If you believe the old proverb, eyes may be the window to the soul, but one’s peepers could also offer a glimpse of something more practical: blood sugar levels. And it may be possible with smart contact lenses. Researchers at Stanford University and South Korea’s Pohang University of Science and Technology (POSTECH) have developed a contact lens that continually monitors the wearer’s blood sugar. So far, the lens has been tested on diabetic rabbits, as well as on one brave human. Researchers say the clinical trial results thus far indicate that the lens is as accurate as a traditional glucometer.

There’s some history here; Google’s health-tech sister company Verity was developing similar technology but scrapped their glucose-measuring contact lens project a few years ago; it turns out getting blood-glucose data from tears is fraught with complications. Past efforts in developing such monitoring systems have run afoul of problems including sensor corrosion, the instability of the enzyme found in tears, slow absorption of tears into lenses, and the fact that the human eye doesn’t produce the volume of tears needed to maintain continuous monitoring.

Until now. Stanford and POSTECH researchers overcame some of the issues faced by others with the use of a porous hydrogel that better absorbs tear fluid. The porosity of this hydrogel is fine-tuned to complement the thin layer of tears that coat the eye, Goldilocks-style. Too porous, and the hydrogel risks compromising the integrity of the lens. Not porous enough, the lens won’t absorb the amount of tears needed to take a measurement. It seems they got it just right. Researchers also turned to catalysts made from non-corrosive gold and platinum nanoparticles, modified with hyaluronic acid to give them long-term stability.

How does the smart contact lens work? When a tear comes into contact with the hydrogel, it undergoes a chemical reaction that creates electricity, which is measured by circuits in the lens. The strength of that electrical current indicates the level of glucose that is in the tear fluid, and thus the glucose level of the wearer’s blood. Those measurements are sent wirelessly to a mobile device, where they’re can be viewed using an app.

There’s a considerable need for simple and accurate glucose monitoring in the United States. According to the American Diabetes Association, over 120 million Americans have diabetes or prediabetes. That number grows with around one and a half million new diagnoses of diabetes every year, a diagnosis rate that has more than doubled over the past two decades. In a paper published in Advanced Materials, the smart-contact researchers say the new lens may soon offer not only a non-invasive tool for measuring glucose, but may even be able to dispense medication to manage blood sugar levels.