Almost every product that you touch in your daily life, from your smartphone to your car, probably has a multitude of laserproduced marks, either directly on it or on the many components inside it. These marks can serve all manner of purposes – such as aesthetic logos, personalisation for marketing, date codes, barcodes, serial numbers or unique IDs for traceability and anti-counterfeiting.

Although almost ubiquitous, there are a few challenging applications where laser marking has struggled for adoption. One of these is the medical device manufacturing market, despite there being a need for lowcost but high-quality, permanent, dark identification marks on stainless steel or titanium metal parts, which do not suffer from fading or corrode over time.

Laser marking: basics, history and background

Since light is absorbed differently based on the material to be marked, a range of laser types with different wavelengths, pulse durations and power levels have been used in laser marking. Historically, the most common laser types employed in marking have been low-power CO2 lasers, DPSS lasers and pulsed fibre lasers operating in the near IR, which are used extensively due to their compact size, low cost and high reliability in constant operation. These three types of laser have proved ideal for marking a broad range of materials, such as metals, plastics, polymers, organic materials, clear plastics, ceramics, composites, painted, coated or anodised parts.

The majority of laser marks are produced by a nanosecond-timescale laser-material interaction that is thermal in nature. Essentially, the energy from each laser pulse is absorbed at the surface causing heating, melting, ablation, oxidation or discoloration of the material, and hence a contrasting ‘heat-affected zone’ is traced across the surrounding untreated material creating the ‘mark’.

In some materials, however, this thermal cycling of the surface material by the laser does not produce a high contrast or aesthetically pleasing mark. In particular, metals such as titanium and alloys of stainless steel have proved notoriously difficult to mark well with nanosecond lasers. It’s thought that their combination of high thermal conduction and inherently passive surface-oxidising properties contribute to the typical low-contrast blue, brown or grey surface marks made with nanosecond lasers.

The introduction of low-cost, compact, high-reliability USP fibre lasers promises to enable UID marking on a broad range of medical products at high speed.

The special needs of the medical device market

Titanium and stainless steels are widely used in the medical device industry for making surgical instruments, endoscopes and implantable devices. These metals are chosen because the devices need to survive for many years, sometimes in vivo, and in the case of surgical instruments or endoscopes, they must survive repeated cleaning and sanitisation in high-temperature autoclaves between surgical procedures. The requirement for reusability and corrosion resistance is usually satisfied by passivation – coating the stainless steel with a chromium oxide to produce a hard surface.

Identifying the problem

In addition to corrosion resistance, the reusable surgical instruments must be catalogued, inventoried, and tracked through the hospital and cleaning processes to ensure their cleanliness and safety. To enable this traceability, and also alleviate the associated liability issues, FDA has mandated that every reusable instrument must have a unique device identifier (UDI) code. These types of UDI marks had to be implemented on all stainless steel surgical tools, and devices that undergo autoclaving and reuse, by 24 September 2018, which created a need for permanent laser UDI marks on a multitude of medical devices.

In addition to the uniqueness of the UDI code, the laser marks must be aesthetically attractive, small or inconspicuous, machine readable, and shallow or subsurface in nature to facilitate effective cleaning and to not trap contaminants or bacteria that could cause infections. Moreover, they must be able to be created in a cost-effective manner that requires minimal postprocessing and no additional passivation steps. To summarise, the key criteria for these UDI laser marks are:

  • machine-readable, high-contrast dark black marks
  • permanent, corrosion resistant and non-fade
  • shallow surface relief so as not to harbour bacteria
  • ideally, no additional passivation step after the mark is written
  • deployable in high-throughput, reliable, low-cost lasermarking system.

Ultrashort pulsed lasers

As previously explained, typical nanosecond pulsed IR lasers struggle to mark polished stainless steel or titanium with good contrast, and the thermal marks that they produce do not offer good resistance to corrosion.

This has driven device manufacturers to explore the use of alternative laser sources with shorter pulse durations in the picosecond regime. This search has yielded some promising results, and device manufacturers have reported that ultrashort pulsed (USP) lasers with picosecond pulse durations and average powers of around 20W can produce excellent corrosion-resistant and high-contrast black marks on these metals with high throughput.

These kinds of marks are resistant to repeated autoclaving because they are created by laser-induced, periodic patterning or nanostructuring of the surface, rather than a thermal effect such as oxide growth.

In a laser-induced, periodic patterned nanostructure on stainless steel formed by an ultrashort two picosecond fibre laser will create a fineetched surface texture with self-organised ridges, having a pitch and height of only a few hundreds of a nanometre. This effectively acts as a light-trapping structure and has a dark appearance to the naked eye.

Ultrafast or USP lasers with picosecond pulse durations have been around for many years in research and scientific applications, and in the past five or so years, we have seen widespread adoption of these lasers in a range of high-end micro-material processing applications. Although picosecond pulsed lasers have been shown to produce excellent black marks on titanium and stainless steel, the historically high price, poor reliability, low rep rates, high complexity and bulky size of traditional free-space-opticsbased USP lasers have prevented them from being widely deployed in marking.

The introduction of low-cost, compact, high-reliability USP fibre lasers promises to overcome this cost of ownership barrier and enable UID marking on a broad range of medical products at high speed.

In conclusion, USP lasers with picosecond pulses have shown the ability to create high-quality, corrosion-resistant, nanostructure surface black marks on polished stainless steel parts. The introduction of new low-cost, highly reliable, ultra-compact USP fibre lasers that operate at high repetition rates, and up to 50W of average power, means that high-throughput marking of UDI marks on stainless steel medical devices at a reasonable cost is now becoming a reality.