ISSUE 115 MARCH 2025 LASER WELDING
THE LASER USER
LASER-BASED HYBRID MANUFACTURE
OF CUSTOMISED STENTS PETER IBRAHIM & MOATAZ ATTALLAH The application of laser technologies in
2. Surface Finishing
biomedical engineering has opened new avenues for the design and manufacture of medical devices and implants. Among these, arterial stents represent a critical sector due to their life-saving functionality and the high precision required for their performance. This article explores the integration of advanced laser techniques, specifically Laser Micro-Machining (LMM) and Laser Powder Bed Fusion (L-PBF), in the fabrication of customised titanium alloy (TNTZO) stents [1], [2]. These methods offer unprecedented design freedom, customisation, and improved material properties, addressing the limitations of traditional stent manufacturing techniques.
Novelty in combining Laser Powder Bed Fusion and Laser Micro-Machining
Among the major challenges in L-PBF is the difficulty in controlling the tolerances and surface finish. Combining L-PBF with LMM allows for the precise cutting and shaping of stents with intricate designs, ensuring compatibility with individual vascular anatomies. LMM provides the ability to create thin strut profiles and complex geometries without introducing thermal or mechanical stress to the material. Simultaneously, L-PBF offers the capability to build thin- walled tubular structures directly from titanium powder, enabling the production of stents with customised diameters and lengths. Together, these laser techniques overcome the design and manufacturing constraints of conventional methods, paving the way for next-generation stent technologies. The hybrid manufacturing technique is tested for manufacturing NiTi (nitinol) stents, a previously filed patent application [3].
Technical Challenges and Solutions
Despite their advantages, implementing LMM and L-PBF in stent manufacturing involves overcoming several technical challenges.
1. Optimisation of Geometries Achieving precise dimensional accuracy and consistent wall thickness during L-PBF is challenging due to factors such as laser energy input and scanning strategy. Optimising these parameters ensures the production of defect- free, uniformly dense tubes.
Figure 1: (A) schematic for different scanning strategies, (B) cross section analysis [2]. 26
The as-built surface roughness from L-PBF often requires post-processing to meet the stringent requirements of vascular stents. Techniques such as electropolishing and laser polishing have been successfully employed to reduce surface roughness and enhance biocompatibility.
3. Performance Validation
Ensuring mechanical integrity under physiological loads is critical. Finite Element Modelling (FEM) and experimental validation, including crimping and bending tests, are essential to assess the structural performance of the stents.
Results and Discussion
The integration of LMM and L-PBF in the manufacturing of TNTZO stents demonstrated significant advancements in customisation and performance. The use of L-PBF enabled the production of thin-walled tubes with optimised wall thickness and minimal defects, while LMM allowed for the creation of complex stent geometries with high precision. Post- processing through electropolishing reduced surface roughness to below 5 μm, enhancing cell attachment and biocompatibility.
1. L-PBF parameter optimisation
This study focused on tuning various parameters of the L-PBF process to improve the build integrity and the surface finish of the custom thin-walled tubes. The sweet spot of laser energy density was found using design of experiments where various combinations of laser power (P) and speed (v) were analysed
for the highest build relative density. Energy density (E) of 0.1 J/mm (P = 150 Watts, v = 1.5 m/s) successfully achieved the highest relative density builds of 99.86%. Lower E resulted in lack of fusion (LoF) defects where the built layers are weakly adhering to one another. On the other hand, higher E resulted in keyhole defects, layer discontinuity and lower dimension stability. Two different scanning strategies were used for building the tubes (see Figure 1A). A continuous laser scanning strategy could build a more robust tube with minimal internal defects compared to using a contour scanning strategy, where there is very clear LoF between the two laser contours forming the tube wall thickness. The contour start/end point also resulted in a weld bead line along the tube height that affected the tube integrity (see Figure 1B).
2. Surface finish
Different surface treatments were applied to the built tubes to improve the surface roughness and remove the partially melted powder particles hanging onto the surface (see Figure 2A). Chemical etching (CE) caused selective etching defects, (called pitting) in the tube surface which increased its surface roughness (see Figure 2B). Both electropolishing (EP) and laser polishing (LP) reduced the surface roughness (Sa) to < 5 μm successfully. However, due to the rapid thermal cycles of LP, many micro cracks were introduced in the tube surface which can deteriorate its mechanical properties, especially its fatigue resistance. Electropolished samples showed the highest metabolic activity after 7 days of testing.
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