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Leuven, Belgium

Ahmadi S.M.,Technical University of Delft | Campoli G.,Technical University of Delft | Amin Yavari S.,Technical University of Delft | Sajadi B.,Technical University of Delft | And 6 more authors.
Journal of the Mechanical Behavior of Biomedical Materials | Year: 2014

Cellular structures with highly controlled micro-architectures are promising materials for orthopedic applications that require bone-substituting biomaterials or implants. The availability of additive manufacturing techniques has enabled manufacturing of biomaterials made of one or multiple types of unit cells. The diamond lattice unit cell is one of the relatively new types of unit cells that are used in manufacturing of regular porous biomaterials. As opposed to many other types of unit cells, there is currently no analytical solution that could be used for prediction of the mechanical properties of cellular structures made of the diamond lattice unit cells. In this paper, we present new analytical solutions and closed-form relationships for predicting the elastic modulus, Poisson[U+05F3]s ratio, critical buckling load, and yield (plateau) stress of cellular structures made of the diamond lattice unit cell. The mechanical properties predicted using the analytical solutions are compared with those obtained using finite element models. A number of solid and porous titanium (Ti6Al4V) specimens were manufactured using selective laser melting. A series of experiments were then performed to determine the mechanical properties of the matrix material and cellular structures. The experimentally measured mechanical properties were compared with those obtained using analytical solutions and finite element (FE) models. It has been shown that, for small apparent density values, the mechanical properties obtained using analytical and numerical solutions are in agreement with each other and with experimental observations. The properties estimated using an analytical solution based on the Euler-Bernoulli theory markedly deviated from experimental results for large apparent density values. The mechanical properties estimated using FE models and another analytical solution based on the Timoshenko beam theory better matched the experimental observations. © 2014 Elsevier Ltd. Source


Amin Yavari S.,Technical University of Delft | Amin Yavari S.,FT Innovations BV | Wauthle R.,Catholic University of Leuven | Wauthle R.,LayerWise NV | And 4 more authors.
Applied Surface Science | Year: 2014

Porous titanium biomaterials manufactured using additive manufacturing techniques such as selective laser melting are considered promising materials for orthopedic applications where the biomaterial needs to mimic the properties of bone. Despite their appropriate mechanical properties and the ample pore space they provide for bone ingrowth and osseointegration, porous titanium structures have an intrinsically bioinert surface and need to be subjected to surface bio-functionalizing procedures to enhance their in vivo performance. In this study, we used a specific anodizing process to build a hierarchical oxide layer on the surface of porous titanium structures made by selective laser melting of Ti6Al4V ELI powder. The hierarchical structure included both nanotopographical features (nanotubes) and micro-features (micropits). After anodizing, the biomaterial was heat treated in Argon at different temperatures ranging between 400 and 600 C for either 1 or 2 h to improve its bioactivity. The effects of applied heat treatment on the crystal structure of TiO 2 nanotubes and the nanotopographical features of the surface were studied using scanning electron microscopy and X-ray diffraction. It was shown that the transition from the initial crystal structure, i.e. anatase, to rutile occurs between 500 and 600 C and that after 2 h of heat treatment at 600 C the crystal structure is predominantly rutile. The nanotopographical features of the surface were found to be largely unchanged for heat treatments carried out at 500 C or below, whereas they were partially or largely disrupted after heat treatment at 600 C. The possible implications of these findings for the bioactivity of porous titanium structures are discussed. © 2013 Elsevier B.V. Source


Thijs L.,Catholic University of Leuven | Montero Sistiaga M.L.,Catholic University of Leuven | Wauthle R.,Catholic University of Leuven | Wauthle R.,LayerWise NV | And 3 more authors.
Acta Materialia | Year: 2013

Selective laser melting (SLM) makes use of a high energy density laser beam to melt successive layers of metallic powders in order to create functional parts. The energy density of the laser is high enough to melt refractory metals like Ta and produce mechanically sound parts. Furthermore, the localized heat input causes a strong directional cooling and solidification. Epitaxial growth due to partial remelting of the previous layer, competitive growth mechanism and a specific global direction of heat flow during SLM of Ta result in the formation of long columnar grains with a 〈1 1 1⌠preferential crystal orientation along the building direction. The microstructure was visualized using both optical and scanning electron microscopy equipped with electron backscattered diffraction and the global crystallographic texture was measured using X-ray diffraction. The thermal profile around the melt pool was modeled using a pragmatic model for SLM. Furthermore, rotation of the scanning direction between different layers was seen to promote the competitive growth. As a result, the texture strength increased to as large as 4.7 for rotating the scanning direction 90 every layer. By comparison of the yield strength measured by compression tests in different orientations and the averaged Taylor factor calculated using the viscoplastic self-consistent model, it was found that both the morphological and crystallographic texture observed in SLM Ta contribute to yield strength anisotropy. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source


Amin Yavari S.,Technical University of Delft | Amin Yavari S.,FT Innovations BV | van der Stok J.,Erasmus University Rotterdam | Chai Y.C.,Catholic University of Leuven | And 10 more authors.
Biomaterials | Year: 2014

The large surface area of highly porous titanium structures produced by additive manufacturing can be modified using biofunctionalizing surface treatments to improve the bone regeneration performance of these otherwise bioinert biomaterials. In this longitudinal study, we applied and compared three types of biofunctionalizing surface treatments, namely acid-alkali (AcAl), alkali-acid-heat treatment (AlAcH), and anodizing-heat treatment (AnH). The effects of treatments on apatite forming ability, cell attachment, cell proliferation, osteogenic gene expression, bone regeneration, biomechanical stability, and bone-biomaterial contact were evaluated using apatite forming ability test, cell culture assays, and animal experiments. It was found that AcAl and AnH work through completely different routes. While AcAl improved the apatite forming ability of as-manufactured (AsM) specimens, it did not have any positive effect on cell attachment, cell proliferation, and osteogenic gene expression. In contrast, AnH did not improve the apatite forming ability of AsM specimens but showed significantly better cell attachment, cell proliferation, and expression of osteogenic markers. The performance of AlAcH in terms of apatite forming ability and cell response was in between both extremes of AnH and AsM. AcAl resulted in significantly larger volumes of newly formed bone within the pores of the scaffold as compared to AnH. Interestingly, larger volumes of regenerated bone did not translate into improved biomechanical stability as AnH exhibited significantly better biomechanical stability as compared to AcAl suggesting that the beneficial effects of cell-nanotopography modulations somehow surpassed the benefits of improved apatite forming ability. In conclusion, the applied surface treatments have considerable effects on apatite forming ability, cell attachment, cell proliferation, and bone ingrowth of the studied biomaterials. The relationship between these properties and the bone-implant biomechanics is, however, not trivial. © 2014 Elsevier Ltd. Source


Moin D.A.,Research Institute MOVE | Hassan B.,Research Institute MOVE | Parsa A.,Academic Center for Dentistry Amsterdam | Mercelis P.,LayerWise NV | Wismeijer D.,Research Institute MOVE
Clinical Oral Implants Research | Year: 2014

Objectives: The aim of this in vitro pilot investigation is to assess the accuracy of the preemptive individually fabricated root analogue implant (RAI) based on three-dimensional (3D) root surface models obtained from a cone beam computed tomography (CBCT) scan, computer-aided designing (CAD), and computer-aided manufacturing (CAM) technology and to measure the discrepancy in congruence with the alveolar socket subsequent to placement of the RAI. Materials and methods: Eleven single-rooted teeth from nine human cadaver mandibles were scanned with the 3D Accuitomo 170 CBCT system. The 3D surface reconstructions of the teeth acquired from the CBCT scans were used as input for fabrication of the RAIs in titanium using rapid manufacturing technology. The teeth were then carefully extracted. The teeth and RAIs were consequently optically scanned. The mandibles with the empty extraction sockets were scanned with CBCT using identical settings to the first scan. Finally, the preemptively made RAIs were implanted into their respective sockets, and the mandibles were again scanned with CBCT using the same scan settings as previous scans. All 3D surface reconstructions (CBCT 3D surface models and optical scan 3D models) were saved for further analysis. 3D models of original teeth and optical scans of the RAIs were superimposed onto each other; differences were quantified as root mean square (RMS) and Hausdorff surface distance. To obtain an estimate of the fit (congruence) of the RAIs in their respective sockets, the volumetric data sets of the sockets were compared with those of the root part of RAIs congruent with the sockets. Results: Superimposed surfaces of the RAIs and the original tooth reveal discrepancy for RMS, volumetric geometry, and surface area varying from 0.08 mm to 0.35 mm, 0.1% to 7.9%, and 1.1% to 3.8%, respectively. Comparing volume differences of the alveolus with the socket corresponding part of the RAI resulted in every case the volume of the socket being greater than the root part of the RAI ranging from 0.6% to 5.9% volume difference. Conclusion: The preemptive CAD/CAM-based RAI technique might offer promising features for immediate implant placement. However, due to the lack of prospective clinical data, further research is needed to fine-tune and evaluate this technique. © 2012 John Wiley & Sons A/S. Published by Blackwell Publishing Ltd. Source

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