Radiopaque FeMnN-Mo composite drawn filled tubing wires for braided absorbable neurovascular devices

Abstract

Flow diverter devices are small stents used to divert blood flow away from aneurysms in the brain, stagnating flow and inducing intra-aneurysmal thrombosis which in time will prevent aneurysm rupture. Current devices are formed from thin (∼25 μm) wires which will remain in place long after the aneurysm has been mitigated. As their continued presence could lead to secondary complications, an absorbable flow diverter which dissolves into the body after aneurysm occlusion is desirable. The absorbable metals investigated to date struggle to achieve the necessary combination of strength, elasticity, corrosion rate, fragmentation resistance, radiopacity, and biocompatibility. This work proposes and investigates a new composite wire concept combining absorbable iron alloy (FeMnN) shells with one or more pure molybdenum (Mo) cores. Various wire configurations are produced and drawn to 25-250 μm wires. Tensile testing revealed high and tunable mechanical properties on par with existing flow diverter materials. In vitro degradation testing of 100 μm wire in DMEM to 7 days indicated progressive corrosion and cracking of the FeMnN shell but not of the Mo, confirming the cathodic protection of the Mo by the FeMnN and thus mitigation of premature fragmentation risk. In vivo implantation and subsequent μCT of the same wires in mouse aortas to 6 months showed meaningful corrosion had begun in the FeMnN shell but not yet in the Mo filament cores. In total, these results indicate that these composites may offer an ideal combination of properties for absorbable flow diverters.

Graphical abstract

Fig. 1

Representative cross sections of the five composite wires used in this study illustrate versatility of the DFT approach. In these images, the outer shell is FeMnN and the darker filaments are Mo.

Fig. 2

Summary of in vivo experiments.

Fig. 3

A) Stress-strain curves of composite wires, compared to FeMnN and Mo. B) Crush resistance force of braided FD's composed of FeMnN, Mo, or DFT configuration D and E wires. *p < 0.05, **p < 0.01, ***p < 0.001.

Fig. 4

A) X-ray image of Ø75 μm Mo compared to stainless steel and other radiopaque materials. B) Grayscale intensity values and statistical significance of the wires in A. C) Grayscale values in relation to density, a key factor in radiopacity. Notably, Mo (red arrow) is above the trendline. D) X-ray image of Ø250 μm wires used in this study, with a Pt-Ta composite for reference. E) Grayscale intensity values and statistical significance of the wires in D. F) Grayscale values in relation to the percentage of each wire's Mo proportion.

Fig. 5

A) Radiograph of commercial flow diverter (FRED) compared to a 32-wire braid of Ø25 μm Wire A. B) Grayscale values of regions of the stents shown in A. C) Radiograph of 16-wire braids of Ø80 μm wires of FeMnN, Wire D, Wire E, and Mo. D) Grayscale values of regions of the stents shown in C.

Fig. 6

In vitro corrosion analysis of the three DFT wires with Wire A, Wire B and Wire C filaments after corrosion over 1, 3, and 7 days. The upper three rows of images are taken with optical microscopy and lower three rows of images are taken with SEM-BSE. Scale bar is 50 μm.

Fig. 7

Corrosion product analysis on in vitro static corrosion tests for wire B in DMEM A) Wire B SEM-BSE and EDS maps at 1 day corrosion and B) at 5 day corrosion. Scale bar is 50 μm.

Fig. 8

Macroscopic view of DFT implants within the abdominal aorta of mice at each implanted timepoint. Yellow arrows show suture points anchoring the wire within the vessel wall. Wire C is shown at 5 months instead of 6 months implantation.

Fig. 9

A) Representative 3D-reconstructed images and 2D μCT of DFT wires A, B, and C after implantation over 3 months. 3D image from left to right: reducing the amount of tissue/product grown onto and into the degradation layer and second cutting further through the wire to reveal additionally cross-sections, 2D scans randomly chosen at degraded parts, wire diameter 100 μm and length up to 1 mm B) Cross-sectional micrograph of wire B, degraded over 3-month, a) metallographic cross-section by light microscopy, b) μCT -scans and c) 3D-reconstructed image.

Fig. 10

A) Representative 3D-reconstructed images (left) and 2D-nanoCT-scans (right) of wires- A (top), B (middle), and C (bottom) after implantation for 6 months. In the 3D images from left to right, the visible contrast limits are modulated to reduce the amount of visible tissue grown onto and into the degradation layer and to reveal additional cross-sections. 2D scans were selected to highlight typical degradation maxima. B) SEM-EDS mapping of wire B from a 6-month implanted sample. In all cases, wire diameter is 100 μm.

Fig. 11

Corrosion progression mechanism of FeMnN-DFT-Mo wires.