CGMs are amazing, and the closed loop makes the diabetics' life vastly better. They can almost pretend they have a functioning (mechanical) pancreas.

But there's more to be done. They don't measure blood insulin, but the interstitial fluid, which responds more slowly. And a new monitor takes an hour to establish its baseline, which is an hour that your closed loop isn't complete. (I gather that the newest Dexcoms take just 30 minutes.)

A version that used just a smartwatch would be amazing, especially if it really was measuring the actual blood level. But making that sure enough to be part of a closed loop is a massive, massive hurdle. It's controlling the delivery of a medicine where both overdose and underdose are dangerous, and that means extremely high levels of precision and proof.