The use of stent to keep open a blood vessel or other body lumen in the human body has become a very effective therapy for treating blood vessel stenosis and lumen obstruction. For example, using one or more stents to treat a coronary or peripheral artery blockage has become a common practice. Stents have been successfully used in keeping passageways open such as the diseased prostate, urethral, the esophagus, the biliary tract, and intestines. There are two types of stents that are widely used and/or studied nowadays: metallic stents and bioabsorbable polymeric stents.
Flexibility of the stent is one of its important characteristics. A stent has to be flexible in their crimped state in order to facilitate its delivery to a targeted lesion, within an artery. In some cases, a stent also has to be flexible in its deployed and expanded state, especially when implanted in a location where the stent is subjected to flexing or bending, axial compressions and repeated displacements at points along its length. This can produce severe strain and fatigue, resulting in failure of the stent.
Various fabrication methods, such as laser cutting, braiding, and thermal forming have been used to produce stents with various structure designs. Once a material is chosen, the mechanical properties of the stents are largely determined by the stent design structures, including the patterns in which the struts and bridges of the stents are linked together.
One of the primary goals of stent designs has been to insure that the stents have sufficient radial strength so that, when it is delivered to the intended treatment location and expanded, they can sufficiently support the lumen. Stents with high radial strength, however, tend to have a higher longitudinal rigidity or lower flexibility. When a stent has a higher longitudinal rigidity than the vessel in which it is deployed, there is a higher chance that the rigid stent will cause trauma to the vessel at the ends of the stent, due to the stress concentration caused by a mismatch in compliances between the stented and un-stented sections of the vessel. Furthermore, when deployed in certain applications that are subjected to substantial flexing or bending, axial compressions and repeated displacements at points along its length, for example, when stenting the superficial femoral artery, longitudinally rigid or less flexible stent can undergo severe strain and fatigue, resulting in fracture or failure of the stent.
Another important aspect of a stent is its capability to be crimped and then expanded without damaging its structure integrity. Stent structure played a vital role in its structure integrity after expansion.