Production of novel medical devices is made simpler--in some cases made possible--by the application of laser technology
By Brian J. Hogan
The success of the laser as a tool for manufacturing medical devices has occurred due to an alignment of its features to that of the industry. Those requirements for high quality, accuracy, miniaturization, and reliability match up well with the laser’s inherent fine resolution, high controllability, and stability. The laser microprocessing area that is relevant to medical device manufacturing encompasses lasers across the wavelength spectrum from 193 nm to 10.6 µm, and pulses from picoseconds to milliseconds for micro welding, fine cutting, drilling, micromachining, and marking.
One example of the application of lasers to medical manufacturing involves the low-cost, high-volume fabrication of microfluidic devices. This work is critical to support a new generation of instruments that provide point of care (POC) blood testing for a host of medical conditions.
Diagnosis and treatment of many medical conditions require testing blood or other bodily fluids. Typically, such tests require large blood samples, and are done in labs using expensive instruments. This approach is costly and time consuming, and often uncomfortable for the patient. A new generation of medical testing equipment makes it possible to economically perform these tests rapidly at the doctor’s office or some other point of care (POC) using only drops of blood.
By using microfluidic devices, the new instruments miniaturize existing methods of testing blood. "Specifically, these are disposable plastic slides containing a three-dimensional structure of tiny interconnected channels as well as electrodes," says Pete Peterson of Coherent Inc. (Santa Clara, CA). "By applying pressure and electrical impulses, an instrument can automatically carry out biochemical and other tests, such as immuno-assays and cell counting under semiskilled, pushbutton control."
"Manufacturing disposable devices in sufficient
volumes at a price point the market will accept can be accomplished through laser digital converting."
These microfluidic devices incorporate features that are too small and complex to machine by conventional rotary die. But high-speed laser processing is very well suited to the task of creating these features.
Most disposable devices are about the size of a credit card and contain up to 15 laminated layers. Manufacturing them in sufficient volumes at a price point the market will accept can be accomplished through laser digital converting. This process involves laser-processing modules, such as the LPM-300 by LasX Industries, mate roll-fed material handling systems, and cost-effective CO2 cutting lasers, together with automated laminating techniques. "The focused laser beam is scanned over the continuously moving roll to create a variety of features," explains John Dillon of LasX Industries (White Bear Lake, MN). "These features may include through cuts, kiss cuts to a liner, via holes, and scores. Typical material thickness ranges from 100 to 250 µm."
Because their infrared output is strongly absorbed by both the polymer substrates and the doubled-sided adhesives that are used as lamination layers, CO2 lasers are preferred for this application. And, just as important, according to Dillon and Peterson, they offer the highest watts/dollar ratio of any industrial laser type. Additionally, they can readily produce the desired feature sizes of the order of 100 µm or larger.
Slab-discharge CO2 lasers, such as Coherent’s Diamond series, are preferred for several reasons, including their small size and low maintenance requirements. For instance, these pulsed lasers deliver fast rise and fall times that produce square pulses in the time domain. Together with their very focusable (TEM00) output beam, this means that fine features can be created with minimal HAZ. This delivers the requisite clean edge free of beading and other issues that could cause the laminated product to leak. In addition, the fast pulsing of these lasers (to 20 kHz) means that a model rated at 400 W can sustain high processing speeds such as cutting at 1–2 m/sec, while producing smooth cuts rather than a scalloped edge. With reels as wide as 12" (305 mm), this high speed supports mass production, with a total processing time of about 4 sec per device.
Fine cutting for medical-device production is another application that recognizes laser technology as an important source of precision tools. The medical device industry has developed from an early adopter to an established user of this type of laser technology.
"The near diffraction-limited beam quality of single-mode fiber lasers makes it possible to achieve focused spot sizes of less than 10 µm," says Andy Appleyard of SPI Lasers (Santa Clara, CA), "which leads to intensities in the range of hundreds of MW/cm² with laser powers of only a few hundred watts. This intensity produces high cutting speeds for thin-section materials which, in many practical applications, are limited only by the motion system capability or cutting process requirements. An additional benefit of the beam quality compared to conventional multimode solid-state lasers is an increase in the depth of focus by an order of magnitude or greater, leading to a proportionately higher degree of process tolerance."
When cutting thin-section materials with conventional solid-state pulsed lasers, the process speed can often be determined by the laser’s repetition. With the fiber laser there’s said to be no such limitation, as the laser can be operated in both CW mode and modulated to several tens of kHz with pulse duty cycles in the range from 10 to 100%. This characteristic enables a process speed to be optimized for the application in a very flexible way. "For example, when cutting a linear section the motion system can accelerate to high speed and the laser operate at a high average power level," says Appleyard. "However when you cut highly detailed sections at slow speed, the average power must be controlled to minimize the HAZ in the material. This can be straightforwardly achieved by modulating the laser with a continuously variable repetition rate but at a constant pulse width to maintain the process conditions."
For manufacturing processes where process control and repeatability are of paramount importance, the output power stability of the laser can be controlled to less than ±1% using a closed-loop power-control circuit. All of the attributes listed above combine to enable the fiber laser to produce high-speed, high-quality precision cutting in a range of materials, including stainless, cobalt chrome, Nitinol, and titanium. When compared to processing with conventional solid-state lasers, productivity improvements of more than 5x in stent and hypo tube-cutting applications have been achieved. The high quality of the laser-cut edge profile reduces the need for postprocess finishing, which reduces the total number of production stages and lowers overall manufacturing cost.
With today’s medical devices getting smaller and more intricate by the day, the edge condition of the laser-cut profile is becoming ever more critical to the smooth actuation of some devices. It’s expected that surface roughness will become a standard part of laser-cut specifications in the future. Fiber lasers enable the device manufacturer to meet the demands involved in producing both surgical instrument components and implantable devices with fully validated processes.
Creating electrical connections is a fundamental requirement for many medical devices. As the size of these devices continues to decrease, there’s a significant challenge for joining technologies to provide highly reliable connections. In many cases, the weld is required between parts with different geometries, such as wire-to-flat or a wire-to-round surface. In addition, the materials are often dissimilar, with the added complexity of accommodating different part platings.
The generic joining options available are crimping, soldering, ultrasonic bonding, resistance welding, and laser welding. Crimping and soldering have disadvantages that include low electrical conductivity connection, weak bonds, and unreliable bond longevity. Ultrasonic bonding is an effective technique for joining copper, but it’s limited by the part’s resistance to mechanical force, joint access, and joint geometry. Resistance welding has been, and remains, a viable option. But with small electrodes, there’s an increasing emphasis on electrode maintenance and potential design limitations due to weld accessibility. "Laser welding offers an autogenous noncontact process with good weld strength with no weld consumables," says Geoff Shannon of Miyachi Unitek (Monrovia, CA) and the ability to scale the size of the weld either up or down according to requirements. However, due to the reflectivity and reflectivity mismatches of dissimilar materials, such as gold, silver, and steel, the 1064-nm laser has struggled to provide a reliable microwelding solution."
In addressing the fundamental issue of high surface reflectivity, a unique green 532-nm pulsed Nd:YAG welding laser has been developed by Miyachi Unitek. This single laser source operates in the same way as the 1064-nm pulsed Nd:YAG welder used in many medical seam-sealing and mechanical microwelding applications. It offers real-time power feedback during each pulse to ensure excellent pulse-to-pulse stability, fine programming resolution of peak power and pulse width to tune the process, a multimode beam output that’s essential for stable microwelding and a robust design that comes with a mature laser technology.
The 532-nm Nd:YAG laser is a highly capable means of microwelding conductive materials; for example: welding 0.004" (0.10-mm) diam gold wire to metalized pads; welding gold-coated 0.002" (0.051-mm) diam steel wire to a steel tube; welding 0.01" (0.254-mm) diam stranded silver wire to a silver-plated copper terminal; welding 0.0007" (0.002-mm) thick nickel-plated copper flat to a gold metalized pad. "When you are developing applications for conductive materials using the green laser, the welding stability is excellent. In short," Shannon states, "the 532-nm Nd:YAG laser welds gold, silver, and copper with the same ease that the 1064-nm laser welds steel and titanium."
The trend in recent years to use new materials and to minimize the sizes of medical devices, implants, and instruments has resulted in new challenges for laser processing in manufacturing. For example, one challenge is the capability to manufacture nonmetal stents.
Stent manufacturers, globally, are working on using the advantages of nonmetal materials. Standard laser-fusion cutting with fiber or pulsed solid-state laser is based on the concept that the focused beam produces melt, which can be blown out of the cutting slot by using high-pressure processing gas. Nonmetals react differently to the focused laser beam, which may impact the melt phase.
With focus intensities of 1012 W/cm² or higher, however, almost every material vaporizes. "This vapor produces a high pressure inside the cutting kerf," explains Juergen Stollhof, program manager, microprocessing at Trumpf Inc. (Farmington, CT), "and blows out the residual material. With decreasing pulse duration, the melt-to-vapor-ratio begins to move closer to vapor. Using a processing gas, like argon or nitrogen, is not necessary, but it can help to increase the edge quality. If the laser delivers a diffraction-limited beam, small cut widths down to 10 and 20 µm are possible."
Another advantage of ultrashort pulses indirectly helps to improve cut quality. Short pulses allow an efficient way to convert the laser wavelength into green or UV. These photons—with two, three, or four times higher photon energy—break the atom-atom-bondings directly. The ablated particles vaporize before they can transfer the excess kinetic energy into heat to the remaining material. The ablation process is supported by this nonthermal light-matter-interaction. HAZ is therefore minimized, and the high quality of the cutting edge allows additional production steps to be minimized, or eliminated.
"The use of ultrashort laser pulses with less than 10 picosec pulse duration is now finding its way into production."
The principles of the ablation process have been recognized for decades. "Because of industrially-proven solid-state laser technology—such as disk technology—the use of ultrashort pulses with less than 10-picosecond pulse duration is now finding its way into production," Stollhof says. An average power of up to 50 W at 1030 nm, 30 W at 515 nm, or 10 W at 343 nm, as well as repetition rates of up to 800 kHz, allow the use of picosecond lasers with economical benefits. The "picosecond" laser can produce a single pulse that physically measures only 0.01" (0.254-mm) long. This ultrashort pulse width enables material processing with minimal thermal damage and extremely high-accuracy processing of both plastics and metals.
But more development work will be necessary. Not only are laser manufacturers being asked to increase power from IR down to deep UV, new beam guidance and management must be developed to use this power—yet still produce advantages regarding improved quality from using picosecond or even femtosecond pulses.
Some of the cornerstone laser applications such as hermetic sealing of implantable pacemakers, cutting of metal stents, and marking steel and titanium parts were derived from a single laser source—the Nd:YAG laser. This source provided a starting point for lasers in the medical device industry. The range and diversity of lasers currently available provide an order of magnitude increase in laser options, ensuring that lasers not only maintain a place in the toolbox for medical device manufacturing but offer a means for continued growth. ME
This article was first published in the May 2011 issue of Manufacturing Engineering magazine. For a PDF of the original article, click here.