Laser Technology Supplement: Lasers Focus on Medical Devices
For small, delicate part applications, lasers deliver stable, accurate energy for cutting, marking, and welding
By Michael Tolinski
Doctors are understandably sensitive about the tools they use—every dimension, joint, or mark can add to or detract from performance, depending on how well the tool features are made. So for medical devices, the pinpoint precision of lasers is valuable for cutting, welding, or marking the smallest of devices.
Lot sizes and part styles for medical parts can vary wildly, from customized implants to relatively mass-produced tools and devices. "Nonetheless, every single instrument is subject to the highest quality standards," says Alexander Knitsch, application specialist for Trumpf Laser (Farmington, CT). For implants in particular, the main priorities include long lifecycles and biocompatibility.
"Medical devices are especially suited to laser processing, because they require extremely tight tolerances and advanced materials processing," says Larry Green, industrial product manager, Spiricon Inc. (of Ophir Optronics Ltd., Logan, UT). "It's also evident that the laser performance must be well characterized for the process to be repeatable, robust, and profitable."
The key to laser-machining small medical applications is to use a laser beam with a narrow, stable, and focused energy profile, even for laser spot diameters under 100 µm. More stringent requirements for stents, pacemakers, implants, catheters, and other medical products require better beam profiling and delivery tools, adds Green. Along with added quality and capability, such tools can improve device manufacturers' cost profile through reduced downtime.
Some problems can be fixed just by checking the beam profile with real-time diagnostics and re-adjusting the laser. Green points to a medical-device manufacturer (the company requested that its name not be used) that laser-marks its product with a logo whose edges were no longer acceptable to the customer. "Since an illegibly marked product cannot be sold, they were discarding perfectly good product because of the poor identification of the part." Here, laser-beam profiling showed that at certain power settings the beam profile had more than one peak, instead of a single peak at the center of its spot. "Readjusting the laser power settings to eliminate the multiple peaks solved the problem."
In another case, a medical manufacturer (which, again, does not wish to be named) traced poor welds to a laser beam spot whose focus location varied randomly over time, causing spots of varying size on the product, says Green. The solution was to change the beam-delivery system from "hard" laser optics to fiber delivery—a common trend in laser/medical applications, as we'll see below.
Bone screws and other medical hardware may look simple, but their manufacturing processes use the most advanced laser technologies. The laser, motion control, part positioning, and even vision software for beam delivery come together to create the small features that doctors need for stabilizing damaged bone or fixing other problems.
"The medical devices industry is now looking for a new generation of lasers that can offer better value and additional functionality over their existing laser technology," says Linda McIntosh, product manager for Virtek Laser Systems North America Inc. (Waterloo, Ontario). This applies to marking lasers, which have been used for years for writing identifying information and other marks on metal and plastic medical tools.
"Laser marking on metals creates a very high quality, permanent mark, and it does not require inks, solvents, etc.—which means lower operational costs." Lasers avoid the FDA-approval issues involved with ink, and they can mark surfaces without creating crevices, a feature to avoid on implants, McIntosh adds.
To improve the marking of bone screws, Virtek's laser-marking systems integrator, FOBA Technology + Service GmbH (Lüdenscheid, Germany) integrates a vision system called Intelligent Mark Positioning (IMP). The system is integrated with the laser system's lens, feeding back data to reduce position errors when marking 0.5-mm characters on 3-mm-diam screw heads. Essentially, the system "puts eyes and intelligence" in the laser, says McIntosh, comparing a model of the part with what it sees for proper positioning. This capability reduces scrapped parts, minimizes fixture costs and laser setup time, and improves machine-to-machine consistency.
Tools used to create the holes for bone screws also require detailed marking, such as depth gaging on the shaft to guide the surgeon. Since these marks must go around a drill or tap's shaft, the tools must be rotated while being marked, a complicated task better suited for automation, according to system supplier Telesis Technologies Inc. (Circleville, OH). The company's system incorporates a six-axis robot for handling, a 100-W Nd:YAG laser, and Telesis software. The system "takes a pallet of 100 parts at a time through the complicated marking cycles in a matter of minutes," says the company's Ralph Villiotti.
Other more common tools, such as surgical hand tools, also require marked numbers or logos—sometimes on different sides of the part. This requires an operator to change the fixturing to turn over the part for the laser. To speed up this process, Telesis supplies its Zenith 10F laser workstation, which can mark on different sides of the part in one operation.
Mounted on wheels, the 10-W laser system also demonstrates portability: it operates off 110 Vac, requires no chiller, and allows users to develop marking patterns off-line. "The system uses an automatic pneumatic door with safety hand controls, and functions in an office cubicle," adds Villiotti. "So the operators can multitask as they meet the FDA restrictions for annealed laser marking."
Catheter parts can use different kinds of laser processing, including welding and cutting. Catheters may be made from different kinds of materials, but they all share a couple of characteristics: their small features and relatively high volumes.
One laser-processed catheter component is a tube around which a thin ribbon of material must be removed in a continuous, progressive spiral cut, of varying angles, along its 31-mm length. The 316L stainless tube is 0.73 mm in diam and 0.075-mm thick, and the cut is made with a pulsed Nd:YAG laser system from Directed Light Inc. (DLI; San Jose, CA). The system cuts the spiral with its 25-µm diam beam, and it's designed to meet a volume of 2000 parts/month.
For cutting the spiral, the system combines X-Y linear motor motion control with a direct-drive rotary axis. DLI says proprietary algorithms were developed for the system's G-code that allow different spiral angles to be cut, and that accurately guide the laser through the two passes required to cut the full width. The laser's focal length was adjusted to prevent burn marks inside the tube opposite the cut. Moreover, a special V-block fixture was also needed to support and align the flexible tube during cutting.
Laser cutting is used with other kinds of medical applications as well, says Ulrich Dürr of Lasag AG (Thun, Switzerland). "In thermalcutting tasks on implants such as stents or instruments like endoscopes, the major advantage of the laser is its flexibility as a tool, which allows complex microcontours." As with the above application, this often requires narrow kerf widths down to 10 µm. "This requirement can be fulfilled by most industrial microprocessing laser systems, whether a lamp-pumped pulsed Nd:YAG system or a fiber laser."
Other catheter parts, and even smaller components, benefit from both laser cutting and welding, using small, integrated workstations that incorporate fiberdelivered laser beams. These small applications include the preparation and welding of fine wire leads for implants and, using the laser in continuous-wave (CW) mode, cut-ting medical-grade steels. Medical devices and implants often use thin, 300-series stainless and face tough size, quality, and durability weld requirements.
For these applications, Crafford-LaserStar Technologies Corp. (Riverside, RI) offers its benchtop FiberStar 7500 workstation, incorporating fiber lasers from SPI Lasers UK Ltd. (Southampton, UK). "The new welding workstations provide a better quality output combined with improved process yield, reduced running costs, and lower maintenance demands," says David Braman, Crafford-LaserStar vice president of engineering.
The workstations are for high-volume, precision assembly environments, says Braman. They're available with laser powers from 20 to 100 W and provide focused spot sizes down to 10 µm. The systems have no optical parts to align or calibrate, reportedly making fiber lasers attractive and practical for shop-floor environments.
There's growing acceptance of fiber lasers for materials-processing applications as a cost-effective alternative to conventional laser design. Braman admits that conventional systems are adequate for a large proportion of laser material processing. However, fiber lasers don't exhibit variation in output power, spot size, or focal point of the sort caused by thermal effects on the glass rods of traditional YAG lasers. "The reason for this is that the generation and transport of the laser beam to the workpiece takes place entirely within the confines of a single-mode fiber."
"In general, the choice of workstation for any application comes down to determining the required performance, followed by a tradeoff between initial outlay, component yield, uptime, and maintenance." While the initial outlay for a fiber laser workstation may be higher than for conventional systems, Braman asserts that a rapid return-on-investment comes from high part yields, near-100% uptime, and near-zero maintenance.
Laser welding is important for implants and devices that require clean, hermetic seals or extremely small welds. System-supplier Miyachi Unitek Corp. (Monrovia, CA) points to several laser-welded medical applications, including spotwelding for pacemaker, battery, and insulin pump cases, and for hypo tubes and orthodontic appliances.
Here, fiber lasers offer several advantages, says Unitek, which just released a new family of fiber laser welders. The lasers' diode pumping provides stable beams, and the units have built-in options for time and energy-sharing beam distribution, customer-replaceable fibers, and integrated scanning and fixed-focus heads.
Generally, solid-state pulsed lasers focus energy consistently without excessive heat buildup—important for delicate medical implants. Alexander Knitsch of Trumpf says his company's Tru-Pulse solid-state lasers are particularly suitable for joining cases of pacemakers, defibrillators, or implanted pumps.
"The overlap spotwelding of the titanium shells ensures that the two halves of the housing are hermetically welded. As the melt-heat spreads from pulse to pulse across the housing, the sensitive inner electronics are never heated to more than 50°C." Welding depth and splatter can also be controlled precisely; with pacemaker shells 0.3-mm thick, the welding depth is adjusted to 0.25 mm.
Overheating during welding is prevented by controlling the shape or profile of the laser pulse. "Wirebonding in pacemakers also has been improved in its reproducibility and reliability by dedicated pulse shapes," says Knitsch. Or, when welding together thin endoscope tubes of stainless or Nitinol shape-memory alloy, the rotation rate of the tubes can be matched with the optimal laser pulse rate to join them as they spin. "The other strategy is to use Lasag's high-speed rotation optics, which rotate the beam around the tubes to be joined."
Laser welding of implants often must be done with exotic metals that are compatible with the human body, adds Lasag's Ulrich Dürr. These range from stainless steel and titanium to tantalum and platinum. Challenging applications for lasers may require joining two different biocompatible materials, such as welding a tantalum marker to a Nitinol stent. "Success in this area would allow [you] to select metals according to functions and cost, and less according to weldability."
This article was first published in the May 2007 edition of Manufacturing Engineering magazine.