With the recent release of the U.S. Food and Drug Administration’s final unique device identifier (UDI) ruling, the race is on for medical manufacturers to comply with the newly proposed mandate. This challenge is not without its fair share of obstacles. Once the technicalities are ironed out and a timeline is put in place, manufacturers must determine the method used to label their products and implement the changes. This raises concerns as the label must sustain throughout the lifecycle of the part, not interfere with the function of their product, be 100% biocompatible, and, all the while, comply with UDI requirements for traceability.

Fig. 1 – The laser marking on this bone saw used in knee replacements shows the required UDI datamatrix mark that fully complies with the new FDA requirements/passivation process.
In a quest for what seems to be an insurmountable goal without compromises, laser marking presents a viable solution. Lasers offer versatility in marking a variety of substrates from metals to polymers, the convenience of no-contact processing, flexibility in marking, and the ease of integration—all as a low-cost industrial solution. On medical components, such as surgical instruments and implantable devices, creating a UDI mark is easy compared to what is required to make that mark withstand post-marking processes. Here, several factors must be considered.

For years, lasers have been used to apply identifying information on surgical instruments and implantable devices critical to supporting the body and sustaining life. A common application is annealing on a variety of stainless steels, cobalt chrome, as well as titanium for implants and instruments. Survivability of a mark and its corrosion resistance are the primary concerns of these manufacturers. This is even more imperative when implementing a means of traceability. Regardless of the laser supplier, the most problematic concern in laser marking metal devices is the ability to generate a mark capable of surviving the passivation process, as well as future sterilization cycles. The process varies from one manufacturer to another and the regulation of passivating medical parts is open-ended. This task becomes even more arduous when the manufacturer needs to apply a coding type. Since the cameras used to read a datamatrix evaluate multiple variables, the criteria used to validate a code leave little leeway for a diminished mark.

Optimizing the Process

In order to optimize the laser marking process, one must possess a generalized understanding of the passivation process. To simplify, we will use laser marking of stainless steels as an example, as this is the most relatable application. Stainless steels are an alloy of iron, chromium, and carbon. The presence of 10.5 percent chromium at minimum is what gives this alloy its corrosion resistance properties. When exposed to oxygen, a chromium oxide layer, often referred to as the passive layer, is formed on the surface of the material and protects the alloy from corrosion. When annealing, the material is heated to just below its melting point, causing oxide layers to form and compromising the integrity of the passive layer. The ratio of chromium to iron changes and facilitates the propensity of corrosion. By passivating the material, the chromium-toiron ratio is improved as free radicals are removed from the surface, and the passive layer is restored.

Process knowledge is the key to ensuring the lifecycle of the mark. Optimizing laser parameters to provide a homogeneous growth of the oxide layer will, in turn, greatly improve the corrosion resistance and durability of the mark. The laser parameter sets created often limit the degree of influence over the ratio of chromium to iron. For success of this strategy, equal energy distribution is necessary. There are multiple ways to accomplish this but, above all, the laser should allow the operator a greater level of control over the energy.

Studies have found the total amount of energy put into the work piece should be no less than 0.4 kJ/cm2. Lowering the pulse energy to the optimum level can be accomplished in a few ways. One way is to use a higher pulse frequency. Multiple passes help to bring out the contrast of this type of mark especially when used in combination with higher scanning speeds. The methodology is the same when using a solid-state or fiber laser. As with all laser marking applications, there is a fine balance between speed and quality. When it comes to corrosion resistance, for example, a short cycle time isn’t always possible. Sometimes a laser with a higher average power can increase the speed of marking, but even that is not without limits.

Laser marking provides an opportunity for permanent marking on substrates where other methods may not. Its high flexibility for marking varying part geometries combined with small spots sizes make it a superior marking method. The level of control in a laser is equally as important, and the benefit of producing high contrasting marks will only enhance your product. In terms of preparation for a UDI rollout, creating the code is a small piece of the puzzle and, with most laser software, an easy task. All in all, time devoted to refining your process means that any UDI code put in place today will still be there tomorrow.

This article was written by Brian Litherland, TruMark Lead Application Engineer, TRUMPF Inc., Farmington, CT. For more information, Click Here .