Biological and biomimetic ceramics: A new frontier

Summary and evaluation of the video-tape of a lecture given at the University of Pennsylvania by Prof. Arthur H. Heuer from the Case School of Engineering of the Case Western Reserve University.

Homework prepared for the class "MSE 566 - Physical Properties of Ceramics", handed in at the 31st of January 1995.
Advisor: Prof. Dawn A. Bonnell from the Department of Materials Science and Engineering of the University of Pennsylvania.

1. Summary

The lecture begins with a definition of the terms used in the title: Biological ceramics are hard tissues occurring naturally, i.e. are produced by organisms, for example bones, shell, and teeth. Biomimetic ceramics are synthesized ceramics that mimic natural tissues. Examples are given to show that there is a vast number of minerals that is being produced by organisms. Most of these are Ca- and Si-based minerals (e.g. quartzite and calcite), but there are also more exotic minerals in the list, for example there are some animals producing magnetite for their teeth. Then the importance of studying the materials science of hard tissues is stressed. The most important reasons are the following:

1) Modeling of ceramic-polymer composites.
2) Self-assembling low-temperature processing.
3) Optimized ceramic-polymer interfaces.

In the rest of the lecture examples of current investigations are given. They concern mainly the structure and mechanical properties of the two CaCO3-polymorphs calcite and aragonite. Some possible biomimetic concepts are briefly outlined.

2. Evaluation

I think that the study of biological and biomimetic principles is very helpful for natural sciences as as well technology, and this is especially true for materials science. Nature has had several million years to perfect some of its concepts, and engineers can definitely learn by trying to understand how organisms solve problems. The video made it quite clear that this concept of studying and copying nature for engineering purposes has, as yet, not been developed far, so that many points are still open to discussion. But the examples of current research given in the talk show a multitude of effects observed in nature that are potentially interesting for engineering.

Concerning the point of modeling ceramic-polymer composites, it should be noted that in nature there are composites containing more than 99% ceramic and less than 1% polymer. This is at the moment not achievable industrially. In nature the polymer is used to control the mineralization of the ceramic. For example some organisms, like scallops, produce calcite for their shells, the polymorph of CaCO3 that is stable at room temperature and pressure. Other organisms, like the pink conch, produce aragonite instead, the polymorph that is only metastable at ambient conditions. This is done by depositing a layer of a protein on which the aragonite mineralizes epitaxially. The aragonite is again covered with protein by the organism, so that a new layer of aragonite can mineralize. This process leads to a lamellar structure, ensures the formation of aragonite, and leads to a fast rate of crystallization. Such a lamellar structure is a recurring motif in nature, although other types are encountered as well, as for example foliated, prismatic, or columnar structures.

Furthermore, the ceramic-polymer composite has very interesting mechanical properties. An example is the aragonite found in shells, which is about twice as hard as the mineral aragonite, and this although the shell has only an addition of 0.5% polymer. The superior mechanical properties of natural composites provide an interesting model for the synthetic processing of composites. It is for example possible to produce enhanced mechanical properties by copying the very common natural layered structure in a composite build up of differing sheets of Al2O3 and metallic aluminum.

Another possibility of using biomimetic principles is for powder processing. Encapsulation and other effects observed in nature provide helpful ideas for the synthesis of powders. Taking this idea to its extreme is the possible production of powders directly by micro-organisms. There are several minerals that are produced by micro-organisms in the form of a powder, so that these micro-organisms could be used on a large scale to synthesize the minerals. It could even be possible to produce minerals not occurring naturally by using the technique of recombinant DNA to alter the micro-organisms╣ genes.

These are just a few examples illustrating the potential impact that the use of biological and biomimetic principles might have to solving engineering problems. The study of the concepts of nature is therefore of utmost importance not only for reasons of a deeper understanding of the natural sciences themselves, but also for practical reasons, to find solutions for difficulties that our technology is facing.

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