Design rules for producing cardiovascular stents by selective laser melting: Geometrical constraints and opportunities

Additive Manufacturing (AM) has risen great interest in biomedical applications, for its flexibility and the possibility of producing patient-customized devices. Customization has high importance to limit inflammation and rejection. Moreover, some occlusions may take place in bifurcation sites, which require the insertion of multiple stents, increasing the risk and complexity of the procedure. Selective Laser Melting (SLM) can be exploited to realize metallic stents with geometries not depending on tubular precursor, allowing the study of devices with new shapes, such as for occlusions in high-tortuosity vessels and in bifurcated arteries. The geometrical flexibility of SLM is largely exploited for large components, while for small implants such as cardiovascular stents it has not been studied in depth. Accordingly, this work analyses the geometrical requirements of cardiovascular stents starting from traditional meshes and identifies design rules for AM. Initially, traditional stent meshes are analysed to assess their feasibility in layer-by-layer production by SLM. An accurate investigation of the limitations imposed by the powder-bed process nature, the powder size and the laser spot diameter is performed, considering the relationship with stent dimensions, cell shape and strut inclinations. Finally, a set of design rules for SLM of stents are defined. These rules are used to design novel meshes producible with an industrial SLM system using cobalt-chromium powder. In particular, tubular stents are produced in expanded and semi-crimped configurations. Additionally, multi-branch stents were designed and produced to prove the capability of the process for bifurcation applications. In conclusion the validity of the defined design rules is assessed by the successful production of cardiovascular stents by SLM and their expansion.