Development, characterisation, and modelling of processability of nitinol stents using laser powder bed fusion

Additive manufacturing (AM) of customised vascular or peripheral stents is of great potential for surgeons and patients, enabling the patients to have customised stents and achieving better outcomes from stenting procedures, with further advantages of having a resource efficient manufacturing process. In this study, the potential for AM of superelastic NiTi-based shape memory alloy (Nitinol) stents was investigated. Two stent designs, which are used for the treatment of complex peripheral artery stenosis in the lower limbs, were studied. Laser Powder Bed Fusion (LPBF) of two stent designs was studied to investigate the impact of the process parameters on the stent geometry, strut size, structural integrity and the phase transformations. The study demonstrated the successful manufacture of Nitinol stents via LPBF, with strut sizes in the range between 250 µm and ≈ 560 µm. The elastic modulus of the stents was between 56 and 73 GPa, which matches well with the elastic modulus of standard austenitic Nitinol. Chemical etching was used to reduce the strut diameter and to remove the partially melted particles. It was shown that the laser energy input has a vital role in controlling the Ni-evaporation and the subsequent changes in the transformation temperatures, as well as the morphology of the stents. The lower energy input results in a reduced Ni-evaporation, maintaining the austenite finish temperature at the expected range, in addition to generating a good build morphology.