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Application Focus: Medical device manufacturing

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New regulations dictate that certain medical devices must be marked with a unique device identification code. Here we explore how different laser technologies can be used to produce these markings

European manufacturers and distributors of medical devices are currently adapting their processes to the latest requirements of the new Medical Device Regulation (MDR)1, introduced in 2017 by the European Union. 

The manufacturers are coming to the end of a subsequent three-year transitional period that has allowed them to ensure their processes, data and documentation meet the requirements of the new regulation, which aims to ensure better protection of public health and patient safety.

One of the demands of the new regulation is that a comprehensive EU database of medical devices be established, along with a device traceability system based on the use of unique device identification (UDI) marks. Similar demands were already made in America towards last year, when on 24 September, under FDA regulations, it was made mandatory that re-usable, non-implantable, class II medical devices bear a permanent UDI mark.

Above image: Pulsed lasers can be used to produce high-contrast UDI markings on medical devices that are resistant to corrosion and repeated sterilisation and cleaning cycles. (Credit: Foba)

UDI-codes applied directly on medical devices, such as surgical instruments and implants, make every single item explicitly traceable throughout its product life cycle, from manufacturing to usage on a patient. In addition to this being important for the safety of patients and the enhancement of product quality, it is also of important – with the increasing digitalisation of the health sector – that these codes be of high contrast, in order to be both human and machine-readable.

Considering the extensive repeated processing in medical practice, these requirements place especially high demands on the lasting stability and reliable machine readability of the UDI mark. Over the product lifecycle, the marking must be reliably resistant against corrosion and fading, especially against the hundreds of rigorous cleaning procedures – for example steam sterilisation accompanied by a high alkaline (pH 14) cleaning process2 – that the devices will have to undergo through everyday clinical use.

Laser marking is ideal for the direct marking of medical devices. It is fast, precise – some parts, such as bone screws2, require markings as small as 0.2mm – economic, automatable, and is well-suited for volume production. 

Ultrafast marking

According to laser manufacturer Coherent3, a problem exists when traditional laser marking applications involving CO2 lasers, solid-state nanosecond (DPSS) lasers, and continuous-wave fibre lasers, are used to create markings on medical devices made from stainless steel.

In a photothermal process, these lasers use a tightly focused beam to deliver intense heat in a highly localised manner, raising the material temperature to induce a change, such as a colour change or an engraving on the surface. A near-infrared output from fibre lasers or DPSS nanosecond lasers, for example, can be used to produce high-contrast black marks. The black appearance of these marks, however, is primarily due to the creation of an outer layer of oxide, which can compromise the corrosion resistance of the surface. Re-passivation of the medical device – the stainless steels alloys used already have a natural passivation outer surface of chromium oxide – is therefore required following this process, according to Coherent, which typically causes this type of mark to fade. In addition, for multi-use products, oxide marks also fade with repeated autoclaving, causing the contrast to eventually reduce below a level readable by certain machines. 

It is therefore recommended by Coherent that picosecond lasers are used for the marking of stainless steel medical devices. This is because the pulse duration they offer is typically shorter than the time for heat to flow out of the laser interaction zone, even in metals, so peripheral thermal effects are vastly reduced, compared to nanosecond lasers. In addition, a much higher portion of the total laser power is used to remove material, rather than produce unwanted heating. Lastly, due to the pulse width being a thousand times shorter than a nanosecond laser, the peak power to average power ratio is around a thousand times higher, which enables unique interactions between the picosecond laser and the substrate, including multi-photon absorption. Here, the material is directly atomised in a relatively cold process, rather than being heated to vaporisation via boiling, as is done when using a CO2 laser for marking applications. 

Coherent has demonstrated that black marks (see above) can be created on 1.4301 stainless steel, using a picosecond laser with an average power of 7W, a pulse width of less than 15 picoseconds, and a maximum pulse repetition rate of 1MHz.

‘At first glance, these marks appear similar to the black [oxide] marks produced using nanosecond lasers. However, their actual structure is quite different,’ the firm remarked. ‘With picosecond laser marking, a major contributor to the high contrast black appearance seems to be a subsurface nanostructural change that results in efficient light trapping and light absorption, without significant change in the material composition.’

In testing, Coherent demonstrated that the created marks are naturally resistant to corrosion during repeated autoclaving and do not require any re-passivation for this purpose. In addition, neither passivation nor autoclaving cause any appreciable fading of the marks. 

‘This extends the lifetime of re-usable devices, lowering the cost of ownership,’ the firm said. ‘It also simplifies and lowers the overall cost of medical device fabrication, as it puts no restriction on the order of when, and in what order, the marking and passivation processes are performed. The bottom line is that these picosecond laser marks are more permanent and less restrictive to use than nanosecond laser marks.’

Coherent's recent decline in quarterly sales, due to challenging economic conditions in Chinawas partially offset by an uptick in medical device manufacturing sales in the region.

'We were...pleased by a bookings increase in medical device manufacturing, including a record 10 per cent contribution from China,' remarked Coherent president and CEO John Ambroseo. 'The Chinese medical device industry is expected to grow by as much as 25 per cent, so this may provide a partial offset to the broader Chinese materials processing market.’

Implant tracking

While UDI-marking is currently only mandatory for non-implantable medical devices, resistant and biocompatible UDI marks could also be used to improve the traceability of medical implants, according to laser marking firm Foba, particularly due to the possibility of revision surgery taking place if an implant malfunctions.

The firm highlighted that the German federal health minister, Jens Spahn, has recently presented a draft law for an official nationwide implant register – based on the already existing Endoprothesenregister Deutschland (EPRD) – that is intended to provide more safety for patients when they need knee replacements, pacemakers and other implants. The register would track where and when an implant was made, in addition to which patient has the implant. This would enable all patients who have had a particular implant installed, to be informed if a problem is discovered with their product. 

Laser marked dental implants. (Image: Foba)

Hospitals and medical professionals are requested to contribute to the current EPRD consistently, with the aim of tracking the lifespan (duration in the body) of registered implants as completely as possible. Patients are also able to report issues with implants.

In 2017, Add’n solutions, a service provider for UDI laser marking on medical devices, in close cooperation with Foba, conducted a long-term study in which laser-marked reusable surgical instruments underwent 500 sterilisation and cleaning cycles in order to prove the durability of the laser markings. 

The markings were proven to withstand the 500 cycles, however the study provided evidence that only by using an additional passivation process – developed to exactly match the marking process – can long-term protection against corrosion be achieved for marks made with both short-pulsed (nanosecond) and ultrashort pulsed (picosecond) lasers. 

Special polymers for welding

Clariant has unveiled medical devices and components made from new variants of its Mevopur polymer material, formulated to improve laser welding performance.

The chemical firm made the announcement at Medical Design & Manufacturing (MD&M) West in Anaheim, at the start of February.

Increasingly, laser welding is preferred in production of medical devices because it provides speed and reliability, can handle complex structures and avoids some of the downsides of other methods, such as solvent residues.

According to Eric Rohr, however, Clariant’s medical and pharmaceutical segment manager for North America, because medical devices are frequently made of transparent or translucent materials, the polymer’s ability to absorb the laser energy often needs to be enhanced using additives.

While Clariant has offered such additives for many years, and in 2016 began using them in compounds used for laser marking applications, welding presents additional challenges to marking, as it involves two polymers rather than one – one polymer that is transparent to laser energy and the another that can absorb it to create the weld. The process can be complicated further by any pigments or fillers, which can change the way the plastic reacts to the laser.

At MD&M West, the firm displayed welded products that appear to involve two parts made of identical materials. However, two different formulations have in fact been developed in order to achieve laser transmission in one, and absorption in the other, enabling them to be welded together reliably.

Another important factor in achieving a good weld, according to Clariant, is the even distribution of the additive throughout the polymer matrix of the final part. In some cases, a concentrate, or masterbatch, can be dosed at the injection-moulding stage of production, where the injection-moulding machine mixes it sufficiently into the polymer melt before moulding. Injection-moulding machines, however, are not always ideal for dispersing the concentrate into the host polymer. In some applications, the machine, the material or the part design may cause inconsistent distribution and lead to unreliable welding.

Clariant addresses this by also offering compounds where the job of distribution of the laser-absorbing additive, along with any other pigments or additives, is performed on highly efficient compounding lines. The injection moulder can use this all-in-one material without further dilution, with the formula and quality control having already been taken care of.

The new laser-friendly materials are manufactured at facilities that are certified compliant with ISO13485-2016, the latest quality management standards for medical devices.

References

[1] Better Patient Safety due to Laser Marking on Medical Implants – Foba Laser Marking + Engraving

[2] Laser marking of reusable surgical instruments mastering multi-process requirements – Foba Laser Marking + Engraving

[3] Permanent Marking of Stainless Steel Medical Devices Without Post-Processing – Coherent

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