Therese Bormann

Tailoring Selective Laser Melting Process Parameters for NiTi Implants

Complex-shaped NiTi constructions become more and more essential for biomedical applications especially for dental or cranio-maxillofacial implants. The additive manufacturing method of selective laser melting allows realizing complex-shaped elements with predefined porosity and three-dimensional micro-architecture directly out of the design data. We demonstrate that the intentional modification of the applied energy during the SLM-process allows tailoring the transformation temperatures of NiTi entities within the entire construction. Differential scanning calorimetry, x-ray diffraction, and metallographic analysis were employed for the thermal and structural characterizations. In particular, the phase transformation temperatures, the related crystallographic phases, and the formed microstructures of SLM constructions were determined for a series of SLM-processing parameters. The SLM-NiTi exhibits pseudoelastic behavior. In this manner, the properties of NiTi implants can be tailored to build smart implants with pre-defined micro-architecture and advanced performance.

Three-dimensional registration of tomography data for quantification in biomaterials science

Abstract The paper shows the benefit of registration tools in the quantitative analysis of tomography data. For this purpose three-dimensional datasets, i.e. target design in rapid prototyping, hard X-ray tomography, and magnetic resonance tomography, were registered with rigid, affine and non-rigid algorithms. Using rigid registration, we aligned individual tomograms to obtain one three-dimensional dataset per specimen, compared human teeth to determine abrasion, and optimized the acquisition parameters in clinical computed tomography. Affine registration was used to determine the sintering-induced shrinkage of hydroxyapatite scaffolds. Using non-rigid registration, we gained insight into tumor morphology imaged by different modes of micro computed tomography. In addition, the fixation-induced shrinkage of tumor tissue and the local deformation of brain tissues as well as the shape changes in nickel titanium alloys were quantitatively determined. Three-dimensional registration is, therefore, a powerful means to count back any influence on morphology of tissues and to quantitatively compare data of different tomography modalities.

Combining micro computed tomography and three-dimensional registration to evaluate local strains in shape memory scaffolds.

Appropriate mechanical stimulation of bony tissue enhances osseointegration of load-bearing implants. Uniaxial compression of porous implants locally results in tensile and compressive strains. Their experimental determination is the objective of this study. Selective laser melting is applied to produce open-porous NiTi scaffolds of cubic units. To measure displacement and strain fields within the compressed scaffold, the authors took advantage of synchrotron radiation-based micro computed tomography during temperature increase and non-rigid three-dimensional data registration. Uniaxial scaffold compression of 6% led to local compressive and tensile strains of up to 15%. The experiments validate modeling by means of the finite element method. Increasing the temperature during the tomography experiment from 15 to 37°C at a rate of 4 K h(-1), one can locally identify the phase transition from martensite to austenite. It starts at ≈ 24°C on the scaffolds bottom, proceeds up towards the top and terminates at ≈ 34°C on the periphery of the scaffold. The results allow not only design optimization of the scaffold architecture, but also estimation of maximal displacements before cracks are initiated and of optimized mechanical stimuli around porous metallic load-bearing implants within the physiological temperature range.

Assessing the morphology of selective laser melted NiTi-scaffolds for a three-dimensional quantification of the one-way shape memory effect

NiTi is promising for the use as bone scaffold, because the pseudoelasticity or the one- and two-way shape memory effect in the physiological window can mechanically stimulate the adherent cells. Such stimuli can enhance osseointegration and might reduce stress shielding associated with load bearing implants. The present study is based on the additive manufacturing technique of selective laser melting (SLM) to fabricate three-dimensional NiTi scaffolds. We demonstrate that the morphology of the scaffolds can be quantified using synchrotron radiation-based micro computed tomography (SRμCT) and sophisticated registration software. Comparing the CAD file with the SLM scaffolds, quality factors are derived. With respect to the CAD file, the overlap corresponds to (92.5 ± 0.6) %. (7.4 ± 0.42) % of material was missing and (48.9 ± 2.3) % of excess material found. This means that the actual scaffold is less porous than expected, a fact that has to be considered for the scaffold design. In order to quantify the shape memory effect during the shape recovery process, we acquired radiographs rotating an initially deformed scaffold in angular steps of 0.2 degree during controlled heating. The continuously acquired radiographs were combined to tomography data, showing that the quality factors evolved with temperature as the scaffold height, measured by conventional thermo-mechanical analysis. Furthermore, the data comprise the presence of compressive and tensile local strains in the three-dimensional scaffolds to be compared with the physiological situation.

Morphology of urethral tissues

Micro computed tomography has been developed to a powerful technique for the characterization of hard and soft human and animal tissues. Soft tissues including the urethra, however, are difficult to be analyzed, since the microstructures of interest exhibit X-ray absorption values very similar to the surroundings. Selective staining using highly absorbing species is a widely used approach, but associated with significant tissue modification. Alternatively, one can suitably embed the soft tissue, which requires the exchange of water. Therefore, the more recently developed phase contrast modes providing much better contrast of low X-ray absorbing species are especially accommodating in soft tissue characterization. The present communication deals with the morphological characterization of sheep, pig and human urethras on the micrometer scale taking advantage of micro computed tomography in absorption and phase contrast modes. The performance of grating-based tomography is demonstrated for freshly explanted male and female urethras in saline solution. The micro-morphology of the urethra is important to understand how the muscles close the urethra to reach continence. As the number of incontinent patients is steadily increasing, the function under static and, more important, under stress conditions has to be uncovered for the realization of artificial urinary sphincters, which needs sophisticated, biologically inspired concepts to become nature analogue.

Damping of Selective-Laser-Melted NiTi for Medical Implants

NiTi exhibits distinct damping properties associated with the martensite-austenite transformation. We fabricated net-shape NiTi parts layer-by-layer using a laser beam that locally melted the NiTi powder. The damping properties of such NiTi parts were analyzed by the decay of cantilever vibrations in comparison to conventionally prepared NiTi. The dynamic modulus as a function of the temperature was derived from the resonant frequency. We found that the two cantilevers showed a damping ratio of about 0.03 at temperatures below austenite start, maximal values of up to 0.04 in the transformation regions and low values of about 0.005 above austenite finish. The results indicate that selective-laser-melted NiTi qualifies for the fabrication of shock-absorbing medical implants in the same manner than conventionally produced NiTi.

Assessing the grain structure of highly X-ray absorbing metallic alloys

Abstract Selective laser melting allows the fabrication of NiTi implants with pre-defined, complex shapes. The control of the process parameters regulates the arrangement of the granular microstructure of the NiTi alloy. We prepared specimens with elongated grains, which build a sound basis for diffraction contrast tomography experiments using synchrotron radiation and for electron backscatter diffraction measurements. Both approaches reveal the orientation and size of the individual grains within the specimen. Still, electron backscatter diffraction is confined to two-dimensional cross-sections while diffraction contrast tomography reveals these microstructural features in three dimensions. We demonstrate that the grains in the selective laser melted specimen, which are oriented along the building direction, do not exhibit a well-defined planar grain orientation but are twisted. These twisted grains give rise to diffraction spots observable for several degrees of specimen rotation simultaneously to the acquisition of tomography data.

Controlling Mechanical Properties of NiTi Scaffolds built by Selective Laser Melting

NiTi, a biocompatible and FDA-approved material, is a promising candidate for load-bearing implants as it combines low stiffness with high strength and exhibits prope rties like the pseudoelasticity, the shape memory e ffect and high damping capacities [1]. Selective laser melting (SL M) allows the fabrication of complex shaped constru cts like open porous parts or filigree lattice structures with st ruts as thin as 200 μm [2]. Using SLM for the fabri cation of complexshaped NiTi-scaffolds, we aim to develop bone impla nts with an improved ossseoinductive performance ow ing to mechanical stimulation of surrounding tissues.

Effects of laser parameters and scanning strategy on structural and mechanical properties of 3D NiTi implants fabricated with selective laser melting

The additive manufacturing method Selective Laser Melting (SLM) allows the creation of open-porous shape memory structure directly from CAD data. However, during the SLM process, laser parameters and scanning strategy must be carefully selected. In this study, we show that they have a significant effect on geometrical and mechanical properties of NiTi scaffolds for the treatment of large bone defects.

Evaluation of oral scanning in comparison to impression using three-dimensional registration

Crown and bridge restorations are one of the main treatment methods in fixed prosthodontics. The fabrication requires data on the patient’s denture shape. This information is generally obtained as a hard copy from an impression mold. Alternatively, one can acquire the data electronically using oral optical three-dimensional (3D) imaging techniques, which determine the surface of the denture. The aim of the study was to quantitatively compare the accuracy of three dimensional scanning with that of conventional impressions and give a statement how far the scanner provides a clinical alternative with equal or better precision. Data from 10 teeth were acquired in the dental office with a polyether impression material and an oral scanner. Data from the impressions were digitalized by means of micro computed tomography. The data were then 3D registered to identify the potential differences between impression and optical scan. We could demonstrate that the oral scanner’s data and the conventional impressions are comparable.