[afarviral]

Would it not be more direct to do gas exchange with blood?

[samus]

Such devices already exist and are used in heart or lung surgery, as well as in intensive care and medical emergencies. However, the potential for complications is much higher since they extract from and return blood to the body.

[analog31]

You still need some permeable membrane with a high surface area, to exchange dissolved oxygen. In this method, the customer supplies the membrane.