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Titanium/polymer sheets designed for biomedical applications

Subject Area Biomaterials
Physical Chemistry of Solids and Surfaces, Material Characterisation
Primary Shaping and Reshaping Technology, Additive Manufacturing
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 406696147
 
Final Report Year 2023

Final Report Abstract

The key goal of this work was to develop CMF implants for PSI applications, with minimal stress shielding effects. SMs were taken as the ideal material structure providing several benefits for biomedical applications, such as tunable mechanical properties, along with great vibrational and thermal insulation properties. To make the implants biocompatible Ti was used as skin material along with PMMA as cores. To avoid toxic epoxies, normally used as adhesives, PMMA was grafted on Ti using the “grafting from” technique. The grafted PMMA was used as an adhesive to achieve a biocompatible SM. The bonding was attained by hot-pressing the monomaterials together in a fitting temperature regime. To make the SMs processing more energy friendly and deal with reduced handling temperatures, a novel PBMA-ran-PMMA copolymer was developed with a strongly reduced Tg (50 °C) compared to PMMA, thus the bonding is possible at a lower temperature. The increase in hot-pressing time showed an excellent improvement in adhesion. The “grafting from” technique seemed a viable approach for grafting large sized Ti sheets without problems. The failure condition (adhesion strength) was found to be much lower for Ti-copolymer SMs (~10 MPa) compared to Ti-PMMA SMs because of early failure of the copolymer. The influence of interfacial adhesion on the tensile properties was studied using tensile tests. The ideal properties for the SMs were calculated via the ROM and compared with the experimental results. The experimental results showed lower values compared to the calculated ones. The reason behind it was the weaker interfacial adhesion, which resulted in early failure. Concerning the thermal and vibrational properties, the SMs revealed their exceptional thermal insulation qualities: Their thermal diffusivity is substantially lower than that of Ti. Moreover, the thermal diffusivity of Ti-copolymer SMs was lower than that of Ti-PMMA SMs. This is due to the lower heat capacity of PBMA. The superior properties of SMs could also be stated in vibrational damping studies using DMA analysis. The SMs showed higher vibrational damping properties than that of Ti as expected and therewith a high damping effect. The shaping possibilities of these SMs for PSI applications were investigated via V-bending and deep drawing tests. As PMMA and copolymer are brittle at RT, early failure for both SMs occured. However, when the V-bending tests were performed at a temperature range where these polymers have sufficient ductility, the SMs were able to bend without failure. Nevertheless, in deep drawing tests, Ti-PMMA SMs were able to attain cup shapes with minimal failure, whereas Ti-copolymer SMs were riddled with earing defects. This was due to the weak copolymer core which also lacks ductility to be able to handle multiaxial stress conditions. For the estimation of formability, the flow limit curve was developed using empirical equations provided by Keller et al. because of lack of sufficient material. The formability data obtained by deep drawing and tensile tests were used to correlate with the estimated FLC. The theoretical FLC proved to correlate with the experimental results with a high degree of accuracy, suggesting a huge efficacy of the theoretical approach in approximating the FLC which can definitely save a huge amount of time. Finally, to investigate the suitability of Ti-PMMA SMs for PSI applications, skull-like regions were shaped using an incremental sheet forming (ISF) process. The results suggested a higher formability of SMs via this approach, as the formability seemed much higher than the expected values via FLC. This was given to the localized deformation in the case of ISF, which enhances the formability of the sheets. In the final stage of the SMs study, a FEM model was developed using Abaqus that could replicate the mechanical properties of Ti-PMMA SMs. The model for RT was prepared in the first phase. The simulations showed that the reason behind the lower tensile properties for the SMs was the weaker interfacial adhesion as suggested earlier. Afterwards, for simulating the Ti-PMMA SMs shaping at elevated (80 °C) temperatures a new model for PMMA was developed using a TLVP model. In the deep drawing simulation of SMs, the behaviour of PMMA was found to be similar to the experimental results suggesting the success of the model in replicating the behaviour of PMMA at elevated temperatures.

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