The latest developments in short-pulse and fiber lasers offer new ways to create today’s innovative medical devices
Lasers cut, weld and mark the metal alloys and polymers used to build the latest medical implant breakthroughs, surgical instruments, and other related medical products. Many key characteristics of today’s lasers make the devices ideal for use in medical applications such as cutting stents and endoscopes, pacemaker components and other medical components.
Laser cutting processes offer many advantages, including being a noncontact method of cutting that has minimal thermal input into the workpiece while delivering extremely precise spot sizes, or tool widths, on components requiring high precision and high quality. Lasers are used to process a wide range of materials from stainless steel to Nitinol, a shape-memory alloy, and titanium, the preferred alloy for implantation into the human body, and polymers used in the high-volume fabrication of microfluidic devices.
Picosecond Lasers Showing Promise
Many lasers currently deployed are either CO2 or fiber lasers, with fiber gaining converts in recent years as prices for fiber lasers have fallen. Both these types of lasers use a fusion cut that melts metals efficiently while using nozzles to deliver a gas, either oxygen, nitrogen, or even shop air, to blow away particles from the cutting area. A newer class of extremely short-pulse picosecond lasers also is starting to be employed, particularly for cutting stents, with a cold process where the laser vaporizes the material instead of melting it.
The manufacturing of stents is an important area in medical device manufacturing, notes Sascha Weiler, program manager, Micro Processing, Trumpf Inc. (Farmington, CT), but lasers are used in many aspects of medical manufacturing. “You can divide that into three applications: welding, marking, and cutting. These can include welding of pacemaker housings, endoscopes, parts that require clean and smooth welds, which go into the human body. And marking, it’s the same story, marking pacemakers, surgical instruments, or hearing aids, which are made of many materials.”
In stent cutting, the state-of-the-art cutting method still is the fusion cut with mostly 100 to 200-W fiber lasers, Weiler adds. “This works well for stainless steel, maybe for Nitinol, because here and there you can allow some postprocessing, and you need the postprocessing because it’s a fusion cut, where the laser melts the metal and you have gas nozzles that blow the metal out of the cutting surface.”
With fusion-cut stents, cutting kerfs typically are 10-20 µm, he notes. Trumpf offers fiber lasers, as well as newer short-pulse picosecond lasers in the green wavelength specifically aimed at cutting stents, which have relatively thin walls and don’t require higher power lasers.
“For stent cutting, we typically use a green picosecond laser. The reason is because it’s more versatile—it can cut a metal stent as well as polymer and nonmetal stents,” Weiler states. “This is very different from the fusion cut. Picosecond pulses are so short that the material is not molten, like with the fusion cut, it’s vaporized. So now we don’t have any metal, we just have vapor, so we just need some kind of nozzle to blow away the vapor—people call it ‘cold cut’ or a cold process, and that eliminates any residual heat in the material, which results in perfect edge quality, and especially for the nonmetallic stents, they cannot be cut with a fusion cut at all, the cutter would just melt the whole thing.”
Picosecond lasers like the Trumpf TruMicro Series 5000 laser line can vaporize metal or nonmetals using 50W of power and a pulse energy rated at up to 250 microjoules. These short pulses of less than 10 picoseconds vaporize the material so fast that no heat-affected zone (HAZ) can be detected. Microprocessing applications for the TruMicro 5000 line include cutting, structuring, ablation and drilling. “That’s a short pulse, actually high intensity, because in the peak of the pulse, you’ve got the power of multiple tens of megawatts,” Weiler observes. “It’s like a very tiny, but powerful hammer. And you’ve got like 800,000 of those hammers a second. This is how it works, this is how you get the productivity.”
The Trumpf TruMicro 5000 line comes not only in different wavelengths, but also different power levels, adds Weiler, noting that the TruMicro 5250 green picosecond laser is the best bet for this type of medical work. Trumpf, which launched its first picosecond lasers in early 2008, is now offering its second-generation picosecond lasers for microprocessing applications. “I’d say for the cutting nonmetals in medical, truly it’s an enabling technology. You couldn’t do that before, with this kind of quality.
“At the beginning, it was a testing element—no one was sure it was going to take off,” Weiler recalls of picosecond lasers. The technology is used not only in medical, but for a wide variety of applications, he adds, including semiconductors. Key to the success of these short-pulse lasers is the quality of the cut. “If the quality isn’t there, it means it must be somehow post-processed, and believe it or not, it’s done manually. There are guys sitting there with a microscope and checking if they see anything on the stent, and then they do sandblasting or brushing, and check again. And it is very time-consuming and a lot of labor.”
Hand finishing of stents involves small sandblaster tools used with microscopes to spot any imperfections in the stent, he notes. “Camera vision is literally impossible to implement. The human eye, you can teach it. You have to somehow hold the stent under the microscope and play with it, and then you see the obstacles. You can do it manually, but it’s very difficult, and a lot of stents are made out of Nitinol, which is 50% nickel and 50% titanium, the shape-memory alloy. It’s a very expensive material, and you waste a lot of money.”
Fine Laser Cutting
Laser cutting technology’s advantages play to medical manufacturing’s requirements for high cut quality in a noncontact, highly flexible process, notes Geoff Shannon, laser technology manager, Miyachi Unitek Corp. (Monrovia, CA), a developer of laser welding, marking and cutting systems. “In recent years, there’s been a lot of movement in medical to laser welding for the obvious reasons—noncontact, very flexible, highly controllable, and you can weld really small parts, which is obviously very good for the medical industry.”
Medical device builders have shown increasing interest in lasers for cutting many different devices, from cannulars, needles, arthroscopic devices, endoscopes and other products, Shannon notes. “It’s all minimally invasive tooling, so the tools are very thin tubing, and normally they need to have some slots or holes for other things to either attach or kind of be fed through,” he says.
In the medical industry, there is some interest in picosecond lasers, Shannon says, adding the technology can offer a unique processing capability for metals and plastics though it is somewhat at the industrial R&D stage. “Companies are looking at it because the picosecond laser offers high quality and fine dimensions that are not attainable with other lasers, but they have really become industrial tools rather than lab devices over the last three or four years.
“One of the considerations with the picosecond lasers is that they are certinaly not cheap, with a system costing you around half-a-million dollars. However, the $64,000 question is does the laser offer such a unique process that the product improvement can justify this level of investment?”
Fiber lasers are the focus for Miyachi Unitek cutting systems, which typically use a 100 or 200-W single-mode fiber laser for medical cutting applications, Shannon says, noting that the lasers offer the ability to cut parts with diameters of about 0.050″ to 0.25″ (0.13–6 mm) with wall thicknesses of about 0.003 to 0.020″ (0.076–0.51 mm). “This represents quite a broad range of cutting capability, and there’s a lot of different features that you can do with lasers,” he says, “such as the single-sided slots, windows, on axes and off-axes features and spirals for many flexible shaft, hypotube, and cannula applications.
“Traditionally the medical industry has used a lot of EDM technology, both wire EDM and sinker EDM, and while wire EDM is still quite prevalent, sinker EDM is normally used for single-sided features,” he adds, “and it is much, much slower than laser cutting—significantly slower than laser cutting.”
With wire EDM, medical manufacturers can cut out teeth on arthroscopic tools for knee surgery, he says. “If you have symmetric teeth on both sides of the tube, wire EDM is great, because it’s like a cheese cutter, and you can rack up 10 or 15 of these parts and you can cut more than one at once. So the advantage with wire EDM is, in some instances, you can gang up multiple parts, so although the cutting speed is slow, you’re cutting 15 parts in one go.”
Lasers can do the same thing, but only on one part at a time, and it offers features that you can’t attain with an EDM process, Shannon says. “It offers the capability to give you different angles on different geometries that they cut, and being able to cut 3-D shapes in the tube without having to take the part out, re-tool it, put it back in, or take it over to another machine,” he adds. “So there are some advantages there for the lasers in multiple areas, particularly in the sinker EDM, which is probably four to five times slower, if not more, than laser cutting. When you look up the price per part, it’s significantly more than laser cutting, and that’s obviously driving a lot of the changeover.”
Price Pinching in Medical
The medical industry has always been cost-conscious but in recent years it has become much more so. “Perhaps four–five years ago, it probably wasn’t quite as aggressive as it is right now,” Shannon notes. “What’s also driving laser cutting is the high-speed/high-quality capability—laser are very flexible and cut lots of materials, thicknesses, shapes and high-mix parts.”
Allan Callander of Prima Power Laserdyne LLC (Champlin, MN) concurs with that assessment, noting medical builders are looking for cost-effective, validated manufacturing processes. “There used to be a perception in medical device technology that price was no object. That has changed,” Callander states. “The engineers have to think of this right from the start.”
Medical device makers need stable, proven manufacturing processes, he says. “It’s absolutely a renewed emphasis on cost, and increasing emphasis on process validation. They work so hard to make sure that the process is validated and can meet FDA requirements.”
For medical, Prima offers its Laserdyne 430 fiber laser system along with its System 94 software and hardware that is validated right from the start, Callander says. “The 430 is designed for lasers that are either directly or fiber-delivered. It is also possible to use a lot of the same system components and deliver the wavelength that best suits the application. For example, a laser with a wavelength in the green is ideal for precision cutting because of the high absorption and ultrafine spot sizes. We can change the optics, but the basics and the machine control don’t change.
“The micromachining of coronary stents was a key aspect of the medical system device world,” he adds. “It opened up a lot of eyes on what could be done with lasers.” While the ultra-short-pulse lasers have appeal, those systems are still coming up short, he notes. “They’re not too cost-effective, because they’re still too slow. A typical 10 or 12-mm stent can be cut in minutes with a fiber laser.”
For most medical applications, high-precision cutting is a given on parts requiring the highest accuracy possible. With the growing numbers of aging Americans requiring ever-increasing joint replacement surgeries, the medical market remains strong, notes Mazak Optonics’ Keith Leuthold.
Mazak Optonics’ STX 44 laser is specifically aimed at medical products that can be accommodated on the unit’s 4 × 4′ (1.2 × 1.2-m) table, he says. “What makes this machine unique is that it’s a precision ballscrew,” he says. “When dealing with customers’ medical components, they not only need high precision but stability.” The STX 44 system features a cast-iron frame made of Meehanite, he says, offering stiffer properties than standard granite. The biggest problem of any machine is the vibration, and the size of a laser beam is about four to six thousandths in diameter,” he states. “The precision on how to focus that beam is crucial. A human hair is about two thousandths.”
Hybrid Lasers Cut Bone Saws
With the latest-generation hybrid HVII laser system from Mitsubishi Laser/MC Machinery Inc. (Wood Dale, IL), one medical customer cuts saws used in knee replacement surgeries, notes Jeff Hahn, Mitsubishi Laser national product manager. The hybrid laser offers better accuracy and a simpler machine tool design, with the stability inherent in the system’s cast frame made from Dianite, which Hahn says is Mitsubishi’s version of Meehanite. “Generally, the hybrid has the best accuracy,” Hahn notes. “Bone saws are generally a very intricate piece, and it’s nothing that you’re going to go fast at. But with the 2-D hybrid 2-kW laser, in this application, it’s cutting thin stainless at about 0.0060″ (0.152-mm) thicknesses, and when you’re using nitrogen-assist, it will give you a low HAZ.”
The HV II machine, which Mitsubishi demonstrated at Fabtech 2011, features the company’s latest M700 series control with a 15″ (380-mm) color touchscreen display. The HVII series is available with either a 2, 3, or 4-kW laser resonator. Travel in the Z axis is two times faster than previous models, according to the company, resulting in cutting part-to-part transition times in half. Larger stroke in the Z axis allows preformed parts to be loaded in and out. The machine also offers an attached tool storage center, a fine-pierce option, and a diminished clamp zone for increasing parts production and quality. ME