Klaus Kleine and Dr Matthias Schulze, Coherent
Femtosecond lasers deliver superior micromachining results in most materials used in medical devices; recent advances in laser performance and cost of ownership facilitate their deployment for a growing range of medical devices ranging from nitinol stents to plastic—for example, polyether ether ketone (PEEK)—catheters.
Micromachining with ultra-short pulse lasers
Medical devices continue to migrate towards more complex shapes, smaller feature dimensions and tighter tolerances. This, in turn, drives the use of next-generation fabrication tools. Micromachining tools incorporating ultra-short pulse (USP) lasers are at the forefront of this trend and are used for drilling holes, cutting slots and grooves, texturing surfaces, and cutting product outlines with difficult shapes.
Most laser processing applications use a tightly-focused beam, in combination with motion of the substrate part, the laser beam or both, to remove material with high spatial precision. The advantages of processing materials with pulsed lasers in this way are well-established across many industries. The method is non-contact with no tool wear; it is very flexible, making it well-suited to 3D part fabrication, and can be configured to produce a variety of parts using CAD/CAM software. Moreover, laser processing is a single-step dry method with no toxic chemicals and is capable of much higher spatial precision than mechanical cutting and drilling methods. Plus, it can be applied to a wide range of materials including metals, plastics and glass (unlike, for example, electrical discharge machining (EDM), which can only work with conductors).
As feature sizes in medical devices are increasingly miniaturised, a challenge in laser processing is to deliver even higher spatial resolution and minimise peripheral thermal damage such as melting or microcracking in surrounding material: the so-called heat affected zone (HAZ). It is also desirable to minimise debris and recast material and create smooth surfaces, as this avoids a requirement for post-processing, such as cleaning.
Traditionally, most laser micromachining tasks used lasers with a pulse duration of 40 to 60 nanoseconds (a nanosecond is a billionth or 10-9 of a second). However, for producing holes and slots smaller than around 10 μm, for processing thin or delicate substrates or for making cuts with very smooth edges (as in some stents), these lasers are no longer optimum.
The advantages of using shorter laser pulses
Lasers with a USP output are a proven method of delivering higher resolution, with correspondingly minimised HAZ. That is why many medical devices are now fabricated using picosecond (10-12 of a second) lasers such as those in Coherent’s Rapid series. With this very short pulse duration, the vaporised material carries away much of the unwanted heat before it can spread into surrounding material and cause a HAZ. Also, the fact that the ejected material consists of very small particles (for example, atoms) means that picosecond laser pulses do not produce recast debris, therefore leaving clean and smooth surfaces.
USP lasers are typically characterised by much lower pulse energies than nanosecond lasers but with very high pulse repetition rates—usually in the 1 to 50 MHz regime. Consequently, each pulse removes a minute amount of material with minimal thermal damage, enabling unmatched depth control. At the same time, the high pulse rate means that overall material removal rates enable practical throughput rates for many device types.
The move to femtosecond lasers
Recently, the interest in using femtosecond (10-15 of a second) lasers for processing some medical devices has increased for three reasons. Most importantly, it meets a growing need for increased miniaturisation and superior edge quality and surface smoothness. This is because the pulse duration (1,000 x shorter than picosecond lasers) further increases the advantages described previously, resulting in an athermal process. This is particularly valuable when processing thin films and delicate materials where no HAZ can be tolerated.
The second reason is the increasing use of mixed and layered materials, for example, bioabsorbable plastics on metal or polyimide on glass. The extremely short pulse width of femtosecond lasers produces very high peak pulse powers, which, in turn, drives non-linear (multiphoton) absorption in the material. Unlike traditional (linear) absorption, this does not depend upon wavelength, meaning that the femtosecond laser can work virtually any material, even if it is transparent, such as glass. This allows coated and laminated substrates to be processed in a single step, enabling streamlined and lower cost fabrication in many cases.
Finally, femtosecond lasers are becoming increasingly attractive to industrial users because of recent improvements in their performance, lifetime, reliability and cost of ownership characteristics. Originally, femtosecond lasers were used exclusively for scientific applications. However, in the last few years, femtosecond laser manufacturers such as Coherent have developed a new laser material, namely ytterbium-doped fibre, which is scalable to much higher power. Also, because the laser material is in the form of a fibre, this new generation of industrial femtosecond lasers has simpler internal design and construction, leading to lower costs and significantly increased reliability.
For example, the Monaco series of lasers from Coherent provides up to 60 watts of processing power in a compact (667 x 360 x 181 mm) sealed package, which given its lower capital cost and increased reliability, makes femtosecond laser processing economically competitive for many medical device applications.
Moreover, these lasers are available at several levels of integration. Options include standalone lasers, laser engines with scanning/focusing optics, complete tools with integrated part handling and even complete solutions with custom software pre-optimised for a specified set of results for particular applications.
Femtosecond lasers in action
As already noted, there are several ways to implement laser processing based on how the parts and/or the laser beam are moved relative to each other.
Fixed beam tube cutting
For devices cut from tubular blanks, common tasks are to make cylindrical cuts as well as intricate patterns of cuts to produce cardiovascular and peripheral stents. Process evaluation in the applications laboratory and at customer sites confirm that the use of femtosecond lasers results in stents with superior strut-to-strut consistency and residual strength. Here, the laser is typically integrated in a workstation in which the blank is mounted to a moving stage with four axes of motion (three translation and one rotation). The use of a femtosecond laser allows for kerf cutting of tube stock or flat stock material with micrometre-scale precision and tolerances as shown in figure 1. Processing is sometimes accompanied by high-pressure co-axial assist gas to help remove vaporised ablation debris when cutting thick-walled materials.
Figure 1: Some examples of different medical device materials processed using Coherent’s femtosecond lasers. Notice the precision, edge quality and clean and smooth surfaces.
2D scanning
A different approach is needed for surface texturing of contoured materials, such as catheter balloons, or surface ablation of flat stock materials. Here, a 2D scanner workstation is usually the optimum solution employing a high-speed two-axis galvanometer scanner to cover a 20 cm radius field. The use of a femtosecond laser enables highly precise results with sub-micrometre depth control as shown in figure 2.
Figure 2: Examples of surface texturing and patterning in polymer and metals using Coherent’s Monaco femtosecond lasers.
Multi-axis scanning and part positioning
Yet another approach has been optimised to perform tasks such as precision hole drilling in irrigated ablation tip catheters with controlled wall taper and precision placement of slots and grooves or to create special shapes in tubes or flat stock material. Here, the workstation contains a five-axis trepanning scan head with co-axial assist gas along with a five-axis motion control system. Again, the femtosecond laser provides sub-micrometre dimensional precision and clean surfaces with no need for post-processing.
Summary
Device manufacturers face a challenge to produce ever smaller and more precise components, while simultaneously reducing cost. USP laser micromachining supports this trend in several ways, since it naturally delivers small features without damaging, heating, cracking or otherwise affecting the bulk material, while its minimisation of debris and recast material largely eliminates the cost of post-processing cleaning.
Coherent