Mechanical properties of open-cell metallic biomaterials manufactured using additive manufacturing

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Assessing and comparing influencing factors of residual stresses in Selective Laser Melting using a novel analysis method

During Selective Laser Melting, the irradiated material experiences large temperature fluctuations in a short time which causes unwanted thermal stresses. In order to assess thermal stresses in a simple and fast way a new pragmatic method is developed, namely the Bridge Curvature Method (BCM). The BCM is used to assess and qualitatively compare the influence of different laser scan patterns, laser parameter settings and more fundamental process changes on residual stresses. The results from the experiments as well as the findings from literature lead to two general conclusions: changes which reduce the high temperature gradient, like using short scan vectors and preheating of the base plate, reduce the thermal stresses. And, thermal stresses in a particular direction can be reduced by optimal choice of the orientation of scan vectors. The experiments indicate the reliability of the Bridge Curvature Method. Statistical analysis is used to check the repeatability of the method and to quantify the uncertainties during measurement.status: publishe

Assessing Influencing Factors of Residual Stresses in SLM using a Novel Analysis Method

Selective Laser Melting (SLM) is a layered manufacturing process that allows to build complex, lightweight and customized parts by consolidating successive layers of powder. Since during the SLM process the powder material is completely molten, a low degree of porosity and good mechanical properties can be achieved. However, during the melting and solidification, the material experiences large temperature fluctuations in a short time. This causes high thermal stresses which can introduce part warpage, cracks or an unwanted decrease in strength of the parts. The goal of the reported investigation is to reduce the thermal stresses by changing standard parameters like the scan pattern or process parameters like the preheating temperature of the base plate, the layer thickness, pre-scanning a deposited powder layer or post-scanning an already scanned layer. To achieve this goal, a new method is defined which enables assessing thermal stresses in a fast way by measuring the curvature of test parts when releasing them from the base plate on which they are built. A review of the existing literature on this topic and the results of own experiments show that changes which lower high temperature gradients reduce the thermal stresses. Also changing the orientation of the scan vectors can reduce the thermal stresses, or can reduce the fatal consequences of the thermal stresses.status: publishe

Study of the influence of material properties on residual stress in selective laser melting

Selective laser melting (SLM) is characterized by highly localized heat input and short interaction times, which lead to large thermal gradients. In this research, nine different materials are processed via SLM and compared. The resulting microstructures are characterized by optical and scanning electron microscopy. Residual stresses are measured qualitatively using a novel deflection method and quantitatively using X-ray diffraction. Microcracking, surface oxidation and the anisotropy of the residual stress are discussed. The different phenomena interacting with the buildup of residual stress make it difficult to distinguish the possible correlations between material parameters and the magnitude of residual stresses.status: publishe

New opportunities for using tantalum for implants with Additive Manufacturing

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Crystal structure and nanotopographical features on the surface of heat-treaded and anodized porous titanium biomaterials produced using selective laser melting

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Mechanical behavior of regular open-cell porous biomaterials made of diamond lattice unit cells

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׳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.status: publishe

Mechanical analysis of a rodent segmental bone defect model: The effects of internal fixation and implant stiffness on load transfer

Segmental bone defect animal models are often used for evaluating the bone regeneration performance of bone substituting biomaterials. Since bone regeneration is dependent on mechanical loading, it is important to determine mechanical load transfer after stabilization of the defect and to study the effects of biomaterial stiffness on the transmitted load. In this study, we assess the mechanical load transmitted over a 6mm femur defect that is stabilized with an internal PEEK fixation plate. Subsequently, three types of selective laser melted porous titanium implants with different stiffness values were used to graft the defect (five specimens per group). In one additional group, the defect was left empty. Micro strain gauges were used to measure strain values at four different locations of the fixation plate during external loading on the femoral head. The load sharing between the fixation plate and titanium implant was highly variable with standard deviations of measured strain values between 31 and 93% of the mean values. As a consequence, no significant differences were measured between the forces transmitted through the titanium implants with different elastic moduli. Only some non-significant trends were observed in the mean strain values that, consistent with the results of a previous finite element study, implied the force transmitted through the implant increases with the implant stiffness. The applied internal fixation method does not standardize mechanical loading over the defect to enable detecting small differences in bone regeneration performances of bone substituting biomaterials. In conclusion, the fixation method requires further optimization to reduce the effects of the operative procedure and make the mechanical loading more consistent and improve the overall sensitivity of this rat femur defect model.status: publishe

Comparison of different scaffold materials and different cell types in a 2D+ bioreactor system by LiMSy, a Live cell Monitoring System

Introduction. The use of a live cell monitoring system (LiMSy) is a valuable tool to investigate cell dynamics in real-time for various application domains. To obtain such a unique real-time monitoring approach, the validation of a 2D+ bioreactor system combined with sensor technology is being conducted. In addition, this 2D+ bioreactor system was used to investigate the influence of scaffold material and cell type in a comparative study. Materials and Methods. Human periosteal derived cells (hPDCs) and immortalized human bone marrow stem cells (ihBMSC) were seeded on either 2D+ titanium (Ti) or tantalum (Ta) scaffolds and cultured in a static or dynamic (bioreactor) condition. The influence of scaffold pretreatment and initial seeding conditions, such as time, seeding volume and cell density, was investigated in the two biomaterials in a static environment. Cell viability and proliferation were determined by measuring the metabolic activity and oxygen consumption in function of the flow rate. Therefore, a Presto Blue metabolic assay and sensor technology such as the spot (PreSens) and needle (Ocean Optics) oxygen sensors were used. Finally, endpoint analyses such as DNA measurements and Live/Dead images were carried out as a control. Results. In static conditions, hPDCs seeded on Ta scaffolds showed a similar or significant increased metabolic activity as compared to cells seeded on Ti scaffolds. The initial seeding density is the main factor influencing proliferation over time for Ti scaffolds, whereas in case of Ta scaffolds this is the initial seeding volume. hPDCs and ihBMSC showed different proliferation characteristics and a different correlation between metabolic activity and cell number. The oxygen consumption of hPDCs seeded on Ti scaffolds in a dynamic bioreactor system showed a positive correlation with the flow rate and was in agreement with the predicted oxygen consumption (as compared to the initially amount of seeded cells). Discussion and conclusion. Cell proliferation and viability differ between different cell types and scaffold materials in a 2D+ bioreactor system. This study provides promising results that in a next step should be combined with data-based modeling to achieve a real-time monitoring system suitable for different cell types and scaffold materials.status: publishe

Effects of bio-functionalizing surface treatments on the mechanical behavior of open porous titanium biomaterials

Bio-functionalizing surface treatments are often applied for improving the bioactivity of biomaterials that are based on otherwise bioinert titanium alloys. When applied on highly porous titanium alloy structures intended for orthopedic bone regeneration purposes, such surface treatments could significantly change the static and fatigue properties of these structures and, thus, affect the application of the biomaterial as bone substitute. Therefore, the interplay between biofunctionalizing surface treatments and mechanical behavior needs to be controlled. In this paper, we studied the effects of two bio-functionalizing surface treatments, namely alkali-acid heat treatment (AlAcH) and acid-alkali (AcAl), on the static and fatigue properties of three different highly porous titanium alloy implants manufactured using selective laser melting. It was found that AlAcH treatment results in minimal mass loss. The static and fatigue properties of AlAcH specimens were therefore not much different from as-manufactured (AsM) specimens. In contrast, AcAl resulted in substantial mass loss and also in significantly less static and fatigue properties particularly for porous structures with the highest porosity. The ratio of the static mechanical properties of AcAl specimens to that of AsM specimen was in the range of 1.5-6. The fatigue lives of AcAl specimens were much more severely affected by the applied surface treatments with fatigue lives up to 23 times smaller than that of AsM specimens particularly for the porous structures with the highest porosity. In conclusion, the fatigue properties of surface treated porous titanium are dependent not only on the type of applied surface treatment but also on the porosity of the biomaterial.status: publishe