Miniaturization and increased complexity are the name of the game in the world of stent design and fabrication. But while stents are being cut down to size thanks to the use of such shape-memory materials as nitinol and such superalloys as cobalt-chromium, the question remains: How do you make such small, demanding devices?
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Strut Their Stuff
“In the early first and second generations of coronary stents, typical feature tolerances were ±0.001 in. or even more,” remarks Steve Burpee, COO of Burpee Materials Technology (BMT; Eatontown, NJ; www.burpeetech.com). “However, such systems could not treat vessels that are less than 2 mm in diameter reliably and consistently, especially if they are diseased.” But things are changing as delivery systems and stents get smaller, Burpee adds. “Thus, we’ve seen Abbott and Medtronic introduce sleeker, mini versions of their workhorse coronary stents in the last three years.”
The next generation of coronary stents, according to Burpee, will have features—including struts and links—that are nearly half the size of present-day stent features. This trend is being driven by the market opportunity created by the ability of coronary stents to access and treat smaller vessels. However, while this same trend is observable in the area of peripheral stents, there is less of an advantage to be gained by reducing the size of peripherals because they are used in much larger vessels than coronary stents.
How are OEMs achieving incredibly shrinking stent designs? The answer is machining, Burpee replies. “There is going to be a premium on faster and more-precise laser machining techniques and improvements in overall machining, which is basically laser cutting plus surface finishing. These processes can create more-uniform and more precisely featured stents than ever before.”
Stent Tasting Menu
Todd Dickson from Lumenous Device Technologies (Sunnyvale, CA; www. lumenous.com) agrees that materials and processes will play a decisive role in future stent design and manufacturing. “It all starts by designing against a specific disease state,” Dickson remarks. “And in a world of smaller and smaller medical devices, specifying the optimal raw materials for performance and durability has become increasingly essential.” To execute optimal designs, he adds, precision laser cutting and postprocessing operations take on greater importance for achieving special product shapes and high-performance surface finishes.
Comparing stent manufacturing to the culinary arts, Dickson notes that a manufacturer should have multiple ‘flavors’ of any given process in order to achieve optimal results. In addition, there should be several ways of parameterizing each flavor in order to meet customer demands. Take laser cutting, for example. “It’s possible to laser-cut metal using a visible-light frequency, an ultraviolet frequency, a near-infrared frequency, short pulses, or shorter pulses,” Dickson says. “One flavor would be the frequency, while another would be the pulse width. And it’s also possible to use an assist gas or a cover gas.” Blending the flavors will ultimately lead to a solution that delivers an optimal product.
“These different dimensions tree out into a wide variety of process inputs,” Dickson says. “The more expertise that prototyping and production teams acquire in each of these different flavor and parameter spaces, the more they will be able to control the production process so that it’s highly robust and reliable.” The goal is to optimize product performance, manage costs, and enhance quality by eliminating irregularities.
As medical devices continue to decrease in size, another important attribute is dimensional stability, or consistency, throughout the product. “With ever-shrinking stent dimensions, expanded process capabilities that can fabricate struts within very small standard deviations are becoming more and more critical,” Dickson says. “And this requirement is compounded by the trend of trying to eke out more performance from a smaller piece of metal. Surface-finish attributes on the outside diameter, inside diameter, and wall surfaces are important factors in the miniaturization trend.
For example, “Nitinol stents are increasingly called upon to perform truly extraordinary tasks,” Dickson comments. “They must stand up to very complicated stress states and cyclic loads that threaten to fatigue and fracture implantable devices. To make the designer’s task more successful in the face of such challenges, Lumenous delivers consistent, accurate strut dimensions—critical for keeping the stress states within a narrow design window—and produces ultrasmooth surfaces to reduce the likelihood of cracks. “This ability,” Dickson adds, “gives the designer an important margin of safety.”
Especially as medical devices continue to become smaller, the manufacturer should ensure that processes stay out of the way of the design as much as possible, Dickson says. Ultimately, they must also stay out of the way of designers so that they don’t have to create against the process but can design against the disease state. Precision laser cutting and postprocessing gives designers the opportunity to focus on improving human function. “What this means,” Dickson concludes, “is that manufacturers have to take certain steps to make sure that they’re as invisible as possible and that they put the lightest footprint possible on the final product from the point of view of variability.”