Various aspects must be considered when deciding upon the best coating technology. Here, Thorsten Agnese, Florian Bang and Thorsten Cech (from BASF), along with Fiorenzo Cembali, Giusi Mondelli and Caterina Funaro (from IMA S.p.A., Active division) describe their recent study comparing three different coating technologies’ efficiency in a pellet coating process.
Thats a wrap
There are various aspects to consider when deciding on a coating technology. Process efficiency, content uniformity of the applied coat, and economical aspects are features of utmost importance, yet vary depending on the type of coating employed. These considerations are particularly vital when dealing with functional coating and/or drug layering.
Looking at the considerations in more detail, we performed a study, which was aimed at comparing three different coating technologies’ efficiency in a pellet coating process. Below, we have provided an overview of our methods and results.
Materials and method
Pellets (20–25 mesh, Emilio Castelli) consisting of sucrose and microcrystalline cellulose were used as substrate. As tracer, riboflavin (BASF) embedded in a film of poly(vinyl alcohol)-poly(ethylene glycol) graft copolymer (Kollicoat IR, BASF), ratio 1:2, was applied as aqueous formulation with a solid matter content of 15%.
The functional coat was based on poly(methyl methacrylate-co-(2-diethylaminoethyl) methacrylate) copolymer (Kollicoat Smartseal 30 D, BASF).1 Further components of the formulation (Table 1) were: FD&C Blue No. 1 (BASF), acetyl tributyl citrate (ATBC, Jungbunzlauer), buthylene hydroxy toluene (BHT, Lanxess), docusate sodium (DS, Cytec), and talc (Merck). The solid matter content of the formulated Kollicoat Smartseal dispersion was 20%.
IMA Table 1
Three different coating technologies were used in this study: a fluid bed coater (FBC) Mylab (Figure 1), with bottom spray configuration, a solid wall drum coater (SWC) GS Evolution Lab (Figure 2), and a side vented pan coater (SVP) Perfima Lab (Figure 3), with wedge wire screen drum. The latter drum has been specifically designed for coating small substrates, such as pellets. Small wedge wires are fitted into the drum wall to allow the passage of process air. The drum maintained the same drum shape, mixing baffles and spray system as the standard equipment for tablet coating. All machines were provided by IMA Active division (Bologna). The coating trials were performed according to schema listed in Table 2.
Figure 2: Solid wall drum coater (SWC), GS HP 25.
IMA Figure 2
To allow a distinct investigation on the coating level, the individual amount of applied coat was determined by photometrical measuring of either riboflavin or the colorant FD&C Blue No. 1.2,3
For dissolution testing, a standard USP Dissolution Apparatus 2 (Paddle) from ERWEKA, equipped with continuous on-line UV measuring (Agilent 8453), was used. Since taste-masking functionality is to be delivered in the oral cavity, phosphate buffer (pH 6.8) was used as dissolution media (700 mL ±1%, 37°C ±0.5 K, n=13).
Results
Firstly, riboflavin was coated onto the pellets as a tracer for the later performance testing of the functional coat (dissolution). In all three coaters, the same coating formulation with the same solid matter content was sprayed onto the pellets for five hours, using optimised settings for each individual technology. The achieved weight gain revealed an excellent performance of the fluid bed coater. However, employing standard deviation as a scale for content uniformity, the solid wall coater presented best results (Table 3).
IMA Table 2
IMA Table 3
Typically, the coating formulation based on Kollicoat Smartseal needs to be applied with a coating level of 3.5 mg/cm² onto tablets to gain functionality.4 When applying the same coating formulation, onto pellets, full functionality could be achieved with distinctively lower coating levels in all coaters. The reason was, most likely, the absence of the critical edge between band and cap, which every tablet bears inherently. Pellets with their spherical shape do not offer such crucial areas.
Figure 4: SEM images (different resolution) of pellets coated in fluid bed coater (SE detector, 5kV, 12 nm Pt).
IMA Figure 4
Pellets, which passed through a fluid bed coating process experienced hardly any mechanical stress. In addition, to the cushioning effect provided by the processing air, the excellent accessibility of the whole product’s surface area allowed perfect drying. This produces coated substrates, with smooth surfaces (Figure 4), already delivering functionality at moderate coating levels (Figure 5). The mechanical impact on the substrate was higher in both drum coaters leading to the surfaces appearing slightly rougher (Figures 6 & 7). Functionality was achieved at similar coating levels in both coaters. Differences were merely found regarding processing time, due to differences in the available inlet air quantities.
Figure 5: Dissolution profiles of pellets bearing three different functional coating levels applied in different coating technologies (values: mean ±SD, n=3).
IMA Figure 5
Figure 6: SEM images (different resolution) of pellets coated in solid wall coater (SE = detector, 5 kV, 12 nm Pt).
IMA Figure 6
Figure 7: SEM images (different resolution) of pellets coated in side vented pan coater (SE = detector, 5 kV, 12 nm Pt).
IMA Figure 7
Conclusion
Expectedly, fluid bed coating allowed the shortest possible processing time thanks to larger quantities of inlet air available. The fluidisation of the particles, inherently provides high drying capacity and mixing efficiency. Consequently, the coating formulations could be applied economically quicker and more homogeneously.
In contrast, both pan coaters offer the advantage of being a more compact installation (smaller footprint). Additionally, access to the processing area is faster and easier. Furthermore, large batch sizes can be coated in smaller sized equipment.
In the current study, side vented pan coating allowed high inlet air quantities resulting in higher spray rates compared to the solid wall one. The dedicated design of the wedge wire screen drum allows high process air volumes, consequently resulting in high spray rates and economical short process times; similar to conventional perforated drum coaters. Compared to the fluid bed processor, this technology provides two main advantages: firstly, it comes with a much smaller footprint, and secondly, the spray guns are readily accessible during the process. Additionally, drum coating processes can easier be controlled simplifying up-scaling procedures.
Solid wall coating generally allows a vast variety of different processes (e.g., tablet coating, powder layering). In order to gain this flexibility, one needs to sacrifice on the amount of inlet air that can be utilised. As a result, processing times were slightly longer but offer the advantage of gaining a higher coating uniformity.
References:
- Kolter, K., et al., ‘Physicochemical characteristics of a new aqueous polymer’, AAPS Annual Meeting, 14–18 November 2010, New Orleans, USA.
- Agnese, Th., et al., ‘Developing a photometric method to determine the amount of film-coating material applied onto individual tablet cores’, 3rd PharmSciFair, 13–17 June 2011, Prague, Czech Republic.
- Agnese, Th., et al., ‘Evaluating different methods to determine the amount of applied coating material onto a tablet’, 3rd PharmSciFair, 13–17 June 2011, Prague, Czech Republic.
- Agnese, Th., et al., ‘Determining the minimum amount of functional coat to be applied to gain taste masking functionality and investigating whether shape or scale is influencing the results’, 3rd PharmSciFair, 13–17 June 2011, Prague, Czech Republic.