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Small-scale laser cutting for stent, tube fabrication

Femtosecond lasers provide alternative for making hypodermic needles, stents

Figure 1
Long laser pulses deliver substantial amounts of heat that spreads throughout the workpiece (left). Postprocess deburring and polishing can require significant amounts of time. Short laser pulses concentrate the heat in the cutting area (right) and reduce the number of postprocessing steps.

Laser cutting for stent and tube applications is nothing new. Named for how the laser beam is developed in the resonator, CO2 lasers have been used in manufacturing for decades and fiber lasers have been in use for a number of years. While these lasers do very well on steel tube and pipe in the common diameters and wall thicknesses for consumer and industrial products, other lasers are designed to cut the materials, diameters, and wall thicknesses used in making medical devices.

Described by both pulse duration and resonator medium, microsecond (μs) fiber lasers have been used successfully for medical device applications like hypodermic tube and stents for many years. While precise and fast, the downside is that the parts require a number of postprocessing operations, which adds to part cost and can damage mechanically delicate parts.

A newer technology, the femtosecond (fs) disk laser (commercially known as Jenoptik's JenLas® femto 10 disk laser) produces pulses that leave no thermal fingerprint on the part. These lasers deliver pulses shorter than 400 fs and use cold ablation cutting rather than the conventional melt-eject process. The tiny beam size allows machining of very fine details, and the resulting cut requires fewer postprocessing steps. Typical applications are catheters, heart valves, and stents.

Femtosecond lasers have been used in institutions and research centers for more than 30 years, but commercial-ready femtosecond technology that can last in an industrial environment that runs 24/7 has been available for less than a decade. Once considered too slow for commercial operations, they recently were the subject of a study that evaluated cutting time per part and postprocessing steps. The study demonstrated that the return on investment for a disk femtosecond laser can be less than 12 months in many cases, especially for high-value components.

Originally used for dicing wafers, scribing solar panels, and making channels in solar panels for electrodes, these lasers are finding additional uses in machining. Many medical devices are excellent candidates, especially given the high cost of the materials.

Femtosecond Laser Basics

Femtosecond light pulses are ultrashort pulses (USPs). One fs equals 10-15 second; a 300-fs pulse has a wavelength of 90 micrometers (µm). Short bursts of laser light impart short bursts of energy into the material, greatly reducing the heat used in cutting (see Figure 1). Because it’s not a thermal process, cutting in the USP range provides several advantages:

  • No heat impact (it induces no thermal tension in the material and therefore doesn’t change the material’s characteristics)

  • No shock waves (no structural changes)
  • No melting effects (no structural changes)
  • No microcracks or recast layer (results in smooth surfaces)
  • No surface damage (no rework or postprocessing)
  • No ejected material or other debris (no cleaning necessary)

The edge quality possible with a femtosecond laser for metals makes it excellent for manufacturing heart, brain, and eye stents; catheters; and heart valves. It is suitable for two common alloys for these applications, nickel titanium (nitinol) and cobalt chrome. 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, even for small-diameter nitinol tube.

Postprocessing steps associated with conventional microsecond laser technology include mechanical, chemical, and electrochemical. Mechanical steps include machining, ID honing (cleaning), and deburring. Then a chemical etch cleans up the edges, followed by electropolishing to finish the entire item. Not only are these steps time-consuming, but they can cause brittleness, microcracking, and deformation. Many parts end up in the scrap bin; yields tend to be in the 70 percent range.

The femtosecond laser is a dry format, using no water or heat to cut the part. After laser cutting, a typical part undergoes an electrochemical process to round the edges (see Figure 2). The process improves part integrity, eliminates several postprocessing steps, and results in yields that approach 95 percent.

Figure 2
Stents made from 100-μm-thick nitinol show little difference in the laser-cut edge quality and the original surface quality. The removal rate was 0.25 mm per second. A femtosecond laser can cut material thicknesses up to 400 μm. (The images displayed here are copyrighted images of Jenoptik Laser GmbH.)

Micromachining Systems

Although micromachining systems can be used to make delicate, high-value components, Amada Miyachi’s femtosecond machines don’t require specialized setups. A typical machine footprint is 84 in. by 44 in. and weighs 1,500 lbs. The machine needs a flat, level surface but doesn’t need a reinforced foundation. The laser system and motion system are isolated from external vibrations, so it can work in a normal shop environment.

The laser resonator requires a 240-V, single-phase connection; the chiller and motion system require 110 V. The usual assist gas is argon.

The machines are delivered in either a Class 1 or Class 4 configuration. Class 1 is fully enclosed and interlocked to comply with ANSI Z136.1 and Center for Devices and Radiological Health (CDRH) requirements. It does not require operators or nearby personnel to wear safety glasses. A Class 4 machine is open and therefore requires operators and any person in the area to wear standard laser safety glasses rated for the wavelength of the laser.

The system uses a proprietary motion control interface, DeltaMotion, which can be configured to work with a variety of off-the-shelf brands of software and hardware.

About the Author
FMA Communications Inc.

Eric Lundin

2135 Point Blvd

Elgin, IL 60123

815-227-8262

Eric Lundin worked on The Tube & Pipe Journal from 2000 to 2022.