Influence of Material Quality on the Performance of Peripheral Stents

This paper presents results of the evaluation of standard- and high-purity (e.g. extra-low inclusions (ELI)) material qualities from four established international tube suppliers. The tube materials were characterized in the as-received and heat-treated condition regarding microstructure (quantitative secondary-phase analysis based on ASTM E45, E1245, E2283, E768; ISO 4967; and DIN 10247), Af-temperature (ASTM F2082), mechanical properties (ASTM F2516) and surface characteristics. It has to be pointed out that the differences are related to a combination of chemical composition of the ingot materials (impurity content such as TiC- and Ti4Ni2Ox particles) and the differences in tube manufacturing processes (e.g. hot- and cold-working steps, degree of cold work, surface finish etc.).

Subsequently, US FDA-approved peripheral Pulsar-18 stents of Biotronik were produced from the different materials, using identical manufacturing processes. The stents were evaluated by measuring Af-temperatures and visual inspection using optical microscopy and scanning electron microscopy (SEM). Additionally, stents were benchmarked in axial fatigue-to-fracture tests (ASTM F2942 and F3211), comparing the fatigue resistance (i.e. number and locations of strut fractures).

This work will help to answer the question, if higher purity material qualities are making a difference on the fatigue performance of an approved medical device, because all previously published papers only looked at raw material or semi-finished products. It has to be noted that the fatigue tests were conducted at hyper-physiological conditions and that the evaluated stent does not fracture under in-vivo conditions within the required life time of 10 years in the patient.