Filtern
Erscheinungsjahr
- 2016 (8) (entfernen)
Dokumenttyp
- Vortrag (4)
- Zeitschriftenartikel (3)
- Beitrag zu einem Tagungsband (1)
Schlagworte
- additive manufacturing (3)
- Large animal model (2)
- Vertebroplasty (2)
- 3D (1)
- Additive Fertigung (1)
- Bone marrow extrusion (1)
- Brushite-forming calcium phosphate cement (1)
- Calcium phosphate cement (1)
- Cement injection (1)
- Computed tomography (1)
Eingeladener Vortrag
- nein (4)
Leistungsstarke Verfahren zur additiven Fertigung wie der 3D Druck und das Selektive Lasersintern basieren auf dem Schichtauftrag eines fließfähigen Pulvers. Bei keramischen Pulvern ist eine gute Fließfähigkeit ab einer Partikelgröße von ca. 40 µm gegeben. Bei Pulvern mit zu feinen Partikeln sind die adhäsiven Kräfte zwischen den Partikeln vergleichbar groß wie die Kräfte, die durch die Gravitation auf die Partikel wirken, was einem gleichmäßigen Fließen des Pulvers und somit einem gleichmäßigen Schichtaufbau entgegensteht. Die Verwendung von feineren Pulvern hat jedoch Vorteile, wie z.B. eine bessere Sinteraktivität.
Neben der nötigen minimalen Partikelgröße besteht für keramische Werkstoffe ein wesentliches Problem der pulverbasierten Verfahren in der Tatsache, dass das Pulverbett, also das durch Schichtauftrag gestapelte Pulver, eine zu geringe Dichte besitzt. Dies bringt grundsätzlich zwei Nachteile mit sich: zum einen verhindert eine zu geringe Packungsdichte des Pulvers den Aufbau kompakter Grünkörper und letztlich deren Sinterung zu einer dichten Keramik, zum anderen wird im Aufbauprozess das Bauteil durch das Pulverbett nicht ausreichend gestützt. Beim Auftragen einer neuen Pulverschicht kann die bereits verfestigte Struktur im Pulverbett verschoben werden, was zu Defekten in der Struktur bis hin zum vollständigen Verlust der Struktur führt. Aus diesem Grunde ist neben dem Aufbau des zu fertigenden Bauteils u.U. der Aufbau von zusätzlich sogenannten Supportstrukturen gefordert. Diese Supportstrukturen verankern das Bauteil mit der Bauplattform und fixieren es somit im Koordinatensystem des Bauraums der Anlage. Das Entfernen der Supportstrukturen erfordert nach dem eigentlichen additiven Fertigungsprozess einen zusätzlichen Prozessschritt, der zeitaufwendig und kaum zu automatisieren ist, und daher einer autonomen Fertigung, wie sie durch die Additive Fertigung realisiert werden kann, entgegensteht.
Es werden unterschiedliche Strategien für die Stabilisierung des Pulverbetts und zur Erzielung einer hohen Packungsdichte der Partikel im Pulverbett aufgezeigt. Neben der Lagenweise SchlickerDeposition (LSD) werden Ergebnisse zur Gas Flow Assisted Powder Deposition vorgestellt.
Purpose – This paper aims to present an additive manufacturing-based approach in which a new strategy for a thermally activated local melting and material flow, which results in densification of printed structures, is introduced.
Design/methodology/approach – For enabling this self-organized relaxation of printed objects by the viscous flow of material, two interconnected structures are printed simultaneously in one printing process, namely, Structure A actually representing the three dimensional object to be built and Structure B acting as a material reservoir for infiltrating Structure A. In an additional process step, subsequent to the printing job, an increase in the objects’ temperature results in the melting of the material reservoir B and infiltration of structure A.
Findings – A thermally activated local melting of the polymethylsilsesquioxane results in densification of the printed structures and the local formation of structures with minimum surface area.
Originality/value – The present work introduces an approach for the local relaxation of printed three-dimensional structures by the viscous flow of the printed material, without the loss of structural integrity of the structure itself. This approach is not restricted only to the materials used, but also offers a more general strategy for printing dense structures with a surface finish far beyond the volumetric resolution of the 3D printing process.
Selective laser sintering is a well-established technology for the additive manufacture of metallic and polymeric parts. For the additive manufacture of ceramic parts some examples for its successful application do exist, also. In this context, the selective laser sintering of SiC ceramic powders has been studied extensively in the past. Despite its low sintering activity, SiC is particularly suited for the selective laser sintering process, because of the possibility to form SiO2 at elevated temperatures in oxidizing ambient. The SiO2 formed can act as a binder within the laser sintering process.
In the present work densely packed powder beds generated by the Layerwise Slurry Deposition (LSD) technology are selectively sintered by a cw. fiber laser. The layerwise slurry deposition is an innovative process for the deposition of layers in additive manufacturing. A slurry with no or very small organic content is repetitively spread as thin layers on each other by means of a doctor blade. During the deposition process, the ceramic particles settle to form thin layers of about 100 µm which have a high packing density (typical 55-60%). This high powder packing density is the result of a slip casting process: When a layer is deposited on a previously dried porous layer, the water is drawn into the pores by capillary forces. The LSD process therefore shares aspects of tape casting and slip casting. An additional benefit of the LSD technology is the free choice of the size of the ceramic particles used. Compared to the processing of dry flowable powders, especially small particles can be very well processed.
This chapter discusses different approaches for the densification of rock salt powders via compaction and sintering, focusing on the use of powder qualities obtained by crunching and milling of the natural rock salt. The chapter also examines the effect of compaction pressure, time and temperature on sintering of rock salt. Natural rock salt samples were crushed and milled in a disc mill to obtain fine powders in an experiment. The hot stage microscope allowed observing the in-situ volumetric shrinkage to be determined during the sintering process by measuring the area of the sample's projection in an optical setup by a CCD camera. Images were collected at a rate of 40 frames per minute. After the application of high pressures, the obtained samples are translucent and show the formation of macroscopic crack. Powder compacts prepared by uniaxial pressing, at different maximum pressures, were annealed at a constant heating rate (10 K min-1) to different maximum temperatures. These experiments were performed in a conventional sintering furnace under ambient atmosphere.
The production of ceramic parts by an additive manufacturing process is still a challenging task. Especially in powder-based technologies, such as 3D printing or laser sintering, the feedstock material is commonly spread out as thin layers of a dry powder/granulate by a roller or a shaker system. As consequence the layers are characterized mostly by a low packing rate. On the other hand, from a ceramic processing viewpoint, appreciable densities can be achieved by the use of ceramic slurries. In this context, the so-called Layer-wise Slurry Deposition (LSD) process has been developed. An overview about the LSD process and latest advances in the development of this technology will be presented.
The use of water based ceramic slurries as feedstock for the additive manufacture of ceramics has many advantages which are not fully exploit yet. In the layerwise slurry deposition (LSD) process a slurry with no or low organic content is repetitively spread as thin layers on each other by means of a doctor blade. During the deposition, the ceramic particles settle on the previously deposited and dried material to form thin layers with a high packing density (55-60%). The LSD therefore shares aspects both of tape casting and slip casting. The LSD differentiates from the classical powder-based AM layer deposition, which typically achieves with a flowable coarse grained powder a low packing density (35-50%) only, consequently hindering the ability of sintering ceramic parts to full density. The LSD is coupled with the principles of selective laser sintering (SLS) or binder jetting, to generate novel processes which take advantage of the possibility of achieving a highly dense powder-bed. Contrary to the LSD process, which requires drying of each individual layer, the direct interaction of ceramic slurries with intense laser radiation is a promising approach for the additive manufacture of ceramics, also. This presentation will provide a detailed discussion of the specific features of the slurry based processes, potentialities and issues connected to the layer deposition and describe the most recent developments in their application to technical ceramics.
Large animal models are highly recommended for meaningful preclinical studies, including the optimization of cement augmentation for vertebral body defects by vertebroplasty/kyphoplasty.
The aim of this study was to perform a systematic characterization of a strictly minimally invasive in vivo large animal model for lumbar ventrolateral vertebroplasty.
This is a prospective experimental animal study.
Lumbar defects (diameter 5 mm; depth approximately 14 mm) were created by a ventrolateral percutaneous approach in aged, osteopenic, female sheep (40 Merino sheep; 6–9 years; 68–110 kg). L1 remained untouched, L2 was left with an empty defect, and L3 carried a defect injected with a brushite-forming calcium phosphate cement (CPC). Trauma/functional impairment, surgical techniques (including drill sleeve and working canula with stop), reproducibility, bone defects, cement filling, and functional cement augmentation were documented by intraoperative incision-to-suture time and X-ray, postoperative trauma/impairment scores, and ex vivo osteodensitometry, microcomputed tomography (CT), histology, static/fluorescence histomorphometry, and biomechanical testing.
Minimally invasive vertebroplasty resulted in short operation times (28±2 minutes; mean±standard error of the mean) and X-ray exposure (1.59±0.12 minutes), very limited local trauma (score 0.00±0.00 at 24 hours), short postoperative recovery (2.95±0.29 hours), and rapid decrease of the postoperative impairment score to 0 (3.28±0.36 hours). Reproducible defect creation and cement filling were documented by intraoperative X-ray and ex vivo conventional/micro-CT. Vertebral cement augmentation and osteoconductivity of the CPC was verified by osteodensitometry (CPC>control), micro-CT (CPC>control and empty defect), histology/static histomorphometry (CPC>control and empty defect), fluorescence histomorphometry (CPC>control; all p<.05 for 3 and 9 months), and compressive strength measurements (CPC numerically higher than control; 102% for 3 months and 110% for 9 months).
This first-time systematic clinical assessment of a minimally invasive, ventrolateral, lumbar vertebroplasty model in aged, osteopenic sheep resulted in short operation times, rapid postoperative recovery, and high experimental reproducibility. This model represents an optimal basis for standardized evaluation of future studies on vertebral augmentation with resorbable and osteoconductive CPC.
Vertebroplasty or kyphoplasty of osteoporotic vertebral fractures bears the risk of pulmonary cement embolism (3.5%–23%) caused by leakage of commonly applied acrylic polymethylmethacrylate (PMMA) cement to spongious bone marrow or outside of the vertebrae. Ultraviscous cement and specific augmentation systems have been developed to reduce such adverse effects. Rapidly setting, resorbable, physiological calcium phosphate cement (CPC) may also represent a suitable alternative.
PURPOSE: This study aimed to compare the intravertebral extrusion of CPC and PMMA cement in an ex vivo and in vivo study in sheep.
STUDY DESIGN/SETTING: A prospective experimental animal study was carried out. METHODS: Defects (diameter 5 mm; 15 mm depth) were created by a ventrolateral percutane-ous approach in lumbar vertebrae of female Merino sheep (2–4 years) either ex vivo (n = 17) or in vivo (n = 6), and injected with: (1) CPC (L3); (2) CPC reinforced with 10% poly(l-lactide-co-glycolide) (PLGA) fibers (L4); or (3) PMMA cement (L5; Kyphon HV-R). Controls were untouched (L1) or empty defects (L2). The effects of the cement injections were assessed in vivo by blood gas analysis and ex vivo by computed tomography (CT), micro-CT (voxel size: 67 µm), histology, and biomechanical testing.