Issue link: https://www.e-digitaleditions.com/i/1035980
26 October 2018 Tablets & Capsules Laser radiation can mark a material surface via several different mechanisms, as shown in Figure 1: • Ablation. The irradiated material evaporates, leaving relatively sharp border trenches on the surface. • Melting. The irradiated material melts and spills from the inside out, creating hills and valleys in the middle of a plain. • Burning. The irradiated material heats up and pro- duces gaseous components that react with atmospheric oxygen, depositing a product of combustion (such as soot) on the surface. • Color change. The material changes color without any other visible surface modifications. • All of the above. Ablation is the cleanest way to inscribe pharmaceuti- cals but provides low marking contrast because the marked area does not change color. Making deeper and wider marks improves legibility but reduces material integrity. One approach is to apply at least one coating layer in a color that contrasts with the tablet color then use a laser to selectively remove the coating and expose the tablet beneath. This leaves a legible mark without destroying the actual tablet. The disadvantage of this method is the requirement for an additional coating layer and a potential problem with long-term durability, since part of the coating—especially around and within the imprint—can chip off and destroy the mark. This may happen when tablets collide with one another during transportation, dispensation, and other processes. The higher the resolution of the mark—a 2D barcode, for example—the weaker the marking integrity, as the remaining coated area becomes smaller and more vulner- able to any structural defect. Melting and burning marking processes are subject to long-term durability problems because the melted mate- rial and burned out deposit may not stick well to the unaffected area. Other issues include tablet integrity and the possible formation of new substances not recom- mended for human consumption. Color-change marking can be an excellent solution on the condition that it provides sufficient contrast, good US pharmacies filled close to 4 billion retail drug pre- scriptions in 2017 [4]. This does not include over-the- counter drug products or products sold outside the US. At least 10 percent of the formulations for these drug products contain TiO 2 . TiO 2 is added to thousands of different formulations of paints, coatings, and plastics to modify optical, physical, and other material properties and is FDA-approved for use in foods and pharmaceutical products. For most industrial applications, it is produced as a powder or slurry with white, submicron particles, but it may also be grown as a single crystal for research and other purposes. Because of its bright white color, TiO 2 is primarily used as a pigment. An optical band gap around 3.1 electron volts accounts for TiO 2 's intense absorption of UV radiation with wavelengths shorter than 380 nanometers [5, 6]. Irradiation with a UV laser permanently turns TiO 2 parti- cles from white to blue/black without altering the mate- rial properties of the pigmented surface. This article describes UV laser marking of TiO 2 - pigmented solid oral dosage forms, focusing on laser-sur- face interactions, and specifically on the color-changing mechanism of TiO 2 exposed to intensive UV laser pulses. Basic considerations A laser marking system consists of a laser coupled with a beam-delivery system synchronized with a parts-han- dling mechanism. Lasers and beam-delivery systems for material processing are the subject of many comprehen- sive reviews and a variety of reference sources and text- books [7-10]. The basic principles and challenges of laser marking of plastics are given in Hoffman et al. [11]. The reaction of a solid surface to laser irradiation depends on many factors including laser wavelength and power, exposure time, and optical properties. For example, a laser beam can be completely reflected from a surface, as a ray of sunlight from a mirror, or propagate unaffected, as a ray of sunlight through a transparent window. The laser does not change or mark either the mirror or the window. To mark a material, at least part of the laser radiation must be absorbed directly on or near the material's surface. Figure 1 Types of laser-surface interaction Laser beam Laser beam Laser beam Laser beam Ablated material Melted material Burned-out material Color-changed material a. Ablation b. Melting c. Burning d. Color change