The blood circulatory system in the human body is associated with a pulsating blood flow which results in cyclic widening of the vessels in the rhythm of the heart beat. For a medical stent, which is supposed to stabilize vessels, this results in cyclic mechanical loading, which can lead to fatigue phenomena, such as fracture of strut elements of the stent. It is important to understand the elementary deformation and damage processes which govern the fatigue of microstents. [1] We have previously shown that fatigue life is affected by surface-related defects such as intermetallic surface particles, laser burrs, and electropolishing artifacts. [2] The purpose of the present work is to document that mechanical cycling of microstents also affects the microstructure. The mechanical experiments performed in the present study are not designed to simulate in vivo conditions of microstents. Instead, the purpose of this study is to contribute to a better understanding of the role of microstructure during mechanical cycling of microstents. Materials and Methods Microstents and Phase Transition TemperaturesPseudoelastic NiTi microstents with a length of 20 mm and an expanded outer diameter of 3 mm were obtained from Abbott Vascular Instruments Deutschland GmbH (Rangendingen, Germany). The stents had a Ni content of 50.8 at.%. Stent struts, the elementary building units of the stents, have rectangular cross-sections of 90 Â 112 mm 2 . The phase transformation behavior of the as-received microstent can be characterized by differential scanning calorimetry (DSC) using a DSC 2920 from TA Instruments. DSC experiments were performed according to ASTM-2004-03 in the temperature range of AE 150 8C at a cooling/heating rate of 10 K Á min À1 , with 3 min hold times at the highest and lowest temperatures. [3] In the as-received condition, the stents showed a two-step martensitic transformation upon cooling from B2 to R phase and from R phase to B19 0 . Upon heating, a two-step reverse transformation back to austenite is observed, as shown in Figure 1. An A f temperature of 21 8C suggests that the material is fully austenitic at room temperature. After 30 million fatigue cycles, the phase transformation temperatures increased slightly and the peaks are broader and less pronounced. Similar observations were made by Grossmann et al. [4] with spring actuators, where it was found that dislocations accumulate in the microstructure duringIn the present study, we investigate the fatigue behavior of Nickel Titanium (NiTi) microstents at 22 8C (room temperature) and 37 8C up to 30 Â 10 6 load cycles. We briefly describe our test procedure, which applies displacement-controlled pull-pull fatigue cycling between displacements corresponding to apparent strains of 5 and 7.5%. The response of the microstents to mechanical loading indicates cyclic softening during 30 Â 10 4 cycles. Subsequently, the maximum load remains constant throughout the remainder of the test. We use transmission electron microscopy (TEM) to clarify the microstructural reasons...