Anatomy of a Test Strip
Each tiny bit of plastic contains big technology
It's all too easy to overlook the humble test strip or balk at its price tag. But these stalwarts of diabetes care are more than mere pieces of plastic—they contain layer upon layer of cutting-edge science and engineering. The story of test strips is also a tale about how technology has made living with diabetes, and staying healthy, easier.
A Brief History
Test strips have come a long way over the past few decades. When blood glucose testing first made its way from the clinic into homes in the 1980s, the technology behind strips was fundamentally different from today. Early test strips measured blood glucose by using an enzyme to convert the glucose in a drop of blood into a proportional amount of dye. The meter measured the amount of dye by shining a beam of light on the test spot and detecting how much of the light was absorbed by the dye. The process worked, but it was tedious. "These meters were difficult to operate," says Selly Saini, the worldwide director of strip products for Johnson & Johnson, who has been developing test strips for 25 years. "There were lots of steps involved. It took a lot of blood and was time consuming."
Test strips underwent a dramatic change in the late '80s and early '90s when they began to feature electrochemistry, the science of turning chemical reactions into electricity. "The breakthrough was getting enzymes to create an electrochemical signal when exposed to blood," says Saini. Electrochemical test strips, the world standard today, also employ enzymes, but instead of making dye, they convert glucose into an electrical current. That electricity speeds through the strip and is read out by the meter as a glucose concentration.
|The test-strip assembly line at a Roche plant in Indianapolis. Top: Plastic is fed through a machine that paints a stripe of glucose-sensing chemicals on it. Bottom: A reel feeds a roll of test-strip plastic to be coated with layers of adhesive to create the blood-sample chamber.|
How a Strip Is Made
Little test strips are big business. At Roche's plant in Indianapolis, giant machines work around the clock to turn out test strips, a projected 4.2 billion in 2012 alone. And that's just one manufacturer. While each company has its own manufacturing process, most test strips are made in roughly the same way because their designs are similar.
At Roche, making strips starts with a 1,000-meter (3,281-foot) roll of plastic. It's as wide as a test strip is long and coated on one side with a super-thin layer of gold. The roll is wrapped around something that looks like a big film reel. A machine spins the reel, feeding the roll across the path of a laser, which cuts the gold into an intricate pattern. This becomes the strip's circuit.
Another machine lays down a chemical brew on the end of the test strip that is destined to wick up the user's blood sample. This chemical cocktail contains all the components needed to turn glucose into electricity. How the chemicals are dried is a surprisingly important part of the process. "You want hydration around the enzyme to keep it active, but not too much because that will lead to degradation," says Saini. "That's a fine balance."
After drying, the roll is transferred to another machine that carves out a space for the blood sample and applies a sticker to protect the strip. The final step is to turn a single 1,000-meter roll into 4 or 5 million individual strips. At Roche, this is accomplished by a lightning-fast guillotine that lops off 1,500 strips per minute. The strips are funneled into a canister, which is immediately sealed to protect them from moisture.
Blood glucose measurements from a home meter must be within 20 percent of the lab-tested value for glucose at 75 mg/dl or higher and within 15 points for measurements lower than 75 mg/dl, though the Food and Drug Administration may soon tighten those standards. While various factors, including user error, affect measurement accuracy, the quality of test strips certainly plays a role. Test strips remain imperfect, but their designers have come up with tricks over the years to enhance their performance.
The enzyme is probably the most finicky and unreliable part of the test strip. Enzymes "are living things," says Bryan Langford, director of the product supply team at Roche. "If you mistreat the enzyme, it will pay you back in bad ways." For example, exposure to humidity or temperature extremes can decrease the activity of the enzyme, reducing accuracy. Strip makers have partly tamed enzymes and increased their life span by incorporating chemicals that stabilize them.
Small imperfections in the strip circuit, the maze-like wire that connects the blood sample side of the strip with the end that's inserted into the meter, can also cause measurement errors. For example, variations in the thickness of the metal that makes up the circuit alter the current and thus the blood glucose results. Some manufacturers have designed the shapes of circuits so that the meter can detect such variations and automatically correct for them while a user is doing a blood glucose test.
Another issue is that the glucose signal can be obscured by other components in blood. Some enzymes can get confused when they encounter, say, other sugars, such as maltose. Test-strip makers select enzymes that ignore all sugars but glucose. They have even tweaked natural enzymes to make them more discriminating and robust.
Other blood components can throw meters off, too, such as the common pain reliever acetaminophen, the active ingredient in Tylenol. Their amounts in blood can vary from person to person and from day to day. "The system has to be intelligent enough to compensate for differences in the blood," says Langford. This is accomplished, in part, by designing the strip's chemical mix so that it can detect the interfering blood components. A computer program that's housed in the meter can use these measurements to correct blood glucose readings.
As scientists and engineers keep trying to make a better test strip, billions of strips are distributed around the world every year to the ever growing number of people with diabetes. "It's great to see people use these to manage their diabetes," says Saini. "It's been an incredible journey."
Test strips can vary from lot to lot, so a test-strip code tells a meter about any quirks in the batch. Some strips are embedded with this code so a meter can detect the information directly from each test strip. Some meters require users to manually enter the code or insert a code "chip."