Steven Hypsh • Jenoptik & Geoff Shannon • Miyachi America
Microsecond fiber lasers have been used successfully for medical device applications, like hypo tube and stent cutting, for years. While precise and fast, the down side is that the parts require a number of post processing operations after they are cut, which adds to part cost and can also damage mechanically delicate parts.
Ultra-short femtosecond (fsec) laser technology, however, produces pulses that leave no thermal fingerprint on the part. These disk-based femtosecond lasers offer sub-400 fsec pulses, plus best-in-class beam quality and peak power well-suited for a 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 minute details.
The process works especially well for production of such medical devices as 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. But perhaps the most interesting potential use is on a whole 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, femtosecond lasers were considered too slow for commercially viable operations. But recent studies evaluated cutting time per part and post processing steps and demonstrated that the return on investment for a disk femtosecond laser can be less than 12 months, especially for high value components. A key aspect of realizing the laser’s potential is the system platform, and to this end, Jenoptik and Miyachi America are jointly developing both stage and scan head platforms that can fully unlock the promise of reaching this new level of quality and precision for micro treatment.
Femtosecond light pulses are ultra-short pulses (USPs). One fsec equals 10 to 15 sec. As a calibration point, a 300-fsec pulse equates to a physical pulse length of 90 µm. Since there is no thermal processing as there would be in nanosecond pulses, USPs have several advantages, including:
- no heat impact—no thermal tension in the material and no change of material characteristics
- no shock waves—no structural changes
- no micro cracks—smooth processed surfaces
- no melting effects—no structural changes
- no surface damage—no rework or after-processing
- no debris—no cleaning necessary
- no ejected material—clean surfaces
- no recast layer—smooth edges
Commercial-ready femtosecond technology that can last in an industrial environment with a 24/7 qualification has been around for about the past seven years.
The femtosecond disk laser can create unique features that were previously not possible due to quality concerns, particularly with polymer processing.
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 minute 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.
Jenoptik developed an ROI tool to illustrate the true cost of post-processing. The calculations demonstrate that femtosecond lasers are actually faster because they alleviate several time consuming post processing steps.
Take the example of a coronary stent, one of the first devices to be manufactured with a fiber laser. First the part has to be machined, then honed, or cleaned out inside with a mechanical tool, and finally deburred. Then a chemical etch process must be performed to clean up around the edges, followed by an electro polishing step. Not only are these steps time consuming, they can also cause the part to become brittle, deformed and can have micro cracks. Yields tend to be in the 70% range, meaning a significant amount of end product is lost.
By contrast, the femtosecond laser is a dry format—no water or heat is introduced in the part. The number of steps is reduced; the part is machined 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%.
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, without causing harm or adverse interactions. Next generation advanced bioabsorbables (also called aspirants) offer an alternative to traditional polymers or metal components and are 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 femtosecond laser is a better choice for the material.
Bioabsorbables are already being used for coronary stents in the EU, although they have not yet received FDA approval for use in the U.S. Mostly composed of polyesters, primarily homopolymers and copolymers of polylactic acid and polyglycolic acid, bioabsorbables show promise for a variety of uses, including cardio stents for patients who may have been stented numerous times and can no longer tolerate a traditional fixed stent. The material is also used to deliver medicines into organs of the body—for example, a plastic material like a sponge is doped with medicine and inserted into the liver, dispensing medicine at a consistent rate and lasting from 6 months to 3 years before dissolving.
Integrating the femtosecond laser into a micromachining tool
The femtosecond 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-axis cuts while maintaining alignment can be a challenge. The optical path design must ensure that such key 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 around Miyachi’s Sigma Tube cutter, as shown in Figure 5.