Equally important, PLA is made by polymerizing lactic acid that is made from corn, a renewable resource. This means we don't have to waste valuable petroleum to make it. A joint venture was created in 1997 by Dow Chemical and Cargill to make PLA.

These aren't perfect materials. A good chemist will ask, "what do plastics like these break down into when they biodegrade?" Sometimes greenhouse gases like carbon dioxide are produced. Also, in some cases more fossil fuels are burned producing biodegradable materials than conventional plastics. The balance of benefits and risks is not always an easy thing to calculate. Materials like PLA are the beginning of a search for an ideal solution, not the end. Reaching the goal will be a big challenge for tomorrow's polymer chemists.

To sum things up, plastics are very useful and often wonderful materials, but like anything else we create, they can turn against us if we're not careful. Unnecessary waste is harmful, whether the waste is plastic, paper, or anything else. Reduce, re-use, recycle" is a good principle to stick to no matter what materials you are using. There are good times to use plastics, and bad times to use them, too. It takes careful discernment to say what is a good use for plastic and what is a not-so-good use. If you're ready to test your powers of discernment, you might be ready for an activity we call The Great PVC Controversy.

Act II: Degradation and Medicine

PLA isn't the only degradable polymer, and plastics aren't their only use. Another polymer, called poly(3-hydroxybutyrate), or PHB, has potential as a biodegradable plastic, and other uses as well. PHB is actually made by certain kinds of bacteria.

poly(3-hydroxybutyrate)

One interesting thing about PHB is that to alter its molecular structure (and therefore its physical properties), scientists alter the DNA of the bacteria that produce it. This creates bacteria that make polymers with a desired molecular structure. In the future, scientists may be able to engineer plants that produce PHB derivatives by splicing pieces of bacterial DNA into plant DNA.

Linneaus C. Dorman researched the use of polymers as biomaterials, and PHB has potential medical uses. So does PLA for that matter. Biodegradable polymers are medically useful because they can be used in the body, and then degraded by the body once they have outlived their usefulness. For example, pins made of degradable polymers can be used to hold the pieces of broken bone together. Once the bone has healed, the pins would degrade. Normally, surgery is required to remove the pins. Degradable polymers could eliminate the need for costly operations.

In addition, a polymer similar to PHB, called poly(4-hydroxybutyrate), or PGHB, is also made by bacteria.

poly(4-hydroxybutyrate)

When it degrades, this polymer produces compounds that have medicinal value. So in theory, a small piece of PGHB could be implanted in a person's body to slowly release medicine over long periods of time. This is called controlled drug release.

Much more research is needed into biodegradable materials. But the potential use of degradable polymers, from plastics to medicine, is large, and makes researching them well worth the effort for scientists of the future.

Next: What Do Those Triangles Mean?

Cargill Dow Polymers LLC — learn more about poly(lactic acid) at the official corporate website.

How Green Are Green Plastics? — an examination of the issues surrounding biodegradable plastics from Scientific American.

References

Sudesh, K., Abe, H., and Y. Doia. "Synthesis, Structure and Properties of Polyhydroxyalkanoates: Biological Polyesters." Progress in Polymer Science, 2000, 25(10), 1503.