# A Review of the Applications of OCT for Analysing Pharmaceutical Film Coatings
## 1 Introduction  
In pharmaceutical manufacturing, film coating on solid oral dosage forms is performed as part of the secondary manufacturing. The film coating process is a widely used unit operation in the pharmaceutical industry for ensuring color uniformity, light protection and taste masking of the final products. Pharmaceutical film coating is a thin polymer-based coat applied to a solid dosage form such as a tablet or pellet. In the majority of cases, a coating formulation contains additives such as a plasticizer, anti-adherents, surfactants and colorants [1]. For advanced drug delivery systems, functional coatings such as modified release (sustained, extended, delayed and osmotic-controlled), active and enteric coatings, are used. Modified release coatings allow formulators to control the rate of drug release as a function of time, whereas enteric coatings will lead to drug release as a response to the environment (e.g., depending on the pH) [1].  
The coating process can be performed either as a batch or continuous operation. Even though the latter is receiving increasing attention in development as part of continuous manufacturing, batch operations are more commonly used commercially. Depending on the size of the solid oral dosage form, coating is routinely performed inside a pan-coater or a fluid-bed apparatus. For example, small drug delivery devices such as granules, pellets and mini-tablets with a diameter <5 mm are typically coated inside a fluid-bed apparatus.  
Even though coating processes have been used for many decades, there are still remaining challenges attributed to the inherent complexity of the process itself. The process ultimately represents a complex interplay between three main physical phenomena: (1) system thermodynamics, influencedby both the drying air and spray conditions that can be considered an adiabatic evaporative cooling process; (2) spray atomization; and (3) tablet mixing, which describes the tumbling of tablets and transfer of coating through contact with neighbouring tablets. These phenomena in turn are influenced by the equipment design and operating parameters, as well as tablet core and coating material properties [2]. A lack of scientific understanding of these complex interactions and how they impact product quality makes it difficult to produce coatings with the desired thicknesses and densities, and with low inter-dosage form and inter-batch variability. As the coating process is stochastic in nature, coating thickness of the final product inevitably will follow a distribution. The realistic aim of a successful coating operation is therefore to make this distribution as narrow as possible, or in other words, to minimise coating variations. Process understanding can be achieved by process modelling, where advances have been made using discrete element modelling, ray tracing [3] and computational fluid dynamics [4]. At the same time, developments in process analytical technology (PAT) have allowed measurements of the dosage forms throughout the process to be acquired, not only useful for quality control, but also for understanding the physicochemical changes of the film during manufacturing. Many analytical techniques have been demonstrated to date for the assessment of the coatings on the dosage forms and extensive reviews on the subject matter have been presented [5; 6; 7]. Optical coherence tomography (OCT) is fast gaining popularity as a non-destructive method for quantitative evaluation of pharmaceutical coatings because it offers both high data acquisition rate and high axial and spatial resolutions. In this review, we will therefore only focus on the use of OCT for analysing pharmaceutical film coatings, highlighting the major achievements, as well as the challenges ahead.