Medical Lasers Make & Mend
Cutting, heating, and healing
By Robert B. Aronson
Physicians use lasers to cut, fuse, and repair elements of the human body. And, because of their unique capabilities, lasers often play a key role in the manufacture of medical equipment. Because of the increase in types, power levels, and operating simplicity of lasers, their application to both medical roles is rapidly expanding.
The trend to minimally invasive surgery has provided an ideal role for lasers. Because of their miniature size and ability to make small cuts and welds, they can be used both in medical procedures, and to make equipment that could not be manufactured any other way. Because of these lasers, it's possible to introduce a medical device through a small incision or artery, instead of making a major incision. The benefits are minimal disruption of tissue, organs, or bones, so recovery is faster and chances of infection reduced.
For example, in dentistry lasers are, in some cases, replacing drills and burrs to vaporize and remove decay from teeth or prepare them for a filling. Lasers can also reshape gums and remove bacteria. A laser with a UV light source is used to harden fillings and tooth facings, and speed the teeth-whitening process. Where there is not direct access to the work area, a system of fibers and mirrors is used to direct the beam where it's needed.
To work on soft tissues with little or no collateral damage, a diode laser is used with flexible fibers that transmit energy to the surgical site. The beam of a CO2 laser is highly absorbed in water, and is good at cutting soft tissue, and is reportedly the best laser to use to control bleeding. When the beam of an erbium-pulsed laser touches hard tissue, it turns the water in that tissue to steam. This generates pressures that ablate the hard tissue. With this process, teeth or bone can be cut, usually without anesthesia, and with a higher precision than that achieved using a drill.
Lasers are used as cutting tools to correct the shape of the eye, in a process commonly known as Lasik surgery. Eximer lasers are often preferred for this type of work because they can supply a small (0.8–2 mm), uniform beam that creates a smooth ablation of the eye. It is also said to be useful in correcting irregular astigmatism. Other lasers are used to remove vision-blocking blood vessels from the retina.
Angioplasty, or plaque removal from an artery, is one of the minimally invasive surgery techniques. A laser and fiber-optic tool are moved to the blood-flow restriction. The laser fires, vaporizing the plaque, then other tools remove the vapor and debris. Similar procedures can remove potentially cancerous tissue.
Lasers can be pulsed or continuous wave (CW), depending on the procedure. Pulses are used for high-power, low-duration needs, and continuous wave for longer, slower operations. A key consideration with CW is to ensure not too much heat is introduced to the tissue.
In one of the "noncutting" laser procedures, the beam is used to trigger chemical reactions needed for some healing processes.
"Aging Baby Boomers are responsible for one of the fastest growing medical-laser applications: cosmetic surgery," explains Bill Shiner of IPG Photonics (Oxford, MA). "Fiber lasers are preferred for much of this work. It's a simple plug-and-play device, and the beam can be adjusted to match the needs of a particular patient."
The properties of laser light allow it to deliver energy that interacts with selectively targeted areas of the skin. Laser light hitting the skin is absorbed and heats the selected area. The goal is to heat the target without causing collateral damage in a process called selective photothermolysis.
The laser's ability to work in micro dimensions makes it popular in medical manufacturing, where it is integral in the production of 3-D parts for use in medical applications. To meet industry needs, Trumpf Inc. (Farmington, CT) recently enhanced its TruLaser Cell 3010. This laser-processing machine cuts, welds, and drills 2-D and 3-D components. "The rotating axis is designed for compact, midsized components up to approximately 8" [203 mm] in diameter." says Juergen Stollhof, of Trumpf's Laser Technology Center (Plymouth, MI). "In addition, the axes can be rotated or pivoted for better workpiece access. Because stability is critical in this type of manufacturing, the machine has a granite base and linear motors that control positioning. "This is a distinct benefit of Trumpf's TruLaser Cell series 3010, which can be combined with pulsed, fiber, or disk lasers with up to 6 kW," he says.
"Our lasers are well known for high stability of the laser parameters," said Stollhof. "Welding a pacemaker housing, for example, requires that beam quality and power have to be carefully controlled." The weld must be deep and wide enough to seal the case and not allow gaps, but not so deep that sensitive electronic components housed inside the unit are destroyed or otherwise compromised by heat generated by the laser process. "The temperature inside a pacemaker should not rise above 60°C. In addition solid-state lasers are delivered with multiple outputs," adds Stollhof. "This offers to the customer distinct benefits, time, and energy sharing. Time sharing means that one laser can be combined with multiple machines. Energy sharing enables welding at the same time at several separate areas on a single workpiece to avoid distortion."
The next level of laser-technology development includes the industrial picosecond laser. It allows for ablation, cutting, and drilling without creating a detectable heat-affected zone. Trumpf's TruMicro series 5000 opens an entirely new application field in the area of medical technology. These lasers generate ultra-short, six picosecond pulses in three different wavelengths (1030, 515, or 343 nm) and with up to 50-W average power. "Our customers use these lasers for cutting of very small parts and nonmetal materials," Stollhof concludes.
Another example of product miniaturization that is made possible by a laser are instruments that can be guided through the body's veins and arteries. A catheter on the tip of a flexible rod is guided to the problem area within the body. Instruments within the catheter are used to make a repair, such as burning away obstructions in a blood vessel.
"Laser welding of wires as small as 0.004" [0.102-mm] diam is one of the key processes in the manufacture of this instrument, explains Tom Noll, president, LSA Laser (Plymouth, MN). "These wires extend from the catheter within the body to the surgeon's hand. Following the movement of the catheter on a fluoroscope, the surgeon pulls a wire by rotating a knob or pulling a trigger. This changes the catheter's direction, just like steering a sled.
"About 98% of our work is for the medical industry in four key areas: welding, tube cutting, plastic machining, and laser marking," explains Noll. "All jobs start out as prototypes or R&D test items and much of our work must be created in a matter of hours, so speed is one of our major capabilities."
Typical products the company makes involve the welding of miniature medical items that have to go into the body, such as heart-monitoring units. "Not long ago the requirement for the laser-beam spot was 0.008" [0.2 mm], now it's typically 0.003" [0.076 mm]," says Noll. "With this size beam we weld 0.0015" [0.038 mm] diam tubing with a wall thickness of 0.0006" [0.015 mm]. Kerf size can be as small as 0.0007" [0.018 mm]. For this work we use 5–100-W, flash-type, fiber, and diode-pumped YAG," he concludes.
"The advent of fiber lasers is the most important advance in laser technology for some time," explains Andrew Dodd, GSI Group (Bedford, MA) "They are simple to use, need no special alignment, and have a wide variety of applications in many fields, including medical device manufacture.
"For this laser, there has been a marked growth in power levels and other improvements are on the way. To say that it has revolutionizes the laser industry is not an under statement."
Although the diode-pumped laser is complex to manufacture, for the user it is basically a plug-and-play instrument. The key is that the beam-producing diodes are imbedded within the fibers, so there is no problem with linking the laser beam source with the delivery mechanism as in most other laser designs. "There is no user alignment. It's a box you never need to open and it just works," says Dodd.
The diode laser operates on a single wavelength in the infrared zone and can generate a continuous beam or short bursts of power, so it does well in cutting, welding, and other metal-related applications.
"It does a good job of making small features because it's spot size is as small as 12 µm, so when cutting, the kerf width is both small and accurate. Stent making is a major use," Dodd concludes.
Getting the laser beam to the workpiece is the job of Laser Mechanisms Inc. (Novi, MI). They provide delivery systems specifically for medical tasks ranging from stent manufacture to tattoo removal. The company's job is particularly critical in medical applications, because these jobs are characterized by minimal operating space and miniaturization. A combination of fibers, lenses, and mirrors are used to get the laser beam where it's wanted.
For example, in the case of stent manufacture: "The delivery system has to have a lot of adjustability," explains Managing Director Pacific Rim, Glenn Golightly. "In making a stent, an operator first manually focuses the beam on the stent material using a high-magnification eyepiece." Once the starting point is established the cutting is automatic. The laser remains stationary and the stent tube is maneuvered in front of the beam. Typically a 100-W fiber laser is used with a 1070-nm wavelength and a 5–25 µm spot size.
"The workpiece can be a small stainless tube about 2-mm diam with 0.15-mm walls," says Golightly. "Although peak power is high, the average power is much lower, about 2% of peak power."
Because of patient safety and liability considerations, laser testing is a critical part of the industry. "It's important to know that the laser is doing what it's supposed to do," says Larry Green, industrial product manager, Ophir-Spiricon Inc., (Logan, UT).
All lasers degrade over time and require periodic calibration. If performance validation isn't done, the power a laser delivers to the patient can be too high or too weak. In addition, it's important to find out if there are any hidden defects such as a cracked fiber, misaligned hand piece, or bad lamps in a flash-lamp laser.
"Because the laser-power controller tells you the laser is operating at a certain power, doesn't mean that is the actual beam power," says Green. "This is particularly dangerous when the control gages are reading lower than actual, and the operator turns up the power to compensate, unnecessarily."
Ophir-Spiricon provides instrumentation to measure beam energy, power, and wave length, plus a very important instrument that measures beam profile. "It discovers any hot spots as you move across the beam that could lead to patient injuries," he concludes.
Many industries use lasers to mark parts, but in the medical industry it is more than a convenience, it is a necessity. The FDA requires part identification as a means of monitoring the performance of devices placed within the body. The marked information can also help the surgeon select the right instrument.
Manufacturers need part identification not only for products used in medical procedures, but those used in all equipment related to the medical industry. This is critical for process control, reliability testing, and failure analysis. Some part data can identify part manufacture down to the day, time, location, machine used, and machine operator.
The two major marking techniques are engraving and coloring. Engraving melts or vaporizes the material, essentially cutting the shape of the data needed into the part. With coloring, the laser causes a color shift in the part material. For example, with metal parts, the laser burns the carbon in the material, essentially "scorching" the information onto the part.
Laser Photonics (Novi, MI) is one of the companies that specializes in medical-part marking. Where space is an issue, the company provides the Fiber-Tower Desktop System, measuring 30 x 27 x 20" (762 x 686 x 508 mm). It marks, anneals, and engraves metals and reacts well with steel and stainless steel. The laser does not contact the material being marked, which reduces errors.
"Many medical device manufacturers use an annealing process that actually changes the composition of the material, bringing the carbon to the surface and essentially burning it," explains Ryan Semmer, industrial product manager. "This is important when working with surgical equipment, where etching or engraving may become dangerous due to the transferring of bacteria."
For cutting operations, Laser Photonics offers the Link PLCS precision fiber-laser cutting system. It cuts highly reflective metals with good edge quality due to the 40 µm beam size. With this cut, manufacturers eliminate postprocessing of the edges.
"Using our 2-kW, fiber-laser system cutting 1/16" [1.6-mm] stainless, we achieved an accuracy of 0.001 mm. Unlike CO2 laser systems, the Link PLCS eliminates consumable gases which reduces operating costs," Semmer concludes.
Moving to Medical
Because of the strong similarity between aerospace and medical manufacturing requirements. Laserdyne Systems (Champlin, MN) is aggressively returning to the medical market. "We have supplied lasers to the medical industry for some time, but for the last 10 years we have had a stronger emphasis on aerospace, particularly turbine engine manufacturing," explains Vice President Mark Barry of Prima North America—Laserdyne Systems and Convergent Lasers.
"Precision positioning and precise verifiable control of the process, which are important to turbine-engine manufacturers, are also medical-manufacturing essentials. We are modifying our Laserdyne Aserdyne 430 workstation to more closely match medical manufacturing needs," says Barry. This flexible machining center cuts, drills, and welds.
It is supplied as a turnkey system with CO2, Nd:YAG or fiber lasers. Fiber lasers are especially interesting because of their capability to produce kerfs as small as 40 µm, and at the same time perform precision welding. "For the manufacturer of small-to-medium lot sizes, the combination of the fiber laser and precision workstation means that laser processing may be a very viable and economical manufacturing option," says Barry.
"We are also looking into the potential of using lasers to provide unique part cladding, texturing, and even part fabrication. With lasers, it's possible to produce a high-purity, highly precise coating," he says. Laserdyne is working with IPG Photonics to develop new applications that take advantage of the fiber laser's properties.
Accurate data handling for both production and quality control are important issues in the medical field. "We already have that capability built into the system software of our workstations and a history of applying this to meet the data handling requirements of our aerospace customers. So, we can provide this, essentially at no customer cost. They, in turn, can generate meaningful reports at little to no cost for their customers," Barry concludes.
This article was first published in the May 2009 edition of Manufacturing Engineering magazine.