How do continuous glucose monitors and "artificial pancreas" systems work?
A sensor under the skin reads sugar every few minutes, and in some systems an algorithm then adjusts an insulin pump on its own.
โถ Start the storyA continuous glucose monitor, or CGM, is a small sensor worn on the body that tracks glucose in real time, replacing the need for periodic finger-stick blood draws. It does not measure blood directly: it tracks glucose in the interstitial fluid, the fluid between cells, which is a valid way to follow blood glucose because the two are highly correlated, despite a lag.
Most CGMs work with chemistry. An enzymatic reaction with glucose in interstitial fluid generates an electrical current proportional to the local glucose concentration, and the reading is sent wirelessly to a display. Where a fingerstick measures glucose at a single point in time, a CGM shows levels continually, and trends over time.
An "artificial pancreas", more formally an automated insulin delivery (AID) system, connects three parts: a CGM, an insulin pump, and an algorithm that uses their data to determine needed insulin adjustments. Despite the nickname, the term has no precise, universally accepted definition, and as of October 2020 available systems could deliver only a single hormone, insulin.
Step 1: Sensor
CGM reads interstitial glucose
Step 2: Algorithm
Uses CGM and pump data to determine needed adjustments
Step 3: Pump
Delivers insulin under the skin
Step 4: Repeat
Glucose changes and the loop continues
The systems come in degrees of automation. Some only halt insulin when a low is predicted. Hybrid closed loops adjust delivery up and down, but users still initiate meal doses. Fully closed loops need no meal announcements. The first approved hybrid closed loop, the Medtronic MiniMed 670G, got FDA approval in September 2016.
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Recap
Sensor, pump, algorithm: three parts that sense glucose, decide and deliver insulin in a loop.
๐ก A trick to remember it ยท Sense, decide, deliver: the thermostat of blood sugar.
Surprising fact ยท CGM readings lag blood sugar by roughly 5 to 10 minutes because they sample the fluid between cells.
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No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.