Medical devices incorporating laser-based processing as a step in their production are becoming increasingly common, especially as industry demands smaller feature sizes that may be impossible or uneconomical to realize using traditional manufacturing techniques. It is fair to say that a large percentage of the processing applications involve punching small holes into some type of plastic or organic material. The material, hole sizes, hole patterns, and other parameters change with the individual application. Typical applications include orifices for drug delivery, liquid or material removal, inflation devices, and analytical devices. Each has its own set of unique requirements and constraints, and, as in all industrial situations, technical objectives must be balanced with fiscal considerations.
This article briefly discusses the advantages of using lasers for manufacturing as well as the most common industrial lasers available.1 Three examples of applying the technologyprofiling metal stents, drilling orifices in medical catheters, and drilling orifices in angioplasty balloonsare presented and discussed.
LASERS AND LASER SYSTEMS
Lasers of some type have been industrial workhorses for more than 25 years, and their use in the manufacturing environment is growing significantly. This is particularly true in the medical product industry, where the use of lasers to manufacture devices is relatively recent and where the possibilities for future use in various areas seem unlimited. Three principal laser types are used in most manufacturing environments. These are the carbon dioxide (CO2) laser, the solid-state (primarily Nd:YAG) laser, and the excimer laser. Each has its own distinct advantages and disadvantages when being considered for manufacturing jobs.
The principal, fundamental wavelengths of emission for these lasers are 10 µm (CO2), 1 µm (Nd:YAG), and 0.2 µm (excimer). It is generally true that thermal considerations in the processing become more important with increasing laser wavelength, and penetration depths are generally greater, allowing for faster bulk material removal at longer wavelengths. Because achievable resolutions become smaller at shorter wavelengths, it is fair to say that longer-wavelength lasers should be used where bulk material removal considerations outweigh either precision or quality requirements, but shorter-wavelength lasers must be used when the highest precision or process quality is required. Table I illustrates these points.
A comparison of hole quality when drilled with the three lasers discussed is shown in Figure 1. The material is polyimide and, in truth, would tend to show the differences between the lasers more dramatically than ceramics or other plastics. However, the same general conclusions can be reached for the processing of most materials: achievable feature sizes are smaller and feature quality is better as the wavelength of the laser decreases.
|Relative operating cost per photon||Low||Low||High|
|Average power||0.3 to 20 kW||0.1 to 2 kW|
|Penetration depth (µm)||>10||1 to 10||0.1 to 1|
|Ultimate feature resolution (µm)||10||>1||0.2|
|Practical feature resolution (µm)||~ 50||~ 25||~ 1|
|Types||CW, pulsed||CW, q-switched, pulsed||Pulsed|
The two issues of primary importance when determining the applicability of any lasers to specific tasks are the material's ability to absorb the wavelength of light used and to carry away excess thermal energy efficiently without causing secondary problems. In general, if the material does not show a fairly strong absorption of the desired wavelength of light, the best course of action is to find a suitable wavelength that does absorb stronglyassuming one can be found. Teflon, for instance, does not really laser machine well at any of the wavelengths under discussion, but it machines well at 157 nm (F2 laser) and can be processed if fillers are added that retain the initial desirable characteristics of Teflon while promoting absorption of the photons.
Many plastics absorb all wavelengths of interest, but their low thermal conductivity prevents the use of longer-wavelength lasers for processing because the heat-affected zone exhibits symptoms of charring, melting, or cracking. This is also true of materials like glass. On the other hand, some ceramics absorb fairly well at all the wavelengths under discussion, and the choice of laser in these cases is dictated by speed, feature resolution, and quality.
It should also be noted that there are optical frequencyaltering techniques commercially available for use with solid-state lasers that will double, triple, or quadruple the fundamental laser frequency. This frequency enhancement comes at the cost of losing some of the original output energy and stability, but provides an alternative way to make "green" and UV photons. In many cases, these new frequency-converted lasersparticularly at the UV wavelengths achieved when frequency tripling or quadruplingmay replace other UV sources in some applications, especially those requiring single-hole drilling of small, round orifices in soft material. As the technology for generating shorter-wavelength light from solid-state sources matures, the relative ease of use and low cost compared to other short-wavelength sources will be quite attractive to potential users.
It is the goal of system integrators to build lasers into micromachining workstations that are cost-effective, highly reliable, and easily operated as well as maintained. A complete laser system will include the laser; beam delivery to get the photons on the target; workpiece tooling or other motion systems for part delivery, manipulation, and exit; integrated and interlocked frame and support; computer control; laser support equipment; and sometimes vision systems or other options to make the system more flexible or increase throughput. Note that flexibility and high throughput do not necessarily correlate in equipment design.
Investment in any laser system is costly and requires attention to facility preparation and education of associated personnel. In cases where either financial or control considerations dictate the placement of a laser system in-house, typical system costs start at $100,000 for a small laser-based system with few options or functions to more than $500,000 for a large, sophisticated system that may require part handling and automated machine vision. Typically, the cost of a small, sealed CO2 laser or small solid-state laser is about $50,000. An integrated system would cost from about $150,000 to $200,000 and would include the laser, camera vision for viewing, motion controlusually at least an x/y table and possibly including rotary motionbeam delivery, computer control, and Class I laser operation (operator-level interface and safety equipment). Excimer laser systems are usually more expensive, with the laser costing about $130,000 and the full system about $300,000. The higher cost for the processing end of the system is due to the fact that, in order to take advantage of the higher resolution capabilities of the excimer laser, better-quality vision, motion control, and other accessories must be used than in a comparable CO2 or solid-state system.
In addition to the above laser system components, most industrial applications require some level of tooling to efficiently and accurately move the parts on and off the laser as well as to position them during processing. The exact level and type of tooling is very dependent on the specific application, so these components are almost always custom made for each user.
If there is a need for laser processing but the initial capital investment for a system is not immediately justifiable, a good option is to work with a reputable contract manufacturer. This is especially true for small-sized or start-up companies. In many cases, device manufacturers can maximize profitability by concentrating on their areas of expertise and outsourcing laser processing, which requires a greater amount of logistics. Areas of concern that need to be addressed to ensure lot-to-stock or other acceptable delivery requirements include process documentation, document and tracking control, and quality control and assurance.
Angioplasty operations are performed on patients who have suffered a major blockage to vessels in the circulatory system. More than 500,000 angioplasty procedures are currently performed each year in the United States alone. The procedure typically involves inflating a balloon in the area of the blockage, which breaks up the accumulated plaque and opens the vessel. While this technique works well in the short term, 30 to 50% of all angioplasty operations performed will need follow-up treatment within six months. This is due to incomplete plaque removal and the formation of scar tissue as a result of irritation of the vessel, known as restenosis. There has been a tremendous push in the health-care industry over the last few years to combat restenosis because repeat operations are expensive, inconvenient, and potentially life threatening. Lasers play an important role in several of the new technologies being developed by both large, well-known medical device companies and small, specialized high-tech firms.
One product that has received tremendous attention in the industry is the metal stent. Most surgeons performing angioplasty operations today insert these stents as a matter of course, since initial results seem promising. These stents are usually machined in intricate patterns to make them more flexible while still maintaining the mechanical rigidity necessary to keep the vessel walls from closing (see Figure 2). The devices are also used to minimize the problem of arterial blockage caused by plaque falling into the vessel after inflation.
Such metal stents are usually made from small-diameter, stainless-steel tubing using a Nd:YAG laser. Other methods used to manufacture the metal stents with high precision include electrodischarge machining (EDM) and etch techniques, but these methods have significant drawbacks and the laser process is the method of choice.
These devices have been used for only a few years and, although initial results look promising, long-term effects have not been fully investigated. It's important to note that, if restenosis should recur, there is no good method to simply remove the blockage, and follow-up treatments may require laser surgery or localized drug delivery using other angioplasty devices.
Because of these potential problems with metal stents, some researchers are looking at polymeric or even biodegradable ones. These materials usually require processing with an excimer laser to avoid thermal problems and heated-affected zones. In some cases stents have even been inserted after being coated in a restenosis inhibitor such as heparin.
An alternative to metal stents is the use of catheters with orifices located near the insertion end. These orifices are generally drilled using UV photons (in most cases from an excimer laser, although frequency-converted solid-state lasers appear to be a good alternative).2,3 The idea in this instance is to inject small amounts of a restenosis- inhibiting drug locally.
Heparin, usually used as an anticoagulant or blood thinner, has been found to be effective in controlling restenosis when used in high concentrations in localized areas. Since this drug is already well known in industry and among regulatory agencies, it has been used as a test case for localized delivery and proof of concept. Other drugs being investigated are more costly and are not as well documented, and systemic delivery can in some cases be toxic at the levels necessary for successful retardation.
Many organic materials are being used, including polyester, polyimide, polyurethane, nylon, and Mylar. Most UV laser processing is done with 248-nm photons, although 193 nm is a good wavelength when materials have a low absorption at 248 nm. Hole sizes range from 10 to about 100 µm in diameter. Most devices require multiple holes, which can be placed along the shaft in many different orientations. The most common arrangement is a set of linear orifices along the catheter shaft at 180°, 90°, or 45° from each other. Another arrangement is a set of orifices spiraling along the length of the shaft. The holes may be drilled in the central lumen of the catheters (many have only one lumen), or into smaller lumens running along the length of the main, central lumen.
While the typical configuration is a series of individual holes, it is also possible to image a mask containing a number of holes and to drill sets of holes at preferred locations along the lumen shafts. Frequentlyespecially when the orifices are drilled into the circumferential lumensblocker material must be used to avoid the laser beam hitting the opposite lumen wall. This blocker is usually a small metal sliver inserted into the lumen during laser processing and then removed. The hole shapes can also vary from the more common circular to square or elliptical shapes.
Another approach is to drill the drug-delivery orifices directly into the inflatable balloon. In this case, the orifices are usually very small in diameter to allow the pressure buildup needed for inflation before actual delivery of the drug. Orifices can cover most of the balloon surface or only portions of it. Some of these devices are drilled by placing the balloon flat and then drilling through both layers simultaneously, while others are drilled by controlling the number of pulses to avoid drilling through the opposing side. In order to accurately control the number of pulses, a consistent and uniform material thickness is required.
OTHER CATHETER-RELATED APPLICATIONS
A large number of high-tech applications for laser-manufactured devices involve angioplasty in some way, but there are also other applications that use lasers in the manufacturing process. One example is the use of CO2 lasers to drill large holes in simple catheters for liquid and material removal. Because quality in these applications is not as great a concern as functionality and low manufacturing cost, these devices can be made in high volumes at low cost.
Another use of lasers is in the manufacture of analytical catheters or devices that are inserted into the body and whose primary purpose is to give analytical feedback.4 These devices are being manufactured for precise monitoring of vital organs and body functions at unprecedented levels of accuracyall made possible by the application of laser technology.
It is clear that the number of applications involving the use of lasers in the manufacturing process is increasing rapidly as advances are made in laser technology as well as in other technologies associated with medical device manufacturing. Two of the largest ongoing efforts by today's scientists and engineers are to "engineer life" (medical and biotechnology) and to make engineered devices more lifelike (microelectronics, robotics, artificial intelligence).5 These two efforts seem to converge on a microscopic scale, if anywhere, and both require novel micromachining capabilities that, in many cases, lasers can provide.
Two of the largest industrial users of precision micromachining lasers are the microelectronics and medical industries. This article has covered only angioplasty and catheter applications for laser technology; there are many more medical applications where devices are already in production or in initial engineering phases.
Of all the applications for laser micromachining, at present it seems as though applications in the medical industry will grow significantly over the next several years. Because of the laser industry's relative infancy and the long lead times associated with incorporating new technology into medical manufacturing, laser use in the device industry is still relatively uncommon. However, this is likely to change over the next few years as the industry experiences a period of rapid growth in which the future looks bright for both device manufacturers and the laser community.
1. Schaeffer R, "Laser Micromachining of Medical Devices," Med Plast Biomat, 3(3): 3238, 1996.
2. Pokora L, "Excimer Lasers and Their Applications in Industrial Technology and in Medicine," Opto-Electronics Rev, 1: 1320, 1993.
3. Gower MC, Rumsby PT, and Thomas DT, Novel Applications of Excimer Lasers for Fabricating Biomedical and Sensor Products, Bellingham, WA, International Society for Optical Engineering, vol 1835, pp 133142, 1993.
4. Highlights, Lambda Physik newsletter, no 8, November 1994.
5. Kelly K, Out of Control, Reading, MA, Addison-Wesley, 1994.