Experimental and numerical characterization of native bone tissue and glass ceramic bone scaffold at small scale

It’s every human being’s right to live with dignity and the absence of suffering during his/her lifetime, but our lack of knowledge on systemic, organ, tissue and cell degeneration due to senescence indicates that the ageing process is often beyond our control. Therefor, plenty of studies are carried out in different laboratories to expand the related knowledge and amend the treating methods. Most models of aging at cellular level are derived from simple in-vitro experiments in addition to animal models ranging from fish to primates. Changing in physiological microenvironment and alternations in signaling between cells and even distant tissues and organs leads to systemic age related diseases such as osteoporosis. This project is an initial step towards a long-term goal of generating reliable biomimetic models of a tissue. The tissue should be applicable for studies of pathological conditions and development of pharmacological strategies reducing animal and clinical testing as well as time and cost associated with them. As a contribution to this wide framework, this study aims at studying the mechanical properties of the bone tissue and of glass-ceramic scaffold at small length scale by making use of the nanoindentation technique. In particular, the study of the bone tissue is focused on the dependence of the mechanical VII properties on the applied load or characteristic size of the experiment. The study was aimed at identifying and quantifying a damaging mechanism occurring in the bone tissue upon loading. The experimental characterization was carried out on both the cortical bone tissue as well as on the trabeculae of spongy bovine bone samples. The damaging mechanisms were further investigated by means of numerical simulation of the nanoindentation experiments at multiple characteristic lengths on the cortical bone samples. The chosen material for bone scaffolds is a glass-ceramic material derived from a highly bioactive glass (called CEL2) (Vitale-Brovarone et al. 2008). It shows very promising biochemical properties and it can be produced with a controlled multi-scale porosity, which is significantly important to improve the scaffold characteristic. Hence, the goal of this study is to perform a mechanical characterization of the glassceramic scaffold at different scales, in particular micro and macro scales. For such a study, a bulk form of the material, non-pores, is examined as well as a 3D Porous material, in order to run a survey on porosity dependency of the mechanical characteristic of the material. In addition, beside the glass ceramic material, the study went through mechanical characterization of trabecular/cortical bone of bovines at the same scale. VIII The mechanical properties at the micro scale are investigated by means of the nanoindentation experimental technique and the mechanical properties at the macro scale are obtained by means of computational tools only. The data found through the experiments is used to feed the computational models for the macroscopic characterization. The 3D structure of the scaffold has been scanned with a micro-CT scanner for further investigation. Based on the scanned images, a binary volume is built up, in which the value 1 is indicating the material and the value 0 is indicating porosity. Subsequently, a finite element model based on the binary volume is developed in order to model the structural geometry of the sample and to estimate the mechanical properties of the scaffold at macro scale. The results of the computational model are checked by result of an analytical model. The model was adopted based on an analytical approach proposed by Zhu et al. (1997). This model is using a simple unit cell to set-up the geometry of the scaffold walls in order to predict the Young’s modulus of the structure as a function of the porosity. A good agreement between the results is found confirming that the prediction from the computational model is an effective approach for above-mentioned purpose. Moreover the computational model results show the anisotropy of the scaffold. This feature is investigated utilizing IX two different approaches. First the structure anisotropy is evaluated by the calculation of the Mean Intercept Length (Whitehouse 1974), that is a standard method to check if the structure is mainly aligned in a specific direction. A degree of anisotropy of 25-30% is found in this way. On the other hand, the effective volume (Quinn 2003) is calculated in order to quantify the unloaded volume, due to inhomogeneity of the stress distribution in the structure during the simulations. According to the results, the structure anisotropy can be explained properly by both anisotropy of the scaffold architecture and inhomogeneity of stress distribution. The comparison between the mechanical response of the glass ceramic material and that of the bone tissue is that the glass ceramic bulk material does not exhibit the typical damaging response with decreasing indentation modulus with increasing indentation load; whilst, a decreasing trend was observed by indenting the walls of the 3D sintered ceramic scaffolds. The decreasing trend found on the scaffold walls was owed to the intrinsic porosity of the sintered ceramic. Consistent values with other investigation techniques carried out by the research group in the Polytechnic university of Turin validated the results found in this study. As a general conclusion, the 3D glass-ceramic scaffolds which are designed and manufactured with the final aim to build “tissue models” to be used in-vitro testing of drugs X simulating health, diseased or aged bone tissues have a good potential to achieve their purpose. The manufacturing process to obtain the 3D scaffolds, which should exhibit mechanical and physical properties consistent with those of the trabecular bone in different aging or clinical conditions, can be tuned so to provide the scaffolds with the desired elasticity and strength properties. The experimental and computational framework reported in this study provides an effective tool to the prediction of the mechanical properties of the scaffold as a function of their physical properties like macro and micro-porosity and 3D architecture