Foundations of tablet coating
Advancements in coating technologies
Innovative coating techniques
Challenges and limitations
Future directions and conclusions
References
Further reading
Tablets were first developed in the mid-1850s, during which sugar coating (SC) was primarily used to improve their surface characteristics and protect pharmaceutical agents from the harsh environment of the gastrointestinal tract. It was not until about a century later that synthetic and semisynthetic polymers began to be used for film coating (FC).
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Foundations of tablet coating
Compared to SC, FC is associated with reduced time constraints in the manufacturing process and improved stability of the pharmaceutical agent. Furthermore, FC offers greater flexibility in drug release designs, as this process can be used to produce conventional-release drugs and modified-release options, including enteric-coated, extended, and delayed-release.
Tablet coatings act as a drug carrier, a system capable of incorporating a specific quantity of nutraceutical, cosmetical, and/or pharmaceutical agents to improve their pharmacokinetic properties1.
Within medicine, tablet coatings protect the active ingredients against biodegradation from their starting point in the oral cavity until they reach their target tissue. Recent technological advances in tablet coatings have increased the efficiency, selectivity, and bioavailability of numerous active pharmaceutical ingredients (APIs)1.
Tablet coating is the procedure in which a granule or tablet is coated with an outer dry film to achieve specific objectives. Some of the most common coating materials used within the pharmaceutical industry today include polymerase, polysaccharides, and other excipients like plasticizers and pigments.
Advancements in coating technologies
In general, tablet coating processes can be categorized into five different types: SC, FC, enteric coating, controlled release coating, and specialized coating. FC involves coating oral particles, granules, or tablets with a thin polymer material between 20 and 100 micrometers (µm) in thickness.
Innovative coating techniques
To date, aqueous-based FC is the most widely used tablet coating process, as it can be used for conventional and delayed-release systems.
Depending on the water's solubility and the type of polymer film used, aqueous FC can be achieved through the solution or dispersion method.
Significant advancements have been made over the past three decades to improve the efficiency, uniformity, and precision of coating processes for pharmaceutical agents.
For example, electrostatic coating involves applying a strong electrostatic charge to the tablet's surface, eliminating the use of solvents or heating processes until the film completely coats the tablet2.
The electrostatic coating can be further classified depending on the charging mechanism, which includes Tribo and corona charging. Tribo charging is a friction-based technique that eliminates the presence of any free irons or electric fields from being present between the spray gun and the grounded substance.
During this process, charged particles are sprayed onto the substrate's surface and allowed to accumulate until electrical forces within the Tribo charging gun provide a repulsive force against the deposited particles to control the coating's uniformity4.
Importantly, many tablet coating processes involve the use of hazardous chemicals and materials like organic solvents, thus necessitating strict regulation and quality control measures to ensure the safety of personnel during the manufacturing process and the protection of the API.
Containment applications ranging from fully welded cabinets to exposed drum rollers are widely used throughout the pharmaceutical industry to protect personnel and the environment from the potential hazards associated with coating manufacturing processes.
Challenges and limitations
Organic solvent-based FC is crucial to overcome the low aqueous solubility of hydrophobic and lipophilic polymers often used to produce tablet coatings. However, organic solvent-based coatings have numerous limitations, including their flammability, potential toxicity of residual solvents, and the risk of drug degradation by hydrolysis5.
Organic solvents are also environmental hazards, as vapors produced during the coating process can cause toxic effects in exposed personnel and increase the risk of an explosion, even in heavily ventilated facilities.
During the solvent-based coating process, coating solution droplets are deposited onto the surface of the tablets to form the coating film using a spray gun. This process is highly complex, thus increasing the likelihood that coating defects such as bridging, cracking, and orange-peel roughness are present on the tablet surface5.
Any alteration in the uniformity of tablet coatings can alter the final quality of these medications, particularly when FC is used to contain the API.
Although aqueous FC is associated with a better safety profile than solvent-based processes, the water evaporation process involved in aqueous coating is both time- and energy-consuming.
Nevertheless, aqueous FC often requires the addition of a suspending agent or plasticizer to achieve a homogenous coating solution when water-insoluble polymers are used.
Future directions and conclusions
Over the past several decades, various technological advancements have been made to optimize tablet coating processes.
Although each of these methods has advantages and disadvantages, process analytical technology (PAT) tools have been incorporated into the coating process to maintain the high quality of final products.
Some of the different PAT tools that have been used to monitor tablet coating processes include spectroscopic techniques such as Raman and laser-induced breakdown spectroscopy, imaging techniques such as magnetic resonance imaging (MRI), terahertz pulse imaging (TPI), and near-infrared imaging, as well as microscopic techniques ranging from confocal laser scanning microscopy to scanning electron microscopy (SEM)5.
Advancements in computational modeling approaches will also improve the understanding, predicting, and troubleshooting of film coating operations.
References
- Salawi, A. (2022). Pharmaceutical Coating and Its Different Approaches, a Review. Polymers 14(16); 3318. doi:10.3390/polym14163318.
- Ganguly, D., Ghosh, S., Chakraborty, P., et al. (2022). A brief review on recent advancements of tablet coating technology. Journal of Applied Pharmaceutical Research 10(1). doi:10.18231/j.joapr.2022.7.14.
- Maderuelo, C., Lanao, J. M., & Zarzuelo, A. (2019). Enteric coating of oral solid dosage forms as a tool to improve drug bioavailability. European Journal of Pharmaceutical Sciences 138. doi:10.1016/j.ejps.2019.105019.
- Different Types of Tablet Coating [Online]. Available from: https://www.lodhapharma.com/different-types-of-tablet-coating.php#:~:text=Tablets%20with%20Electrostatic%20Coating&text=An%20electrostatic%20charge%20is%20introduced,to%20allow%20the%20proper%20direction.
- Seo, K., Bajracharya, R., Lee, S. H., & Han, H. (2020). Pharmaceutical Application of Tablet Film Coating. Pharmaceutics 12(9); 853. doi:10.3390/pharmaceutics12090853.
Further reading