Application of the UV laser printing technique to soft gelatin capsules containing titanium dioxide in the shells

Abstract

Aim: The purpose of this study was to examine application of ultraviolet (UV) laser irradiation to printing soft gelatin capsules containing titanium dioxide (TiO2) in the shells and to study effect of UV laser power on the color strength of printing on the soft gelatin capsules.Methods: Size 6 Oval type soft gelatin capsules of which shells contained 0.685% TiO2 and 0.005% ferric dioxide were used in this study. The capsules were irradiated pulsed UV laser at a wavelength 355 nm. The color strength of the printed capsules was determined by a spectrophotometer as total color difference (dE).Results: The soft gelatin capsules which contained TiO2 in the shells could be printed gray by the laser. Many black particles, which were associated with the printing, were formed at the colored parts of the shells. It was found that there were two inflection points in relationship between output laser energy of a pulse and dE. Below the lower point, the capsules were not printed. From the lower point to the upper point, the capsules were printed gray and total color difference of the printing increased linearly in proportion with the output laser energy. Beyond the upper point, total color difference showed saturation because of micro-bubbles formation at the laser irradiated spot.Conclusions: Soft gelatin capsules containing TiO2 in the shells could be performed stable printing using the UV laser printing technique. Color strength of the printing could be controlled by regulating the laser energy between the two inflection points.