Issue link: https://www.e-digitaleditions.com/i/1035980
Tablets & Capsules October 2018 27 durability and long-term stability; does not affect the effi- cacy of the medicine; and does not create unwanted chem- ical components. UV laser marking of TiO 2 -pigmented tablets and capsules satisfies all these requirements. Experimental data and theoretical estimates Some commonly used pharmaceutical additives such as titanium oxide change color upon intensive short-pulse UV laser exposure. The same effect occurs when those additives are embedded into a substrate. Ideally, laser radiation does not react with the outer material and passes freely through to the substrate surface. The pig- ment particles distributed in the substrate absorb the laser energy, which modifies the particles' structure and appearance, including color. This practically irreversible change provides a mark with both chemical and mechanical permanency. Due to strong absorption, most of the beam's energy dissipates within a few surface layers, so only a small fraction of pigment is modified. Typically, the concentration of such additives is minimal, so the total amount of modified material in the substrate is extremely low. The depth of marking is inversely proportional to the additive concen- tration and usually does not exceed 100 microns. Short nanosecond laser pulses prevent regular heat exchange between the additives and the surrounding material, lim- iting any structural and/or chemical modifications to the pigment particles themselves. Figure 2 Typical UV laser imprint on a soft gelatin capsule a. Entire capsule 2mm d. Further-magnified image of portion of letter E c. Magnified image of letter E b. Imprint cross section 0.5mm 0.5mm 0.2mm Figure 2 shows UV laser marking of a soft gelatin cap- sule under different magnifications. The marking depth is less than 100 microns, while the thickness of gelatin film