Investigation of Shear Strength and Microstructure Formation of Joined Ni Superalloys Using Ni Nanopastes.

By using Ni nanoparticles, the bonding of Ni base superalloys can be achieved with shear strengths well above 200 MPa in a joining process at comparatively low temperatures between 675 °C and 975 °C. This is enabled due to the high surface-to-volume ratio of nanoparticles, which leads to distinctly lower melting and sintering temperatures than those of the corresponding bulk material. The nanoparticles in this study are employed in high-metal nanopastes, whereby different chemical compositions of the pastes and different sizes of Ni nanoparticles were investigated. The results for the joining of Ni base superalloys showed that both size and composition had a significant influence on the achievable strengths. In addition, an extensive examination was conducted to reveal the influence of the process parameters joining temperature, holding time and joining pressure on the shear strengths as well as microstructure. It was shown that the temperature exerted the most influence on the strengths and the microstructure. The joining pressure also had a significant influence. The holding time, on the other hand, did not have a major influence on the strengths and in some cases even showed an unexpected behavior, as the values decreased for some combinations with longer holding time.

Transapical mitral valved stent implantation: computed tomographic evaluation of different prototype designs.

AIMS: The evaluation of in vivo shaping of mitral valved stent prototypes using cardiac computed tomography (CT) was the focus of this study. METHODS AND RESULTS: Twelve pigs received a self-expanding mitral valved stent, composed of an atrial element connected to a tubular ventricular body at a modified angle (45°, 90°, 110°) resulting in three designs. Cardiac CT was performed three weeks after implantation, with focus placed on stent design-related parameters: possible left ventricular outflow tract obstruction and stent shaping. CT was successfully conducted in 11/12 animals showing correct stent position within the mitral annulus and no obstruction of the left ventricular outflow tract in 9/11 animals. Minor radial deformations of the stent body were detected. At the atrioventricular junction, deformations of the stent structure were observed in all cases. Stents with a 45° angle exhibited the greatest deflection (≤56.4°±14.5°). CONCLUSIONS: The effectiveness of cardiac computed tomography in the development process of valved stents to provide essential information and quantitative data about the in vivo stent geometry was demonstrated. The in vivo mechanical deformations of the stent were quantified, identifying critical design areas: a larger preset angle leads to less deflection and improved alignment and hence reduces the mechanical load.

Experimental microablation of choroidal tissue for autologous retinal pigment epithelium-choroid translocation with the pulsed electron avalanche knife (PEAK-fc).

AIM: To test the microablation of excess graft choroidal tissue with the pulsed electron avalanche knife (PEAK-fc) in an in-vitro model of autologous retinal pigment epithelium (RPE)-choroid translocation. METHODS: Choroidal tissue of porcine RPE-choroid explants was ablated with the PEAK-fc. Tissue morphology was assessed by light microscopy (LM) and scanning electron microscopy (SEM). The amount of ablated choroidal tissue was analysed as a function of three PEAK-fc parameters: (1) amplitude of biphasic voltage (70-100%); (2) distance between choroidal tissue and tip of the PEAK-fc (0-300 µm); and (3) exposure time (2-8 s). RESULTS: LM and SEM showed a smooth plain within the ablation area with well defined cutting edges and preserved adjacent tissue structure. The mean amount of ablated tissue correlated linearly with applied voltage (range 79-120 µm, r=0.34) and distance between choroidal tissue and PEAK-fc tip (range 10-100 µm, r=0.74). The mean amount of ablated tissue increased with exposure time between 2 and 4 s (36-88 µm, r=0.4) and remained constant between 4 and 8 s. CONCLUSION: The PEAK-fc accurately microablates choroidal tissue in-vitro. The adjacent choroidal tissue structure and Bruch's membrane are preserved. Patient studies are required to test the PEAK-fc in RPE-choroid translocation surgery.