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Hydroxyapatite Ceramics: Hierarchical Porosity and Mechanical Characterisation

3 febbraio 2020
9:30 am
San Francesco Complex - Classroom 2

In the field of bone replacement, there is the need of developing materials with a high load-bearing capacity, but also showing good osteoinduction, osteoconduction, and osseointegration, for which high biocompatibility is required. To this purpose, biomorphic ceramics based on hydroxyapatite crystals (which is the main constituent of the mineral part of bones) were obtained by many researchers through bio-templating procedures that may or may not involve sintering [1-4]. Bio-templating processes exploit the inner structure of materials on which the mineral settles down trough inflation stages in the manufacturing. As a drawback of this process, the resulting material has a small level of cohesion due to the low interaction forces between the hydroxyapatite crystals. In addition to that, some researchers relied on sintering in their procedures. Sintering is a process which requires high temperature and alters the microstructure leaving almost no control over the porosity of the final product, a feature that can highly affect material biocompatibility. Recently, Tampieri et al. [5-6] put forward a novel chemical process which, rather than exploiting the base material as a template for gas inflation, aimed at transforming it (rattan wood, in particular) into a highly porous hydroxyapatite ceramic while avoiding sintering. The material obtained by such procedure, called BioApatite (henceforth BA), boasts great cohesion (crystal-to-crystal) and thus outstanding mechanical properties. Furthermore, the free-from-sintering transformation leads to a material in which the spatially organized porosity lends optimal osteoinduction and osteoconduction. 

In order to fully characterize the mechanical behavior of BA, we have performed several tests on samples of different shapes and dimensions. From these experimental tests, the BA material has shown many peculiar behavior/ several interesting features: (i) strength in tension may exceed that in compression, (ii) failure in compression involves complex exfoliation patterns which results in high toughness, (iii) differently from sintered porous hydroxyapatite, fracture does not occur ‘instantaneously', but its propagation may be observed as it exhibits tortuous patterns following the original fibrillar structure of wood, (iv) the anisotropy in elastic stiffness and strength leads to unprecedented values for cases of stress parallel to or orthogonal to the channels [7-8]. We have produced Ashby charts in order to compare BA, a candidate for bone replacement, and different types of human bones. From these charts, one can easily assess the unusual position for the ceramic BA and the good match in terms of mechanical properties between this material and bones [7-8]. 


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7. Sprio, S., et al., Hierarchical porosity inherited by natural sources affects the mechanical and biological behaviour of bone scaffolds, J. Eu. Ceram. Soc., In press.

8. Bigoni, D., Cavuoto, R., Misseroni, D., Paggi, M., Ruffini, A., Sprio, S., Tampieri, A., Ceramics with the signature of wood: a mechanical insight, Materials Today Bio, 2020, Vol. 5, 100032. 

Riccardo Cavuoto, University of Trento