Faster Laser Processing for Medical DevicesNov 19, 2014
Microsecond (ms) fiber lasers have been used successfully for medical device applications like hypo tube and stent cutting for many years. While precise and fast, the downside has been that the parts require a number of post processing operations after they are cut, which add significantly to part cost and may damage mechanically delicate parts.
Ultra-short femtosecond (fs) laser technology produces pulses that leave no thermal fingerprint on the part. These disk-based femtosecond lasers offer sub-400fs pulses, plus “best in class” beam quality, and peak power that enable an extremely high quality cold ablation cutting process rather than a melt ejection process. The resulting cut requires minimal post processing and the smaller beam size allows machining of very fine details.
The process works especially well for producing medical devices, like catheters, heart valves, and stents; for medical and glass cutting and marking applications; as well as for 3D-structuring of ceramic material for dental implants.
Perhaps the most interesting potential for fs laser technology is on a new class of bioabsorbable materials, polymers that safely remain in the body for controlled lengths of time before absorbing, which are being developed as an alternative to traditional polymers or metal components.
In the past, fs lasers have been considered too slow for commercially viable operations. Recent studies evaluated cutting time per part and post processing steps, demonstrating that the return on investment for a disk femtosecond laser can be less than 12 months, especially for high value components. Jenoptik and Miyachi America are jointly developing both stage and scan head platforms that may be the foundation of this new level of quality and precision in micro treatment.
Femtosecond basics — Fs light pulses are ultra-short pulses (USPs). One fs equals 10-15 seconds, and as a calibration point a 300-fs pulse equates to a physical length of pulse of only 90 micrometers (µm). Because there is no thermal processing, as there would be in nanosecond (ns) pulses, USPs have many advantages, chief among them: a) No heat impact (no thermal tension in the material and no change of material characteristics); b) No shock waves (so, no structural changes); c) No micro cracks (smooth processed surfaces); d) No melting effects (again, no structural changes); e) No surface damages (so, no rework or after-processing); f) No debris (no cleaning is necessary); g) No ejected material (thus, clean surfaces); and h) No recast layer (i.e., smooth edges). These effects are illustrated in Figure 1.
Femtosecond laser technology has been used in R&D for more than 30 years, but commercial-ready fs technology that can last in an industrial environment with a 24/7 qualification has been around for less than a decade. Originally used for wafer dicing and scribing of P1, P2, P3 solar panels or creating channels in panels for electrodes, fs lasers are advancing into machining applications, and many medical devices are excellent candidates, especially given the high cost of the components machined.
In addition to the ROI justification of minimized post processing, the fs disk laser can create unique features that were previously not possible due to quality concerns, particularly with polymers processing. Figure 2 shows the comparison of a nanosecond 355nm source and a 1030nm fs disk laser source processing polypropylene. The appearance of the disk fs hole shows little taper, no melting or heat effect distortion around the hole. This enables product design freedom to maximize functionality with little or no compromise to the manufacturing process.
ROI of fs lasers for medical devices — The edge quality possible with a femtosecond laser for metals and plastics makes it excellent for machining of heart, brain and eye stents (both Nitinol and cobalt-chrome), catheters, heart valves, and polymer tubing. The nearly cold cutting process means very fine feature sizes can be cut into the thinnest material, while still maintaining mechanical and material integrity. No internal water cooling is needed for even the smallest Nitinol diameter tube.
The quality improvements and promise of reduced post-processing with fs laser technology were only theoretical until some manufactures (including Jenoptik) developed an ROI tool to demonstrate the true cost of post-processing. The tool can be used to factor in overall costs, including laser equipment purchase, post-processing capabilities, machine time, and handling time. The calculations demonstrate that femtosecond lasers are actually faster, because they alleviate several extremely time consuming post processing steps.
Figure 3 demonstrates this approach using the example of a coronary stent, one of the first devices to be manufactured with a fiber laser. The part has to be machined, then honed or cleaned inside with a mechanical tool, and finally deburred. Then a chemical etching process must be performed to clean around the edges, followed by an electro polishing step. These steps are time consuming and also may embrittle or deform the part, or induce micro cracks. Yields tend to be in the 70 percent range, meaning a significant amount of end product is lost.
By contrast, fs laser processing is a dry format (no water or heat is introduced), so the number of steps is drastically reduced. The part is machined and then undergoes an electro chemical process to round the edges. The integrity of the part is improved, several time consuming steps are eliminated, and yields can be closer to 95 percent.
The femtosecond laser is also the only current technology appropriate for machining medical products out of new bioabsorbable polymers, which can be safely implanted in the body for controlled lengths of time before absorbing, without causing harm or adverse interactions. Bioabsorbables (aka “aspirants”) provide an alternative to traditional polymers or metal components and are being designed to meet precise degradation rates and other specifications.
The bioabsorbable material can be machined into any profile that can be used for stents. However, it must be machined correctly and without inducing heat: Failure to do so might lead to crystallization in the material, which would degrade its structure and affect its lifespan and ability to dispense medicine at the correct rate. Also, because bioabsorbables dissolve they cannot be cleaned like most plastics, nor can they be touched with any liquid solutions, another reason the fs laser is a better choice for the material.
Figure 4 shows an fs laser cut of a bioabsorbable stent. Several firms are awaiting FDA approval to introduce such products in the U.S., and several have been qualifying fs laser equipment to gear up for the precision micro-machining required.
Integrating fs lasers into micromachining — The industrial robustness of the disk fs laser needs to be matched to an equivalent system to deliver daily reliability that the medical device manufacturing demands. Note that the fs laser cannot currently be fiber-delivered and therefore is directed and delivered to the focusing optics by fixed mirrors. Thus, designing a beam delivery system for a 4-axis tube cutter that can make off-axes cuts while maintaining alignment is a challenge. The optical path design has to ensure that such critical optical tools as the beam expander and fine-tuning attenuator are easily accessible as needed for process development. The system design requires full mechanical isolation, and in some cases ambient temperature stability, to provide a system foundation for process repeatability.
To gain the system integration capabilities needed to move the femtosecond laser capability into the marketplace, Jenoptik teamed with Miyachi America. The first developed platform was based on Miyachi’s Sigma Tube cutter, as shown in Figure 5.
Miyachi is taking full ownership of the systems, providing the first line of support, including sampling processing, quoting, and building of the work cell, as well as installation, training, service, and warrantee. The work begins with understanding the end user’s process to determine a specific application’s system needs.
Precision micro machining — Designing precision micro machining systems may appear to be a question of determining the degree of mechanical stiffness and isolation required, but there is more to it: Determining how delicate parts and materials will be repeatably positioned or clamped, implementing in-system part inspection, and incorporating real-time optical beam diagnostics also are critical.
For example, the beam is directed through the system by mirrors, so maintaining optical alignment is important, but this is only the first step. Ensuring that the beam profile and power levels are maintained requires the use of optical diagnostic tools – and these tools must be in line and non-intrusive, providing real time information. The tool is usually mounted directly after the laser and the last turning mirror in the beam path to enable deviations to be isolated to the laser or the optical beam path. Being in line and non-intrusive enables data collection during processing that can be time and date stamped as part of the manufacturing data.
The femtosecond disk offers exceptional process capability with excellent beam quality and high peak powers. To maximize the process capability for production the laser is integrated into a system that enables high quality and repeatable processing.
Stephen Hypsh is the vice president of vice president of Jepoptik AG’s Lasers - North America business unit. Contact him at Stephen@jenoptik.us.
Geoff Shannon, Ph.D. is the laser technology manager for for Miyachi Unitek Corp. Contact him at Geoff@muc.miyachi.com.