13

Sept

Lasers put flex in stiff microparts

November/December 2013 Volume 6 Issue 5

Ronald D Schaeffer - PhotoMachining

Ronald D. Schaeffer
CEO, PhotoMachining, Inc.
rschaeffer@photomachining.com

Many devices need to be made from a hard material that can flex. Manufacturing such a device usually requires cutting strategically placed, narrow slits into the material. Lasers are well-suited for the job because they cut precisely and have a narrow kerf. Moreover, lasers can process a host of materials cost effectively.

One area where lasers are increasingly being used for this purpose is the medical device market. Perhaps the best-known application is implantable stents. They come in a variety of materials—primarily metals—and in balloon-expandable and self-expanding styles.

A question some people ask about stents is, “Instead of cutting a pattern in a tube, why not use a spring or some other naturally flexible device?” There are several reasons. One is recoil. When deployed, recoil must be minimal or the stent strut will not properly hold open the artery. For the same reason, foreshortening (how much a stent retracts when deployed) must be minimized. In a nutshell, a spring doesn’t possess the characteristics needed for a stent. Laser-cut tubing does.

Lasing allows the production of stents that can be crimped on a balloon and then expanded radially to four or five times their original cut diameter. In addition, thin stent struts promote endothelialization (tissue growth on the stent). Struts with a 40µm thickness can be shaped with a laser beam.

laser machined medical part

The spiral pattern cut into this metal shaft makes it flexible. Image courtesy Miyachi Unitek.

Types of lasers used

A few years ago, most stent cutting was done with pulsed neodymium-doped yttrium-aluminum-garnet lasers. For the most part, though, fiber lasers have replaced Nd:YAGs in stent manufacturing. Nd:YAGs and fiber lasers have wavelengths close or identical to one another, but a fiber laser’s outstanding beam characteristics and low cost make it more attractive. Some metal stents, especially those made from exotic metals, are manufactured with ultrashort-pulse lasers (USPLs). They leave a better edge than nanosecond-pulse lasers, which minimizes or eliminates post-laser processing.

Many metal stents are implanted today. One downside to using metal is that if a problem arises, the stent must be surgically removed. This has led to some stents being made from bioabsorbable materials that dissolve in the body after a few weeks. They must meet design requirements similar to those for metal stents, including recoil, foreshortening, longitudinal compression and radial force.

laser stent

Lasers are used to cut patterns in cardiovascular stents, like this balloon-expandable stent from Boston Scientific. Image courtesy Boston Scientific.

Bioabsorbable materials melt at a relatively low temperature, which, given that they’re formulated to dissolve in the body, isn’t surprising. Because of their low temperature threshold, bioabsorbable materials must be cut with a “cool” laser beam. There’s a growing use of ultrashort-pulse femtosecond lasers for this application.

All stents are cut with a fixed-beam laser fitted with a lens having a short focal length. A 50mm focal length is typical, but, in practice, manufacturers use the shortest length the application permits. This results in a cleaner cut. It also minimizes the possibility of the beam striking the tube wall opposite the beam entry-point and cutting through the wall, making a mark on it or compromising its integrity in some other way.

A fixed-beam system also allows application of a high-pressure assist gas, which further promotes clean cuts. The type of gas used depends on the material, the laser and the cut quality required.

Other ‘flex’ apps

Many devices need to be made less rigid with a post-processing operation. Among them are the conductive circuits used in aerospace components. Because of the extreme temperature, pressure and humidity variations experienced in flight, a circuit’s substrate experiences a lot of stretching and shrinking that, over time, can cause its copper traces to break. Lasers are used to make cuts in strategically located areas of the metal traces that relieve the stresses that cause them to fail.

laser machined circuit

Potomac Photonics developed a process for laser cutting 10µm-wide circuit channels and filling them with a conductive material. Image courtesy Potomac Photonics.

An interesting technology developed by Potomac Photonics takes a different approach to circuit flexibility. The technique, which the Lanham, Md., shop calls “mill and fill,” combines the production of a circuit’s channels and the fabrication of conductive traces in a single setup. An ultraviolet laser first cuts the channels and drills vias in a substrate. Then, the voids are filled with a nanoparticle silver paste to form 3-D traces, some of which are narrower than 10µm.

The “forms we can produce are no longer limited to 2-D,” said a Potomac Photonics spokesperson. “We can make a variety of shapes and geometries, and even create electronic devices on conformal surfaces. Potomac’s laser fabrication technique allows for easier miniaturization of flex circuits, essentially making [a circuit] a ‘smart’ interconnection device.”

Besides circuits and medical devices, lasers can add flex to many other types of products. And, lasers are well-suited to similar tasks, including trimming pressure sensors, wire trimming and stripping, and cutting capillary burst valves in microfluidic devices. µ

— R. Schaeffer