Medeling and Rapid Prototyping of Cellular Solids

Cellular solids form the basis of many biomaterials (e.g. bone and wood), since they can be optimized in numerous ways and are adaptable to a wide variety of environmental conditions. Using cellular solids, nature has developed lightweight structures which can withstand large loads, additionally these materials can adapt their structure dynamically in order to adapt to changing conditions. This project investigates these concepts using computational numerical methods. Goal is to achieve a better understanding of the mechanical properties of cellular solids. Besides numerical investigations, these properties will also be investigated experimentally. This will be done by using Rapid Prototyping to fabricate physical prototypes of the computer generated models. Recently developed techniques allow the fabrication of complex structures on a size scale that corresponds to the model of natural origin. Besides these scientific topics, this project will also try to answer more application-oriented questions, since RP is an manufacturing technique which frees the designer from most constraints regarding geometry. It therefore offers new ways to fabricate scaffolding materials which are needed in tissue engineering.

Both nature and technology have long since been aware of the benefits of cellularly structured materials, namely a high degree of strength at low weight. In a research project at the Vienna University of Technology these cellular materials are investigated by examining the crucial effect that cell geometry has upon these benefits. This is done by examining how the choice of cell geometry augments material strength while maintaining the same weight. The key factor for the project`s success was the fact that Rapid Prototyping (RP) was used in combination with finite element modelling. RP is a technique which enables the fabrication of physical prototypes starting from virtual models. By this way theoretical models regarding the mechniacl properties of material-structures can be avaluated experimentally. Starting from structures with different unit cells loaded in different directions it could be shown by modelling and simulation that strength and stiffness vary by a factor of three, even when the specific weight is kept constant. Irregular structures, as they are observed in nature, exhibit a smaller strength and stiffness, but they are significantly less prone against error loads, a situation that, for instance, can come about during a fall. In a next step cellular structures with biofunctional properties where manufactured. For this purpose, moulds were fabricated using RP. The mould wast then filled with a special ceramic slurry. After removing the mould and thermally treating the so called green part a ceramic structure with defined pore size and geometry is obtained. The utilized ceramic base material was hydroxyapatite which forms the inorganic basis of natural bone. It is therefore possible to fabricate scaffolds which are made of a material that serves as suitable substrate for bone- forming cells. In cooperation with the Ludwig Boltzmann Institute of Osteology it could be shown that osteoblasts - the cells which build up bone tissue - grow and proliferate on these scaffolds.