Above: Some of the structures we have observed for polymeric myelins, viewed by confocal laser scanning micrography using the fluorescent dye Rhodamine B octadecyl esterperchlorate to pick out the polymer membranes.
Phospholipids are simple molecules composed of a polar (water loving) head group, and a hydrocarbon (water hating) tail. When added to water these molecules therefore self assemble into structures that maximize the interactions between head groups and the water, and minimize interactions between the water and the tails. Biologically the most important of the structures is the membrane.
Phospholipid membranes are essential to all life on earth, encapsulating living cells, and forming sub-cellular structures and machinery such as organelles and vesicles. Technically, membranes are very difficult to use, however, as they are dynamic structures: The cell has to spend considerable time and energy maintaining and controlling the exact chemistry of its membranes, as these allow the basic processes of ‘life’ to continue.
Recently chemists have found that many of the properties of biological membranes can be duplicated using amphiphilic copolymers. Technologically these materials have many advantages – the attractive and repulsive interactions are multiplied in long chain polymer molecules, resulting in robust structures. Further, we are able to synthesize diverse families of molecules, with modifications to suite different tasks.
Structures formed from large, amphiphilic copolymer molecules also change slowly, as the molecules must be disentangled before any structural change can occur. This allows us to study the phase properties and dynamics of these membranes, watching processes such as fission and fusion of vesicles (the central process governing), and the phase changes that accompany dilution of these molecules. This ability is helping us to understand many of the fundamental properties of their natural analogues.
The ability of these materials to form vesicles, and the comparative stability of these structures, allows us to develop technically useful compartmentalization strategies – either nano-scale chemical reactors, scent delivery vehicles in cosmetics and clothes washing liquids, or drug delivery capsules. Coupled with suitable synthetic strategies we can prepare ‘smart’ vesicles that release their contents only under certain environmental conditions – of pH, temperature, etc.
Above left: Unilamellar vesicle formation monitored by confocal laser scanning microscope. The dye Rhodamine B octadecyl esterperchlorate, has been added to highlight the membranes (orange).
Vesicles are not the only structure that can be formed, however. These synthetic membranes can form structures with a very broad range of scales – from 100nm to 100μm. Amongst the larger structures are myelins, tubular structures similar to nerve sheaths. Such structures may be useful physical supports for catalysts or tissue engineering projects.