Tomasz Brynk

Post Processing and Biological Evaluation of the Titanium Scaffolds for Bone Tissue Engineering

Nowadays, post-surgical or post-accidental bone loss can be substituted by custom-made scaffolds fabricated by additive manufacturing (AM) methods from metallic powders. However, the partially melted powder particles must be removed in a post-process chemical treatment. The aim of this study was to investigate the effect of the chemical polishing with various acid baths on novel scaffolds’ morphology, porosity and mechanical properties. In the first stage, Magics software (Materialise NV, Leuven, Belgium) was used to design a porous scaffolds with pore size equal to (A) 200 µm, (B) 500 µm and (C) 200 + 500 µm, and diamond cell structure. The scaffolds were fabricated from commercially pure titanium powder (CP Ti) using a SLM50 3D printing machine (Realizer GmbH, Borchen, Germany). The selective laser melting (SLM) process was optimized and the laser beam energy density in range of 91–151 J/mm3 was applied to receive 3D structures with fully dense struts. To remove not fully melted titanium particles the scaffolds were chemically polished using various HF and HF-HNO3 acid solutions. Based on scaffolds mass loss and scanning electron (SEM) observations, baths which provided most uniform surface cleaning were proposed for each porosity. The pore and strut size after chemical treatments was calculated based on the micro-computed tomography (µ-CT) and SEM images. The mechanical tests showed that the treated scaffolds had Young’s modulus close to that of compact bone. Additionally, the effect of pore size of chemically polished scaffolds on cell retention, proliferation and differentiation was studied using human mesenchymal stem cells. Small pores yielded higher cell retention within the scaffolds, which then affected their growth. This shows that in vitro cell performance can be controlled to certain extent by varying pore sizes.

Micromechanics of bioresorbable porous CEL2 glass-ceramic scaffolds for bone tissue engineering

Abstract Owing to their stimulating effects on bone cells, ceramics are identified as expressly promising materials for fabrication of tissue engineering (TE) scaffolds. To ensure the mechanical competence of TE scaffolds, it is of central importance to understand the impact of pore shape and volume on the mechanical behaviour of the scaffolds, also under complex loading states. Therefore, the theory of continuum micromechanics is used as basis for a material model predicting relationships between porosity and elastic/strength properties. The model, which mathematically expresses the mechanical behaviour of a ceramic matrix (based on a glass system of the type SiO2–P2O5–CaO–MgO–Na2O–K2O; called CEL2) in which interconnected pores are embedded, is carefully validated through a wealth of independent experimental data. The remarkably good agreement between porosity based model predictions for the elastic and strength properties of CEL2-based porous scaffolds and corresponding experimentally determined mechanical properties underlines the great potential of micromechanical modelling for speeding up the biomaterial and tissue engineering scaffold development process—by delivering reasonable estimates for thematerial behaviour, also beyond experimentally observed situations.

Fatigue Crack Growth Rates and Tensile Strength of Titanium Produced by Means of Selective Laser Melting

Mini-samples technique was utilized to determine mechanical properties of technically pure titanium produced by means of selective laser melting (SLM). Full-field digital image correlation (DIC) measurements and inverse method were applied for crack tip position and stress intensity factors calculations in the case of fatigue crack growth rate tests. DIC was also used for strain measurement during tensile tests on sub sized samples. There was studied the influence of samples orientation on the mechanical properties of mini-samples. Samples were cut out from rectangular cubes and were oriented with 0°, 45° or 90° angle to the direction of laser beam travel. There were also tested samples directly produced via SLM. Additionally microstructure observations were performed to verify the quality of SLM processed materials and explain mechanical properties variations.

The influence of chemical polishing of titanium scaffolds on their mechanical strength and in-vitro cell response

Selective Laser Melting (SLM) is a powder-bed-based additive manufacturing method, using a laser beam, which can be used to produce metallic scaffolds for bone regeneration. However, this process also has a few disadvantages. One of its drawbacks is the necessity of post-processing in order to improve the surface finish. Another drawback lies in the removal of unmelted powder particles from the build. In this study, the influence of chemical polishing of SLM fabricated titanium scaffolds on their mechanical strength and in vitro cellular response was investigated. Scaffolds with bimodal pore size (200 μm core and 500 μm shell) were fabricated by SLM from commercially pure titanium powder and then chemically treated in HF/HNO3 solutions to remove unmelted powder particles. The cell viability and mechanical strength were compared between as-made and chemically-treated scaffolds. The chemical treatment was successful in the removal of unmelted powder particles from the titanium scaffold. The Youngs modulus of the fabricated cellular structures was of 42.7 and 13.3 GPa for as-made and chemically-treated scaffolds respectively. These values are very similar to the Youngs modulus of living human bone. Chemical treatment did not affect negatively cell proliferation and differentiation. Additionally, the chemically-treated scaffolds had a twofold increase in colonization of osteoblast cells migrating out of multicellular spheroids. Furthermore, X-ray computed microtomography confirmed that chemically-treated scaffolds met the dimensions originally set in the CAD models. Therefore, chemical-treatment can be used as a tool to cancel the discrepancies between the designed and fabricated objects, thus enabling fabrication of finer structures with regular struts and high resolution.

Fatigue Crack Growth in Fe Mini-Samples Consolidated by Means of Impact Sintering

The paper presents the results of fatigue crack growth rate test of iron sinters. The samples were produced by means of the impact sintering. Applied production method allowed to obtain dense sinters with fine grain size resulted from large shear strains. Due to the limited size of the final products the mechanical tests were carried out in mini-samples. Optical, non-contact method of displacement measurement, namely Digital Image Correlation (DIC), was applied for determination of the displacement fields near the crack tip at the maximal force of selected loading cycles. The results of DIC measurement were utilized in the calculations of stress intensity factors and crack tip coordinates by means of the iterative procedure based on inverse method. These parameters were used for measuring crack development rate. There were investigated two types of materials produced by the consolidation of two different kinds of Fe powders and sintered in different temperatures. The results of crack growth rate tests were correlated with the microstructure changes, as well as yield and ultimate strength of the materials.

Magnesium AZ91 Alloy Cast Mechanical Properties Measured by the Miniaturized Disc-Bend Test

Miniaturized Disc-Bend Test (MDBT), also called the Small Punch Test (SPT) is used for characterizing the mechanical properties of metals, when only a small volume of material is available. This study was dedicated to investigating the mechanical properties of AZ91 magnesium cast alloy. The casts were prepared via gravity sand casting and have sections with different wall thickness. The examined samples were cut out of 30 mm and 10 mm thick walls. The correlation between results obtained from the tensile tests and MDBT was determined.

Mini-samples technique in tensile and fracture toughness tests of nano-structured materials

Samples dimensions defined in standards for tensile and fracture toughness tests may be too large in the case of modern materials produced in small volumes, e.g. nano-structured metals. Also, dimensions of irradiation tests samples are frequently not appropriate for standardized samples. In such situations so called mini-samples need to be used. The paper presents methodology and the results of tensile and fracture toughness tests which were carried out on mini-samples with a few millimeter dimensions. These samples were made of nano-structured metals processed by hydro extrusion (HE) and equal channel angular pressing (ECAP). Due to small size of specimens optical method of strain measurement - digital image correlation (DIC) - was applied. DIC twopoint- tracing mode was used as an optical extensometer in the tensile tests. The inverse method was applied for precise determining stress intensity factors and crack tip positions from displacement fields acquired by DIC. Comparison of the results of the tensile and fracture toughness tests which were carried out on standardized and mini-samples made of the same materials is also presented and the results are discussed in terms of the applicability of mini-samples technique in the studies of nano-metals.

DIC Assisted FCG Testing for Materials Used in Shale Gas Mining

Limited mineral resources amount induce research in natural gas sequestration from non-conventional reservoirs with shale rock being a prominent example. CO2 based fracturing is promising technique for both shale rock fracturing and greenhouse effect causing gas underground storage, however CO2 in the presence of water might be corrosive for traditional materials used in natural gas wells limiting their performance in static and dynamic loading conditions. The aim of this work is to develop minisamples based technique for Fatigue Crack Growth (FCG) rate testing on casing pipe material used in conventional gas mining. 3-point bending type samples of three different W dimension has been selected for testing with FCG standard based approach as well as with optical, noncontact displacement measurements made with Digital Image Correlation (DIC) near the propagating crack tip. Results of different testing techniques for P110 steel have been discussed and conclusion drawn out giving general guidelines for proposed method usage in pipeline materials investigation. Additionally scale effect on FCG results has been revealed.

Influence of Low Temperature on Mechanical Properties of Carbon Steel P110 Estimated by Means of Small Punch Test

As recommended by international standards, conventional methods of determining the mechanical properties of materials require relatively large test samples. This can create difficulties when only a limited amount of material is available for testing. The small punch test is one of the techniques that can replace traditional testing methods in such conditions.

Fatigue Crack Growth of Thin Wall Magnesium AZ91 Alloy Cast

The paper presents the results of fatigue crack growth rate tests performed in magnesium AZ91 alloy cast of different wall thicknesses. Due to the limited size of investigated cast the tests were carried on mini-samples of 4x4x1 mm gauge section dimensions. The samples were cut from the 5, 10 and 30 mm thick walls. The optical displacement measurement technique, namely Digital Image Correlation, and inverse method were applied for determination of the stress intensity factors and the crack tip coordinates during tests. There were also performed uniaxial tensile tests in mini-samples for the determination of the mechanical properties changes related to different cast thicknesses. The relation between the microstructure and the results of mechanical tests was discussed.