Optical coherence tomography for non-destructive analysis of coatings in pharmaceutical tablets

Tablet coating is a common pharmaceutical technique to apply a thin continuous layer of solid on the top of a tablet or a granule containing active pharmaceutical ingredients (APIs). Coating thickness and homogeneity are critical parameters regarding the drug release rate, and consequently a direct or indirect monitoring strategy of these critical process parameters is essential. With the aid of Optical Coherence Tomography (OCT) it is not only possible to measure the absolute coating thickness, but also to detect inhomogeneities in the coating or substrate material. In this work the possible application of OCT as in-line method for monitoring pharmaceutical tablet film coating is studied. Firstly, the feasibility of OCT for analysis tablet coating is examined. Seven different commercially available film-coated tablets with different shapes, formulations and coating thicknesses were investigated off-line. OCT images were acquired by two different spectral-domain OCT systems operating at center wavelengths of 830 and 1325 nm. Since the images of both systems allow the analysis of the coatings, the OCT system employing the shorter wavelength and thus providing a higher axial resolution was selected for the further experiments. The influence of a moving tablet bed on OCT images was analyzed by considering a static tablet bed and moving the sensor head along the tablet bed. The ability to analyze the coating homogeneity is limited to a speed up to 0.3 m/s. However, determining the coating thickness and inter-coating uniformity is still possible up to a speed of 0.7 m/s.