I guess its the engineer in me but I’m looking for an explanation for how the Tandem t:slim X2 determines how much insulin is in the cartridge.
I’ve see the videos on the delivery side but can’t find anything on the cartridge side. I’m coming from using a Medtronics 670 where the stepper motor’s position could be used to determine the level in the reservoir.
It’s done via pressure sensors.
If it helps, there are teardown images posted online:
The teardown images aren’t really that helpful in answering my question.
From what I can see, the cartridge contains a “bag” that is filled with insulin. There is what appears to be a probe at the “bottom” of the cartridge which I assume is used to pressurize the inside of the cartridge forcing the insulin in the bag to be pulled out by the micro delivery apparatus at the opposite end.
So this explains how the pump empties the cartridge. What it doesn’t explain is how the pump determines how much insulin is in the cartridge initially, and how it gets “more accurate” after 10 units are delivered.
This my opinion only, I have no inside information as to how Tandem actually manages the cartridge. It is only my assumptions and how I might design such a system.
Having pulled a cartridge apart, I think there is an air pump that puts a known pressure on the bag inside the cartridge. As insulin is dispensed, the pressure in the cartridge goes down and periodically an air pump has to add air to the cartridge to make up for the insulin used. If you kept track of how much air, or how long an air pump has run, you could track the remaining capacity in the cartridge. Seems logical but again, I have no real knowledge as to how Tandem implements this system.
It can’t be air because then you would have air injected instead of insulin.
I am pretty sure there is a pressure sensor in there and it estimates.
The pumping mechanism is really a tiny bilge pump. The pump it self just moves a metal cam back and forth. The valve in the cartridge has a one way valve that pulls insulin out but prevents air getting back in.
This way the bag creates a vacuum. I think this negative pressure is what is used to determine how much insulin there is.
That makes sense because you need at least 50 units to start a cartridge. The more insulin that’s in there the more accurate it will be in detecting it.
So it calibrates on being full, then it just subtracts as you use it.
There is always insulin left after it says empty. It’s not a perfect system. They did all that engineering just to save space.
It can be air because the insulin is in a sealed flexible bag and the air would be injected into the plastic cartridge housing outside the bag.
I’m with Timothy on this (as an medical device engineer). I suspect they measure the pressure in the space around the insulin bag to determine change in volume of the bag. I think it would be too expensive to have a air pump and believe they used the bilge pump idea to get around existing “using a syringe” patents.