Matthias Niemeyer

Biocorrosion of magnesium alloys: a new principle in cardiovascular implant technology?

Objectives: To develop and test a new concept of the degradation kinetics of newly developed coronary stents consisting of magnesium alloys. Methods: Design of a coronary stent prototype consisting of the non-commercial magnesium based alloy AE21 (containing 2% aluminium and 1% rare earths) with an expected 50% loss of mass within six months. Eleven domestic pigs underwent coronary implantation of 20 stents (overstretch injury). Results: No stent caused major problems during implantation or showed signs of initial breakage in the histological evaluation. There were no thromboembolic events. Quantitative angiography at follow up showed a significant (p < 0.01) 40% loss of perfused lumen diameter between days 10 and 35, corresponding to neointima formation seen on histological analysis, and a 25% re-enlargement (p < 0.05) between days 35 and 56 caused by vascular remodelling (based on intravascular ultrasound) resulting from the loss of mechanical integrity of the stent. Inflammation (p < 0.001) and neointimal plaque area (p < 0.05) depended significantly on injury score. Planimetric degradation correlated with time (r = 0.67, p < 0.01). Conclusion: Vascular implants consisting of magnesium alloy degradable by biocorrosion seem to be a realistic alternative to permanent implants.

Degradation of tungsten coils implanted into the subclavian artery of New Zealand white rabbits is not associated with local or systemic toxicity

OBJECTIVE To assess whether corrosion of tungsten coils is related to residual shunting and to evaluate whether elevated tungsten serum levels are associated with local or systemic toxicity. METHODS Tungsten coils (SPI, Balt, France) were implanted into the subclavian artery of New Zealand white rabbits leading to a residual high-flow shunt in 5/10 rabbits. Serial serum tungsten levels, complete blood count and clinical chemistry were analysed prior to the implantation as well as 15 min, 2 and 4 months thereafter. After 4 months the rabbits underwent repeat angiography before they were sacrificed and the internal organs were evaluated histopathologically. RESULTS Mean tungsten levels rose from 0.48 microg/l prior to the implantation to 12.4 microg/l 4 months post-implantation. The rise in serum tungsten levels was neither associated with residual shunting present at the time of implantation nor with residual shunting at the time of explantation. One animal had to be sacrificed because of non-resolving palsy of the upper extremity. The remaining animals had an uneventful clinical course with no signs of toxicity of the elevated tungsten levels. Histological examination revealed no evidence of local or systemic toxicity of the tungsten coils. CONCLUSION Tungsten coils corrode and lead to a steady increase in serum tungsten levels starting as early as 15 min after implantation. Residual shunting does not seem to influence the kinetics of corrosion of tungsten coils. Despite markedly elevated serum tungsten levels 4 months after implantation degradation of tungsten coils is not associated with local or systemic toxicity.

Untersuchungen an magnesiumbasierten Legierungen als neue Materialien in der Implantologie

Magnesium-based alloys show as well a fitting Youngss Modulus (45 GPa) referring to the corticalis as high physico-chemical biocompatibility due to its essential character. The biodegradation of Magnesium-alloys could be used to obtain temporary implants. The corrosion resistance of commercial alloys as AM20 and AZ31 and testing materials with higher biocompatibility, basing on cph Mg-Li, is tested in a body ullage.

Entwicklung und Eigenschaften von Magnesium‐Lithium‐Legierungen

Der Einsatz von Lithium in Magnesium-Legierungen erweitert den Eigenschaftsbereich dieser hinsichtlich geringerer Dichte und deutlich gesteigerter Duktilitat. Hiermit konnen Forderungen gerade von Seiten der Automobilindustrie nach schadenstoleranten, besser verformbaren Magnesiumbauteilen begegnet werden. Untersuchungen an hexagonalen, niederlithiumhaltigen Magnesium-Lithium-Legierungen zeigen vielversprechende Ergebnisse, die nicht nur ausgewogenere mechanische Eigenschaften sondern auch ein verbessertes Korrosionsverhalten beinhalten. Development and Properties of Magnesium-Lithium-Alloys The application of Lithium in Magnesium alloys extends the properties of these regarding smaller density and clearly increased ductility. Hereby can be met to demands even on the part of the automobile industry for damage-tolerant, better ductile magnesium components. Investigations of hexagonal Magnesium Lithium alloys show promising results, which contain not only more balanced mechanical properties but also an improved corrosion behavior.

Tracing thermal process of permanent mould casting

Accurate process control is the most effective means to ensure product quality. Online information about the operating conditions has to be gathered and evaluated in order to adjust parameters. For manufacturing processes with nonstationary characteristics, manifold variables and multimode heat transfer phenomena, one is confronted with severe problems, if targetted control is at stake. Thus mould or die casting, for example, are production technologies which are mostly controlled by foundrymen, who set and change the operating parameters empirically. The occurrence of defects in produced parts is sometimes detected by post process evaluation with much delay, causing economic harm. Thermography may be used to identify feeding performance and to monitor the instantaneous process conditions during fabrication. These results may be used to either control the cooling rate of the mould or to specify best design of cooling ducts in new dies. An experimental set-up and evaluation technique for thermography at permanent mould casting is given, revealing the potential for automated adjustment of process control.

Solidification process and infrared image characteristics of permanent mold castings

Interdependence between the development of temperature gradients at the solid-liquid interface during solidification of metals and the formation of local defects demands for thermal investigation. In foundry practice thermocouples are used to control the dies overall cooling-rate, but fluctuations in product quality still occur. Capturing FIR- thermograms after opening the die visualizes the state, when most thermal throughput has already flattened the temperature gradients in the mold. Rapid dissipation of heat from liquid metal to the mold during solidification forces further approach of the process investigation by slowing down the heat flux or the use of transparent mold material. Aluminum gravity casting experiments under technical vacuum conditions lead to decelerated solidification by suppression of convection and image sequences containing explicit characteristics that could be assigned to local shrinkage of the casting. Hence relevant clusters are extracted and thermal profiles are drawn from image series, pointing out correlations between feeding performance from the sink heads and the appearance of local defects. Tracing thermal processes in vacuum casting can scarcely be transferred to image data in foundry practice, since only little analogies exist between atmospheric and vacuum casting. The diagnosis of the casting process requires detection of the still closed mold using a transparent silica- aerogel sheet as part of the die. Hereby thermograms of the initial heat input are recorded by adapting a NIR-camera in addition to the FIR-unit. Thus the entire thermal compensation at the joint face for each casting is visualized. This experimental set-up is used for image sequence analysis related to the intermediate casting phases of mold filling, body formation and solidification shrinkage.

Thermal investigation of compound cast steel tools

Tools for hot forging are exposed to complex stresses during their life-cycle. Therefore, forging dies should have a high wear resistance and toughness on the surface, combined with excellent thermal conductivity in the die body. Hot-work tool steel is appropriate for this application except from its thermal conductance. Hence, a tool consisting of hot-work tool steel in the area of contact and heat-treatable steel as die body is favorable. A smoothly graded microstructure in the joint zone between the two steel alloys is needed to match with the requirements. Fabrication of such functionally graded dies by sand casting exhibits high sensitivity to temperature and geometry dependent parameters. To melt on the inlays surface must be ensured without destroying this region according to overheat coarsening and mixing of alloying elements. Instead of empirical methods to optimize the process parameters, a thermographic CCD-device is used for visualization of the heat flow while pouring the melt on the inlay. In fact the molten metal flow can be directed homogeneously across the bonding surface at adequate temperatures after evaluation of thermography data. The use of a silica-aerogel sheet as opaque window beneath the inlay in the mold enables systematic development of gating and risering, whereas undesirable scaling of the inlay due to the change of emissivity is retarded. Infrared image sequences clearly demonstrate the influence of different ring gating systems concerning the filling properties. Non-joined cavities may even be classified from image data. Compound cast steel tools have been manufactured and examined in forging trials validating life-cycle prolongation.

Innovative magnesiumgerechte Gießtechnik

Als ein Haupthinderungsgrund fur die breiter gefacherte Anwendung von Magnesiumlegierungen als Konstruktionswerkstoff gilt die problematische giestechnische Verarbeitung. Das hohe Reaktionsvermogen mit der Umgebungsatmosphare und verschiedenen Kontaktwerkstoffen, der volumenbezogen geringe Warmeinhalt und die bei kleinem Erstarrungsintervall meist verhaltnismasig hohen Liquidustemperaturen eigenschaftsoptimierter Werkstoffe erfordern angepaste Giesprozesse und Anlagentechniken. Die Zielsetzung des Teilprojektes B10 im Rahmen des SFB 390 besteht in der Optimierung bekannter und der Entwicklung neuer Giesverfahren zur Herstellung fehlerarmer Magnesiumbauteile mit hohen gewichtsspezifischen Eigenschaften. Im Institut fur Werkstoffkunde (IW) der Universitat Hannover werden neben Entwicklungen im Bereich der verunreinigungsarmen Erschmelzung, Schmelzehandhabung, Schmelzezufuhrung und Formfullung Grundlagen fur Giesprozesse geschaffen, die auf die spezifischen Eigenschaften auch hochreaktiver Magnesiumschmelzen abgestimmt sind. Innovative Magnesium adepted Casting Technology A main reason for the relatively small use of magnesium alloys as construction material is the demanding casting technology. The reactivity with ambient atmosphere and several other materials, the low specific heat values like heat capacity and heat of fusion, the small melting range and comparatively high liquidus temperatures of alloys with improved properties demand the optimizing of excisting and development new casting processes. In the scope of the Thrust Research Program SFB 390 “Magnesium Technology” founded by the German Research Council (DFG) the project B10 “Casting Technology” is engaged in the development and optimisation of casting techniques for Mg-alloys. The emphasis of the developments in the Institute of Materials Science (IW) of the Universtity of Hanover is set on reliable, low contaminated processes of melting, melt handling and mould filling for the production of low defect structural components, even dealing with alloys difficult to handle.

Gießtechnisches Herstellverfahren für Magnesiumschäume

Metallschaume gewinnen durch ihre besonderen Eigenschaften zunehmend Beachtung als Strukturbauteil. Auch in der Medizintechnik konnten diese Vorzuge der Metallschaume positiv genutzt werden, da bisher genutzte bioresorbierbare Implantate nur uber eine geringe Festigkeit verfugen und somit als stabilisierendes Bauteil nur bedingt in Betracht kommen. Magnesiumschaume, die als Implantate in den Korper eingebracht wurden, sind nicht bioinert wie zum Beispiel Titanimplantate, so das sie nach einer Zeit der stutzenden Wirkung vom nachwachsenden Knochenmaterial zunachst in der Porenstruktur durchsetzt werden, um dann vor dem nachwachsenden Knochen resorbiert, d. h. abgebaut zu werden. Bekannte Herstellverfahren [1, 2, 4], die zumeist die Erzeugung von Aluminiumschaumen zum Ziel haben, sind nicht zur Fertigung bioresorbierbarer Magnesiumschaume geeignet, da die Biovertraglichkeit hochreine Materialien voraussetzt, und somit der Einsatz von Partikeln zur Viskositatssteigerung und Treibmitteln zur Aufschaumung nicht moglich ist. Die Herstellung von hochreinen Magnesiumschaumen erfolgt daher in einem giestechnischen Verfahren unter Verzicht auf die oben genannten Hilfsmittel. Zur Aufschaumung wird dem Schmelzestrom ein inerter Gasstrom zugefuhrt, der in einem statischen Mischer dispergiert wird, so das ein flussiger Magnesiumschaum entsteht. Durch genaue Temperaturfuhrung der schmelzefuhrenden Komponenten wird die Viskositat der Schmelze so eingestellt, das der Metallschaum stabilisiert wird und anschliesend in einer Kokille in endkonturnaher Form erstarrt werden kann.

Advances in gear wheel manufacturing

Precision forging of gearwheels is an economical alternative to the conventional machining operations for the manufacturing of gearwheels. Furthermore, the efficiency can be increased, if the heat treatment of the gears takes place directly after the forging. Especially for this purpose, a fully automated quenching device was developed. In addition, if the new technology of compound forging is used, significant economical and ecological benefits can be achieved.