# A Review of the Applications of OCT for Analysing Pharmaceutical Film Coatings
## 4 Conclusions and Outlook  
Studies to date have clearly demonstrated the potential of OCT for the off-line and in-line analysis of pharmaceutical coatings. However, the field is currently facing some challenges and limitations: (1) A number of typical coating formulations (in particular cosmetic coatings) cause strong scattering thus limiting the penetration depth of OCT to tens of \(\upmu\)m. (2) Window fouling during in-process control may either affect the data quality or, in the worst case, render the data useless as the signal strength of the back-reflections from the sample is not sufficiently high to detect the interfaces between the coating layer and the core material. (3) The integration of the sensor in the process is a critical aspect, which may have a significant effect on the quality of the measurements. This is particularly important for the integration of an OCT probe in a fluid-bed coater, where the particles typically do not have defined and uniform movement. In this case, the sensor needs to be positioned where the pellets/mini-tablets pass the sensor with a low but constant speed. However, (2) and (3) are challenges that are applicable to almost every measurement technology that is used to monitor a product quality attribute. OCT has the advantage compared to most other techniques, in particular Near-infrared (NIR) and Raman spectroscopy, that challenges (2) and (3) can be relatively easily detected. Window fouling will result in a built up of material on the OCT probe causing the disappearance of the tablets in the OCT image. Misalignments of the probe with respect to the samples of interest can be detected by comparing the contrast from the coating interface from in-line data to that from off-line measurements. The majority of OCT systems used for assessing the quality of film-coatings operated at a central wavelength of 700-900 nm in order to improve axial resolution rather than penetration power. The A-scan rate is particularly important for the in-line monitoring of the coating quality. There is great potential to adopt technologies from fields beyond pharmaceutical sciences, which have demonstrated A-scan acquisition rates > 1 MHz [43]. Finally, like any other PAT, data quickly builds up, especially operating at acquisition rates up to 1 MHz, leading to the need to address the issue of data management or optimal sampling strategy.  
[PERSON], [PERSON] and [PERSON] wrote the manuscript. All authors proof read the manuscript.  
**Acknowledgments:** The authors acknowledge the financial support from UK Engineering and Physical Sciences Research Council Research Grant EP/L019787/1, EP/L019922/1 and EP/R019460/1.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**Measurement**} & \multirow{2}{*}{**Light Source**} & \multicolumn{2}{c}{**Central**} & \multirow{2}{*}{**Axial**} & \multirow{2}{*}{**Lateral**} & \multirow{2}{*}{**Acquisition**} \\  & & & **WFWIM** & & & & **Resolution** & **Resolution** \\  & & & **Bandwidth** & & **(nm)** & **(nm)** & **Rate** \\ \hline [PERSON] et al. (2009) [22] & Off-line & / & / & / & / & 5 & / \\  & & Spectral radar OCT & 930/100 & 6.5 & 9 & 15 KHz \\ [PERSON] et al. (2009) [25] & Off-line & \
\begin{tabular}{c} **S**uperp source OCT \\ (Thorlabs) \\ \end{tabular} & 1325/130 & 12 & 15 & 16 KHz \\ [PERSON] et al. (2009, 2010) [26, 27] & Off-line &
\begin{tabular}{c} Tungsten Halogen Lamp \\ Supercontinuum Lasers \\ (NKT Photonics) \\ LED \\ TELST/10