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(Pocket-lint) - A major feature coming to the Apple Watch may have just been revealed in an SEC filing for UK-based startup Rockley Photonics.

Rockley Photonics designs sensors for monitoring a person's blood using infrared light. The sensors could be used for monitoring blood glucose and alcohol levels. According to an SEC filing for the company, spotted by the Telegraph, Rockley is preparing to go public in New York and has subsequently revealed that Apple is one of its "few large customers". Two of its largest customers accounted for 100% of its revenue in 2020 and 99.6% in 2019.

It's unknown if Apple is the first or second-largest customer, but one can add two and two together and assume Apple is interested in Rockley's sensors or sensor technology, possibly even for improving its own products such as the Apple Watch. Rockley is said to have a "supply and development agreement" with Apple, after all, and most of its revenue seems to come from engineering fees for development work related to future products.

Rockley is working on a basic sensor and an advanced one. The basic one supports heart rate, O2 saturation, blood pressure, hydration, and core body temperature, while the advanced one adds blood sugar, carbon monoxide, lactate, and alcohol level. Rockley is pitching the advanced module for “high-end smartwatch” use. It expects the technology to be available in consumer products by 2022, though it's unclear if that includes Apple Watches.

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Keep in mind Apple has been rumoured to be developing glucose monitoring for its smartwatches for a long time, with recent speculation concluding the Apple Watch Series 7 could gain such a feature. Apple has also been filing patent applications for components that could be used to monitor glucose levels.

Writing by Maggie Tillman. Originally published on 3 May 2021.