Lasers Cure Ills in Medical Manufacturing
They “soothe the pain” caused by new materials, tighter tolerances, more stringent regulations, and greater cost pressures
By James R. Koelsch
Conventional processes don’t always cut it at Stryker Leibinger Micro Implants (Portage, MI). Look at the production of the medical-device manufacturer’s titanium bone plates. Not only can the titanium material be tricky to process by conventional methods, but the 3-D geometries of the plates also are difficult to produce with the surface quality necessary for biocompatibility. The surfaces must be smooth—free of both burrs that can damage tissue and crevices that can provide places for germs and residual materials to collect.
And the challenges don’t stop with the stringent specifications. Stryker also faces the normal pressures to produce quality products at competitive prices. Because surgeons use the company’s titanium plates to mend broken bones of various shapes and sizes, another challenge for its craftsmen is to produce the complex, smooth surfaces economically in a large mix of sizes in lots as small as one.
To do the job, they need tools that are more accurate and flexible than those used in conventional processes like punching, flame cutting, or nibbling. Consequently, Stryker has become one of the many medical manufacturers to replace some of its conventional cutting and welding processes with lasers. Not only do lasers boost the quality of cuts and welds, thereby eliminating the need for secondary clean-up operations, their flexibility also makes it easier to keep pace with frequent changeovers, continual technological advancements, stringent governmental regulations, and growing pressures to control costs.
To cut its system of titanium bone plates, Stryker uses a five-axis LASMA laser from Trumpf Inc.’s Laser Technology Center (Plymouth Township, MI). Using flexible laser-light cables to transmit the beam, the machine manipulates the cutting head in three linear axes and two rotary axes, directing the beam along the cutting path over the plates’ contoured surfaces. It generates a beam with Trumpf’s 100-W (average power) HL101P resonator and focuses it into a 75-µm spot on the cutting path. Because the cutting action is fast, the heat-affected zone falls within 5 µm, and does not need to be removed.
The laser produces fine, burr-free cuts much faster than conventional mechanical processes could for the small lots of the various sizes that Stryker produces. Thus no secondary finishing operations are necessary to remove burrs or smooth sharp edges. Changeover is fast, usually involving clamping the workpiece and selecting the appropriate laser program. “The variety of part geometries and angles that can be laser-cut is nearly unlimited,” says Alexander Knitsch, an application manager at Trumpf.
Lasers can cut and weld a variety of workpieces quickly and cleanly because they direct large amounts of energy onto very small areas. Although spot diameters for q-switched lasers can be as small as 10 µm with fixed optics and 25 µm for scanning optics, “typical spot sizes for medical applications are in the range of 100–400 µm,” says Knitsch. So, he estimates kerfs to range between 15 and 50 µm and welded seams to be less than 100 µm.
The extremely high energy density of the beam melts and vaporizes the material so fast that there is not enough time for much heat flow into the surrounding regions. Consequently, heat-affected zone is typically 5–30 µm for welding and 2–15 µm for cutting, which permits processing temperature-sensitive materials like Nitinol (nickel titanium alloys) and working near electronics and other heat-sensitive components.
“Another positive effect is that lasers allow radial and nonradial cutting of little tubes with diameters of 0.2–0 mm and wall thicknesses of 0.02– to 0.3 mm without damaging the opposite wall,” adds Knitsch. Although a little distortion can occur during welding, it is difficult to quantify. “It depends on weld width, weld depth [which depends on spot size, power and energy per pulse], material, and part geometry—among other factors.”
Lasers also have the twin benefits of being both highly accurate and flexible. Not only can narrow laser beams cut or weld intricate details containing small corners, but they also can be positioned within a micron with one-micron repeatability, making accuracy and repeatability dependent mostly on the machine’s positioning mechanism. Lasers can process most of the materials used in the medical industry—stainless, platinum, gold, and titanium. Being nothing more than highly focused monochromatic light, lasers lend themselves to computer control, and they accommodate high mixes of small lots.
Over time, these characteristics have elevated the status of lasers in the medical industry. “Both manufacturing and design engineers no longer consider lasers to be a last-choice process,” notes Mark Barry, vice president at Laserdyne Systems (Champlin, MN), a unit of Prima North America Inc. “For early applications, we were often given problems that could not be accomplished by conventional machining or welding. Today people come to us with manufacturing challenges, looking to lasers as a first choice.” Users and vendors alike are drawing on the experience that they have gained since the early days of applying the technology.
Sometimes, an innovative application in manufacturing unleashes the creativity of designers. “Once manufacturing engineers can show that they can do a particular process, design engineers will say, hey, if you can do that, let’s do it on this,” explains Barry. The classic example of this kind of progressive improvement is the 3-D machining required to create textured surfaces on the portions orthopedic implants that surgeons insert into bones. The texture allows the surrounding bone to grow into the implant’s surface and fuse the two pieces together. Early patterns were simple designs. Designers are now challenging Laserdyne’s engineers with progressively more intricate designs on complex contours.
The texturing operations are almost exclusively done by YAG drilling lasers, but Laserdyne’s engineers specify both YAG and CO2 lasers in other 3-D cutting applications, such as machining the various sizes of bone routers and other surgical instruments that vendors carry in their catalogs. Although one style of laser usually works better in an application than the other, often the difference is not dramatic. So deciding between them often depends on other considerations, such as deciding on how universal the user wants the system to be, or accommodating the user’s preference.
“A good rule of thumb is that if you have and understand one particular system, you’ll continue with that,” offers Barry. The operators and maintenance technicians may already know a machine, and sticking with the same technology reduces spare-parts inventories. Moreover, “your quality people have seen this process, these types of parts before. It’s easier to write and understand the specifications for quality if you understand the system.”
Stent manufacturers also have put builders of laser cutters through their paces. They want narrower kerfs, tighter corner radii, smoother cuts, and smaller heat-affected zones to improve products, reduce postprocessing costs, and boost production rates. To respond to these demands, builders of pulsed solid-state lasers have had to boost pulse repetition rates, improve beam quality, and improve the pulse stability for the small amounts of energy necessary for cutting thin walls. They also had to develop cutting strategies for preventing damage to the opposite walls and maneuvering through complex contours quickly on CNC machines.
Lasag Industrial-Lasers (Buffalo Grove, IL) was one of the builders that responded with a new generation of lamp-pumped cutting lasers. On its KLS FC laser cutter, the pulse-repetition rate can be as high as 5000 Hz, which satisfies stent designers’ desire for faster cutting speeds and finer finishes. Because the pulse length can be as short as 0.01 msec, the heat-affected zone can be as small as 0.02 mm. The company’s researchers also solved the stability problem for the few millijoules of output, and improved beam quality to the point where it can produce kerfs that are less than 0.015-mm wide in walls of thicknesses typical for stents, according to Ulrich Duerr, Lasag’s manager of market development, innovation, and technology (Thun, Switzerland).
Because the new lasers cut faster, builders of stent-cutting machines have had to find ways to take advantage of the newfound speed. For example, many are deploying high-speed CNC systems that can follow the increasingly complex contours. Another tactic is to avoid drilling starting holes and then accelerating the movement to the final cutting speed on contours consisting of many closed subcontours. “So-called ‘cutting-on-the-fly’ strategies have reduced production times appreciably,” says Duerr. “For more demanding metals like NiTi and thick walls, this strategy requires high power density, which high-beam-quality pulsed lasers can easily provide.”
Welding lasers have promoted product development, too. Their flexibility and accuracy in welding complex geometry in three dimensions, making hermetic seals, and performing overlap spot welding have made the technology appealing to medical manufacturers. Because lasers let very little heat flow into the workpiece beyond the seam, they also have been enablers for miniaturization. As pacemakers, for example, shrink in size, the seams move ever closer to the internal electronics, which are susceptible to damage from the heat introduced by conventional welding processes.
As one might expect, quality and long-term stability of the welds are other important requirements for welding biocompatible metals in implants. The welds on pacemakers, stents, and other such devices must withstand not only the cleaners and heat used for sterilization, but also the caustic conditions inside body. Moreover, the seam and surrounding surfaces must be smooth to eliminate crevices for microbes and debris to accumulate, aid cleaning before sterilization, and prevent irritation of the surrounding tissue once the welded device is implanted inside a body.
“For a long time, it has been known that temporally forming the welding pulse in spot welding or seam welding could improve weldability and welding quality,” says Lasag’s Duerr. “Temporally forming a pulse means that during the pulse the pulse power is changed. A typical such pulse has a high power at the beginning of the pulse to overcome the welding threshold of the metal, then one tries to keep the temperature on the surface constant to avoid overheating and ‘slowly’ reduces the power in the cooling phase to optimize solidification and reduce stress in the welded areas. This all lasts a few milliseconds.” Developing successful strategies for using the technique to seal titanium pacemaker housings, for example, requires research and development. In fact, research was necessary to develop efficient strategies that require less energy and create smoother seams on pacemakers. For this reason, Duerr recommends working with vendors that are experienced and knowledgeable, who understand both the metallurgy behind welding and the interaction of laser radiation with metals.
He adds that temporally formed welding pulses open the door not only to weld advanced materials like titanium very reliably, but also dissimilar metals like NiTi and Ta, the latter being a marker material. “In lamp-pumped lasers, the boost came with current controlled-welding lasers,” he says. “These lasers not only allow pulse forming, but also offer pulse on demand and extremely long, stable pulses of up to 100 msec.” Stable pulses allows users to modify metallurgy, and permit experts to establish new strategies like single-pulse, high-speed welding, which Lasag has used to weld the tubes in endoscopes.
Trumpf’s Knitsch reports that Nd:YAG lasers typically outperform their CO2 counterparts in welding instruments for endoscopic surgery. For these complex 3-D applications, YAG lasers usually produce the smallest heat-affected zones, incur the least costs, and occupy the least floor space.
To reap these benefits, Aesculap Inc. (Center Valley, PA) uses a five-axis Lasma 443 laser from Trumpf to weld supply tubes to its ventriculoscope trocars. The machine generates the beam with an HL124P pulsed resonator and delivers it through a light cable. Positional tolerance is 0.02 mm throughout the machine’s three linear and two rotary axes of motion. Although the maximum traveling speed is 30 m/min, the combination of spot size, spot overlap, and pulse frequency limits overlap welding speeds to less than 1 m/min. The resulting smooth joints prevent debris and germs from accumulating, and withstand the high temperatures used in sterilization processes.
The number of welds in each model in Aesculap’s line of trocars depends on the number of supply tubes and other variable components. “Here again, we have a wide variety of welding tasks in combination with small lot sizes,” notes Knitsch. Laser welders keep changeover time and costs low because changeovers are usually a matter of installing the appropriate clamping mechanism and loading the right laser program. Hard tooling is unnecessary, and is another ill cured by welding and cutting with lasers.
Lasers Leave their Mark
Traceability is a huge concern among medical manufacturers for controlling the quality of their products and proving their compliance with governmental regulations. Consequently, these manufacturers need a quick, reliable method of marking their products permanently without creating burrs or leaving residue. These marks must withstand sterilization procedures and sometimes the caustic conditions inside the body. Because laser markers can focus their spot diameters down to 25 µm, they can produce a 20-digit data matrix in a 2 by 2-mm area, according to David Havrilla, product manager at Trumpf Inc.’s Laser Technology Center (Plymouth Township, MI).
Reliability and repeatability are crucial, so medical manufacturers demand markers that are repeatable, easy to focus, and consistent from machine to machine. “Many medical devices are extremely expensive,” notes Linda McIntosh, product manager, FOBA (Waterloo, Ontario), a Virtek company. So rejecting parts because the marks are misaligned or incorrect is unacceptable. Consequently FOBA has integrated machine vision into its laser marker to create what it calls Intelligent Mark Positioning, or IMP, to ensure that the right mark is in the right place.
The IMP technology uses through-the-lens technology to eliminate the need to re-position or re-align parts. “If operator intervention is required to optimize parameters, in many cases, re-validation must be done,” notes McIntosh. “This is very time consuming and reduces product throughput significantly.” Because orientation is irrelevant for the IMP, the machine will read, verify, and etch without having to hold the parts in costly fixtures. The only adjustment at changeover is refocusing the laser, which the through-the-lens vision system allows the operator to do in a minute or so by looking at the monitor.
To boost the efficiency of its MicroLase laser markers, Geo. T. Schmidt Inc. (Niles, IL) has replaced traditional lamp-pumped YAG resonators with more advanced diode-pumped technology. The builder has seen the 2–3% efficiency that it had been getting from the lamp-pumped unit jump to 30–50% when it converted to the solid-state diode rack.
Lamp-pumped lasers also require more maintenance. “When a lamp fails, the system is down until a qualified technician can replace the lamp and assure the reflectors are clean and chilling water is purified of any contaminants,” explains Matt Beatty, Schmidt product manager. “By contrast, the energy of a diode-pumped laser will diminish slowly over time—not catastrophically—allowing for planned maintenance to replace the diode rack, a 30-min procedure. The only required regular maintenance is to clean the focusing lens.”
This article was first published in the May 2006 edition of Manufacturing Engineering magazine.