Alexander Woesz

The effect of geometry on three-dimensional tissue growth.

Tissue formation is determined by uncountable biochemical signals between cells; in addition, physical parameters have been shown to exhibit significant effects on the level of the single cell. Beyond the cell, however, there is still no quantitative understanding of how geometry affects tissue growth, which is of much significance for bone healing and tissue engineering. In this paper, it is shown that the local growth rate of tissue formed by osteoblasts is strongly influenced by the geometrical features of channels in an artificial three-dimensional matrix. Curvature-driven effects and mechanical forces within the tissue may explain the growth patterns as demonstrated by numerical simulation and confocal laser scanning microscopy. This implies that cells within the tissue surface are able to sense and react to radii of curvature much larger than the size of the cells themselves. This has important implications towards the understanding of bone remodelling and defect healing as well as towards scaffold design in bone tissue engineering.

Fabrication and moulding of cellular materials by rapid prototyping

Many biological materials (e.g. wood, cork, bone, etc) are based on cellular designs, since cellular architectures offer the possibility to optimise the properties (stiffness, density, strength, etc) of a structure according to the environmental conditions the structure is exposed to. By using rapid prototyping, it is possible to fabricate cellular materials on a similar size scale as in natural material-structures. By using appropriate moulding techniques, these structures can be fabricated out of a wide variety of materials (polymers, ceramics, composites). In this work, several RP techniques are investigated regarding their suitability for the fabrication of cellular solids. The main focus is on using direct light projection (stereolithography) in combination with gelcasting as moulding technique. Besides using commercial light-sensitive resins, a class of newly developed water-soluble resins has been evaluated regarding its usability as sacrificial mould material.

A finite element study on the effects of disorder in cellular structures

The susceptibility to deformation localization of simple cubic arrangements of struts, which are a simple approximation of the micro-architecture in cancellous bone, is analyzed. The coherence between structural disorder and the tendency towards deformation localization is investigated and its relevance from a biological point of view is discussed. A systematic study on the spatial deformation distribution of regular and disordered open cell structures is carried out. To this end, finite element models are employed which account for elastic-plastic bulk material and large strain theory, and a methodology for the estimation of the degree of deformation localization is introduced.

Physical and materials aspects of photonic crystals for microwaves and millimetre waves

Abstract Experimental and numerical results on photonic crystals are presented for the frequency range 26–60 GHz. The material used is alumina where two techniques have been applied for fabricating the photonic crystals: manual assembly of alumina rods and rapid prototyping. The observed positions of the fundamental and higher photonic band gaps are in excellent agreement with the calculated results. A new type of defect in the 3D woodpile structure, is created by inserting interstitial rods. As a new 2D structure a square parquet lattice is investigated. The concept of a negative index of refraction is adressed including a model calculation and an experimental demonstration by the transmission through a slab of a 2D photonic crystal.

Regular, low density cellular structures - rapid prototyping, numerical simulation, mechanical testing

Cellular solids form the basis of many biological and engineering structures. Most models use the relative density and the mechanical properties of the bulk material as the main parameter for the prediction of the mechanical properties of such structures. In this work the inuence of the architecture of periodic cellular solids on the mechanical properties is investigated numerically and experimentally. Using computer aided design, structures with 8x8x8 base cells are designed and fabricated. The physical prototypes which are tested experimentally are made from thermosetting and thermoplastic polymers by employing Rapid Prototyping (RP) techniques. Various RP techniques are compared regarding their suitability for the fabrication of cellular materials. For numerical simulation of the cellular structures, linear Finite Element analysis is employed. Three-dimensional models are set up using higher order beam elements. In a rst step, the structure is treated as an innite medium and homogenization via a ’periodic micro-eld approach’ is used. The entire elastic tensors for different relative densities are evaluated, from which the directional dependencies of the Young’s moduli are derived. In a second step, simulations of nite structures are performed for direct comparison with experiments. Samples consisting of several basic cells are modeled which leads to a better correspondence to the experimental setup. Finite structures of different numbers of cells are modeled to study the inuence of the sample size. The experimental and numerical results correspond very well and form a consistent picture of the problem. The multi-disciplinary approach leads to a comprehensive view of effects which govern the mechanical behaviour of the investigated cellular structures.

Influence of defects and perturbations on the performance of 3D open cell structures

Regular and irregular highly porous open cell structures with a relative density of 12.5% are investigated by the Finite Element Method. The three-dimensional models are based on beam elements and account for the material distribution and the constrained deformation in the vertices [1].

The influence of the thermal treatment of hydroxylapatite scaffolds on the physical properties and the bone cell in growth behaviour

The material bone consists of a biopolymer matrix (collagen) reinforced with mineral nanoparticles (carbonated hydroxylapatite), forming a natural composite which builds up a dense shell on the exterior and a network of struts with a mean diameter of 200µm in the core of many bones. The architecture of the foamy inner part of bones (spongiosa) is determined by loading conditions. The architecture strongly influences the mechanical properties of cellular solids together with the apparent density and the material it consists of. In addition, the ingrowth of bone cells into porous implants depends on pore size, size distribution and interconnectivity. From this it is clear that the possibility to design the architecture of a bone replacement material is beneficial from a biological as well as a mechanical point of view. Our approach uses rapid prototyping methods, ceramic gelcasting and sintering to produce cellular structures with designed architecture from hydroxylapatite and other bioceramics. The influence of sintering temperature and atmosphere on the physical properties of these scaffolds was investigated with x-ray diffraction and scanning electron microscopy. Furthermore, the cell ingrowth behaviour was determined in cell culture experiments, using the praeosteoblastic cell line MC3T3-E1, derived from mouse calvariae. The cell ingrowth behaviour was evaluated during a culture period of two and three weeks, by light microscopy and afterwards by histology after embedding and Giemsa-staining. The phase composition of the material was found to change with increasing sintering temperature and its surface characteristics was influenced by the sintering atmosphere. These changes also affected the cell ingrowth behaviour. In some experiments, the osteoblasts-like cells were found to cover the whole external and internal surface of the scaffold. The cells produced extracellular matrix consisting of collagen, which eventually filled nearly all the pores. In particular, the cells had the tendency to fill any crack or opening in the scaffolds, and to generally smooth the surfaces. In conclusion, rapid prototyping and ceramic gelcasting allows the freeform fabrication of porous bioceramics with controlled architecture. Such structures made of hydroxylapatit were found to support the growth of mouse osteoblasts.