A new nanoindentation protocol for identifying the elasticity of undamaged extracellular bone tissue

For the determination of the Young's modulus of bone tissues several methods are widely in use, among them quasi-static mechanical tests, ultrasound tests, and nanoindentation tests. However, the key question of how an elastic modulus can be reliably retrieved from such tests, is surprisingly unsettled. In this Master's thesis, a new method for determination of the elastic modulus of extracellular bone matrix from very many nanoindentation results is developed, based on an earlier contribution in the field of brittle ceramics used in bone tissue engineering (Kariem et al., 2015). 576 nanoindentation tests were performed on carefully polished bovine femoral bone samples, and the results were statistically analyzed,by fitting a number of Gaussian distribution functions to the histogram made up by all indent-specific elastic moduli, each of them being retrieved from Oliver and Pharr's solution for the elastic half space. The fitting procedure was based on an evolutionary algorithm (Weicker, 2007; Jaindl et al., 2009), and revealed the existence of several material phases with distinct expected values for their corresponding elastic moduli, according to the premises of the statistical or grid nanoindentation technique (Ulm et al, 2007). The stiffest of these moduli refers to the undamaged elastic modulus of extracellular bone tissue, while all other moduli reflect influences of microcracks in the vicinity of the indent, or directly branching off from the microcracks; this was explicitly confirmed by a preliminary nanoindentation test series performed under a Scanning electron microscope (SEM). The value obtained with our new method for the undamaged extracellular femoral bovine bone matrix, amounting to 31.4±2.5 GPa, appears remarkably well to the results obtained from unloading quasi-static compression tests on single-micro-sized micropillars which were SEM-FIB-milled from the same type of bone (Luczynski et al., 2015); and to predictions of a carefully validated micromechanical model for bone (Morin and Hellmich, 2014). This is regarded as major step toward reliable determination of the elastic properties of bone5

A new Nanoindentation Protocol for identifying the elasticity of undamaged extracellular bone tissue

\u3cp\u3eWhile the quest for understanding and even mimicking biological tissue has propelled, over the last decades, more and more experimental activities at the micro and nanoscales, the appropriate evaluation and interpretation of respective test results has remained a formidable challenge. As a contribution to tackling this challenge, we here describe a new method for identifying, from nanoindentation, the elasticity of the undamaged extracellular bone matrix. The underlying premise is that the tested bovine bone sample is either initially damaged (i.e. exhibits micro-cracks a priori) or that such micro-cracks are actually induced by the nanoindentation process itself, or both. Then, (very many) indentations may relate to either an intact material phase (which is located sufficiently far away from micro-cracks), or to differently strongly damaged material phases. Corresponding elastic phase properties are identified from the statistical evaluation of the measured indentation moduli, through representation of their histogram as a weighted sum of Gaussian distribution functions. The resulting undamaged elastic modulus of bovine femoral extracellular bone matrix amounts to 31 GPa, a value agreeing strikingly well both with direct quasi-static modulus tests performed on SEM-FIB-produced micro-pillars (Luczynski et al., 2015), and with the predictions of a widely validated micromechanics model (Morin and Hellmich, 2014). Further confidence is gained through observing typical indentation imprints under Scanning Electron Microscopy (SEM), which actually confirms the existence of the two types of micro-cracks as described above.\u3c/p\u3