Charlie Cunningham, Colorcon, says continuous coating by far outperforms traditional batch coating and is fast catching on with pharmaceutical customers.
Continuous coating is rapidly becoming more than a niche technology for high volume products. In March 2015, at the 50th American Association of Pharmaceutical Scientists (AAPS) Arden Conference, focus was set on the continuous manufacture of tablets, capsules and other oral solid dosage (OSD) forms. Speakers from many of the largest global pharmaceutical companies as well as the US Food and Drug Administration discussed the benefits and challenges of continuous manufacturing and initiatives supporting this bold shift in the manufacture of OSD forms.
Aligned with this trend, Colorcon has seen a dramatic increase in customer requests for film coating support in continuous coating technologies. Historically, a handful of dietary supplement (DS) and over-the-counter (OTC) manufacturers in the US have utilised continuous coating. We are now helping customers develop, optimise and implement continuous coating processes not only for DS but also for pharmaceutical applications in the US, Europe and Asia.
There is rapid advancement in the equipment used for continuous film coating, with technology employing new mechanisms to eliminate start-up and shut-down product losses. The scale of equipment to meet product capacity demands is variable, and some manufacturers have developed novel approaches to coating in both continuous and semi-continuous modes.
While there is plenty of information on traditional batch coating technology, published over the past decades, new information and understanding is needed to support this evolution in continuous coating technology. Colorcon continues to work with several leading equipment manufacturers, including Thomas Engineering, O’Hara Technologies, DRIAM and GEA, to progress a deeper understanding of the processes and how optimisation can provide the highest product quality and efficiency.
Film coating formulation technology is also advancing with the utilisation of lower viscosity polymers that provide significantly improved process efficiency and enhanced moisture protection compared with traditional hypromellose-based coating systems. A benefit of new low viscosity coating formulations is the ability to apply at significantly higher solids concentrations compared with traditional coatings, resulting in shorter process times. Other benefits resulting from low viscosity film coating systems may include more efficient droplet atomisation, a smoother finish to coated tablet surfaces and fewer process related issues such as spray gun nozzle blockages. In order to realise the full advantages of these low viscosity coating formulations, the limitations of traditional batch-oriented coating pans must be addressed.
In traditional batch coating, pilot-scale batches regularly run in a coating pan of 15 to 24 inches in diameter, and production-scale pans often extend to 60 inches in diameter or more. This increase in pan diameter greatly affects the tablet bed depth and the difference is significant. By increasing tablet bed depth in scale-up, the turnover of tablets through the spray zone is dramatically decreased, and the weight of the bed increases to a point that may induce tablet defects not previously seen in smaller scale trials.
On the other hand, rather than increasing pan diameter, continuous coaters have maintained the diameter of a typical pilot-scale coating pan and elongated it to a length as long as 15 ft (Figure 2). This produces a tablet bed depth that is more consistent with laboratory or pilot scale batch coating pans. A 24-inch continuous coater is able to produce up to 1,000 kg/hr of coated tablets compared with 300 kg of tablets over two hours in a 60-inch batch coater.
The elongated spray zone and shallow bed depth of the continuous coater ensure that the tablets are presented with high frequency to the spray zone. This results in the faster development of colour uniformity, reduced process times and shorter exposure of the tablets to the adverse thermal and mechanical stresses of the coating process. However, individual tablet movement and transit times through these elongated pans is less understood, as well as coating variability related to any potential variation in forward movement.
At the aforementioned AAPS Arden Conference, Colorcon presented a study focused on the effect of tablet residence time and uniformity of tablet progression on coated tablet weight variation in a Thomas Engineering FLEX CTC continuous tablet coater. The coating uniformity of a typical production scale batch coating process was also evaluated for comparison with the continuous coating process.
Quantification of coating variability and tablet movement were studied to gain further understanding of this type of coating equipment. Tablets progressed through the continuous process with relatively small variability in transit times and exhibited superior uniformity compared with tablets coated in a traditional batch coater. Higher tablet throughput rates were also achieved utilising low viscosity coating systems, enabling application at >20% solids concentration.
Colorcon continues to work with the industry evaluating a variety of continuous and semi-continuous coating machines, and further developing coating formulations to provide greater productivity benefits, specifically suited for these applications.