Mohammad Sanami

The main aim of this project was to assess auxetic (negative Poisson's ratio) materials for potential in biomedical devices. Specifically, a detailed comparative indentation study has been undertaken on auxetic and conventional foams for hip protector devices; radially-gradient one-piece foams having auxetic character have been produced for the first time and shown to have potential in artificial intervertebral disc (IVD) implant devices; and auxetic honeycomb geometries have been assessed for the stem component in hip implant devices.
For the hip protector application, combined compression and heat treatment of conventional polyurethane open-cell foam was used to produce monolithic auxetic foams. The foams were characterised structurally using optical microscopy, and mechanically using mechanical testing combined with videoextensometry. Static indentation using six different indenter shapes on each of the six faces of the foam specimens has been undertaken. The key conclusion here is that the enhanced indentation resistance for the converted foam is not a consequence of increased density accompanied by the usual significant increase in foam stiffness. The enhanced indentation resistance is consistent with the auxetic effect associated with the increased density, providing a localised densification mechanism under indentation (i.e. material flows under the indenter). At higher indentation displacement the Poisson’s ratios for both the unconverted and converted foams tend towards zero. In this case, the increase in foam stiffness for the converted foams at higher strain may also contribute to the indentation enhancement at high indentation displacement.
New radially-gradient foams mimicking the core-sheath structure of the natural IVD have been produced through the development of a novel thermo-mechanical manufacturing route. Foam microstructural characterisation has been undertaken using optical and scanning electron microscopy, and also micro-CT scans performed by collaborators at the University of Manchester. Detailed x-y strain mapping using combined mechanical testing and videoextensometry enabled the local and global Young's modulus and Poisson's ratio responses of these new materials to be determined. In one example, global auxetic response is achieved in a foam having a positive Poisson's ratio core and auxetic sheath. It is suggested this may be a more realistic representation of the properties of natural IVD tissue.
Analytical and Finite Element (FE) models have been developed to design honeycomb geometries for the stems in new total hip replacement implants. FE models of the devices implanted within bone have been developed and the auxetic stems shown to lead to reduced stress shielding effect.