Biomaterials: New Polymers and Novel Applications
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Introduction Over the last 21 years I have been involved in examining and studying, and in some cases synthesizing new polymers and biomaterials, looking at how they might be able to solve two problems. One set of problems involves drug delivery; a second involves creating new tissues. Controlled drug release is a relatively young field that has involved a fundamental change in the way drugs are delivered. Transdermal drug delivery, for example, uses five layers of polymers, yet the whole system is 0.2 mm thick. One such polymer contains nitroglycerine, which is slowly released for one day from a patch through the skin into the blood circulation. This process provided a novel way to treat angina for patients with chest pains. First introduced in 1982, such drug-delivery systems saw a usage rate of over 500 million last year. Norplant systems, which are silicone capsules placed under the forearm, slowly release a birth-control drug for over 2,000 days, or five years, from what are actually six small implants about the size of match sticks. These two examples illustrate that drugs can be made to slowly diffuse through polymers over a long period of time. Background on Drug Delivery My interest in drug-delivery polymers started in 1974 when the field was just emerging. I studied the issue of whether a large molecule could be released slowly through these polymers. At that time, such a question was purely of academic interest, and it was a very difficult chal-
lenge. Large molecules taken orally would not work since they are usually destroyed by enzymes during swallowing and cannot be absorbed. They are too large to pass through the skin transdermally, and when injected, they are rapidly destroyed by the body. Very often they do not last for more than 20-25 minutes. In the 1970s and 1980s, however, the field of genetic engineering and biotechnology emerged. Genetic-engineering companies began making large-molecular-weight drugs, and these drugs sometimes faced serious delivery problems. Many of the large molecules needed a way to be delivered in unaltered forms and yet be protected from harm. In the 1970s, researchers in the polymer community believed that large molecules could not be slowly released from a biocompatible polymer.1
Figure 1. A 5 ^m-thick section of ethylene vinyl acetate copolymer with no drug or protein in it. No pores were found, and molecules of 300 daltons or greater were unable to diffuse from one side of the thin polymer matrix section to the other.
It was against this background that our research group began studying this problem. We discovered that certain very hydrophobic polymers, like ethylenevinyl/acetate or lactic glycolic acid copolymers, could be dissolved in certain solvents and mixed with powdered protein (often at a very low temperature); then, depending on the fabrication procedure, these polymers could be made into small microspheres or other dosage forms. About 19 years ago we found that we could release different proteins ranging from lysozyme (14,000 daltons) to catalase (250,0
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