Lasers and steel
Creutzfeldt-Jakob disease is no longer making headline news. Yet, in health care, combating this deadly disease has triggered significant consequences for everyday clinical practice. Using highly alkaline cleaning and subsequent sterilisation of surgical instruments at a minimum of 134°C has eliminated the risk of contamination. However, these more aggressive cleaning methods pose new challenges for the medical device industry.
Permanent traceability of medical devices is vital and laser marking has become the established procedure for this. This process offers high precision, reproducibility even in places difficult to access, and does not require acids or solvents. Marking speeds of more than 100 mm/s and flexibility of marking content without refitting time add to these advantages. Yet, there is an increased risk of corrosion and possible fading or flaking of the laser mark after cleaning processes (Figure 1).
Medical instruments and many implants are primarily made of stainless steel with at least 10.5% chromium (Cr) to provide sufficient corrosion resistance. An industrial consortium undertook the goal of eliminating the last uncertainties concerning the effects of different laser parameters on instrument steels. The Natural Science and Medicine Institute (NMI) at the University of Tübingen, Germany (www.nmi.de) conducted the study in cooperation with Trumpf, various medical engineering companies and the Competence Centre of Minimal Invasive Medicine and Technology (Tübingen-
The objective of the study was to establish optimised parameter sets for various laser marking procedures and improve corrosion resistance in the laser marked area. The team investigated the resistance of the laser mark after passivation and clinical preparation with highly alkaline cleaning and sterilisation.
Probes fabricated from low alloyed stainless steel 1.4021 and the highly alloyed steel 1.4301 with shiny, electro-polished and matt brushed surfaces were lasered with a predefined economically sensible parameter set (one giving good mark quality with low cycle times). These were then characterised, evaluated and documented. Half of the laser marked probes were treated with a spray cleaning passivation procedure (Borer Chemie, www.borer.ch). The other half was treated using an ultrasound procedure CitriSurf 2250 (MKK, www.mkk-gmbh.de).
To properly reflect clinical procedure, the probes were investigated after undergoing a fivefold washing and sterilisation cycle in which machine decontamination was followed by steam sterilisation in a cycling belt machine. The chemical composition, and from this the ratio of Cr to iron (Fe), were determined using photoelectron spectroscopy to investigate how susceptible the surface is to corrosion. A second test, conducted before and after passivation and clinical preparation, assessed the physiological contrast of the marked areas. Unlike technical contrast measurement, physiological contrast measurement takes into account human intensity of perception, which is comparable to how medical devices are identified in everyday clinical practice. Because the passivation and preparation processes can reduce contrast, only the marked areas that were clearly visible by physiological contrast measurement were used for further investigation of the Cr/Fe and Cr-oxide/Fe-oxide ratios. Even after treatment, good contrast was found on all the materials.
Figure 2: REM images of stainless steel surfaces.
(click image to enlarge)
Because of its beam quality and pulse-to-pulse stability, the diode-pumped laser marking system, Vectormark compact VMc5, (Trumpf ) was employed for the investigations. Two different laser marking procedures were used: annealing and engraving.
Annealing warms the material locally to a temperature below its melting point. This generates oxide layers on the surface of the workpiece and metallic annealing colours are created. The contrast depends on the thickness of the oxide layer. This method is favoured in medical engineering because the surface quality is retained. In engraving, marking is applied through material removal. This technique is suitable for ceramics and plastics as well as metals. The power density of the laser beam in this procedure is so high that the material melts and partly evaporates and a cavity is formed in the material. As a result of the interaction of the melted base material with the oxygen in the air, oxides form, which can increase the contrast. After annealing, the Cr to Fe ratio is lower in treated stainless steel than in untreated. Better ratios can be found in engraving. The study investigated annealing parameters that exhibit acceptable Cr to Fe ratios.
The study found evidence that the energy brought into the material must exceed a certain value, and the total energy density must be at least 0.4 KJ/cm² to ensure that an acceptable ratio between Cr and Fe is retained. From the investigations, parameters for all surfaces were determined for use as modified annealing parameters with a gently melted surface, or for a dedicated engraving procedure. It is also possible to define an annealing parameter that does not change the surface structure. Figure 2 shows a comparison of the procedures. The laser mark survives undamaged after clinical procedures of sterilisation and disinfection as well as the preceding passivation process. Annealing still ensures good visibility of the mark, even after the passivation process. It is true that the susceptibility to corrosion initially increases with laser marking. However, the CirtriSurf passivation process after marking reduced susceptibility to corrosion and the Cr to Fe ratio improved. For this reason, passivation after annealing is advisable.
Using the investigation results, the quality of laser annealing and engraving can be significantly improved. However, it is always advisable to perform durability and corrosion susceptibility tests, even after small changes in processing methods.
Birgit Faisst, Dr of Engineering, is Director of the Marking and Microprocessing Application Department, Trumpf Laser GmbH + Co. KG, Schramberg, Germany, tel.+ 49 7422 515 443, e-mail: email@example.com; www.trumpf-laser.com