Polymers In Advanced Technology Applications In Health applications: Part 3

IMPLANTED POLYMERS FOR DRUG DELIVERY
Biodegradable polymers are good for the environment, but they may be good for cancer patients, too. Efforts are now under way to design polymer implants that will slowly degrade inside the human body, releasing cancer-fighting drugs in the process.

Such an implant would need several specific properties. It would have to degrade slowly, from its outside surface inward, so that a drug contained throughout the implant would be released in a controlled fashion over time. The polymer as a whole should repel water, protecting the drug within it—as well as the interior of the implant itself—from dissolving prematurely. But the links between the monomers—the building blocks that make up the polymer—should be water-sensitive so that they will slowly fall apart. Anhydride linkages—formed when two carboxylic-acid-containing molecules join together into a single molecule, creating and expelling a water molecule in the process—are promising candidates, because water molecules readily split the anhydride linkages in the reverse of the process that created them, yet the polymer molecules can still be water-repellent in bulk. By varying the ratios of the components, surface-eroding polymers lasting from one week to several years have been synthesized.

These polymer disks are now being used experimentally as a postoperative treatment for brain cancer. The surgeon implants several polyanhydride disks, each about the size of a quarter, in the same operation in which the brain tumor is removed. The disk contains powerful cell-killing drugs called nitrosoureas. Nitrosoureas are normally given intravenously, but they are effective in the bloodstream for less than an hour. Unfortunately, nitrosoureas are indiscriminately toxic, and this approach generally damages other organs in the body while killing the cancer cells. But placing the drug in the polymer protects the drug from the body, and the body from the drug. The nitrosourea lasts for approximately the duration of the polymer—in this case, nearly one month. And the eroding disk delivers the drug only to its immediate surroundings, where the cancer cells lurk. The polymer degradation method of drug delivery is making good progress toward approval by the Food and Drug Administration.

There are many challenges in designing polymers for controlled-release applications. These polymers must be biocompatible, pure, chemically inert, nontoxic, noncarcinogenic, highly processible, mechanically stable, and sterilizable. The polymers in use today in drug delivery are also mostly borrowed from the chemical industry and in many cases lack the exact required properties. Novel polymers designed and synthesized to provide optimal properties and characteristics will be required to take full advantage of the emerging technologies.

SEASICKNESS PATCHES

A polymer-based "transdermal patch" proved to be an effective solution. The thickness of a playing card and less than three-eighths of an inch in diameter, the patch is applied like an adhesive bandage and does not break the skin. The skin behind the ear is the most permeable, and from there the scopolamine rapidly diffuses into the blood vessels just below the surface.

The patch consists of several laminated layers of different polymers, each one designed for a different function. The topmost layer is a polyester film, colored to match the skin. Adhering to the polyester's underside is a film of vapor-deposited aluminum to protect the drug from sunlight, evaporation, and contamination. Then comes a polymer adhesive that binds the aluminum to the rest of the patch. The next layer, the reservoir, is made of a polyisobutylene skeleton filled with mineral oil that contains a 72-hour supply of the drug in a special skin-permeable formulation. Between the reservoir and the skin is a polypropylene membrane riddled with microscopic pores. The pores are just the right size to ensure that the drug seeps out at a rate less than it can be absorbed by the most permeable skin. This feature ensures a constant dose rate, regardless of the skin's permeability. The patch's bottom layer is an adhesive formulation of polyisobutylene and mineral oil. This mineral oil also contains the drug, so that it saturates the skin as soon as the patch is applied and minimizes the time lag before the scopolamine takes effect. (Even so, it generally takes about 4 hours to kick in.) The adhesive layer is protected before use by a peel-off backing of siliconized polyester.

The transdermal patch technology transformed an otherwise unmanageable drug into the most effective motion sickness treatment available, and one good for three days. Yet this seemingly simple patch—a glorified sticker/Band-Aid—employs at least six layers of carefully chosen polymers, each of which has a specific function and each of which must be compatible with the neighboring materials. Designing and testing the patch required attention to complex issues of drug dosage and behavior as well as the challenge of fabricating a pharmaceutical product in a radically different and untried form.

source and credit: https://www.nap.edu/read/2307/chapter/4#40