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Improving Laser Technologies Driving Big Changes in Little Stents

 

Multibillion-dollar stent industry is seeing a shift toward bioresorbable stents that favor ultrafast, pulsed lasers

 

By Ilene Wolff
Contributing Editor

For about 20 years, medical device manufacturers have used continuous wave lasers to make permanent metal stents, first out of stainless steel and then out of alloys like nitinol and cobalt chrome. Doctors implant the devices to prop open blood vessels or help with other medical conditions. 

But during the last two decades ultrafast, pulsed lasers have been perfected that can also produce stents made of polymers, which are bioplastics that dissolve in the body once they’ve done their job.

Because permanently implanted stents can cause inflammation, clot formation, new vessel plaque formation, and other unwanted side effects, bioengineers and other medical professionals are looking for medical technology that can reduce or eliminate these problems. Bioresorbable stents may just be the solution, but there’s no definitive answer because researchers are still testing them in humans.

Laserage, a job shop, has 15 dedicated laser systems for cutting stents.

While medical professionals design and test new kinds of stents in people—whether permanent implants or bioresorbables—laser makers are working to improve technology for making both types. “All the manufacturers of lasers that are going into industrial and medical applications, they’re all touting stents,” said Michelle Stock, president of mlstock consulting in Ann Arbor, MI. “Truly where the impact will come is when we get to the newer materials.”

That’s because there are so many uses for stents, and because there’s an unmet need for unusual configurations and smaller sizes.

  


How Small Can They Get?

Sascha Weiler, program manager for micro processing at TRUMPF Inc. (Farmington, CT), said that while it’s possible to cut a kerf 10–12 microns wide with good cutting quality, it’s impossible to get the waste material out of a space that small.

“The cutting kerfs are so narrow that the material won’t fall out,” he said. “With the laser you could focus down even tighter, but it doesn't give you any advantages.”

Blood vessels are the most common sites for stents, whether near the heart, in the carotid artery in the neck, or in a peripheral location such as the leg. But stents are also used in the fallopian tubes, the bladder and kidneys, the esophagus, the gut, the sinuses, the arm (for hemodialysis access), and more. Different body sites call for different designs, sizes and materials. For example, while the protected area near the heart doesn’t need nitinol’s shape-retention properties, a leg stent that is subject to everyday wear-and-tear does.

As the need for stents grows, so does the money to be made from designing and manufacturing them.TRUMPF Inc. said the green wavelength is best for cutting stents such as this.

According to ABMRG (Bristol, TN), a global authority on medical technology market intelligence, the global market for coronary stents alone is expected to top $13 billion by 2019.

“Small to medium-size companies from the developing economies are ambitiously conducting clinical studies to win the approvals and expand the use of devices for new indications,” said Tammy Thomas, ABMRG’s lead pipeline analyst.

Although small to medium companies may be frantically working on bringing new stents to market, Abbott Laboratories (Abbott Park, IL), Medtronic (Minneapolis) and Boston Scientific (Natick, MA) already dominate the increasingly global business. These three lead the global stent market, whether permanent or bioresorbable.

And even as medical device manufacturers continue producing metal stents, it appears that they are turning toward bioresorbables as their Holy Grail.

“The interest in reabsorbable stents is increasing,” said Dan Capp, vice president of sales development for Laserage, a job shop in Waukegan, IL, with 40 laser systems working on all types of devices. “Everybody is making them, from the three-man little companies to the big monsters.”

Absorbable stents have become so ubiquitous that in 2013 worldwide standards organization ASTM International (West Conschohocken, PA) established a new standard for testing them that addresses changes in their physical and mechanical properties, such as radial strength from initial manufacture to final degradation in the body. ASTM F3036, Guide for Testing Absorbable Stents, addresses pretest conditioning requirements, handling requirements before and during the test, and time-dependent mechanical property evaluations.


Metals

Despite the buzz over bioresorbables, manufacturers are still using lasers to make metal stents and doctors are still implanting them. Lasers work so well in the industry because they produce clean edges that reduce or eliminate the need for postprocessing. They also do away with the need for tooling for complex components, are able to process a wide variety of materials and are faster than other methods.

Of Capp’s 40 laser systems, 15 are dedicated to stent-making and run 24/5. He said he does have a favorite machine for cutting the tiny metal tubes.
“YAG lasers are excellent lasers for cutting metal stents,” said Capp. “We still use YAGs every day to cut stents.”

But some companies use a different laser that reduces some other, downstream costs that YAGs do not deal with.

Weiler said his company can cut metal stents with ultrafast, pico- and femtosecond lasers that operate without producing a heat-affected zone, which can cause burrs or other clot-inducing defects, if not outright rejection for use. That eliminates the need for postprocessing, which is very costly.

This bioresorbable PLGA stent, with a 200-micron strut width, was fabricated with a Spectra-Physics Spirit femtosecond laser.

Why is postprocessing so expensive? Weiler offers a suprising reason.

“It was explained to me that only the human eye in combination with the human brain can be trained to look at the stent under various angles and identify a burr or another feature that needs to be taken care of,” he said.

In other words, postprocessing for the metal stents has to be done by hand.


Temp Workers

The polymer-based bioresorables that manufacturers are now making break down inside the human body to water and naturally occurring chemicals such as lactic acid. Not all bioresorbable stents are made from polymers; they can also be made from zinc, magnesium or iron.

Commonly used polymers are polylactic acid (aka polyactide or PLA); poly(lactic-co-glycolic acid) (aka PLGA); and poly (L-lactide) (aka PLLA). They have the added advantage of being able to release medication inside the body as they degrade.

Because of their temporary nature, these stents are often called scaffolds. The term “bioresorbable” is interchangeable with “bioabsorbable” and “dissolvable.”

Now that companies like Abbott are conducting large clinical trials on dissolvables, laser manufacturers are racing to keep up with the demand for ultrafast, pulsed lasers that can work on all manner of new polymers. And the laser-makers are not shy about pushing their new machines.

Laserdyne’s Capp said: “They’re spending a ton of money on it, they really are. So we get beat up all the time, why aren’t you buying our pico-laser, why aren’t you buying our femtolaser?”

But it’s not just about buying new machines when switching from metal to polymer production. It also involves a learning curve for the stent manufacturers.

“I think the main challenge is the polymers tend to melt, so they can be damaged easily,” said Herman Chui, senior director for product marketing at Spectra-Physics (Santa Clara, CA). His company was the first and is now the No. 1 producer of ultrafast, pulsed lasers, which can solve that problem, Chui said.

While longer-pulse lasers melt the material they’re focused on, shorter-pulse lasers avoid that problem by driving electrons into a plasma plume. The laser energy enters the material and departs almost instantaneously, before it can be transferred within the material as heat. As a result, machining with an ultrafast, pulsed laser is often referred to as “cold machining” or “cold ablation.”

In fact, the new approach is crucial to the emerging dissolvables industry: The polymers have such low melting points that before the newer generation of ultrafast, pico- and femtosecond pulsed lasers were available and reliable, scaled production was all but impossible.

The machines have steadily improved. Ultrafast, pulsed lasers had a reputation for reliably breaking down through the early 2000s, which led many to predict they’d never be used on a shop floor. But by the mid-2000s ultrafast lasers had been developed to meet the demands of industrial production.

“They’ve come a long way,” said TRUMPF’s Weiler, who said his company’s best polymer tube-cutting machine is the TruMicro 5250. “There’s no difference now in reliability.”

There are other considerations too, including wavelength.

Weiler said:  “As it turns out, these polymers absorb the green wavelength the best.”

He agrees that there’s a formidable learning curve, even after purchasing the right laser.

Chui said proper process techniques—moving the laser beam quickly and using the right assist gas, if necessary, to cool the work area and remove debris in the form of a vapor plume—are critical.

And while laser makers tout elimination of postprocessing, there are no guarantees.

“Almost nobody wants to postprocess bioabsorbable stents, because by their very nature they’re very susceptible to their environment,” said Capp, pointing out that the metals can corrode and the polymers degrade.

Capp explains that by using the correct energy, beam shape and pulse characteristics, an operator can reduce or eliminate the need for postprocessing.

But if you don’t do it right, “You end up with [evidence that] the laser was here,” said Capp.


Cost, Size and Speed

Just like their slower cousins, ultrafast, pulsed lasers have their pros and cons, including cost considerations, size, and processing speed. Not surprisingly, each technology has its proponents and critics.

While costs have come down, there’s a big jump in price from nanosecond lasers to the ultrafast, pulsed picosecond and femtosecond lasers, said consultant Stock. How big a jump? The newer type lasers are about 10 times the cost of nanosecond lasers, she said.

Darren Kraemer, president of Attodyne Inc., a Toronto-based picosecond laser maker, said: “The reason they are typically more expensive than their nanosecond cousins is because of additional components.”

Spectra-Physics determined that using a green femtosecond laser with helium-assist gas produced the best results for machining PLLA.

For example, Kraemer said, a femtosecond laser’s added chirp pulse amplifier stretches a short pulse and then compresses it, an unnecessary step in his view.

“With a picosecond laser you don’t need to stretch and compress,” he said. “As long as your laser pulse is less than 10 picoseconds, it doesn’t make a big difference.”

In addition, Attodyne has reduced the cost of its picosecond lasers by using the best of fiber-based and diode-pumped, solid-state technology without a lot of extra components that drive up price.

“Our goal is to essentially bring the cost of picosecond lasers down to or below the cost of an ultraviolet, nanosecond laser,” said Kraemer.

Weiler agrees that the cost for the faster lasers will always be higher due to additional components needed to create the pulse; however, he has a strong rationale for why his company uses them anyway.

“Yes, the equipment may be more expensive, but due to the savings you get from [eliminating] postprocessing, the cost is actually lower [per part],” he said.

Also on the upside, ultrafast, pulsed lasers are more compact (especially fiber-based ones) and more efficient (especially fiber-based, which are 10% more efficient than crystal-based lasers, said Stock). The size of a femtosecond laser, for instance, has gone down from that of an old-fashioned luggage trunk to about the size of an inkjet printer, she said.

The smaller size is important when it comes to issues related to integrating the laser into a closed platform for industrial use.

The fastest lasers are also quite versatile.

“They basically can work with any material,” said Stock. “That’s one of the best things about the ultrafast laser.”

Ironically, however, processing stents with an ultrafast, pulsed laser requires patience.

“It goes pretty slowly,” said Stock. “You’re just taking little bites.” That can be a problem because, “When you’re manufacturing anything, speed is important.”

Weiler agrees that speed is important, but said the new technology has caught up: As recently as three years ago, manufacturing with fiber lasers was two to three times faster than with the new, ultra-quick machines, “but now they’re the same speed or even faster.”

Capp, meanwhile, insists that cutting a nitinol stent would take longer on the ultrafast pulsed systems, perhaps three to five times longer then the traditional Nd:YAG or fiber-laser system.

But the beauty of eliminating postprocessing has Laserage customers asking that their stents be cut on the newer technology.

“I tell them no,” he said. “The little bit of money you save on postprocessing isn’t worth it.”

Capp said the answer to bringing down the process time is to increase the laser’s energy density.

“Tunability—now you’re talking about a crescent wrench where you can change the diameter.” ME


This article was first published in the May 2014 edition of Manufacturing Engineering magazine. Click here for PDF


Published Date : 5/1/2014

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