A comparative study of microstructures and nanomechanical properties of additively manufactured and commercial metallic stents

Abstract

Additive manufacturing emerges as an innovative technology to fabricate medical stents used to treat blocked arteries. However, there is a lack of direct comparison of underlying microstructure and mechanical properties of additively manufactured and commercial stents. In this study, additively manufactured and commercial 316 L stainless steel stents were investigated comparatively, with electrochemical polishing being used to improve the surface finish of the former stent. Microstructural characterisation of both stents was carried out through optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Their hardness and elastic modulus were studied using Berkovich nanoindentation, with an emphasis on the effect of grain orientation. In addition, spherical nanoindentation was used to generate indentation stress-strain curves based on load-displacement responses. The obtained results showed that electrochemical polishing was effective in diminishing the average surface roughness, with a reduction of Ra value from 8.45 µm to 5.96 µm. The additively manufactured stent demonstrated the hierarchical grain microstructure with columnar grains and cellular sub-grains, as opposed to equiaxed fine grains and twins in the commercial stent. The hardness and modulus of additively manufactured stents were higher than those of the commercial ones. The grains close to the (111) orientation exhibited the highest hardness and elastic modulus followed by (101) and (001) orientations. The indentation stress-strain curves, yield strength, and hardening behaviour were similar for the additively manufactured and commercial stents. This work provides a fundamental understanding of the microstructure and properties of the additively manufactured stent and represents an important step towards innovative manufacturing of stents.