(M1130-02-12) Establishing the Mechanism of Immediate Release Film Coating Dissolution Using Terahertz Measurements

Purpose: The film coating dissolution is an indispensable part of many immediate release tablets disintegration and dissolution since the dissolution medium needs to penetrate this polymer barrier before entering the tablets solid matrix. Using terahertz pulsed imaging (TPI), a process analytical technology, we established that the mechanism of film dissolution is not only determined by the coating polymer, thickness and surface area, but is also dependent on the tablet’s composition, shape and debossing. TPI offered real-time investigation on the fundamental interactions between water and coating polymers, where the insights would have profound impact on the development of predictive modelling of tablet disintegration and dissolution.

Methods: Tablet cores were directly compressed using a compaction simulator (HB50, Huxley Bertram Engineering Ltd, Cambridge, UK) (Table 1) and film coated with Opadry II (85F or 47U, Colorcon, Dartford, UK) based on polyvinyl alcohol (PVA) or hydroxypropyl methylcellulose (HPMC) using a spray rig. The coating dissolution experiments were performed using room temperature water via a customized immersion cell integrated with a commercial TPI system (TeraPulse 4000, TeraView Ltd, Cambridge, UK) (Ma et al., 2025).

Results: The TPI precisely tracked the water-front position as the film coated tablet was hydrating (Figure 1(b)). The kinetics plots clearly demonstrated that the film coating acted as a temporary water transport barrier that needs to become water-permeable enough before the hydration in the tablet core can commence. For each datapoint in Figure 1(b), its terahertz amplitude, indicating the water concentration, was also recorded. Thus, a plot of amplitude against time (Figure 1(a)) showed the real-time film coating dissolution process. It is evident from the data shown in the figures that reducing the tablet diameter (hence the coating surface area) did not affect the time required for the film coating dissolution. However, when the surface was convex, the dissolution became much faster due to a thinner coating close to the center. If this convex surface was also debossed, the dissolution was found to be the most rapid as the debossing introduced significant local thickness variations. Hence, the spot with the thinnest coating would dissolve first and resulted in tablet disintegration. Similar TPI data analysis can be conducted to investigate the impact of core formulation on the film coating dissolution (Figure 2). Including a small amount of disintegrant (3%wt CCS, Formulation C) in the core led to rapid core erosion once the coating was fully hydrated, which disrupted the core/coating interface and thus fully exposed the tablet core at a much earlier stage compared to the case without the disintegrant CCS. In the latter case, solid-state lactose monohydrate can be stabilized in the core containing anhydrous lactose (Formulation B) because of water molecule diffusion through film coating (Ma et al., 2025). The gelling of HPMC in the coating formulation slowed down the coating dissolution and prolonged the diffusion. Quantitative analyses in Table 2 reveal that the coating dissolution had significant involvement in the tablet disintegration or dissolution, underlining the need to accurately model the coating dissolution.

Conclusion: The film coating dissolution not only depended on the film coating properties but was also associated with the tablet shape and formulation.

References: Ma, M., Powell, D., Nassar, M., Teckoe, J., Markl, D., Zeitler, J.A., Impact of immediate release film coating on the disintegration process of tablets, J. Control. Release, vol. 373, pp. 533–546, 2024.
Ma, M., Nassar, M., Wang, S., Zhang, R., Teckoe, J., Korter, T. M., Zeitler, J.A., Utilising terahertz pulsed imaging to analyse the anhydrous-to-hydrate transformation of excipients during immediate release film coating hydration, Int. J. Pharm., vol. 670, no. 125119, 2025.

Acknowledgements: Mingrui Ma would like to acknowledge funding from Cambridge Commonwealth, European & International Trust.


Table 1. Tablet formulations (%: percentage by weight)
Table 2. Summary of dissolution and hydration durations of the PVA-based film coating (Thickness: 80-120 μm).


Figure 1. The film coating dissolution profiles of tablets with different diameters or shapes captured by TPI. (a) The changes in terahertz amplitude; (b) The water transport kinetics in the core. All tablets have 20% porosity and contain only MCC.


Figure 2. The impact of tablet formulations on the film coating dissolution profiles studied by TPI. All 13-mm flat-faced tablet cores have 20% porosity.