Microstructural and mechanical characterisation of metallic stents manufactured with selective laser melting

Coronary artery disease (CAD) is a prevalent group of cardiovascular diseases and one of the main causes of mortality globally. Stent implantation has become one of the standard medical procedures to treat CAD patients. Currently, stents are produced by laser micromachining of micro-tubes, resulting in stents with a uniform structure in terms of diameter, length, and design. However, the lesion is very complex and differs considerably from patient to patient, and the stents with uniform design make it difficult to produce desirable outcomes. So, there is a need to develop stents with personalised designs for much improved clinical outcomes, and they can be fabricated by additive manufacturing (AM). However, AM process has a great influence on the microstructure, surface finish and mechanical properties of the stents and this has not been well studied yet. Thus, this thesis aims to investigate the microstructure and the mechanical properties of metallic stents produced through selective laser melting (SLM), especially a head-to-head comparison with commercial stents.

In this study, the microstructures of additively manufactured and commercial stents were investigated through optical microscopy, scanning electron microscopy, and electron backscatter diffraction. The surface roughness measurements before and after electrochemical polishing were acquired using non-contact focus variation microscopy. To characterise the mechanical properties, nanoindentation tests were performed at different load levels, with specific emphasis on the effect of grain orientation. In addition, spherical nanoindentation was used to generate indentation stress-strain curves based on load-displacement responses as well as to study the depth recoverability ratio for the nitinol stent. Also, crimping and expansion behaviours were investigated for 316L stainless steel stents using a balloon catheter with a measurement of diameter change against pressure.

The study revealed that the SLMed stent has a hierarchical grain microstructure composed of columnar grains and cellular sub-grains, as opposed to equiaxed fine grains in the commercial stent. Electrochemical polishing allowed for the removal of un-melted powders without altering the cell structure, although the surface of the SLMed stent was still rougher than that of the commercial stent. The hardness and modulus of the SLMed stents were higher than those of the commercial ones, with a grain orientation dependency. Also, the crimping and expansion process of the SLMed stent, in terms of stent diameter change against pressure, foreshortening, recoiling, and dogboning effects, were comparable to those of the commercial stent, indicating a good starting point and promising behaviour in terms of plastic deformation. The depth recoverability ratio was lower for the SLMed nitinol stent when compared to that of the commercial one, indicating a lack of pseudo-elasticity.

To summarise, the microstructures of the SLMed stent need to be refined and optimised in order to achieve mechanical properties comparable to the commercial stent. The cellular sub-grains contribute to the strength of the SLMed stents, which is beneficial for radial support of the stent after expansion. The SLMed nitinol stent also needs to be heat-treated to improve the most important pseudo-elastic behaviour required for a self-expandable stent. The data in this study can be used to support the further development of AM process for innovative manufacturing of future stents.