The first modern surgical stent was inserted into the coronary artery of a patient in Toulouse in 1986. The implant – a wire mesh inserted and inflated inside the blood vessel by means of a catheter and a small balloon – fulfilled a very basic, if delicate function: widening an arterial passage to prevent further clots. This function had to be balanced against the risk of restenosis that could accompany emplacement, raising the patient’s susceptibility to thrombi still further.

Since then, stents have been put to work for a variety of urological, oesophageal and digestive applications, with some even capable of drug elution. Their manufacture, however, like that of all medical implants, requires the utmost care so as to minimise the potential for instigating new infections. Thankfully, the emergence of new manufacturing technologies is allowing OEMs to fine-tune their uses and thus enhance patient safety.

Eduard Fassbind is the founder and CEO of Swisstec, a manufacturing firm that utilises ultrashort-pulse (USP) lasers to fashion intricate devices and tools for the aerospace, automotive, and medical devices sector. "We have specialised in micromachining with lasers for more than 15 years," he says. "Our machining techniques are in great demand across multiple industries, including aerospace, electronics and life sciences. In particular, we have seen demand leap for micromachined parts among medical device OEMs. Almost every day, we’re producing components and devices ranging from stents and heart valves to endoscopes and needles."

Retaining the ability to cut through a variety of materials with a tolerance of only several microns, lasers are superior machining tools when it comes to achieving clean, dross-free cuts. Such precision is especially valued in the fashioning of blades and biopsy needles, where the sharpness of the cutting edge is essential in assuring quality of care for the patient. Conversely, lasers can also prove invaluable in ensuring jagged edges are reduced or eliminated wholesale in a device or implant – this is essential in the case of tools fashioned out of wire mesh, like stents.

Point of light

Multiple steps are required, however, in order to manufacture a usable tool or implant out of a tube. Firstly, it is important to combine all machining to form a single system. "It’s useful to complement the laser cutting or welding stages with the addition of mechanical machining stages," says Fassbind. "The integration of shaping units or high-speed grinding spindles, for example, expands the field of application of the laser, which is already very wide. Combining several lasers within a single system, such as for welding, cutting, drilling, ablating and marking, also creates a particularly broad application spectrum."

A single system, however, can entail more than the traditional two axes, depending on the device you need to have fashioned. "Most applications, such as fashioning a stent or a heart valve, only require two axes, because they just involve cutting through the centre of a metal tube," Fassbind explains. "More complex medical devices, such as biopsy needles, require up to eight axes. These are highly dynamic and flexible systems that facilitate almost any cutting geometry. Their precise NC-driven linear axes are able to operate to an accuracy of less than one micron."

USP lasers fire in periods of picoseconds or femtoseconds… The material is vaporised directly at the site of machining without it melting, leaving behind burr and slag-free edges and surfaces. 

Regardless of the ultimate complexity of the device, a high level of product quality is crucial. In the case of stents, it must be the cutting edges and surfaces of these very delicate and complex mesh-like structures must remain burr and slag-free, lest they cut the arterial wall after insertion and initiate the growth of scar tissue. To achieve this, the material in the vicinity of the cut made by the laser into the pre-formed stent must be protected, by minimising the size of the area affected by heat.

This is all dependent on which laser the producer chooses for the task at hand. Generally, that choice is between what is known as a femto laser – or USP laser – or a fibre laser. "We have always prided ourselves in remaining at the forefront of laser machining," says Fassbind. "We integrated femto lasers into our operations and sold devices fashioned using those techniques years before many of our competitors. Although we might be a small company, we’re very flexible in which laser technologies and machining solutions we use for our customers."

As Fassbind has found out, both techniques have their advantages. Since 2004, Swisstec has utilised fibre lasers as the workhorse for much of its machining needs, as is common throughout the industry. This is partly down to the low capital investment required in installing and operating a fibre laser machining production line. Compared with older techniques, fibre lasers also boast higher processing speeds.

"High rates of repetition and effective process optimisation have enabled a cost-efficient cutting speed to be achieved for manufacturers across the medical devices sector," says Fassbind. "This application is rapidly becoming more efficient than conventional processes, especially in view of the fact that product quality cannot be achieved in any other way and post-processing steps are no longer required."

The modular design of the fibre laser also enables micromachining systems to be expanded individually, and is therefore unrestricted by mechanical machining stages, laser sources or the limitations of linear axes. "Automating the process at Swisstec through the addition of tube loading systems and the automatic removal of finished components now enables three shifts to operate, seven days a week. The expansion of wet cutting also improves the quality and speed by cooling the tube and protecting the material on the opposite side."

Short pulses

There is a disadvantage in relying solely on fibre lasers for your machining needs: such as the intensity of the beam generating unwanted heat in the material it is meant to cut. "It uses a long pulse during its operation," says Fassbind, "and if you have that, you also run the risk of developing slack and micro-cracks in the surface of your device. Ultimately, it will fail if it is used to cut materials like polymer, nitinol or magnesium."

This drawback led Swisstec to embark on a concerted research and development effort to find an alternative laser-machining technique. Eventually, scientists at the firm advocated adoption of a new technique that avoided heat generation altogether: the USP laser.

High rates of repetition and effective process optimisation have enabled a cost-efficient cutting speed to be achieved for manufacturers across the medical devices sector. 

"Generally speaking, the cutting properties and the quality required can usually only be achieved by using what is known as ‘cold’ machining," explains Fassbind. "USP lasers fire in periods of picoseconds or femtoseconds. By doing this, the material is vaporised directly at the site of machining without it melting, leaving behind burr and slag-free edges and surfaces in the machining area."

This has allowed Swisstec to expand the number of materials they are able to cut using lasers alone. "Laser radiation in the green wavelength range enables the laser to process a larger number of materials, including temperature-sensitive materials," he says. "The reason for this is that many of the latest polymers used in medical device technology demonstrate particularly good absorption properties in this wavelength spectrum. This makes the machining of these products using a green USP laser particularly attractive."

Depending on the laser and optical instruments used, very fine and complex structures with gap widths of 5µm and web widths of 20µm can now be manufactured. Furthermore, the general lack of melt-free ablation drastically reduces the amount of time spent fine-tuning the device after machining, and therefore the long-term manufacturing costs for customers. "Time and cost outlays can be reduced by up to 70-80% using this technique," asserts Fassbind.

In the short term, however, the utilisation of USP lasers remains expensive. So far, manufacturers have proven unwilling to end their widespread reliance on fibre lasers for the new technology. But Fassbind is confident that certain trends in the sector, like the incorporation of polymers in newly developed components, will mean a gradual uptake in the technique.

"The basic principles behind the laser technology we currently use have been known for many years," says Fassbind. "But now, I think it is really the time to invest in the technology. For me, fibre lasers are akin to a light in your home: you can turn it on or off at the push of a button at a relatively low cost. You can’t yet do this with a USP laser. However, when you take into account the total cost of the parts and the technique’s ability to reduce the need to tweak the device after its initial fashioning by the laser, I think you’ll see costs decrease."