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Making proteins isn't easy. The body uses a complicated system
of DNA, RNA, and other proteins to make new protein molecules. Every natural
protein has a specific sequence of amino acid monomer units in its backbone
chain. It's hard for chemists to synthesize proteins like these, though it
can be done with enough patience and money. When Linneaus Dorman began
making the polymers that would become a synthetic bone material replacement,
he simplified the whole process by using a polymer made from only one kind
of amino acid. He made a lot of these, so we'll show you one for an example,
poly(methyl glutamate), which you see on the right. This was rather
creative, as it is the polymer of an amino acid that the body doesn't even
use in making proteins! There are two important things that Dorman did to
make a strong material from his new polymer, and we're going to talk about
them now.
Composites
Dorman did two things to make a strong material from polymers like this. First, he combined them with calcium phosphate. This was inspired by nature, as our bones are made from natural proteins combined with calcium salts.
Materials like Dorman's bone replacement that are made of more than one component are called composite materials. Those fancy balloons you get when you're in the hospital are made of a composite of polyester film and aluminum foil sandwiched together. A lot of cars have body parts made of composites. These materials are usually some kind of polymer reinforced with a strong fiber such as carbon fiber or Kevlar®. Everything from golf clubs to airplane parts are made from fiber-reinforced composites.
But Dorman's composites were different from the sandwich composites of balloons or fiber-reinforced composites. In these materials you can easily see the separate components. But Dorman's material was made of a polymer combined with calcium phosphate particles that are far too small to be seen without a microscope. Composites like this are sometimes called nanocomposites. If you could see what these materials looked like on the microscopic level, you might see tiny chunks of calcium phosphate dispersed in a mass of tangled polymer chains, like meatballs in a plate of spaghetti. You can imagine it as looking something like this:
Why would Dorman or anyone else go to the trouble of putting two materials together to make a composite? We make composites to get a material that has all the best properties of both materials. Calcium phosphate can support a lot of weight, but it isn't particularly flexible. The polypeptide is more flexible, so when the two materials are combined, you get something that not only supports weight, but it can bend a little bit, which helps keep it from breaking. This combining of the best properties of two or more materials is the reason for making all kinds of composites, whether they are nanocomposites, fiber-reinforced composites, or sandwich composites.
Crosslinking
The second thing he did was to crosslink the polymers. Dorman didn't invent crosslinking, but he put it to good use in his bone material replacement. Now you may be asking, "what is crosslinking?" Since you asked, crosslinking is the joining of a lot of polymer molecules together to form a single supergiant molecule. Remember, polymer molecules are already big...that's why we call them macromolecules, of course. But when we crosslink a sample of a polymer, we join all the polymer molecules together to form a single molecule big enough to see and pick up with your hands! Here's a picture to help you understand what's going on:
Polymer chains. |
Polymer chains joined
together |
By joining all the molecules together like this, you can make a material that is very, very strong. In this case, the material is strong enough to repair broken or damaged bones. A lot of things that need to be strong are made of crosslinked polymers. Most of the things made of composites that we talked about earlier, you know, the airplane parts, the golf clubs, car body panels, etc., are made from crosslinked materials. So are bowling balls. So if anyone ever asks you how many molecules are in a bowling ball, you'll know the answer: one!
Most rubber is crosslinked, too. This helps the rubber keep from getting gooey in hot weather and brittle in cold weather. If you're ready to try crosslinking a rubbery polymer for yourself, try the activity: The Glüg Recipe
Engineered Materials
Try to grasp the bigger picture here. Dorman created a material made of a polymer with tiny particles of calcium phosphate scattered throughout that polymer. What's more, the polymer was crosslinked, locking everything into place. The polymer chains are locked together, and the calcium phosphate particles are locked in their places, too, caged in by a molecular jail.
This result is a very complicated piece of chemistry and engineering. Scientists like Dorman who create these materials are in many ways architects as well as chemists. Not only do they make new materials with desired molecular structures, but they also combine different materials using specific desired architectures to get the properties they want. Being able to deliberately build materials this complex at the microscopic level is truly amazing. It's hard to grasp that scientists like Dorman can assemble tiny molecules the way kids play with building blocks to get exactly the structures they want. But that is just one of the many amazing things that is possible through chemistry.
- Next: A
Liquid Crystal
For more information, at other Web sites...
- Composites — part of The Macrogalleria from the
University of Southern Mississippi.
The Crosslinking Page — part of The Macrogalleria from the University of Southern Mississippi.
Leo Hendrik Baekeland — a biographical sketch of the inventor of a famous crosslinked polymer, Bakelite. Part of Chemical Achievers from the Chemical Heritage Foundation.
References
- Dorman, Linneaus C. and Meyers, Paul A. United States Patent
4,525,495; 25 June 1985.