MSL), Curiosity, captured the world’s imagination, provided a steady stream of new discoveries, and spawned a series of TV documentaries on the National Geographic and Discovery channels.

This week, NOVA’s Ultimate Mars Challenge—airing Nov. 14 on PBS at 9pm (ET)—joins the fray, not only chronicling the engineering necessary to make the landing, subsequent exploration, and scientific discoveries possible, but placing it in context with preceding Mars missions. (The program spells out what we learned from past missions that informs the current one, as well as what we hope to learn from this one that, budgets allowing, will inform the next.)

“One of the reasons Mars is still exciting is because it hasn't turned out to be a boring story at all,” said MSL project chief engineer Rob Manning during a NOVA press conference. “Every time we've turned a page, we've learned something more rich about this planet, that maybe there's something more to go. Now, we're going to start exploring Mars over the coming two Earth years. And a lot of it is, `What are we going to learn?’ And of course, we'll have to debate and duke it out with everyone else to figure out the right thing to do with limited resources.”

Challenge producers Gail Willumsen and Jill Shinefeld culled more than 60 hours of raw NASA footage and their own interviews for the hour-long documentary. They note that getting the project finished was a challenge. With discoveries continually flowing in from Curiosity, they were making changes right up to the day they needed to submit it to NOVA.

Billions of years ago, “Mars and Earth had similar conditions for life,” says Willumsen, who also directed the documentary. “It arose on Earth, but we don’t know if it arose on Mars. What we do know is that Mars changed radically: something happened and it went in a different direction. The exciting thing about the landing site, Gale Crater, is that it has Mount Sharp, which is a mountain of layered rock that is a chronology of Mars’ history. So they’re going to, theoretically, go through and read those layers and perhaps find out why it became the cold, dry planet of today. It’s a massive form of climate change.”

Among the more frustrating decisions the pair had to make in streamlining the documentary's focus, was cutting one interview with Nathalie Cabrol, a senior research scientist at the SETI Institute in Mountainview, Calif. 

“The thing she said that really struck me was that the need to explore is rooted in all of life, not just Homo sapiens,” says Willumsen. “The impulse to explore is part of the survival instinct.”

Stanford University announced Monday that a team of chemists and engineers created a flexible, self-healing, conductive material. Led by chemical engineering professor Zhenan Bao, the researchers combined a plastic consisting of chains of molecules joined by hydrogen bonds, with rough nanoparticles of nickel that one of the researchers, Benjamin Chee-Keong Tee, describes as mini-machetes. The bumpy edges, Tee said, concentrate the electrical field and make it easier for current to flow from one particle to the next. Twisting or pressing on the material changes the distance between the metal particles and therefore the resistance; such changes can be translated to measurements of pressure.

When sliced with a scalpel and then pressed back together, the material recovers 75 percent of its mechanical strength and electrical conductivity in seconds; 100 percent in about half an hour.

The team envisions prosthetic arms that can detect the pressure of a handshake or the degree of bend in a joint, as well as electrical wires that can repair themselves when broken.

Caption: A researcher cuts a piece of the self-healing "skin". Photo: Stanford University.

The unveiling of Oak Ridge National Laboratory’s Titan Cray XK7 supercomputer knocks every other computer in the world down one notch in the Petaflop Hall of Fame. The Top500 Supercomputer Sites website has just posted new rankings noting that there are now 23 systems with performance better than a petaflop per second, “just four-and-a-half years after the debut of Roadrunner, the world’s first petaflop/s supercomputer.” (Los Alamos National Laboratory's Roadrunner is now 22^nd on the list.)

Top500’s 12 November announcement  includes an analysis of today’s trends in supercomputing. Some highlights:

• Multi-core systems dominate, and more systems (including Titan) are using processor/co-processor architectures.
• Intel provides 76 percent of the supercomputer processors, followed by AMD (12 percent) and IBM (10.6 percent).
• The threshold for membership in the Top 100 list has moved up to 241.3 teraflop per second, up from 172.7 Tflop/s just six months ago.

So here are the world's 10 most powerful supercomputers—for now: 

• Titan (Cray XK7)—Oak Ridge National Laboratory, USA:  560 640 cores, 17 590 teraflop per second, 8 209 kilowatts.
• Sequoia (IBM BlueGene/Q)—Lawrence Livermore National Laboratory, USA:  1 572 864 cores, 16 325 Tflop/s, 7890 kW
• K computer (Fujitsu K computer)—RIKEN Advanced Institute for Computational Science, Japan:  705 024 cores, 10 510 Tflop/s, 12 660 kW
• Mira  (IBM BlueGene/Q )—Argonne National Laboratory, USA: 786 432 cores, 8162 Tflop/s, 3945 kW
• JUQUEEN (IBM  BlueGene/Q)—Forschungszentrum Juelich (FZJ), Germany: 393 216 cores, 4141 Tflop/s, 1970 kW
• SuperMUC (IBM iDataPlex DX360M4)—Leibniz Rechenzentrum, Germany:  147 456 cores, 2897 Tflop/s, 3423 kW
• Stampede  (Dell PowerEdge C82207)—Texas Advanced Computing Center, USA: 204 900 cores, 2660 Tflop/s , (power n/a)
• Tianhe-1A ( NUDT YH MPP)—National Supercomputing Center in Tianjin, China: 186 368 cores, 2566 Tflop/s,  4040 kW
• Fermi  (IBM BlueGene/Q)—CINECA, Italy:  163 840 cores, 1725 Tflop/s, 822 kW
• DARPA Trial Subset  (IBM Power 775)—IBM Development Engineering, USA:  63 360 cores, 1515 Tflop/s, 3576 kW

See the Top500 site for the full list, highlights, analysis and a poster showing the evolution of big iron.

Images: Titan, Oak Ridge National Laboratory (top). Sequoia, IBM

The unconscious brain may not be able to ace an SAT test, but new research suggests that it can handle more complex language processing and arithmetic tasks than anyone has previously believed. According to these findings, just published in the Proceedings of the National Academy of Sciences, we may be blithely unaware of all the hard work the unconscious brain is doing. 

In their experiments, researchers from Hebrew University in Israel used a cutting-edge "masking" technique to keep their test subjects from consciously perceiving certain stimuli. With this technique, known as continuous flash suppression, the researchers show a rapidly changing series of colorful patterns to just one of the subject's eyes. The bright patterns dominate the subject's awareness to such an extent that when researchers show less flashy material to the other eye (like words or equations), it takes several seconds before the brain consciously registers it. 

This masking technique is "a game changer in the study of the unconscious," the scientists write, "because unlike all previous methods, it gives unconscious processes ample time to engage with and operate on subliminal stimuli."

To study the unconscious brain's ability to process language, the researchers subliminally showed the subject short phrases that made variable amounts of sense: For example, subjects might see the phrase "I ironed coffee" or "I ironed clothes." The researchers gradually turned up the contrast between the phrase and its background, and measured how long it took for the phrase to "pop" into the subject's conscious awareness. As the nonsensical phrases popped sooner, the researchers hypothesize that the unconscious brain processed the sentence, found it surprising and odd, and quickly passed it along to the conscious brain for further examination.  

To determine the unconscious brain's mathematical abilities, the researchers presented a simple subtraction or addition equation (for example, "9 − 3 − 4 = ") to a subject, but took it away before it could pop into consciousness. Then they stopped the masking pattern and displayed a single number, asking the viewer to pronounce the number as soon as it registered. When the number was the answer to the subtraction equation (for example, "2"), the subject was quicker to pronounce it. The researchers argue that the viewer was "primed" to respond to that number because the unconscious brain had solved the equation. Oddly, they didn't find the same clear effect with easier addition equations. 

Why is IEEE Spectrum covering this? We could argue that until we understand the workings of consciousness in the human brain, we'll never be able to build an artificial intelligence that can be described as conscious and aware. Or we could admit that we just thought the study was nifty. 

Images: Wikimedia Commons; Ran Hassin et al. 

The potential of thin film bulk acoustic wave resonators (FBARs—no sniggering from the back row, please) to measure mass at the nanogram level has marred by one persistent obstacle.

An FBAR membrane is electronically driven to vibrate at a characteristic resonant frequency. When a particle—a protein molecule, perhaps—adsorbs to membrane, the resonant frequency drops; the frequency change is proportional to the glued-on mass. So, measure the frequency drop, measure the mass. And because the frequency shift for a given mass is proportional to the square of the initial resonant frequency, the higher the initial resonant frequency, the more sensitive the measurement and the smaller the mass you can expect to measure.

And now for the obstacle. The FBAR’s acoustic characteristics change significantly with temperature. The speed of sound changes, and so does the thickness of the membrane. And this temperature sensitivity grows along with its measurement sensitivity as the resonant frequency climbs.

Up until now, solutions to this gravimetric problem have required two separate measurements—either measuring the frequency change in two distinct environments (an isolated test environment and the field environment) or building a second temperature sensor into the device.

But a collaboration among researchers from four British Universities (Cambridge, Manchester, Sheffield, and Bolton) and from Korea’s Kyung Hee University—has yielded a new twist on an old solution to the problem of temperature effects on precise measurement.

Temperature changes have dogged metrologists at least since John Harrison labored through much of the 18^th Century to construct the first accurate clocks. Balance springs sag as temperatures rise. Iron pendulum rods stretch.  Time seems to slow down as the weather warms up.  The solution is to combine materials to counteract these effects. Harrison devised a “compensation curb” (similar to the bimetallic spring at the heart of many home thermostats) to “rewind” the balance spring as temperatures rose. He also invented the gridiron pendulum, in which brass or zinc compression beams lift the pendulum weight to counteract the iron rod’s expansion. (See Dava Sobel’s Longitude or this Royal Society inventory of Harrison’s innovations.)

Though others had suggested a Harrisonian two-material solution to FBARs’ problems, this is the first time it’s been made to work over a broad range of real-world temperatures. To do this, Cambridge’s Luis Garcia-Gacendo and the team built a two-material device that resonates in two characteristic modes—arranged so that one of the composite's resonant modes increases in frequency as temperature rises while the other decreases in frequency. By measuring the shifts in both modes simultaneously, scientists can calculate both the temperature at the device and the mass of adsorbed particles.

The prototype FBAR device “for parallel sensing of temperature and mass loading” consists of a 2-micrometer-thick piezoelectric film of crystalline zinc oxide sputtered onto a 2 μm layer of silicon dioxide, which sits atop a silicon wafer sandwiched between chromium-gold electrodes. The prototype measurement tool showed native resonant modes at 754 megahertz and 1.44 gigahertz. Both silicon dioxide and zinc oxide expand with temperature (though at different rates), so both layers get thicker as the temperature rises. This would normally mean that the resonant frequencies of both modes would fall with temperature. In this case, though, rising temperatures cause the acoustic wave velocity to increase in the silicon dioxide layer and decrease in the zinc oxide layer. So a one-Kelvin change in temperature causes a 79.5 parts per million frequency increase in the SiO₂ and a 7 parts per million frequency drop in ZnO.

Thus, measuring frequency changes in both modes simultaneously indicates both temperature and mass.

The team successfully measured loads of human fibrinogen and bovine serum albumin in concentrations of about 1 to 1000 micrograms per milliliter (The total mass measured is unstated; the calibration tests measured masses on the nanogram level). Note, though, that the protein solutions were deposited on the membrane and then dried. The device as built relies on thickness longitudinal mode data, and cannot be applied to direct sensing of masses in liquids. The authors note, however, that repositioning the electrodes could enable thickness shear mode sensing in a successor device, opening up, for example, opportunities for direct sensing in biological or biotechnology applications. 

Images: Top: L. Garcia-Gacendo, Cambridge University. Bottom: Public Domain.