Fiber Lasers Used in Medical Stent Manufacturing
Anne L. Fischer
The use of medical stents is now common practice in minimally invasive surgery. Inserted into the body through a catheter and left in place, they are employed in procedures such as the treatment of peripheral vascular disease, in which the stent is used to open clogged arteries, to administer drugs or to help make a diagnosis.
Stents look like a woven mesh and are made of materials such as stainless steel, cobalt chrome and nitinol. They present unique challenges in fabrication to achieve the required geometries. Stent manufacturers often take a tube 0.5 mm in diameter and remove 90 percent of it, leaving a very thin support structure. This involves detailed cutting that must be of the highest possible precision because of the value of the end product.
A 0.013-in.-diameter stainless steel tube with a 0.0004-in. wall thickness is seen magnified 100X. The kerf is 20 µm wide and is cut as a variable-pitch spiral. Courtesy of LPL Systems Inc.
Stent manufacturers used conventional flashlamp-pumped solid-state lasers for this application. The complex task could take minutes to complete, however, and the lasers did not offer the stability to ensure the yields necessary for this high-cost, high-volume process.
Thus, LPL Systems Inc. of Mountain View, Calif., which produces laser-based machine tool systems for the fabrication of medical devices, turned to fiber lasers from SPI Lasers plc of Los Gatos, Calif., to cut stents, primarily to reduce maintenance costs. Advantages of fiber lasers over flashlamp-pumped solid-state lasers include high stability, a small footprint, low maintenance and energy efficiency.
Fiber lasers typically exhibit a fluctuation in power of less than 1 percent. Because stents tend to be manufactured in small laboratories, the 19-in.-or-smaller footprint of a fiber laser is an advantage over the solid-state laser, which includes a separate laser head and power supply and occupies up to four times the space.
The fiber laser can make extremely intricate cuts, as shown by this 20-µm cut seen magnified 1500×.
In cutting performance, a fiber laser is not faster than a flashlamp-pumped solid-state laser, but it does cut more cleanly, said Russell Denton, LPL’s director of operations. There is less “slag” after cutting, greatly reducing the postprocessing time required to clean a stent, he explained. Most stents take at least four or five minutes to cut, but in some applications, LPL customers were taking as long as 45 minutes. By using the fiber laser, they increased their speed by three to five times.
The company has had success cutting all the major material types from which stents are manufactured. Nitinol, for example, a nickel/titanium alloy shape memory metal discovered in 1959 by William J. Buehler of the US Naval Ordnance Laboratory, is one of the most challenging for a laser to cut, but the company has demonstrated good results with a fiber laser.
Cost savings are found in the reduced need for consumables in fiber lasers. Flashlamps, for example, have a typical lifetime of 1000 hours, so they must be changed several times a year at a cost of $275 to $350 per lamp, according to Denton. This also results in machine downtime, an added expense. Some customers using pulsed lasers must change lamps every day on a rotation basis to maintain their production schedule. This is a considerable maintenance and cost issue.
Fiber lasers also do not require the lenses, telescopes or other optical add-ons that flashlamp-pumped lasers need to focus the beam to small spot sizes. However, LPL President Richard Press noted that some additional optics often must be used with a fiber laser to get the variable spot size necessary when following the complex geometry of particular stents. Because of this, LPL has not abandoned flashlamp-pumped lasers.
Press sees energy savings as one of the greatest advantages of the fiber laser. Energy efficiency comes from the diodes, which are spliced directly into the pumping fibers. These provide a much more efficient pumping source than flashlamps because the power does not have to be ramped up over time to maintain constant output power, as often happens with traditional lasers. At LPL, the flashlamp-pumped lasers consume 220 V at 30 A, whereas the fiber lasers use 110 V at 6 A or less.
Also, flashlamp-pumped lasers must be constantly water-cooled, while fiber lasers require only air- or water-cooling to keep the pump diodes at a consistent temperature so that their output wavelength is in the optimum range for pumping the lasing fiber.
As a result, in real-world use, the company has worked with 50-W diode-pumped fiber lasers that Denton said equal the performance of 100-W flashlamp-pumped solid-state devices.
The bottom line with stent cutting is product reliability. Fiber lasers meet the challenge of producing a consistent product, and the up-front cost is comparable to that of a flashlamp-pumped laser. The savings are found in the reduced postprocessing time, and in lower maintenance and energy costs down the road.
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