X-Ray propagation imaging of a lipid bilayer in solution

We have used X-ray propagation imaging to visualize a less than 5 nm thick native lipid bilayer membrane freely suspended in aqueous solution. Contrast is formed by free space propagation of hard X-rays, with the membrane illuminated by a nano-focused, partially coherent synchrotron beam, at a controllable distance (defocus) behind the focal spot. Quantitative fitting of the magnified Fresnel fringes shows the transition from membranes swollen with solvent to the native bilayer, containing structural information at near-molecular resolution along the dimension perpendicular to the bilayer. We show first applications of this hybrid technique of propagation imaging and near-field diffraction to the investigation of ultra-thin organic films formed in micro-fluidic devices, namely the formation of a lipid bilayer by the adhesion of two constitutive monolayers.