Apple is reportedly developing sensors that would make blood sugar level monitoring for diabetes patients pain-free, sources with knowledge of the matter said.

The breakthrough, which was envisioned by the late Apple co-founder Steve Jacobs, is dubbed as a "holy grail" for life sciences, as developing such non-invasive technology is a challenge. John L. Smith, one of the top experts in the industry, described is as "the most difficult technical challenge I have encountered in my career."

The iPhone maker has allegedly hired a small team of biomedical engineers to work on the initiative. It has also reportedly carried out feasibility trails at clinical areas across the Bay Area and has sought the help of medical professionals to determine regulatory pathways, CNBC reported.

The project has been going on for at least five years, according to the sources. Jobs envisioned wearable devices like smart watches to help monitor vital signs. One of the sources said that Apple plans to develop optical sensors that can measure glucose level by shining a light through the skin.

One source said that Apple had at least 30 people on the team secretively working on the technology. But that number could have changed as the company has snapped several biomedical experts from other firms such as C8 Medisensors, Masimo Corp, Medtronic, and Vital Connect.

According to the World Health Organization, diabetes is one of the world's fastest growing diseases, affecting one in 11 adults worldwide. Patients suffering from diabetes have to undergo several finger prick blood test daily, as out of control glucose levels could risk damage to the eyes, kidneys, and heart.

Google said three years ago that it would also develop contact lenses with sensors to help monitor glucose levels. But the company has not released that product yet.

CNET said Apple did not respond to its request for comment as well.