Shells as Medical Specialists from the Sea
 

Scientists at Leibniz Universität Hannover, Hannover Medical School and TU Braunschweig are Testing Mother-Of-Pearl as a Model for Innovative and Exceptionally Hard-Wearing Artificial Joints.

As souvenirs, shells have long had a special place in the hearts of seaside holidaymakers. In future they could find a place in other parts of the human body, too. Artificial mother-of-pearl is to be used as an implant for medical prostheses. A research team headed by Prof. Berend Denkena from the Production Technology Centre (PZH) at Leibniz Universität Hannover is investigating the natural material gained from seashells and applying their findings to artificially produced varieties. First results imply that mother-of-pearl is in many ways superior to conventional materials. So, for example, knee and hip implants could be made even more break-resistant and hard-wearing. The lifecycle of artificial joints would thus be considerably prolonged.

The innermost layer of the shells of various molluscs such as mussels and some types of snails is made of mother-of-pearl. Here aragonite, a modification of calcium carbonate, is layered and piled up with organic components. This structure of alternating soft organic parts and hard calcium makes mother-of-pearl particularly break-resistant and tough. Humans have long recognised the value of the material and known how to make use of it: Because the light is refracted and partly reflected, mother-of-pearl shimmers in all the colours of the rainbow. Decorative items such as buttons, jewellery and carvings have been made out of it since olden times.

In the meantime scientists have also developed a stronger interest in this exciting material, as it is a further example of an energy-elastic composite developed by nature for areas under high mechanical stress. These composites are frequently far superior to their individual components. The break-resistance of a seashell is some 3,000 times higher than that of pure aragonite. In order to make technical use of these materials, the precise composition of the relevant material must be deciphered so that biomimetic materials can be developed using this model. Biomimetic is the name given to materials that imitate their natural models without being identical to them. The advantages of the synthetic varieties lie in the fact that they can usually be produced in larger quantities and their properties can be finely adjusted.

Mother-of-pearl could for example also revolutionise the development of building materials. Extremely light composite materials with so far undreamt-of stability are conceivable. In the case of the current research activity being carried out at Leibniz Universität Hannover, Hannover Medical School and TU Braunschweig, medical engineering in particular is set to benefit from the natural model of mother-of-pearl. Conventional materials are not yet optimal. “Ceramic implants, for example, are hardly subject to wear, but they are not sufficiently malleable. In other words: they break easily,” reports Analía Inés Moral, who works as a research assistant at the Institute of Production Engineering and Machine Tools (IFW) under Prof. Berend Denkena. “The disadvantage of materials made of metal or based on plastics, on the other hand, is that their lifecycle is limited.”

Imitating the qualities of mother-or-pearl in synthetic materials such as polymers and inorganic nanoparticles is the job of chemists in the team headed by Prof. Henning Menzel at the Institute of Chemical Engineering at TU Braunschweig and in Prof. Peter Behrens’ team at the Institute of Inorganic Chemistry at Leibniz Universität Hannover, in cooperation with Prof. Birgit Glasmacher’s group (Institute of Multiphase Processes at Leibniz Universität Hannover). In the laboratory for biomechanics at Hannover Medical School, Dr.-Ing. Christoph Hurschler is testing the properties of the material in a variety of situations, including using a knee joint simulator.

Synthetic mother-or-pearl will then have to prove its worth in further tests at IFW. It must be malleable, while showing the desired characteristics of the endoprostheses – in other words it must also have a high mechanical load capacity. For this, Analía Inés Moral from the Institute of Production Engineering and Machine Tools is first conducting experiments with genuine mother-of-pearl. The results are to lay the foundations of a more complex processing of artificial mother-of-pearl. If the material is successful, it could serve as an implant in endoprostheses, i.e. inside the body, for example in knee or hip joints. “To produce endoprostheses we need top-quality surfaces,” reports the 27-year-old Argentinean Moral. “Our findings indicate that the natural composite mother-of-pearl can help here, too. The synthetically produced implants that we hope to develop would thus be particularly break-resistant and hard-wearing.”

So far little research has been done into how the material can be artificially produced in larger quantities from tailor-made polymers and adapted inorganic nanoparticles. This project is therefore also part of the collaborative research centre SFB 599 “Innovative Bio-Resorbable and Permanent Implants made of Metal and Ceramic Materials”. “First indications with synthetics show that the structures of the mother-of-pearl can be produced in the laboratory at least as thin layers,” according to Moral. “We are now turning our attention to the production of larger basic structures. Even if we still have a long way to go before we can realise this ambitious goal, we and our project partners will have created the essential conditions for developing appropriate biomimetic materials.”

Meldung vom 04.08.2008