The laser micromachining of polymer-based medical devices typically involves tiny components with feature sizes and tolerances that range from 1 micron to 1 mm, with depths not exceeding 1 mm. Here are basic guidelines on laser types, wavelengths, and machining methods.

First, it is helpful to know that, in general, the choice of laser source depends on the material being processed. For metals, CO² and Nd:YAG are used because they operate at longer wavelengths, from 1.064 to 10.6 micron. Long wavelengths make laser micromachining a “thermal process” where electrons absorb the laser light and cause the metallic bonds to vibrate. This generates heat and results in the melting of the metal. Or, put simply, the material goes from a solid to a liquid phase. The liquefied material is then flushed away with liquid or gas assist. This approach is often used in the laser machining of metal stents to cut intricate patterns from nitinol, stainless steel, cobalt-chromium, or precious metal tubes.

Long wavelengths melt or discolor polymer materials, which makes them unsuitable for devices such as catheters or balloons. Therefore, lasers operating in ultraviolet wavelengths (193, 248, 266, and 355 nm) are preferable. As a general rule, shorter wavelengths produce higher photon energies for more efficient covalent bond breaking and ionization of the polymer. This “photochemical ablation” mechanism makes the material go directly from a solid to gas phase.

The laser wavelength choice depends on the material interaction and the needed machining quality. A laser source operating in the UV range minimizes the melting or discoloration of the polymer and allows the cutting of feature sizes less than 100 micron. Traditional mechanical methods such as drills, scalpels, or punches may not meet quality requirements due to excess burrs, flash, or inconsistencies from tool wear and breakage.

Excimer and DPSS lasers

In choosing a laser machining method, short UV (193 nm) excimer lasers work best to minimize melting or heat-affected zones (HAZ) for materials such as nylon, ETFE, PTFE, and other fluorpolymers or bioasborbables. Other materials such as polyurethane, polyimide, and parylene require more than one laser wavelength (193, 248, 266, or 355 nm), so more than one type of laser source can be used.

Excimer lasers, which typically operate up to 200 pulse/sec, use what’s called mask projection, where the laser source illuminates a noncontact mask and mask features are imaged by a lens downstream. This is similar to the semiconductor lithography process except that it is a material removal rather than a resistexposure process. For the ablation of polymers, the image on target is typically demagnified five to 10 times, as compared to the mask, to generate enough energy density (J/cm²) to ablate the material.

The raw excimer beam is quite large (typically 25 × 8 mm). Therefore, the beam can illuminate a mask etched with a large array of complex features. This lets all the features on the mask be printed at the same time on the medical device. Applications include drug-eluting balloons, embolic filters, nebulizers, drug delivery catheters, and membranes.

In contrast, diode-pumped soldstate lasers (DPSS) operate using a direct-write process. A DPSS laser has a small round Gaussian-distributed beam (typically 2 to 4 mm in diameter) and the beam is focused by a lens to a small circular spot of about 10 to 15 microns in diameter.

DPSSs operate up to 100,000 pulse/sec (or higher). Similar to an entertainment laser light show in the sky, the beam is directed by high speed galvanometers (XY spinning mirrors) to trace the machining pattern. For example, when the laser drills a hole of 200 microns in diameter, the small beam spirals within the circumference to etch away the material. DPSSs suit singulation processes, performing as a cookie cutter to cut out the device profile from the mother substrate. Applications include the singulation of medical sensors, catheter skiving, balloon trimming, and tube cutting.

Tolerances and feature sizes

As a rule of thumb, the hole diameter tolerance is proportional to the radius of the spot size. If the spot side is 20 microns, then the obtainable machining tolerance is 10 micron. For an excimer laser, feature tolerances can be as small as 1 micron. As in any application, the final choice often comes down to a trade-off between cost and quality. DPSS lasers have a lower operating cost, but excimers have a better hole quality.

In general, the theoretical smallest feature size is proportional to twice the laser wavelength. In practice though, for a CO² laser operating at 10.6 micron, the smallest feature size is 50 micron. For an excimer laser operating at 193 and 248 nm, the smallest feature size is 1 micron. With DPSS lasers operating at 266 or 355 nm, the spot size diameter determines the smallest feature size (typically 10 to 15- micron diameter).

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