Application of green laser marking in medical industry

Marking is critical for the medical device and pharmaceutical industries to enable product tracking and identification and combat counterfeiting. Usually, people use ink to print labels. In recent years, infrared lasers have been used for marking. But these methods have shortcomings. Ultraviolet diode-pumped solid-state lasers (DPSS) have truly overcome the drawbacks of other technologies, but were rarely used in the past due to their high cost. However, the substantial reduction in investment cost and acquisition cost makes UV laser marking attract more attention for medical applications. This article reviews the characteristics of UV laser marking and shows how UV laser marking can be used in certain medical and pharmaceutical industries.

medical marking

Compared with other industries, medical products have more unique requirements for marking. Pills are taken orally, and many other medical products (such as catheters, stents, etc.) are applied externally or implanted in patients. Therefore, it is generally required that the marking itself cannot be a source of contamination or contain chemical components that could cause allergic reactions. In addition, it is generally required that the marked surface be smooth after marking to avoid tissue damage and to avoid making the mark a breeding ground for bacteria.

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Medical markings are often also required to include batch numbers, serial numbers, or other identifying information that can identify when and where a particular product was manufactured. Therefore, if a product is found to be defective, users can easily confirm that the product they purchased is from the same batch.

Batch and source marking can also help address a growing problem in the pharmaceutical and medical industries, namely counterfeit and "marketplace" products. Sometimes counterfeit products simply rip off the label from the bottle and put a new one on. But Western countries are increasingly inundated with counterfeit medicines (often made in Asia) that look exactly the same but may contain the wrong dose or fail to meet the necessary quality control standards.

Therefore, the ideal medical mark should be difficult to erase, easy to identify, difficult to copy or replace, contains unique serialized information, and does not alter the functionality of the product.

traditional marking method

For the marking of medicines, medical devices and their packaging, the mainstream method has always been ink printing (inkjet or pad printing). Pills are usually stamped using gravure offset printing. For manufacturers, this printing method is chosen because of its relatively low investment cost. However, consumables (ink) costs are usually higher.

The main disadvantage of printing in medical applications is that the printed indicia are always easily removed or altered (especially if printed on paper labels). This means that after transportation, handling and storage, printed marks can become difficult to identify and easily counterfeited by those with ulterior motives. Print quality is also limited, creating problems if manufacturers want to squeeze more information, including QR codes, into a small area. For pill printing in particular, it is difficult to apply to the increasingly popular "soft gel capsules" because of the stress on the product during lithography.

Even though the inks used to print medicines and medical devices are non-toxic, the printing equipment itself is often "dirty", using airborne lubricants and solvents that can contaminate the printed product. In addition, printing equipment is often extremely complex and requires downtime for cleaning and maintenance.

Laser marking is a non-contact marking method that avoids contamination issues and requires no consumable costs. In addition, laser marking typically supports small print areas with high contrast and high resolution marks, and can be applied to curved or contoured surfaces.

Laser marking typically uses CO2 or solid state lasers in infrared radiation. The marking process itself is a thermal process, where the material is heated until it whitens, carbonizes or ablates to create color contrast. Almost all plastics directly absorb the heat output of far-infrared CO2, sometimes using absorbing additives, coupled with near-infrared solid-state lasers to facilitate this process. However, heating will change the chemical structure of the HAZ material and cause some surface bumps. This provides a home for bacteria and is difficult to clean.

UV laser marking

Compared with infrared lasers, ultraviolet lasers and materials are basically difficult to interact. In particular, the UV (355 nm) output of frequency tripled diode-pumped solid-state lasers absorbs much more than in long wavelengths. It then undergoes luminochemical (rather than photothermal) interaction with fillers or pigments in the plastic. Most plastics are white, and the pigment is titanium dioxide (TiO2), which absorbs UV light heavily and then changes its crystal structure. This causes the substance to darken in color, creating smooth, highly legible marks inside the material rather than on the surface.

Since the mark is actually inside the material, there is no breeding ground for bacteria, and it is almost impossible for the mark to be altered or damaged without damaging the material itself. Furthermore, since this is a cold working process, there is essentially no heat-affected zone and no changes to surrounding materials. Also, the high absorption rate of UV light means that the material can be processed using lower laser power. Finally, because UV light can be focused more tightly than infrared light, UV lasers support the marking of complex, high-resolution marks such as QR codes.

Given the above advantages, why haven't UV lasers been widely used in medical marking in the past? The answer is simple: cost. However, over the past decade, Coherent has made substantial progress in UV laser lifetime, reliability and output power. These are achieved through improvements in laser design, materials, and the use of stringent cleanroom procedures in the production process. In addition, economies of scale created by automated assembly methods and increased sales volumes helped reduce the price of UV lasers by 5%.

marker effect

The Coherent Applications Laboratory (Lübeck, Germany) has used a 355nm diode-pumped solid-state laser (MATRIX 355) to mark representative materials used in a variety of medical applications. Some of the most relevant results are as follows:

High-density polyethylene (HDPE) is a plastic widely used in pharmaceutical packaging and food, as well as in the production of water bottles. Inkjet markings on HDPE can be removed using a solvent, allowing the product to be relabeled after packaging. Also, ink can cause contamination.

In the test, a laser was used to mark the curved surface of the vial with a QR code (Figure 1). The 355nm laser uses a spot size of 30μm, and uses a galvanometer galvanometer system to scan the surface of the part. In this configuration, a high-contrast 8x8mm barcode pattern can be generated in 2 seconds.

Figure 1: Medicine bottle marked with QR code

The MATRIX 355 laser is particularly suitable for marking such marks on HDPE because its Pulse EQ (PulseEQ) mode keeps the pulse energy at a constant value, even if the repetition rate changes. And Q-switched lasers can't do that. The changing repetition rate allows the color, shade and pulse overlap of the mark to change rapidly without affecting other scan parameters, making it easier to stay within the operating range of the mark.

Another plastic widely used in the medical field is silicone rubber, which is transparent or white. Silicone rubber is commonly used to make catheters for intravenous fluids to patients or to make cannulas. Usually, hoses need to be marked with their diameter size and storage date (by law, these hoses must be used within three years). In this way, the laser is focused on the inner surface of the catheter (Figure 2), so that the marking does not change the texture of the outer surface at all (since the outer surface is in direct contact with the patient).

Figure 2: Silicone rubber tubing marked with white letters on the inner diameter.

The lab conducted marking tests on different types of soft and hard capsules to determine the fastest possible marking speeds (Figure 3). When marking soft capsules with a height of 1.5mm, the fastest speed can reach <0.024s/character. All markings have excellent legibility. The two-dimensional code marking is completed in 0.2 seconds on a 1×1mm hard capsule. In contrast, ink coding requires 1-2 seconds of drying time after printing to process the pills to prevent smearing of the marks.

Figure 3: UV laser marked soft capsules (a) and hard capsules (b).

Another type of gelatin is also used to make blister packs for some medical products (Figure 4). Then there is a need to produce a clear mark with a maximum penetration depth of 30% of the packaging material (specifically, the entire thickness is 0.58mm, and the penetration depth needs to reach 0.17mm). The laser pulse energy was 100 μJ, and the scanning speed was 1.3 m/s. The laser was intentionally defocused on the work surface to produce feature lines with a width of 160 μm. The color-changing markings exhibit good contrast without causing ablation of the material.

Figure 4: Marking gelatin blister packaging.

Testing in the Coherent Applications Laboratory has shown that UV diode-pumped solid-state lasers are an effective tool for rapidly marking medical devices and pharmaceutical products with high definition and resolution. These permanent markers are better than printed ones. Compared to long-wavelength lasers, UV lasers have the advantage of being applicable to a wider range of materials, including plastics and paper that cannot withstand thermal processes.