Layers break apart in controlled way. (Making Polymers That Self-Destruct).

Scientists in Japan have created a polymer film that can chew itself apart, making it a candidate for the controlled delivery of therapeutic drugs.

A major thrust of modern medicinal research is to regulate the release of drugs from pills and biomedical implants so that the therapeutic molecules are liberated over a long period or target selected organs. One of the first steps in developing these drug-delivery methods is the design of materials that can discharge molecules at desired rates. The new multilayer film created at Japan's Kagoshima University might ultimately serve that purpose, says team member Takeshi Serizawa.

The material contains 17 alternating layers of two polymers. One of these is a positively charged synthetic polymer and the other is DNA, a negatively charged biological polymer. To construct the film, the researchers apply a layer-by-layer assembly method developed several years ago. to build thin films (SN: 11/11/00, p. 312). The researchers dip an experimental substrate, such as a piece of quartz, into a solution of one polymer and then into a solution of an oppositely charged polymer. Electrostatic charges hold the layers together.

To the surface of their film, the Kagoshima researchers added their version of a self-destruct button: a negatively charged layer of the DNA-snipping enzyme DNase I. In the March 10 Angewandte Chemic International Edition, the researchers report that this enzyme remains inert while stuck to the surface of the positively charged polymer. When the film encounters a solution of positively charged calcium and magnesium ions, the enzymes break free and begin to chew through successive layers of DNA, disrupting the intervening synthetic polymer layers in the process.

The concentration of calcium and magnesium ions regulates the rate at which the film degrades, says Serizawa. Because the abundance of these ions varies throughout the body, the researchers aim to create targeted drug-delivery systems, which might include biomedical implants such as artificial blood vessels and artery-opening stents.

The Kagoshima work puts a new and elegant twist on the design of biodegradable, layered films, says Michael Rubner of the Massachusetts Institute of Technology, who designs similar materials. Like other biodegradable layered films, however, the new film is far from a sure thing, he notes. The material has yet to be tested for toxicity and effective drug delivery.