Micromechanics of Biomaterials

Research focus

The Peer review has evaluated this group as Excellent

The research work in this topic started four years ago and its main focus is the numerical modelling of ceramic materials and ceramic-based composites for biomedical applications. Within the general class of ceramic-based biomaterials, the Functionally Graded Materials (FGMs) are of great interest. FGM are spatially varied microstructures or chemical compositions created by non-uniform distributions of the phases or components of the material in a continuous or a step-wise manner. FGMs can be manufactured through the production of ceramic/ceramic and ceramic/metal composites which are particularly relevant in the development of innovative and performing biomedical devices. The specific fields of wear applications in orthopaedic devices and the applications related to osteointegration with specific reference to dental implants have been targeted. For wear applications the Alumina and Zirconia ceramics and their Functionally Graded composites have been studied. This specific type of FGM achieves enhanced mechanical performances of structures by resorting to two main factors: i) the spatial variation of the elastic properties and ii) a suitably designed residual stress field owed to the mismatch between the coefficient of thermal expansion of the constituent materials. Indeed, it has been shown that a suitably introduced spatial gradient in the Young modulus, can lead to components exhibiting better mechanical performances; moreover, surfaces with an increased wear resistance can be obtained if the tensile stress state in the superficial layers is lowered. In fact, a residual compressive stress on the surface can enhance the wear resistance of the surface, particularly for ceramic surfaces. Both theoretical and numerical approaches have been used for these studies. In particular, theoretical studies are aimed at determining the overall mechanical and physical properties of ceramicbased composite materials starting from the properties of each single constituent and their interactions. The numerical approaches are aimed at determining the overall mechanical performance of graded materials and devices. Moreover the research is developed at different length scales, namely micro and macro scale, in order to study the peculiar characteristics of FGMs and their manufacturing processes at the scale of the microstructure of the materials and at the macro scale of the device. For wear applications, the mechanical performances of thin hard layers like Titanium Carbides and Alumina thin coatings deposited on metal (typically titanium alloys) substrates have been studied from both modelling and manufacturing points of view. This latter topic has been developed in collaboration with partner institutions having manufacturing capability (other departments of the Politecnico di Milano). Applications in the field of osteo-integration for dental implants has been approached through numerical studies of titanium alloy/hydroxiapatite functionally graded composites. In a dental implant the material demands differ from position to position: at the top the implant is heavily loaded and a strong material is needed to cope with the stresses while at the bottom of the implant more hydroxyapatite would be useful to encourage bone integration. A composite with a gradual transition from mainly hydroxyapatite to pure titanium would be a very interesting material. In the neverending quest for better biomaterials, the titanium-hydroxyapatite composite is now subject of several study groups around the world. The expected properties of this composite material are better osseointegration properties than titanium and good mechanical stability. A PhD thesis is currently ongoing on this specific topic. 19 Another class of biomaterials studied in this research topic is that of shape memory alloys. In particular, super-elastic as well as the shape recovery capabilities of Nickel Titanium alloys (NiTinol) have been studied. A numerical and experimental approach has been undertaken. Mechanical characterisation of NiTinol wires through mechanical laboratory tests and finite element simulations of mechanical behaviour of the alloy have been carried out at the LaBS.

Dipartimento di afferenza

Dipartimento di Ingegneria Strutturale (DIS)

Docenti afferenti

Full Professors
Roberto Contro
Assistant Professors
Pasquale Vena
Lorenza Petrini