Fibre lasers are used for a wide range of precision engineering applications within the medical sector. These lasers are able to cut, weld and mark a large variety of materials, both metal and non-metal, and bring many benefits such as remote material processing, accurate process control, reproducibility and high-energy efficiency in a compact, highly reliable product.
Control of the fibre design enables the beam properties and other performance characteristics to be optimised for different applications, for example the very fine focused spot sizes achievable with single-mode fibre lasers have enabled them to become the benchmark standard for the cutting of stents and other medical devices. Low-moded or multi-moded lasers in both pulsed and continuous wave (CW) product types, meanwhile, can be optimised for welding, marking and joining of metal, glass and plastic components.
There are a range of laser processing applications for medical devices. The largest applications group is laser marking followed by welding, drilling and micro-level processing. Each of these laser applications will require different process characteristics. Two key process parameters, focal spot intensity and interaction time, vary within the group of applications under consideration.
The interaction time for a pulsed laser is typically considered to be the pulse duration, while for a CW laser it is the contact time between the laser beam and a particular area of the work-piece, so for example if a 200µm laser spot processes a work-piece moving at 500mm/s, the interaction time would be 400µsecs.
Focal spot intensity is considered in units of W/cm², which is dependent on both the instantaneous power output of the laser and the size of the incident laser beam at the work-piece, and can vary considerably across the range of available lasers. For example, a pulsed fibre laser with an average power of 20W when operating with a pulse repetition rate of 25kHz gives a pulse energy of 0.8mJ with a pulse duration of 35nsecs (full width half maximum).
The high beam quality enables the beam to be focused to spot diameters, which can be smaller than 30µm, resulting in typical power densities at the work-piece greater than 108 W/cm² with a peak power of >12kW, making this an ideal type of laser for laser marking applications.
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Single-mode and low-moded lasers with output powers up to 400W can be focused to spot sizes <15µm, generating power densities in excess of 200MW/cm², enabling a process capability that ranges from high-speed precision thin metal cutting to thicker section cutting and metal welding. On the other hand, fibre-coupled laser diodes with output powers of up to 400W generate a larger focal spot size due to the lower beam quality, so the power density at the work-piece is in the region of 105W/cm², making this type of laser ideally suited for welding and joining applications, particularly for plastic materials.
Fine cut with fibre lasers
There are many instances within the production of medical devices where very high-precision cutting and trimming of metal components is required with high-quality, dross-free edges. Both preformed sheet parts and tube components can be processed, and in the medical industry today the trend towards less invasive intervention leads to miniaturisation that requires tighter tolerances, which in turn demands the use of fibre lasers.
Joe Lovotti, president of medical device component manufacturer Reliance Laser, says: "Without our fibre lasers we could not meet the requirements of today's surgical instrument manufacturers. We routinely receive prints with feature tolerances of +/-0.001.
"To hit a process capability index of 1.33Cpk (a measure of process capability indicating 99.99% process yield) requires approximately +/-0.0004in as the process variation during validation, and that is only if statistically the process is well centred. Fibre lasers allow us to meet these demands and manufacture surgical instrument components with fully validated processes. The high quality of the laser cut edge profile reduces the need for post-process finishing, which in turn reduces the number of production stages and lowers overall costs."
Depending on the material, a single-mode 100–200W laser is typically used for cutting thin section metal up to a few hundred microns thickness. With today's medical devices becoming smaller and more intricate by the day, the edge condition of the laser cut profile is becoming ever more critical to the smooth actuation of some devices. It is expected that surface roughness will become a standard part of laser cut specifications in the future.
The other key parameter is cutting speed. The high power output combined with the small focal spot size enabled by the high beam quality of these lasers allows high-quality cut edges to be made with very little dross and very low heat-affected zone (HAZ). The high power density and small focused spot size result in fibre lasers having unparalleled cutting speed capabilities for thin section metals.
Even with the fastest motion systems available the fibre laser is capable of cutting at least two or three times faster than the maximum speed that the part can be moved. In addition, the laser pulse repetition rate can be continuously varied to control the average output power of the laser in order to avoid overheating the work-piece in highly detailed regions. Figure 1 shows some examples of fine laser cutting using fibre lasers.
Stent cutting using lasers
There are many medical situations where an artery, a blood vessel or another duct needs to be held open using a stent. These components are usually supplied as a tube-shaped metal mesh typically manufactured by taking a solid-walled tube and removing the unwanted material by profile cutting.
Many of the critical parameters for the stent profile cutting process relate to the surface quality of the processed edges where it is desirable to have narrow, burr-free cuts with minimal HAZ. Fibre laser cutting of stents provides a non-contact cutting solution with low levels of thermal stress, which can currently achieve 10µm cut width (known as a kerf width) in 2mm diameter stent tubes with 0.2mm wall thickness.
When combined with a precision motion system, highly repeatable parts can be manufactured with short cycle times, thus enabling significantly reduced stent manufacturing costs.
Lasers that can produce stable short pulses of 5–10µs pulse duration at continuously variable repetition rates between 1 and 50kHz enable a level of process control and processing speed not available with other laser technologies.
In the future, new designs for stents will involve narrower tube diameters, thinner walls and more complex designs, and tighter material property specifications. This will lead to tighter restrictions on the HAZ that occurs during the laser-cutting process, and it is anticipated that some materials may require cutting parameters in the nano or picosecond regime.
Making a mark
Laser marking is widely used within medical device manufacture and can be used to generate many kinds of indelible marks including serial numbers, barcodes, alphanumerics, logos, date codes and graphics. It can enable unique identifiers to be added to individual components allowing them to become permanently traceable.
There are many laser marking techniques for a wide range of materials, but only some are applicable to the manufacture of medical devices because certain marking methods, particularly those that involve ablation or engraving of the work-piece, cannot be used due to the localised change in surface profile. The marks must also not increase the susceptibility to corrosion and must be able to withstand many cycles of autoclave sterilisation without degradation.
There are several preferred marking techniques that allow a mark to be made without altering the surface quality of the component. For metals, particularly stainless steels, the preferred technique is anneal marking where the thermal input of the laser beam only causes changes in material properties without the need for generating melt, thus leaving the surface profile unchanged.
Figure 2 shows a typical use of this technique, where a 2D barcode has been marked onto a pair of surgical scissors. For polymers the preferred marking method is carbonisation, where controlled thermochemical changes produce dark marks on the surface of the component.
For many medical device marking applications pulsed fibre lasers bring many process benefits and provide a significant improvement to the quality of the mark due to the fine pulse controllability of the laser source. In addition, the small spot sizes that can be achieved with this type of laser as a result of the high beam quality allow the creation of fine marks using narrow line widths.
Fibre laser welding has many applications within the manufacture of medical devices including seam and spot welding of sheet metals, orbital welding of tubes and welding of fine wires for the manufacture of guide wire components as well as stents and other cardiovascular devices.
In many instances, particularly for implantable devices, laser welding is the preferred joining method due to the ability to simultaneously join and seal components with consistent, high-quality results where the weld properties must be non-porous and hermetically sealed. The high yield makes this welding technique suitable for use with high-value components as the highly stable laser output leads to welds with very little deviation in weld depth over the length of the weld.
The ability of single-mode fibre lasers to operate in closed loop power control mode, whereby the output power of the laser is continuously monitored and regulated, enables outstanding output stability and allows in-line process information of each laser pulse to be recorded in a straightforward manner.
Fibre laser technology also enables small or thin parts to be joined and many types of medical metals such as titanium and stainless steel can be welded. In such cases the temporal pulse profile of the laser output can be controlled by the user to generate a specific thermal profile to ensure that the joint is made with optimum heating and cooling conditions.
In some cases it is also possible to join components made of different types of metal. Figure 3 shows an example of stainless steel endoscope welding where a thin, formed tube is joined to an end-plate. Figure 4 shows examples of microwelding of narrow diameter stainless steel tubes – note in both cases the smooth weld surface and consistent weld quality around the tube diameter.
Certain welding applications require a larger spot size than can easily be achieved using a single-mode laser. A low-moded M2~4 laser with a modified 'flattened top' beam profile compared to the pure Gaussian mode of the single-mode laser can address these requirements. A low-moded M2~4 laser offers the process engineer a unique combination of high power density and optimised beam profile not previously available and enables significant improvements in process tolerance (depth of field) or working distance in comparison with conventional highly multi-moded beam sources.
For applications such as cell culture flasks, catheters, ostomy bags and micro-fluidic cartridges, laser welding using fibre-coupled diode lasers bring many benefits to the production process. The technique is clean-room friendly, with no surface contamination of the welded regions, no generation of particles and is also fume-free since no glue or solvents are required.
Laser processing enables outstanding flexibility in the production environment due to the laser's ability to simultaneously seal and join parts, weld close to thermally sensitive components and access restricted areas. In addition, the finished welded components have a good visual appearance. Laser plastic welding for medical components allows accurate, active process control, resulting in excellent process repeatability, and can be integrated into traceability process monitoring.
The range of potential applications of fibre laser technology within the area of medical device manufacture is enormous, and as many current applications approach maturity and are accepted into standard best practice, the expectation is that the number and scope of applications will continue to grow.
A typical example of a laser welded, laser marked implantable component is shown in Figure 5. This sector will always have product quality as the main concern, and fibre laser technology is ideally suited to these applications due to the very high stability of the laser output.