Rumors are flying that Apple is developing some kind of wearable that would continuously track the user’s blood sugar without breaking their skin. For people with diabetes, this would be a huge improvement over the somewhat invasive or downright painful options they currently rely on. But experts warn that if the rumors are true, Apple will be facing a scientific and technological battlefield littered with decades of other companies’ failures.

If Apple is chasing a needleless blood sugar monitor, it wouldn’t be that surprising. (Apple declined to comment.) After all, the market would be massive. About 30 million Americans have diabetes, a disease caused when there’s too much sugar, or glucose, in the blood. People with diabetes have to carefully titrate their food intake, or even inject the hormone insulin in order to keep their blood sugar from spiking or dropping to dangerous levels. So regularly measuring blood glucose is key.

Right now, it’s also unpleasant. People with diabetes have to prick their fingers to draw blood, or wear a monitor that inserts a tiny tube beneath their skin to continuously measure glucose in the fluid between cells (the same fluid that spills out when you pop a blister). So a needleless device — preferably one that continuously monitors glucose levels and spits them out in real time — would be a huge upgrade.

A needleless device would be a huge upgrade

“That is the holy grail,” says Eric Topol, the director of the Scripps Translational Science Institute who also sits on the board of glucose monitor manufacturer Dexcom. And that’s why so many before Apple have made the attempt. Google tried to develop a contact lens to detect glucose in tears, but ever since pharmaceutical giant Novartis licensed the technology in 2014, the project’s gone quiet. (A spokesperson for Alcon, a division of Novartis, told The Verge in an email: “We’re working together to initiate larger clinical trials in the near future, but do not have a definitive timeline to share today.”)

“It’s an incredibly difficult problem,” says Mark Rice, an anesthesiologist and diabetes expert at Vanderbilt University. “Everybody thinks they have a way to do it, and everybody, so far, has failed.” Why? The hurdles are many — so let’s talk about a few of them.

Glucose is actually a tough molecule to measure

The first problem is that there’s only about a sugar packet’s worth of glucose floating through the blood, writes expert John L. Smith in his book The Pursuit of Noninvasive Glucose: Hunting the Deceitful Turkey. So, there’s not a lot of glucose to measure from the outset. The second problem is that glucose is actually a pretty boring molecule. It’s colorless, small, and it doesn’t have many distinguishing features. That’s why the current glucose tests use a chemical reaction to convert glucose into molecules that are easier to track, either because they have a color, or because they can generate electrical currents.

“It’s an incredibly difficult problem.”

These tests can be run at a clinical lab or, since the 1970s, in the comfort of your home using a glucometer. In both cases, you have to draw blood — for glucometers, this is done by pricking your finger and pressing a drop to a test strip. More recent continuous glucose monitors use a wire inserted beneath the skin that takes measurements every few minutes, and can deliver the results onto a smart device, like your phone or Apple Watch. Users still need to prick their fingers, though, to calibrate the device with the more accurate measurements from a glucometer.

In fact, both tests are not always perfectly accurate: the test strips can go off if they’re not stored correctly; common drugs like Tylenol can interfere with the chemical reaction; and microorganisms and tissue can gunk up the sensor in a continuous monitor. And that’s dangerous. Inaccurate results could drive people to take insulin when they don’t need it or miss the signs of dangerously low blood sugar — which can lead to coma or even death.

It’s hard to accurately test blood sugar without breaking the skin

The problem with a device like Google’s contact lens is that the body likes to hold on to sugar. Sure, if blood sugar levels are too high, glucose will spill over into tears, spit, sweat, or urine. (In the 1600s, a physician who taste-tested diabetic pee reported that it was "wonderfully sweet as if it were imbued with honey or sugar.") But as blood sugar levels drop, the body holds on to this precious source of energy and stops leaking it into other fluids. This dooms the prospects of monitoring glucose from the bodily fluids you can access without puncturing the skin.

Taste-testing pee: the original glucose test

Some have tried to get to that blister-filling liquid under the skin without using a needle. In the early 2000s, the FDA approved a device called the GlucoWatch. It used a low electrical current to draw glucose right out of the body, where it was measured by sensors on the back of a tight-fitting watch.

The problem was that the device was a pain to use: it caused a bad rash underneath the watch for 80 percent of the people who used it, according to an evaluation by ECRI Institute, a nonprofit research organization. It also took three hours to warm up before it could take a measurement; people with hairy arms needed to shave a clear patch for the sensor; and it didn’t work if the user got too sweaty. (This last one is a big design flaw, because sweatiness can be a symptom of dangerous drops in blood sugar.) The GlucoWatch is no longer on the market.

So, is a needleless glucose sensor even possible?

Technically yes, but again, it’s hard — right now, there aren’t any FDA-approved noninvasive glucose trackers. The two main methods attempted so far have been unsuccessful. One way to measure glucose through the skin is to monitor something that changes as result of blood sugar fluctuations — like how much light reflects off of your skin, for example. If you eat lots of sugar, your skin will get warmer or it might get shinier, Rice says. So you could theoretically have a device that monitors these physiological changes.

“That’s how everybody gets fooled.”

The problem is that plenty of other molecules can also cause these changes. And so measurements wouldn’t be very reliable day to day. “There are many technologies that correlate with glucose going up and down — and that’s how everybody gets fooled,” Rice says.

Another more direct approach is to measure how much light the actual glucose molecules either scatter or absorb to figure out how much is in the blood. “There are thousands of patent applications to do this,” Rice says. He admits: “Some are mine.”

So, for example, you could have a device that shines a near-infrared light through a thinner region of skin, like an earlobe, and measure the wavelengths that come out the other side. But the problem is that glucose is just one tiny, nondescript component of the complex chemical makeup of human tissue. So the main challenge with getting this method to work has been pulling glucose’s signal out of all the other molecular background noise.

“That’s the whole secret,” says analytical chemist Mark Arnold at the University of Iowa, who has been working on this method in his lab. And he seems to have figured it out — at least, on rat skin. There’s only one problem with the setup: “It’s about the size of a small refrigerator,” he says. “Not a device that could work on somebody’s wrist.”

FDA approval could take years

Even if Apple settles on a strategy that somehow manages to noninvasively bypass the skin’s barrier, even if it develops a device that reliably and accurately measures blood glucose — there’s still the hurdle of FDA approval. “It’s very different to be a medical device manufacturer than a consumer product manufacturer,” says Brad Bonnette, a medical device evaluator at ECRI Institute. “They could certainly do it, but it would be a pretty big change.”

“You can get it to market, and still not stick.”

A noninvasive glucose monitor would probably fall into the category of a Class III medical device, Bonnette says. That’s reserved for products unlike anything else on the market that could seriously harm a patient if they failed. And it requires that a company get what’s called pre-market approval, a process that can take years.

Then, once the device is on the market, there are still challenges to overcome. Bonnette guesses that a noninvasive monitor would initially only be available with a prescription. So that means that the company making the device will need to convince insurers to pay for it. “You’d have to prove to the insurers that it’s worth doing,” Bonnette says.

So it might be worth it for Apple, or any other company entering this space, to consider the lesson of the GlucoWatch. “It’s not enough to get past all of these hurdles, to still be able to make money and have a successful device,” he says. “You can get it to market, and still not stick.”