My guess would be this: > Another possibility, which could prevent the need for immunosuppressive drugs, is to encapsulate the transplanted cells within a flex

My guess would be this: > Another possibility, which could prevent the need for immunosuppressive drugs, is to encapsulate the transplanted cells within a flexible device that protects the cells from the immune system. However, finding a reliable oxygen supply for these encapsulated cells has proven challenging. Hemoglobin is a large molecule. It is difficult to create a barrier that allows it to pass without also allowing in anti-bodies. Trying to extract the oxygen outside of the barrier and that pass just the oxygen through is theoretically another approach. But at that point, doing the same thing with water instead of hemoglobin is just easier.