Phospholipid polymer films put on surfaces A pair of methods that coat solids with suitable phospholipids and then polymerize the lipids into a membranelike coating are being developed independently by U.S. and British laboratories. The resulting coated solids have many properties that are determined not by the bulk material, but by the properties of the outer phospholipid membrane. The simplest of these methods is being developed by chemistry associate professor Steven L. Regen, along with Piotr Kirszensztejn and Alok Singh, all at Marquette University, Milwaukee [Macromolecules 16, 335 (1983)]. The other method, though not as easy to perform, has the advantage of being able to form either single or multiple layers of lipid in the polymerized coating. It is being developed by a team of chemists and biochemists at the University of London that is headed by Dennis Chapman [Biochem. Biophys. Acta., 687,165 (1982)]. Though the two techniques are quite similar, they were developed with different goals in mind. The British group is trying to develop a model for cell membranes, which can be thought of as phospholipid bilayers that have certain proteins and glycoproteins embedded in them. They hope to study processes like blood coagulation and protein adsorption, which appear to be controlled by the surface properties of membranes. The Marquette chemists, on the other hand, hope to use the technique to modify the surface properties of solid organic polymers to make them more useful in specific applications. Properties like wetting, adhesion, dye adsorption, and biocompatibility are controlled by the surface conditions of polymers, rather than their bulk properties, Regen explains. Thus, by providing a uniform coating of a lipid with suitable properties, it may be possible to extend the usefulness of many solid organic polymers. Fundamentally, the techniques developed by both groups are similar. Both use the film-forming properties of lipids in oil/water mixtures to coat a solid support with a layer of phospholipid molecules. In each case
the phospholipids have been previously modified to contain polymerizable side groups. The second step is to irradiate the lipid coating, causing the film to polymerize into a membranelike coating. The Marquette chemists simply place strips of prepared solid polymer in a vesicle dispersion of the desired phospholipid, irradiate the system, and then gently rinse the excess lipid off the coated polymer. In their tests they have used lowdensity polyethylene film as the solid polymer and phosphatidyl choline as the phospholipid that forms the coating. One reason for choosing this particular system is that earlier work with polymerized vesicles of phosphatidyl choline indicates that this material does not cause blood platelets to aggregate—a promising indication, Regen says, that materials coated in this way might have good biocompatibility properties. Pharmacologist Rudy Guliano at the
Lipid monolayer forms easily on solid polymer
x = Polymerizabie group y = Polymerizable group or hydrogen
Health Science Center of the University of Texas medical school in Houston is collaborating with the Marquette group to examine this possibility. "We think we have something approaching a polymerized monolayer on a polymer surface," Regen says. Although characterization of such surfaces is difficult, he thinks they can rule out two other types of reactions that might have occurred. These are reactions of the phospholipid molecules with molecules below the surface of the polymer or the building up of chains of phospholipid molecules sticking out from the surface in a process called tethering. The Marquette work initially was sponsored by the National Science Foundation and the National Cancer Institute. Current funding, however, comes primarily from 3M Corp., a major adhesives manufacturer. It is the potential applications of the technique to adhesives thai the Marquette group is focusing on now, Regen says. What the chemists would like to do is to put a monolayer onto a film and a complementary layer on a second film and then study the interactions of the two layers. The London scientists use a device called a film balance to spread a lipid layer of controlled thickness on a water surface. They then raise and lower their solid base material—in their case, slides made of glass, quartz, steel, or Teflon—through the lipid layer. The more times the solid is dipped, the thicker the resulting coating. With the right phospholipids, the researchers have built coatings up to 18 layers thick. The multilayers are then polymerized by irradiation with ultraviolet light. The lipid molecules stack head-to-head and tail-to-tail, so the number of layers in the coating determines whether the outer layer contains the phosphatidyl choline end of the molecules or the acyl chain end. By careful manipulation of this property, the researchers can obtain surfaces that are either hydrophilic or hydrophobic and can build model membranes of both types. Their work is supported by the Wellcome Trust, Science Research Council, Deutsche Forschung Gemeinschaft, and Leverhulme Trust. D February 21, 1983 C&EN
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