I am aware that I know next to nothing about the topic and that I am ignorant of countless complications. With that said, your pancreas also does not know that you plan to work out, at least I would assume so. So what is the crucial difference between a pancreas and insulin injections? That the response is delayed because the injection is not into the blood stream or does the pancreas also have better sensors to figure out how much insulin to release, or maybe even release things other than insulin? If it is the former, what obstacle is there for injecting into the blood stream? The infection risk of having a permanently open port into the circulatory system? If so, is there a similarly effective way to deliver insulin that is not into the blood stream and with less risks? Or could you have some kind of membrane in the port that only allows insulin to pass through but that is impermeable for pathogens? Or would that get clogged immediately?
The difference is the pancreas reacts immediately to changes and has no practical delay for its effects, same concept as PID controllers in engineering.
So the limiting factor is the subcutane delivery, i.e. if the insulin could be delivered into the blood stream, then continuous glucose monitoring and controlled precise insulin doses could work as well as the pancreas?
The subcutaneous part is not really solvable, but even if it were, "real" insulin is much faster than even the fastest insulins (lispros) that we currently have. So there is a delay between delivery and action. There is also a delay on the sensing side; the pancreas always knows what's up, but even cutting edge CGM technology has significant delays, mostly related to that subcutaneous issue from my understanding.
There are faster acting insulins creatively named "faster aspart" and "ultra rapid lispro" with the latter beginning to appear in the bloodstream at 2 minutes and reaching 50% effect around 20 minutes.
With that said the difference between CGM/pumps and the human body's mechanisms of blood glucose regulation is not purely due to pharmacokinetics, regulation of blood sugar is very complex and we still don't fully understand them but there are multiple hormones and factors affecting blood glucose regulation.
As one example some incretins are released in response to ingested food content and stimulate insulin secretion before blood glucose levels rise. We can't replicate that just by measuring blood glucose (not that we necessarily have to).
The homeostatic mechanisms of the human body are fascinatingly complex.
It's a huge oversimplification, but essentially yes.
The current external artificial pancreas solutions operate subcutaneously, and your pancreas gets to work more directly with your blood. Subcutaneous blood sugar readings lag about 20 minutes compared to actual blood (finger pricks), and subcutaneous insulin delivery lags behind a healthy pancreas by even longer.
There isn't much of a difference between injecting insulin (being diabetic) or getting your body to produce it. The main difference lies in having no automated mechanism for regulating blood sugar levels. Due to this - you have to calculate the correct amount of insulin you need for your meals and measure your blood sugar regularly to make sure it stays within balance. If your blood sugar levels tend to be higher than normal (i.e. you aren't injecting enough insulin), that isn't good. It is OK in the near term - but can be devastating in the long term due to health complications (i.e. blindness / nerve damage / etc..). If your blood sugars are fall too low - you can have severe consequences in the near term (i.e. die or have a seizure within 1 hour of injecting insulin). In other words - you need to always keep your blood sugar in check in order to survive and to be in good health. Some people find this extremely hard to do - some people like me embrace it.
Your pancreas actually does know things like that you've ingested a lot of carbohydrates before they enter the bloodstream, etc. Certainly there is also a response to exercise that occurs before you coiod easily detect it with a CGM. There are very complex brain-gut-pancreas interactions mediated by a variety of molecules/hormones.
