Alexander Dejaco
Hydroxyapatite, Finite Element Analysis, Polycrystal Mi-cromechanics, Computed Tomography.
Hundred micrometers-sized porous hydroxyapatite glob- ules have proved as successful tissue engineering strategy for bone defects in vivo, as was shown in studies on hu- man mandibles. These granules need to provide enough porous space for bone ingrowth, while maintaining suf- ficient mechanical competence (stiffness and strength) in this highly load-bearing organ. This double challenge mo- tivates us to scrutinize more deeply the micro and nanome- chanical characteristics of such globules, as to identify pos- sible optimization routes. Therefore, we imaged such a (pre-cracked) granule in a microCT scanner, transformed the attenuation coefficients into voxel-specific nanoporosi- ties, from which we determined, via polycrystal microme- chanics, voxel-specific (heterogeneous) elastic properties. The importance of the latter and of the presence of one to several hundred micrometers-sized cracks for realistically estimating the load-carrying behavior of the globule under a typical two-point compressive loading (as in a “splitting” test) is shown through results of large-scale Finite Element analyses, in comparison to analytical results for a sphere loaded at its poles: Use of homogeneous instead of het- erogeneous elastic properties would overestimate the struc- ture’s stiffness by 5% (when employing a micromechanics- based process as to attain homogeneous properties) - the cracks, in comparison, weaken the structure by one to two orders of magnitudes.
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