Just spoke to the GORE rep and got this explanation. Hope it helps others in a similar situatuion
The graft is microporous and what you are seeing is air inside the ePFTE. It takes a bit for the graft to be ‘wetted in’, up to two weeks.
Here is ePTFE under an electron microscope (from google), the air is trapped in these tiny gaps, but it poses no risk to the patient or the graft. Over time, the air dissipates and the gals are filled- however I’m not 100% sure if it’s with tissue or fluid. If you like, I can ask the question to my colleagues to see if they know?