Assessment of technological characteristics of functional coatings of solid dosage forms as indicators of their quality (review)

Introduction. This review presents modern methods for measuring the thickness and homogeneity of functional coatings on solid dosage forms, such as tablets and spherical granules (pellets). The quality of these coatings determines product stability, protection of the pharmaceutical substance from degradation in the acidic environment of gastric juice, and the timeliness of its release in the gastrointestinal tract. The authors highlight the shift of the pharmaceutical industry from traditional batch production, based on selective quality control with destructive methods (dissolution testing, optical microscopy), to continuous production and quality control with non-destructive real-time monitoring. Text. This review systematically analyzes various measurement methods. Indirect spectroscopic methods, such as near-infrared and Raman spectroscopy, are suitable for in-line monitoring but require complex chemometric models and yield averaged data. Direct imaging methods, such as terahertz pulsed imaging (TPI) and optical coherence tomography (OCT), are considered the most promising. TPI is excellent for measuring thick coatings and multilayer structures, while OCT enables high-speed, high-resolution analysis of thin coatings. A key advantage of TPI and OCT is that they require no calibration and need only the refractive index of the coating material. The authors note that a combination of TPI and OCT can provide comprehensive information over a wide range of coating thicknesses (20—300 μm). Conclusion. The future of quality control of solid dosage forms lies in the implementation of direct, non-destructive, and rapid methods, such as TPI and OCT, integrated into coating equipment. This makes real-time quality analysis possible and aligns with regulatory initiatives such as Process Analytical Technology (PAT), that is, the transition from end-product testing to continuous quality assurance throughout the manufacturing process.