My local supermarket sells generic paracetamol for 50p (about $0.70) but I always buy the caplets, which cost almost twice as much – I don’t like throwing money away, but the caplets are coated to mask the bitter taste, and they’re easier to swallow. Perhaps not all tablet coatings justify a near 100% price increase, but my willingness to double my expenditure on something I’m just going to quickly swallow goes a long way in helping my own understanding of the importance of tablet coating technology, when it comes to patient acceptance and compliance.

Why coat?

Tablet coating is a process by which an outer layer is applied to the surface of a dosage form in order to confer specific properties. Most obviously, the coating makes the tablet more palatable and easier to swallow. However, it can also protect the dosage form from stomach acids, or conversely it can protect the stomach lining from aggressive drugs (e.g. enteric coated aspirin). Coatings can also control the delayed release of a medication. In these ways and others, the coating of a tablet can have a profound effect on drug pharmacokinetics and pharmacodynamics. 

What coat?

Of course, tablet coating is not new. Some medical historians date the process back to ancient Egyptian times. Certainly, in medieval times, people are said to have coated pills with slippery plant substances in order to make them easier to swallow. I have also heard that emperors would coat their pills in gold and silver (although they would have passed through the digestive tract unchanged, so can’t have been an effective solution!) Suffice to say that tablet coating is probably one of the oldest pharmaceutical processes in existence.

Today, four main types of coating are used:

1. Sugar coating: compressed, dry tablets are sealed and coated with a water-soluble layer of sugar that dissolves easily to release the drug substance. Interestingly, the first sugar coatings for medicinal tablets were invented as early as the 19thcentury.

2. Film coating: involves the application of a thin film of a protective polymer, with added plasticizers and pigments. Film coating is lighter, faster, more flexible and more chip-resistant than sugar coating, and represented a significant advance when it was invented in the 1950s. 

3. Compression coating: also referred to as press coating or dry coating, a fine dry granulation is compressed onto a tablet core. This is usually achieved using a specially designed tablet press, and the process has been used to separate two incompatible active pharmaceutical ingredients (one contained in the tablet core and the other in the coating). Although less popular than film coating, compression has gained interest in recent years as a means of creating specialized modified-release products.

4. Microencapsulation:  involves the application of a relatively thin coating to tiny particles (below 800 μm diameter), but is similar – in principle – to film coating. 

The base formula for any coating is optimised for aesthetics, flavour, texture, hardness, solubility, shelf-life and numerous other properties, depending on the coating’s purpose and requirements. Such formulation techniques are a fascinating science of their own. However, some of the greatest innovations are seen not just in the coating formulations themselves, but also in the equipment used to apply them. 

How to coat

Traditionally, tablets were coated using a rotating coating pan, which uses a circular motion to tumble the tablets within the pan and cover them with the coating solution. Modern tablets tend to be spray coated rather than rolled in solutions (with ever increasing refinements to nozzles and aerosols), but recent advances have seen a revolution in new technologies used for tablet coating. Coatings have been the subject of remarkable developmental efforts, aiming to ensure and enhance the quality of tablet dosage forms.  

In the latest technologies, materials are coated directly on to the surface of the dosage forms, using techniques such as electrostatic dry coating and magnetically assisted impaction coating (MAIC). Unlike conventional techniques, electrostatic dry coating and MAIC avoid the use of solvents or water (or at least reduces the volume of water to a very small amount) so the limitations associated with liquid coating technologies – such as time, energy, drying and drug stability issues – are overcome.  Other solventless coating techniques include powder coating, hot melt coating, supercritical fluid coating, and plasma enhanced chemical vapour deposition. 

Solventless coating technologies have numerous advantages over conventional liquid coatings. For example, as well as reduced process time and energy (e.g. for drying/evaporation), they do not emit volatile organic compounds, nor do they produce as much hazardous waste. For the finished product, they can achieve a thicker coating if required, but it is achieved more quickly and at a lower cost than conventional techniques. 

Looking ahead

Such is the interest in these technologies that MarketsandMarkets recently projected that the tablet coatings market would reach $969 million by 2025 (up from $739 million in 2020, at a CAGR of 5.6%). Therefore, the acquisition of new tablet coating technologies rightly warrants press releases from CDMOs, as service providers and healthcare supply companies invest in equipment and technologies that are expected to be useful to their pharma industry customers. Most recently, earlier this year, French CDMO Skyepharma announced that it was extending its range of tablet coating technologies that will allow them to provide tailored film coatings, even to small batches of oral solid doses.

Many of the new coating technologies are still niche, which makes them a high value offering for CDMOs. Most techniques still use a rotating coating pan, fluidized bed coater or spray coater, with minor modifications. Only MAIC and electrostatic coating require very specialised equipment. However, as with any investment in new equipment, understanding your (or your customer’s) pipeline, establishing likely future needs, and evaluating the suitability of different techniques (e.g. for scale up, pharmacokinetics, pharmacodynamics) is a crucial undertaking in making sure you are not being left behind in the pharma race.

Author biography

Sarah Harding, PhD

Editorial Director of Chemicals Knowledge

Sarah Harding worked as a medical writer and consultant in the pharmaceutical industry for 15 years, for the last 10 years of which she owned and ran her own medical communications agency that provided a range of services to blue-chip Pharma companies. She subsequently began a new career in publishing as Editor of Speciality Chemicals Magazine, and then Editorial Director at Chemicals Knowledge. She now focusses on providing independent writing and consultancy services to the pharmaceutical and speciality chemicals industry.