Design, Manufacturing and Validation of Cellular Zn Alloys as Biodegradable Implant Materials

Some implants only have a temporary function and are therefore preferably made from a biodegradable material which is removed from the body by dissolution after several months or years. Zn and its alloys are suitable for this application, both from a biological and degradation rate point of view. However, little is known about Zn and Zn alloys in cellular (porous) form. In addition, one of the manufacturing processes which is most promising for the production of (orthopedic) implants with porous volumes, laser powder bed fusion or Direct Metal Printing (DMP), has not been used for Zn and its alloys yet. Therefore, the aim of this thesis was to develop the DMP process for the production of cellular Zn alloys, compare their mechanical properties with those of cellular Zn alloys produced by two spaceholder-based production processes, and study the influence of the meso- and microstructure on the mechanical properties. NaCl was used as a spaceholder for the production of cellular Zn alloys with Field Assisted Sintering Technology (FAST) and Liquid Metal Infiltration (LMI). This leads to a limited mesostructural control, combined with a sintered microstructure for FAST and a cast microstructure for LMI. The mechanical properties of these materials were tested in compression for a porosity ranging from 75 to 88%. After optimization of the process parameters, the DMP process was used to produce non-cellular and cellular Zn, the latter with a closely controlled mesostructure and DMP microstructure. Non-cellular DMP Zn was studied by X-Ray Diffraction to measure its crystallographic texture and its mechanical properties were measured by tensile testing and impulse excitation. Cellular DMP Zn with five different unit cells - diamond, dodecahedron, octet truss, face centered cubic and Kagome - and a porosity ranging from 49 to 80% was tested in compression. This research showed that FAST is the least suitable production process for cellular Zn, as the oxide skin, originally at the powder surface, causes a brittle fracture of the sintered struts. Both LMI and DMP resulted in ductile deformation and the influence of the mesostructure on the plateau stress was shown to be more important than the influence of the microstructure. The plateau stress is 6±0.6 MPa for LMI cellular Zn with a porosity of 76% and 8±0.2 MPa for DMP cellular Zn with 74% porosity and a bending-dominated mesostructure. In contrast, the plateau stress for DMP cellular Zn with a more stretching-dominated mesostructure, the Kagome unit cell, is 14±0.4 MPa for 77% porosity. The use of alloying elements can also increase the strength, as the plateau stress for LMI cellular Zn1.5Mg is 10±0.5 MPa for 77% porosity. The mechanical properties of non-cellular Zn with a density of 98.5% produced by DMP are anisotropic due to the preferential orientation of the crystallographic direction in the building plane. The yield strength and stiffness in the building direction are 78±0.4 MPa and 110±0.2 GPa, respectively, while in the building plane they are only 55±0.7 MPa and 81±0.4 GPa. Elongation at break is around 10%, independent of the orientation. Overall, these results show that the plateau stress of cellular Zn and Zn1.5Mg is at the lower end of the strength range for bone (5-200 MPa). The strength of DMP cellular Zn is higher than of LMI cellular Zn and Zn1.5Mg: up to 48±3 MPa for 66% porosity (Kagome unit cell). A further increase in strength for a given density is not trivial for DMP, as manufacturing inaccuracies prevent the experimental strength from reaching its theoretical value. In addition, the DMP process requires a full powder characterization and process parameter optimization in case alloying elements would be added to further increase the strength.status: publishe

Direct metal printing of zinc: From single laser tracks to high density parts

The production of zinc by Direct Metal Printing (DMP) is explored with an aim to understand the influence of different DMP process parameters on the melting, evaporation and solidification of zinc powders. The study of single track (1D) scans with and without powder enabled a first estimation of a stable process window. In the 2D experiments, the hatch spacing was chosen to balance track overlap and smoke formation. The quality of the layers after DMP depends greatly on the flowability of the powder. DMP of simple cubes (3D) of relative density >99.70% was achieved, showing that the DMP of zinc into personalized biodegradable implants and other components is feasible.status: publishe

The gender gap in student engagement:The role of teachers’ autonomy support, structure, and involvement

BACKGROUND: The gender gap in education in favour of girls is a widely known phenomenon. Boys generally have higher dropout rates, obtain lower grades, and show lower engagement. Insight into factors related to these academic outcomes could help to address the gender gap. AIMS: This study investigated, for Dutch language classes, (1) how boys and girls differ in behavioural engagement, (2) which teacher support dimensions (autonomy support, structure, involvement) may explain gender differences in engagement (mediation hypothesis), and (3) whether and which of these teacher support dimensions matter more for boys' as opposed to girls' engagement (moderation or differential effects hypothesis). SAMPLE: A total of 385 Grade 7 students and their 15 language teachers participated in this study. METHODS: Teacher support was assessed through student reports. Student engagement was measured using student, teacher, and observer reports. By means of structural equation modelling, the mediating role of the teacher support dimensions for gender differences in behavioural engagement was tested. The potential differential role of the teacher support dimensions for boys' and girls' engagement was investigated through multigroup analysis. RESULTS: Boys were less engaged than girls and reported lower support from their teacher. Autonomy support and involvement partially mediated the relationship between gender and behavioural engagement. Autonomy support was demonstrated to be a protective factor for boys' engagement but not for girls'. Structure and involvement contributed equally to engagement for both sexes. CONCLUSIONS: Although involvement and autonomy support partly explained the gender gap in engagement (mediation hypothesis), more support was found for differential effects of autonomy support on boys' versus girls' engagement (differential effects hypothesis)

Rationally designed meta-implants: a combination of auxetic and conventional meta-biomaterials

Rationally designed meta-biomaterials present unprecedented combinations of mechanical, mass transport, and biological properties favorable for tissue regeneration. Here we introduce hybrid meta-biomaterials with rationally-distributed values of negative (auxetic) and positive Poisson’s ratios, and use them to design meta-implants that unlike conventional implants do not retract from the bone under biomechanical loading. We rationally design and additively manufacture six different types of meta-biomaterials (three auxetic and three conventional), which then serve as the parent materials to six hybrid meta-biomaterials (with or without transitional regions). Both single and hybrid meta-biomaterials are mechanically tested to reveal their full-field strain distribution by digital image correlation. The best-performing hybrid metabiomaterials are then selected for the design of meta-implants (hip stems), which are tested under simulated-implantation conditions.Full-field strain measurements clearly show that, under biomechanical loading, hybrid meta-implants press onto the bone on both the medial and lateral sides, thereby improving implant–bone contact and potentially implant longevity.Biomaterials & Tissue Biomechanic

Additively manufactured metallic porous biomaterials based on minimal surfaces : A unique combination of topological, mechanical, and mass transport properties

Porous biomaterials that simultaneously mimic the topological, mechanical, and mass transport properties of bone are in great demand but are rarely found in the literature. In this study, we rationally designed and additively manufactured (AM) porous metallic biomaterials based on four different types of triply periodic minimal surfaces (TPMS) that mimic the properties of bone to an unprecedented level of multi-physics detail. Sixteen different types of porous biomaterials were rationally designed and fabricated using selective laser melting (SLM) from a titanium alloy (Ti-6Al-4V). The topology, quasi-static mechanical properties, fatigue resistance, and permeability of the developed biomaterials were then characterized. In terms of topology, the biomaterials resembled the morphological properties of trabecular bone including mean surface curvatures close to zero. The biomaterials showed a favorable but rare combination of relatively low elastic properties in the range of those observed for trabecular bone and high yield strengths exceeding those reported for cortical bone. This combination allows for simultaneously avoiding stress shielding, while providing ample mechanical support for bone tissue regeneration and osseointegration. Furthermore, as opposed to other AM porous biomaterials developed to date for which the fatigue endurance limit has been found to be ≈20% of their yield (or plateau) stress, some of the biomaterials developed in the current study show extremely high fatigue resistance with endurance limits up to 60% of their yield stress. It was also found that the permeability values measured for the developed biomaterials were in the range of values reported for trabecular bone. In summary, the developed porous metallic biomaterials based on TPMS mimic the topological, mechanical, and physical properties of trabecular bone to a great degree. These properties make them potential candidates to be applied as parts of orthopedic implants and/or as bone-substituting biomaterials. STATEMENT OF SIGNIFICANCE: Bone-substituting biomaterials aim to mimic bone properties. Although mimicking some of bone properties is feasible, biomaterials that could simultaneously mimic all or most of the relevant bone properties are rare. We used rational design and additive manufacturing to develop porous metallic biomaterials that exhibit an interesting combination of topological, mechanical, and mass transport properties. The topology of the developed biomaterials resembles that of trabecular bone including a mean curvature close to zero. Moreover, the developed biomaterials show an unusual combination of low elastic modulus to avoid stress shielding and high strength to provide mechanical support. The fatigue resistance of the developed biomaterials is also exceptionally high, while their permeability is in the range of values reported for bone