Femtosecond laser processing

Microsecond (ms) fiber lasers offer precise and fast hypo tube and stent cutting but require a number of post-processing operations, which add to part cost and can damage delicate parts mechanisms.

In recent years, ultra-short femtosecond (fs) laser technology has been introduced, producing pulses that leave no thermal fingerprint. Disk-based femtosecond lasers offer sub-400fs pulses and peak power for a high-quality, cold ablation cutting process opposed to a melt-ejection process. The resulting cut requires minimal post-processing and the smaller beam size allows fine detail machining.

The process works well for medical devices such as catheters, heart valves, and stents; for medical marking applications; and 3D-structuring of ceramic material for dental implants. The most interesting potential use is a new class of bioabsorbable materials – polymers that safely remain in the body for controlled lengths of time before absorbing – developed as an alternative to traditional polymers or metal components.

In the past, many have considered fs lasers too slow for commercial viability. Recent studies however, evaluated cutting time per-part and post-processing steps, demonstrating return on investment of less than 12 months, especially for high value components. A key aspect of the fs laser’s potential is the system platform, and Jenoptik and Miyachi America are jointly developing both stage- and scan-head platforms to reach a new level of quality and precision for micro treatment.

Femtosecond laser technology has been widely used in institutions and research centers for more than 30 years. Commercial-ready fs technology that can last in an industrial environment with a 24/7 qualification has only been around for the past 7 years. Originally used for wafer dicing and scribing of P1, P2, and P3 solar panels or creating channels in panels for electrodes, fs lasers are advancing into new machining operations and many medical devices are excellent candidates.

In addition to the ROI justification of minimized post-processing, the fs disk laser can create features that previously weren’t possible due to quality concerns, particularly with polymers processing (see picture on page 42). Holes cut in a polypropylene disk with a fs laser show little taper and no melting or heat-effect distortion around the hole. This enables product designers freedom to maximize functionality with little or no compromise to the manufacturing process.

Fs lasers for medical devices

The edge quality possible with a fs 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 allows fine feature sizes to be cut into thin material while maintaining mechanical and material integrity. Internal water cooling is not needed even for the smallest Nitinol diameter tube.

Companies, including Jenoptik, have developed an ROI tool to illustrate the true cost of post-processing. The tool can factor in overall costs, including laser equipment purchase, post-processing capabilities, machine time, and handling time. The calculations demonstrate that fs lasers are actually faster, because they alleviate several time consuming post-processing steps.

For example, the ubiquitous coronary stent, one of the first devices to be manufactured with a fiber laser, has to be machined, honed or cleaned inside with a mechanical tool, and finally deburred. Then, a chemical etch process cleans up around the edges, followed by an electro polishing step.

These steps are time consuming, and can cause the part to become brittle, deformed, or have micro cracks. Yields tend to be in the 70% range, with a significant amount of end-product loss. The fs laser is dry format – no water or heat is introduced in the part. The part is machined and undergoes an electro chemical process to round its edges. The integrity of the part is improved, several time-consuming steps are eliminated, and yields can be closer to 95%.

Bioabsorbable materials

The femtosecond laser is also the only current technology appropriate for machining medical products out of new bioabsorbable polymers. Next generation advanced bioabsorbables, or aspirants, provide 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 profiles used for stents, but must be machined without inducing heat. Failure might lead to crystallization in the material, that would degrade its structure and affect its lifespan and ability to properly dispense medicine. Because bioabsorbables dissolve, they cannot be cleaned like most plastics or touched by any liquid solutions, steps that fs laser production does not use.

Bioabsorbables are already being used for coronary stents in the European Union, although they have not received U.S. Food and Drug Administration (FDA) approval. Mostly composed of polyesters, bioabsorbables are showing promise for a variety of uses, including cardio stents for patients who can no longer tolerate a traditional fixed stent. The material is also being used to deliver medicines into body organs – a plastic material can be doped with medicine and inserted into the liver, dispensing medicine at a consistent rate for 6 months to 3 years before dissolving.

After years of clinical trials, several firms are waiting approval and planning for the new innovation to hit the U.S. market. Several have been qualifying use of fs laser equipment to gear up for the precision micro-machining required.
 

Fs laser micromachining tool

The industrial robustness of the disk fs laser needs to be matched to an equivalent system to deliver reliability. It is worth nothing 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-axis cuts while maintaining alignment can be a challenge.

The optical path design has to ensure that key optical tools – the beam expander and fine tuning attenuator – are easily accessible for process development. The system design requires full mechanical isolation, and in some cases ambient temperature stability, to provide a foundation for process repeatability.

The beam is directed through the system by mirrors, so maintaining optical alignment is important. 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. In-line and non-intrusive ability enables data collection during processing that can be time-and-date stamped.

To gain the system integration capabilities, Jenoptik teamed with Miyachi America. The first platform developed was based on Miyachi’s Sigma Tube cutter. 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 warranty. The work begins with understanding the end user’s process to determine a specific application’s system needs.
 

Future of fs disk

The fs disk offers in-class process capability with excellent beam quality and high peak powers. To maximize the process capability for production, the laser must be integrated into a system that enables high quality and repeatable processing. The combination of a highly robust femtosecond product with an experienced micro systems provider with in-house parts processing capabilities results in a partnership that can develop production system solutions for high value medical products.

Amada Miyachi America
www.amadamiyachi.com

Jenoptik
www.jenoptik.com

About the authors: Stephen Hypsh is the vice president, Business Unit Lasers – USA & North America at Jenoptik and can be reached at stephen.hypsh@jenoptik-inc.com. Geoff Shannon is the laser technology manager at Amada Miyachi America and can be reached at geoff.shannon@amadamiyachi.com.