Laser microprocessing of metallic stent for medical therapy☆
Introduction
In medicine, coronary artery obstruction is a major problem, which leads to complicate problems of haemorrhage eventually leading to bypass surgery. In order to treat this coronary artery obstruction, usually a standard angioplasty technique is applied. But its clinical efficacy is limited by acute vessel occlusion and restenosis problems in the first 6 months. To reduce these deficiencies a new clinical therapy has been introduced with the implantation of metallic cardiovascular stent with adequate radio opacity. Metallic stent is typically a hollow cylindrical tube (d = 2–4 mm; l = 15–20 mm) with a patterned slit structure in two or three segments. It maintains the flow of blood in the damaged vessel, thus avoiding it from being ruptured. The study shows [1] that stent geometry, i.e. diameter and mesh density produced with different porosities, defined as a percentage of metal free unit area per total unit area of the stent are most important for long term successful results. Typically, the porosity varies between 76 and 85% values and actually offers regulatory resistance to the blood flow in the vessel. These mesh/fine slit structures are usually created with the electric discharge machine, which takes considerable processing time, but recently with the application of laser technology, the process has become relatively much faster. Various biomaterials, such as stainless steel, nitinol, platinum, titanium and tantalum alloys and gold or polymer coated stents have been used for such applications. The present paper describes the precision fabrication of metallic stent from stainless steel (SS316L) by using short pulse Nd-YAG laser.
Section snippets
Fabrication technology
Until now several methods [2] have been used for metallic stent processing, but the laser processing methods has various advantages over the conventional etching and electroforming techniques. In the etching technique, the formation of ‘etching lip’ may lead to undesirable side effects, whereas in the laser processing method which produces the straight edge with desired taper [3], results in a uniform fluid flow in the blood vessel. In the recent, past several configurations of Nd-YAG lasers in
Experimental procedure
The experiments were performed using a short pulse Nd-YAG laser as shown in Fig. 1. The typical process parameters are given in Table 2. Prior to processing, CAD data of the stent with the desired configuration is generated, which is further analysed by FEM for its optimum performance, considering the material characteristics, fluid flow and various forces acting at its nodal points. Finally, this data is fed to the laser processing system. To fabricate the metallic stent (SS316L), first the
Dross adherence and removal process
During the laser processing of the stent, one encounter with the problem of dross, i.e. burrs and spatter adherence to the underside of the cut (Fig. 3B). This is due to the three reasons: (a) temperature gradient caused by the laser beam from top to bottom surface of the work-piece; (b) beam divergence from top to bottom results in larger kerf width underneath as a result more material is melted in the lower side and (c) with the increase in depth of the cut, gas jet becomes turbulent and its
Heat affected zone and roughness
Conventionally photo-etching, electroforming or micro electric discharge machining (μEDM) produced stent, offers burr free smooth cut-quality with low roughness, but in the first two techniques it is not free from ‘etching lip’ which may lead to undesirable side effects. Besides that, the process is slow and cost expensive compared to the laser technique. In the laser processing method which produces the straight edge with desired taper, results in a uniform fluid flow in the blood vessel. But
Conclusion
In summary, it is concluded that metallic cardiovascular stent is now a widely accepted therapy to treat coronary artery obstruction. To achieve this goal, the precision fabrication of metallic stent using the short pulse Nd-YAG laser has now become a viable technique to create such fine mesh structures with slit width of 0.05–0.1 mm which are required for various applications in stent therapy. It was shown that the processing of stent with desired taper and quality shall still be preferred by
Acknowledgement
The author wishes to express his gratitude to Prof. S. Sugiyama of Ritsumeikan University for his kind support.
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Part of this work was presented at the 17th International Congress in Laser Medicine: Laser Florence, October 28–31, 2002, Florence, Italy.
- 1
Formarly at: Laser X Co. Ltd., Chiryu-shi, Japan.