Apple CEO Tim Cook has been wearing a prototype glucose tracker that pairs with the Apple Watch, according to a report from CNBC. The revelation adds weight to a previous report from CNBC, which said last month that Apple has hired a team of biomechanical engineers to develop a noninvasive device to monitor the blood sugar levels of people with diabetes.

Citing unnamed sources, CNBC reports that Apple’s Palo Alto-based team has already begun feasibility tests with the tracker, which connects to the Apple Watch. Glucose trackers currently on the market use sensors that penetrate the skin. Cook told students at the University of Glasgow in February that he had been wearing a glucose tracker, and that it helped him understand the impact of different foods on his blood sugar levels.

"It's mentally anguishing to stick yourself many times a day to check your blood sugar," Cook said, according to CNBC. "There is lots of hope out there that if someone has constant knowledge of what they're eating, they can instantly know what causes the response... and that they can adjust well before they become diabetic."

More than 29 million Americans suffer from diabetes, according to recent figures from the Centers for Disease Control and Prevention (CDC). Among those who have the disease, one in four are unaware of their condition, according to the CDC.

Steve Jobs envisioned using wearables to monitor blood sugar and other vital signs during his tenure as Apple CEO, and the company’s glucose tracker has been in development for at least five years, CNBC reported last month. But developing an accurate and noninvasive technique has proven challenging. Speaking to CNBC last month, biomedical expert John L. Smith said developing such a device has been “the most difficult technical challenge I have encountered in my career.”