Here we present three different types of mechanically stable nanometer-sized hollow capsules. The common point of the currently developed systems in our laboratory is that they are liposome based. Biomolecules can be used to functionalize lipid vesicles to create a new type of intelligent material. For example, insertion of membrane channels into the capsule wall can modify the permeability. Covalent binding of antibodies allows targeting of the capsule to specific sites. Liposomes loaded with enzymes may provide an optimal environment for them with respect to the maximal turnover and may stabilize the enzyme. However, the main drawback of liposomes is their instability in biological media as well as their sensitivity to many external parameters such as temperature or osmotic pressure. To increase their stability we follow different strategies: 1) polymerize a two-dimensional network in the hydrophobic core of the membrane; 2) coat the liposome with a polyelectrolyte shell; or 3) add surface active polymers to form mixed vesicular structures.

We employed UV-induced template polymerization to create hollow nanometer-sized polymer capsules. Homogeneous, unilamellar liposomes served as a two-dimensional template for the cross-linking of either butyl methacrylate or hydroxyethyl methacrylate with the bifunctional ethyleneglycol dimethacrylate. Different molar ratios of lipid/hydrophobic monomer/bifunctional monomer/photoinitiator were tested and dynamic light scattering revealed negligible changes of size at a defined molar ratio of 2/1/10/20, respectively. Cryo-transmission electron microscopy provided clear evidence that incorporation of the methacrylate monomers into and polymerization in the hydrophobic bilayer phase does not disrupt vesicle integrity. Moreover, after solubilization of the lipids, the polymethacrylate nanocapsules were stable at conditions needed for negative staining and could be visualized by atomic force microscopy. In contrast to previous findings, the nanocapsule size and shape did not change considerably after removal of the template phase, and the size distribution remained strictly monomodal. The employed method is not only an advance to fortify liposomes, but the nanocapsules themselves can be functionalized.