Cover Story
Sensors keep watch on Diabetes
Clever sensor design turned a 100-year-old medical observation into a new kind of glucose monitor
Joseph Ogando, Materials Editor -- Design News, June 4, 2001
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Redwood City, CA —If Dick Tracy had diabetes, he'd probably wear the GlucoWatch® Biographer, a new glucose monitor from Cygnus Corp. Rather than sitting on a table somewhere, this monitor straps to the patient's wrist and uses a patented electrochemical sensor to measure glucose levels through the skin. More than just a gizmo, the device could revolutionize the way diabetics manage their disease. "It's a godsend," says Dr. Steven Edelman, director of diabetes research at the University of California San Diego and founder of Taking Control of Your Diabetes, a patient-education organization.
Unlike home glucose monitors, which provide a snap shot of blood sugar levels only as often as patients choose to prick their fingers and draw blood for analysis, the GlucoWatch works both continuously and non-invasively. The device displays a new glucose reading every 20 minutes and sounds an alarm if blood sugar gets too high or low. It also stores up to 4,000 readings worth of historical information, revealing blood sugar fluctuations that intermittent monitoring could easily miss. "We've never been able to collect this kind of continuous trend information before," Edelman says. And for diabetics, reliable blood sugar information can matter almost as much as insulin itself. According to Edelman, a sketchy picture of overall blood sugar levels makes it difficult to control diabetes by maintaining blood sugar levels within a healthy range. Left uncontrolled, diabetes can bring on blindness, amputation, heart disease, or worse. "I've seen people die when they don't have a handle on their blood sugar," he says.
The GlucoWatch is a blood sugar monitor that patients wear.
Though it just won FDA approval at the end of March and only recently went on sale in the United Kingdom, the GlucoWatch actually got its start almost 100 years ago with the observation that an electric current can selectively transport chemicals through human skin. This transport phenomenon, called "iontophoresis," has historically been seen as a one-way street, a way to get chemicals into the body. Cygnus scientists and engineers took the opposite tack, creating a device that reverses iontophoresis to get the glucose out. "A lot of substances can be measured through reverse iontophoresis, but we felt there was a great unmet need for glucose monitoring," says Dr. Russell Potts, a biochemist and Cygnus' vice president of research.
With a boxy appearance best described as "early digital," the 2 × 1 × 0.5-inch GlucoWatch won't soon be mistaken for a Rolex—or even a Timex. But the underside of the watch houses an innovative biosensor that handles the important business of collecting glucose, breaking it down to produce an electrochemical signal, and measuring that signal before calculating a blood sugar reading. This disposable sensor, which snaps in place and lasts for 12 hours of continuous monitoring, handles these three tasks with two sets of screen-printed electrodes and enzyme-packed hydrogel collection discs.
Sugaring time. Each 20-minute analysis cycle starts as the sensor's silver-silverchloride iontophoresis electrode applies a 300-microamp current to the skin. For the next three minutes, positive and negative ions travel through the patient's skin to GlucoWatch's side-by-side collection discs, which serve as an anode and cathode during glucose extraction. This ion migration brings glucose along for the ride, depositing it at the cathode. "We ignore what happens at the anode," Potts notes.
Next, enzymes in a cathode disc chemically break down the glucose, producing a nanoamp-sized electrochemical signal that the device's platinum biosensing electrode measures over a seven-minute period. "This signal correlates extremely well to the glucose levels in the blood," Potts says. The GlucoWatch then applies proprietary algorithms to transform the raw signal—which the device integrates over the seven-minute sensing period—into a glucose measurement. For the next ten minutes of the cycle, the GlucoWatch repeats the same steps but with the sensor's polarity reversed. This way, the disc that acted as the anode in the first half of the cycle becomes the cathode in the second, in order to prevent the plating that would otherwise shorten sensor life.
The final glucose reading displayed by the GlucoWatch averages the two ten-minute cycles. Clinical studies conducted by Edelman show these readings to be about 15 to 20% accurate, inclusive of human calibration errors. "That's comparable with what most people get from home glucose monitors," he says.
In designing the biosensor, Cygnus engineers took pains to minimize complexity. One stroke of integration genius enabled them to reduce the number of electrodes that operate at any given point in the cycle from four down to three. They devised an iontophoresis electrode that does double duty as a counter electrode for the platinum "working" electrode as soon as the glucose extraction stops. A third reference electrode completes the three-electrode set. Each sensor has an identical pair of these electrode sets—one behind each hydrogel—in order to accommodate the polarity reversal.
Not so noisy. A lot of work, and several patents, went into maximizing the sensor's signal to noise ratio. Sensor efficiency counts for a lot because the GlucoWatch has to overcome the inefficiency of the glucose extraction through the skin. Potts notes that iontophoresis only summons up a tiny bit of glucose—about 1/10,000th the amount in a drop of blood. And that tiny sample produces a barely noticeable signal. "It's a formidable detection problem," he says, explaining that glucose breakdown produces a current of only a few hundred nanoamps. "At first we had a lot more noise than signal."
To come up with a sensor capable of working at nanoamp resolution, the company's engineers fought the noise with a patented electrode design that minimizes signal loss by distributing electrode surface area into discrete, electrically insulated pockets. Another efficiency boost for the sensor comes from physical "masks" that control how the glucose meets the sensor face. Located between the hydrogel discs and the electrodes, these barriers ensure that the glucose enters the biosensor normal to its face rather than radially. "The masks let the biosensor obtain accurate readings more quickly by reducing the distance glucose has to travel," Potts says.
The GlucoWatch's disposable sensor consists of ring-shaped electrode and hydrogel collection disc layers.
Other sources of noise turned up in the materials used to make the sensor, forcing Cygnus to wage a war on contamination. "We had found that some of the noise was chemical in nature," Potts says. Antioxidants and other additives used in early formulations of the hydrogel, for example, had their own electrochemical signatures, which skewed glucose readings. So did some of the solvents used in the screen-printing process for the electrodes. A big push toward purity in both materials sourcing and manufacturing steps dramatically reduced noise.
Ordinary innards. As for the GlucoWatch hardware, it consists almost entirely of common electrical components. "Our strategy was to miniaturize using off-the-shelf components," Potts says. Early on, the design team even considered and rejected optical methods for measuring glucose because it would have required more custom components than an electrochemical system. Yet rather than designing solely for miniaturization, the GlucoWatch engineers also had to balance size against affordability and ease-of-use.
In this case, smaller would not have been better because vision loss often accompanies uncontrolled diabetes. "We could have made the watch smaller, but too many diabetics would have trouble using it," Potts says. Cygnus engineers determined the right size of the buttons and screen by putting prototypes in front of focus groups.
The need to make the monitor and disposable sensor affordable further tempered any desire to create a svelte device. To keep costs low, the watch uses off-the shelf ASICs rather than a custom design that could deliver the same functionality in less real estate. And a standard AAA battery powers it even though more costly batteries would have taken up less space. "We wanted to use a battery that's both cheap and readily available," Potts says.
The GlucoWatch does have some ease-of-use limitations. The device needs a three-hour warm up period. It shouldn't be immersed. Too much sweat can change skin conductivity, triggering a temporary disruption of readings. And finally, the FDA approved the prescription-only GlucoWatch as an adjunct to home glucose monitoring systems, which must be used to provide an initial calibration reading.
But Edelman stresses that the GlucoWatch really represents the first, not the final, step in the evolution of continuous blood sugar monitoring. "Someday people will look back at this device and think of it as the Model T of glucose monitoring," says Edelman. "But for now, it's a Porsche."
