Additive manufacturing and post-processing of superelastic NiTi micro struts as building blocks for cardiovascular stents

Laser powder bed fusion (PBF-LB) can be potentially used for producing patient-specific biomedical devices based on micro struts such as cardiovascular stents. NiTi alloys are an appealing choice for their superelastic behaviour, while their processing with PBF-LB poses several challenges. The final component is expected to have the correct transition temperature and mechanical properties along with an acceptable surface finish. All of these properties require a complete manufacturing cycle from PBF-LB for the production of the geometrical form, to the heat treatment for the adjustment of the mechanical properties, and finally to the chemical etching for the reduction of surface roughness. Therefore this work studies a complete manufacturing cycle composed of PBF-LB process parameters, heat treatment recipes, and chemical etching conditions for producing NiTi micro struts with superelastic behaviour. Transition temperature measurements and tensile tests were applied to verify the influence of each manufacturing process on the material properties using ad-hoc designed micro strut geometries. The results showed previously unreported tensile superelasticity with micro struts at the end of the manufacturing cycle, once the PBF-LB process parameters were correctly selected, followed by heat treatment and chemical etching stages. The determined additive manufacturing chain was then used to produce demonstrator stent mesh with variable diameter and 130 µm strut thickness.