I believe that you believe you are properly filling the cartridge, but something in the process is out of balance. You are an imperfect human being. or nature, physics, is fouling you up - or both.
I vote for physics because its easier for me to understand and work with than people.
There will always be occasional small bubbles because air can dissolve in insulin. It’s a function of air pressure and temperature.
There is a 20 degree F difference between “room temperature” and your skin under clothing. There is also a lag in the temperature level of the surface of a container vs its contents. (Like the difference between interstitial glucose and blood glucose level.)
Raising fluid temperature will cause some dissolved air to escape. But like a glass of beer, some air won’t bubble out until I drink it.(Hopefully some won’t)
I don’t see any ways to avoid those things, so if it were me, I’d look carefully at what I might be doing or not doing that is fighting physics. I want my insulin to go with the flow.
Ho could air get into a cartridge? How could air get into the insulin?
Either air wasn’t completely withdrawn before introducing the insulin, or it is introduced during the filing.
If the air in that cartridge is removed slowly, it will not be trapped. It will move to toward the low pressure area where you are drawing it out. If I do it too fast, the bladder may not completely collapse, trapping air.
If I inject insulin into the cartridge by introducing it faster than the cartridge can accept it, turbulence will cause micro-bubbles to form (cavitation).
In an empty sealed container like an insulin vial or cartridge a change in fluid within it will cause it to try to expand or contact to match. If there is some air in it, and the container can’t expand fast enough the pressure differential will cause air in the cartridge and air from outside to be pulled into it through any crack, no matter how small.
How? It could happen along the sides of the needle where it penetrates the filler port. It could happen in the syringe between the cylinder wall and the plunger seal. Very tiny bubbles will be dissolved in the insulin. When pressure is relieved bubbles will form and some will stay in suspension a long time, as foam.
That’s why I was taught to roll insulin vials, not shake them.
Eventually the micro-bubbles will join into larger bubbles and rise toward the upper surface. An infusion set has a very large interior surface that promotes micro bubbles joining.
The way to avoid both issues is to take my time when filling the cartridge.
- Before I start drawing insulin, I’ll move the syringe plunger in and out several times. The lubricant can separate and pool during storage. The action re-lubricates the seal.
- I’ll avoid any tilting of the syringe relative to the cartridge which could create a weaker seal between needle and filler membrane.
- After adding the insulin, I’ll tap the cartridge can dislodge any air that was trapped or stuck to the bladder wall as bubbles.
- I’ll draw back on the plunger half way through the fill. That will cause much of the trapped air to be pulled into the syringe with some insulin. If it appears, I’ll withdraw the needle, invert the syringe and urge the air, then resume.
- I won’t shake the reservoir or pump between filing it and filling the infusion set
- I’ll keep the reservoir and pump oriented with filler port up as much as possible through each step of filling the infusion set.
These will give any residual trapped air the best chance of escape, and me the best chance of success.