Mechanical properties and microstructure of polycrystalline phospholipid monolayer shells: Novel solid microparticles

A polycrystalline phospholipid monolayer self-assembled at the surface of an air microbubble in aqueous solution represents a novel material structure: in essence, a solid shell of wax with micrometer-scale dimensions and a thickness of only a single molecule. Micropipet manipulation of these microparticles revealed the dependence of the mechanical properties of the lipid shells, specifically, yield shear and shear viscosity, on the composition, grain microstructure, and thermal processing of the material, in particular the cooling rate of the shells from the melt. Properties were measured as a function of the (1) lipid composition at a fixed cooling rate and (2) cooling rate at a fixed lipid composition. Epifluorescent microscopy and transmission electron microscopy revealed that the morphology of the 1,2-distearoyl-sn-glycero-3-phosphatidylcholine monolayer microstructure, which develops upon freezing from the melt, is dependent on the cooling rate through the lipid transition temperature T-m, with larger micrograins being formed at slower cooling rates. Mechanical properties of the lipid shell follow micrograin size, with the coarse grain structure exhibiting a higher resistance to shear deformation than the fine grain structure does, which is behavior consistent with that of more traditional bulk crystalline materials.