Introduction

This reading describes a polypeptide and calcium phosphate composite material invented by Linneaus Dorman and other scientists at Dow Chemical Company. This synthetic bone material replacement involves a great deal of organic chemistry. Choose how much detail you feel your students are capable of understanding when teaching this lesson. The nature and preparation of Dorman's composites are complicated, but they are also elegant and amazing when fully grasped.

Major Concepts

1. Proteins are natural polymers, polyamides, and their structure, that also inspired Carothers' nylon, inspired the synthetic polypeptides that Dorman used in his materials.

2. A composite material is any material made from more than one component. Composites combine the best properties of two different materials, and in doing so compensate for any weaknesses that any of the component materials may have when used alone.

3. Crosslinking is the chemical joining of polymer chains to form a single giant supermolecule. An object made of a crosslinked material contains one single molecule. Crosslinking greatly enhances the physical strength of a material.

Polymer Synthesis and Composite Preparation

The monomer synthesis begins with an amino acid. In the example in the student version, the monomer is methyl glutamate. Note: This is not one of the twenty amino acids used by the body to make proteins. Rather, it is a derivative of one, namely, glutamic acid. The first step in the synthesis involves its reaction with phosgene to produce the N-carboxyanhydride.

N-carboxyanhydrides polymerize spontaneously at room temperature in the presence of mineral salts such as calcium phosphate. The polymerization is a condensation reaction, with carbon dioxide produced as the polypeptide is formed.

As the polymerization is carried out in a suspension of the calcium phosphate particles, the composite material is produced in situ.

The polypeptide can also be crosslinked using agents such as N-(2-aminoethyl)piperazine carbamate. However, the mechanism of crosslinking is not described in the patents.

N-(2-aminoethyl)piperazine carbamate

Teaching Tips

To demonstrate the effects of crosslinking, divide the students into two or three groups, but leave a few students out, as they'll be needed later. Have each group line up and hold hands. Then ask the groups to try walking around. Next, have the extra students who aren't in any group stand among the groups, and ask each to hang on to two different groups, and to stay in one spot. Now ask the groups to try to walk around. With motion much more difficult, the students should get some idea of how crosslinking affects the behavior of polymer chains.

The students can also experience the effects of crosslinking firsthand with the activity The Glüg Recipe.

Relevant National Science Education Standards

Unifying Concepts and Processes — A complex system of a composite material is the theme of the reading.

Physical Science — The atomic nature and the molecular nature of matter are central in the chemical composition of the components of the composite examined. The reading explores the interaction of matter and energy with regard to how the design of the composite allows the material to respond to mechanical stresses without breaking.

Science and Technology — The reading explores the technology of using scientific knowledge to design sophisticated materials.

Science in Personal and Social Perspectives — The reading shows science as used to face a health challenge, the repair of badly damaged bones.

Next: A Liquid Crystal

References

Dorman, Linneaus C. and Meyers, Paul A. United States Patent 4,525,495; 25 June 1985.