In order to maintain their asymmetric lipid distribution, cellular membranes have a number of means to translocate lipid molecules across the bilayer. On top of the well-studied active flipping processes carried out by the flippases & floppases, there are also passive transfer mechanisms which facilitate the lipid migration even in protein-free bilayers. This process, acting against the protein transport systems, is considered to be the significant natural process that cells must overcome in order to sustain 'normal' cell asymmetry. Despite this, the mechanism by which this occurs in a protein independent manner is poorly understood and warrants investigation leveraging our work with ISIS developing improved membrane models and deposition techniques. By selectively deuterating the bilayer we will be able to observe the rate of lipid exchange, significantly improving our understanding.