A Predictive Model for the Elastic Properties of a Collagen-Hydroxyapatite Porous Scaffold for Multi-Layer Osteochondral Substitutes

Damaged articular cartilage can be substituted by porous scaffolds exhibiting tailored mechanical properties and with a suited layer-based design. Reliable predictive models are able to provide a structure-property relationship in the design phase is still an open issue which is of prominent relevance. In this paper, a bottom-up homogenization approach is presented having the purpose to determine the elastic properties of each single layer of a osteochondral porous three-layers scaffold: a top cartilage chondral layer and two mineralized layers: an intermediate and a subchondral bone layer. For the cartilage top layer, dry and wet conditions are considered; while, for intermediate and bone layers only dry conditions are considered. The homogenization model is based on the porosity of each layer and on the elastic properties of the constituent materials, i.e., water, hydroxyapatite (HA) and collagen. The elastic moduli predicted for the mineralized layers are compared with available literature results. The model results obtained on the cartilage layers are validated through flat punch micro-indentation tests carried out on wet and dry samples. The results have shown that the elastic modulus of the mineralized layers is of the order of magnitude of few GPa; whereas, the elastic modulus of the cartilage layer which exhibits porosity higher than 90% is as low as 50 kPa and 300 kPa in wet and dry conditions, respectively. The above results show that the knowledge of the mechanical properties of the basic constituents which are universally known and the porosity of the layers are sufficient information to obtain a reliable prediction of the elastic properties of both mineralized layers and of cartilage layers.