TY - BOOK

T1 - Mechanical integrity of cranial implants produced via material extrusion-based additive manufacturing

AU - Petersmann, Sandra

N1 - no embargo

PY - 2022

Y1 - 2022

N2 - Additive manufacturing processes are becoming increasingly popular for the development of components, especially in areas where customisability plays an important role, such as medicine. In particular, 3D-printing of bone replacement materials such as cranial implants is currently being intensively researched. Among the different process variants, additive manufacturing based on material extrusion is widespread for polymers. However, process-induced defects can strongly affect the material properties and thus the load capacity of printed components. This must be considered during designing, especially for critical applications. Otherwise, a false sense of security could be created. In the field of cranial implants, premature failure could even endanger human lives. Hence, the materials used in critical applications and their processing-dependent as well as application-related material behaviour should be systematically characterised before use. The aim of this work is therefore to characterise selected medical polymers across scales using a mechanical testing pyramid, as originally developed by NASA for composite materials, based on the example of a cranial implant for material extrusion. The pyramid was adapted for the additive manufacturing process and starts with the material selection/filament level and goes through test specimens and subcomponents to component testing. In parallel, changes in morphology and microstructure are considered. In the course of the first level of the test pyramid, both medical and processing criteria were used for the material selection. In addition, the general temperature dependence of the mechanical properties of the selected polymers (polyetheretherketone, polylactide, poly(methyl methacrylate), glycol-modified poly(ethylene terephthalate), poly(vinylidene fluoride) and polypropylene) was analysed on filaments. It was found that the mechanical properties of some selected materials already change significantly between standard temperature (23 °C) and application temperature (approx. 37 °C). This illustrates the importance of characteristic values determined under actual application conditions. At the specimen level, strain rate-dependent tensile tests (between quasi-static and impact) were carried out to generate situation-dependent material data for the simulation-aided design of components. This means that not only static or monotonic loads, as usually presented in the literature, but also abrupt loads can be taken into account. Furthermore, the influence of preceding cleaning and sterilisation processes was analysed. The results showed that after correct selection of the processes, no significant influences on the printed component are to be expected. In addition to the above-mentioned medical or application-relevant investigations, the process-dependent morphology and its influence on thermal and mechanical properties was also investigated. This is of particular importance for semi-crystalline polymers. It was shown that, depending on the printing speed, nozzle temperature, but also the selected printing path, on the one hand almost homogeneous and on the other hand strongly anisotropic structures, including clearly pronounced shear-induced crystallisation and shish-kebab structures, can be produced. This confirms that the printing path suggested by the slicer software should not be chosen arbitrarily, but can be used for targeted adaptation of the morphology. Based on these findings, subcomponent tests were conducted. The aim was to find the optimal filling structures for cranial implants under impact loading. Internal 3D-honeycomb structures with 70% infill and rectilinear structures with 100% infill yielded the best combination of permissible maximum deformation, maximum tolerable force and absorbed energy until fracture. The results could not be surpassed by stiffness-based topology

AB - Additive manufacturing processes are becoming increasingly popular for the development of components, especially in areas where customisability plays an important role, such as medicine. In particular, 3D-printing of bone replacement materials such as cranial implants is currently being intensively researched. Among the different process variants, additive manufacturing based on material extrusion is widespread for polymers. However, process-induced defects can strongly affect the material properties and thus the load capacity of printed components. This must be considered during designing, especially for critical applications. Otherwise, a false sense of security could be created. In the field of cranial implants, premature failure could even endanger human lives. Hence, the materials used in critical applications and their processing-dependent as well as application-related material behaviour should be systematically characterised before use. The aim of this work is therefore to characterise selected medical polymers across scales using a mechanical testing pyramid, as originally developed by NASA for composite materials, based on the example of a cranial implant for material extrusion. The pyramid was adapted for the additive manufacturing process and starts with the material selection/filament level and goes through test specimens and subcomponents to component testing. In parallel, changes in morphology and microstructure are considered. In the course of the first level of the test pyramid, both medical and processing criteria were used for the material selection. In addition, the general temperature dependence of the mechanical properties of the selected polymers (polyetheretherketone, polylactide, poly(methyl methacrylate), glycol-modified poly(ethylene terephthalate), poly(vinylidene fluoride) and polypropylene) was analysed on filaments. It was found that the mechanical properties of some selected materials already change significantly between standard temperature (23 °C) and application temperature (approx. 37 °C). This illustrates the importance of characteristic values determined under actual application conditions. At the specimen level, strain rate-dependent tensile tests (between quasi-static and impact) were carried out to generate situation-dependent material data for the simulation-aided design of components. This means that not only static or monotonic loads, as usually presented in the literature, but also abrupt loads can be taken into account. Furthermore, the influence of preceding cleaning and sterilisation processes was analysed. The results showed that after correct selection of the processes, no significant influences on the printed component are to be expected. In addition to the above-mentioned medical or application-relevant investigations, the process-dependent morphology and its influence on thermal and mechanical properties was also investigated. This is of particular importance for semi-crystalline polymers. It was shown that, depending on the printing speed, nozzle temperature, but also the selected printing path, on the one hand almost homogeneous and on the other hand strongly anisotropic structures, including clearly pronounced shear-induced crystallisation and shish-kebab structures, can be produced. This confirms that the printing path suggested by the slicer software should not be chosen arbitrarily, but can be used for targeted adaptation of the morphology. Based on these findings, subcomponent tests were conducted. The aim was to find the optimal filling structures for cranial implants under impact loading. Internal 3D-honeycomb structures with 70% infill and rectilinear structures with 100% infill yielded the best combination of permissible maximum deformation, maximum tolerable force and absorbed energy until fracture. The results could not be surpassed by stiffness-based topology

KW - Additive manufacturing

KW - Material extrusion

KW - Mechanical integrity

KW - cranial implant

KW - morphology

KW - micro-structure

KW - Additive Fertigung

KW - Materialextrusion

KW - Mechanische Prüfpyramide

KW - Mechanische Integrität

KW - Schädelimplantate

KW - Prozess-Struktur-Eigenschafts-Beziehung

KW - Morphologie

KW - Mikrostruktur

M3 - Doctoral Thesis

ER -