The microstructure of the modulated MIL-116(Ga) is not a solid sphere, but a hollow-sphere from a druse-like crystal assembly. Apparently, during our synthesis these hollow spheres must form from a loose particulate, which later grows into a polycrystalline shell, consisting of individual needle like crystallites. We think that shell-induced Ostwald ripening leads to dissolution of the nanoparticles for the sake of the micron-sized shell, leading to the druse-like morphology as shown in Figure . These druse-like crystals offer interesting features for MMMs, as shown in the schematic in Figure . For He, diffusion is enhanced in the middle of the particle, offering a highway for gas diffusion, as the average pathway over the particle is decreased. The outer surface of the MIL-116(Ga) particle is assumed to offer H-bonding to PSU.