Light structured stents
In recent years, bioabsorbable and drug eluting stents have come onto the market made from new and exotic materials. The laser – and particularly the ultrafast laser – has emerged as an effective means of processing medical stents. There is even work ongoing to structure the stents with lasers to promote or discourage cell growth, depending on where it will be implanted inside the body.
The introduction of ultrafast, high-power UV and fibre lasers has offered the capability to process a broader range of materials at a higher quality. ‘The increase in laser source types and powers − particularly of the ultrafast lasers − over the last decade has maintained the ability for the laser to profile many more materials than we were dealing with before, when stents were principally based on stainless steel,’ according to Dr Martin Sharp, a leader of the Photonics in Engineering Research Group at Liverpool John Moores University in the UK, and president of the Medical Special Interest Group (SIG) at the Association of Industrial Laser Users (AILU).
Depending on the properties of the material, there are different laser machining methods, according to Dirk Müller, director of product line management at Coherent: ‘The one category the laser generates a local heat, and with this heat the material is quickly melted and evaporated away. This process is OK for anything that can tolerate some thermal impact.’
Ultrafast lasers are more suited to processing the newer biodegradable materials, Müller explained: ‘Then there is a second class of laser machining… and that is in the ultrafast range. These types of lasers are used whenever the material is quite delicate, or when you are trying to make sure that there is no heat affected zone (HAZ); that there are no micro cracks or structural changes to the material,’ he said. ‘These ultrafast pulses, for example, are used when you are trying to machine biodegradeable stents.’
Ultrafast lasers can also create new features on medical instruments, which were previously not possible. This includes functional surfaces, whereby laser technology is used to texture the surface of a medical implant in order to create a certain cell response. ‘You can now create a functional surface that either enhances or discourages tissue growth,’ described Müller. ‘Some implants you may plan to remove from the body again later on − in that case you want to discourage the body tissue from growing very strongly on the implant so that removal becomes easier.’
There is also the potential to encourage growth of specific cells using structured surfaces produced with ultrafast lasers, such as in the case of hearing aids. ‘The creation of spikes makes only neuronal cells grow on hearing implants,’ said Müller.
While no such functional surface is currently being machined onto stents, this is a research area that Dr Martin Sharp and the Photonics in Engineering Research Group at Liverpool John Moores University in the UK are focusing on. The researchers are trying to create a surface that will discourage smooth muscle cells in the artery from proliferating, as these types of cells are associated with blood clots, and encourage endothelial cells to grow and heal the artery.
For this project, a range of laser sources has been investigated, Sharp told Laser Systems Europe: ‘We are doing a lot of our work with a simple infrared 1,064nm nanosecond fibre laser. Picosecond and femtosecond lasers also have a role, because they can often offer more precision and less heat input. With pico- and femtosecond lasers, sometimes it is just purely cost and speed, whereas a high power nanosecond produces a surface that cells respond better to,’ Sharp said. ‘So there is a broad range of laser tools that always have their benefits and drawbacks.’
It is not only developments in materials and surface texture of medical devices, but their size. ‘I see an enormous amount of potential in micro fluidics, where you are creating very small channels for liquids inside of either a polymer or glass. Now, you are able to do an enormous amount of diagnostic tests on a very small sample size,’ said Coherent’s Müller. These types of rapid developments within the medical industry is something the laser industry has managed to keep up with until now, according to Müller: ‘I would say that the medical industry is working very closely with the laser industry in exploring new tools for processing medical devices.’
One way of achieving this is through application labs, where researchers with specialised designs can test different laser sources to see if a device can be produced. ‘In these application labs they have a whole range of laser sources,’ Müller pointed out. ‘A customer can come in and present the material they want to machine, and see if there is any laser in the lab that can be used to process it. So they don’t have to first make a £200,000 investment only to find out that their laser tool is not ideal. They can continue with their work of making medical devices, and not have to study the laser in action − it is our job to guide the customer to the right laser and laser process.
‘People are figuring out how to use lasers to create new features,’ Müller continued. ‘I think the laser technology is already there, it is more about learning the process, because there are so many ways that the laser can interact with the material that you have to study the best way to use the laser in order to create the desired features.’
However, although there is a strong relationship between the medical device and laser processing industries, Sharp believes that lasers are not always thought of as an option to manufacture a medical device because of a lack of understanding. The Association of Industrial Laser Users’ (AILU) Medical Special Interest Group (SIG) is trying to train advising bodies on how to educate people about the potential of laser processing. ‘We believe that every mechanical/manufacturing engineer coming out of a British University should understand that lasers can do things for them,’ Sharp explained. ‘A medical device may be developed in association with a clinician, and at some point a mechanical, design or a manufacturing engineer will sit down and work out how to manufacture it. That is the point where we want them to say: “How can we cut this? Can we weld it? How do we process it?” And they should be thinking of lasers.
‘A design engineer who is thinking of injection moulding parts of plastic should be aware of laser welding as a possibility. You go to a hospital and look around at the number of plastic devices; there are very specialised tools nowadays that use plastic. Welding transparent plastics together, welding dissimilar plastics to each other, being able to weld inside assemblies by shining a laser through the plastic to get to the joint line, are all innovative ways that a laser enables innovative design.’
Making a mark
But laser cutting is only one of the several stages a medical device undergoes during its production. ‘A stent is a good example showing that the laser cutting is just one part of the process − the stent manufacturing line is quite long,’ said Sharp.
The introduction of fibre lasers into the market has not only benefitted the manufacture of stents, but has allowed for higher quality laser marking. ‘Fibre lasers definitely have a big role to play in marking − probably 50-60 per cent of the laser systems we sell are fibre lasers now,’ said Tom Goodnow, Foba Laser. ‘Fibre lasers have a sharper beam profile, so you can create a better controlled mark,’ added Faycal Benayad-Cherif, Foba Laser.
And, the use of UV lasers has meant that a wider variety of materials can now be marked. ‘Traditional lasers were around 1,000nm, which do a good job marking certain plastics, but some coloured plastics do not mark that well with 1,000nm lasers − [the plastics do not] absorb the energy very well − but then sometimes these [plastics] absorb well with the 355nm UV lasers,’ Goodnow explained. ‘The best results we have had have been with UV lasers of 355nm.’
In the medical industry, traceability is of particular importance and is required by law. Laser marking is carried out at the end of production to apply information, such as batch and serial numbers, to ensure each instrument is traceable. ‘You should be able to go to a stent and then be able to find out which laser produced that stent, what parameters were put in place, and other features of that process,’ explained Sharp.
And, by 2016, both the USA and Europe require that all medical devices must contain a 2D code to further regulate the traceability of instruments used in the medical industry. ‘In the USA it is a requirement from the FDA that a 2D code must be put onto every component that is manufactured so that by 2016 every part will have a 2D component on it,’ said Benayad-Cherif. These 2D codes, which can fit more information into a smaller area, often have a more complex design and go down to a few millimetres in size.
A medical device is normally marked at the very end of its production, so any error during the laser marking stage would result in a significant loss for the manufacturer. ‘In the production process for a hip implant, for example, the marking is the seventh step, just before it gets cleaned and packaged,’ Benayad-Cherif pointed out. ‘There is a lot of engineering that goes in between − from the prototyping to the machining, coating, washing, measuring and so on. So, the laser marking is really a part of the process where you cannot make mistakes.’
Therefore, vision analysis tools are starting to be incorporated into laser systems, to prevent errors, ensure quality control, and to help manufacturers comply with the industry’s standards and requirements for codes.
As part of Foba’s MarkUS software, a camera is integrated into the scan head of a laser to find the part and accurately align the laser mark. This removes the lengthy process of trial and error to align the laser to the part, and also ensures that the position is correct before any mistakes are made. ‘The user doesn’t have to figure out where the part is − they can see the part, and bring the mark to the position where they want it on the part,’ explained Benayad-Cherif. ‘System integrators that are building a laser system… are usually responsible for very accurate part placement.
With our system, the positioning of the part doesn’t have to be anything near as accurate,’ added Goodnow.
It is not just inaccuracies in mark placement where errors can occur. In some cases, such as the marking of knee implants, subtle differences between the length of the left and right parts are hardly noticeable to the human eye, which could result in the wrong part being marked, or the same part being marked twice. ‘If you put one [knee implant] in each hand, there would be no way you could tell which one was longer unless you measured them,’ said Benayad-Cherif. With the part validation feature in the software, the vision system compares both ends of the implants, and if one is longer or shorter than expected, it will reject the part and not mark it. ‘It ensures that the operator hasn’t put the incorrect part in the machine before it gets marked,’ Benayad-Cherif detailed. ‘Even if the wrong part is placed into the machine, the system would flag it and not mark it because it is not what [the machine] is expecting.’
After the laser has marked the device, it is also important to verify that the mark is correct, contains all the necessary information, and that it meets standards. The 2D codes that will be required on every medical device by 2016 can sometimes degrade, according to Benayad-Cherif: ‘One of the problems with these 2D codes is because of contaminants, surface preparation, or because the laser is degrading or losing power, the code itself could degrade,’ he explained. To help ensure that the code is of an acceptable quality, manufacturers have to abide by national standards relating to the quality of the mark. The vision system analyses the mark, and if the contrast of the mark has dropped such that it doesn’t fall into the highest grades of the standard, the software will then flag that part up to the user, said Benayad-Cherif.
Another feature of the vision system is Optical Character Verification (OCV), which is used to ensure that the characters that make up serial or lot numbers are readable. Bone screws, for example, are marked with characters the size of a few hundred microns, and sometimes even a small scratch on the screw not visible to the human eye can stop the character from being read as it should be. ‘Hundreds of these screws are manufactured at a time, so you could end up marking a thousand screws and only realising the mistake at the end,’ Benayad-Cherif said. The vision system is capable of analysing each character individually, and if a character is not readable it can be spotted straight away. ‘With this system, you know exactly what happens and exactly when it happens so that the customer can correct it and prevent wastage,’ Benayad-Cherif added.
The use of vision analysis for the marking of medical devices, as well as during laser processing, is becoming ever more useful for quality control in the production of medical devices. ‘The overall message of adding the vision capability to marking is that you have complete control over the marking process − you are not depending on operators to inspect the parts, and you can set the process parameters in such a way that you would be alerted the moment when something changes,’ Goodnow noted.