Coating the solid dosage form, such as tablets, is considered common, but it is a critical process that provides different characteristics to tablets. It increases the value of solid dosage form, administered orally, and thus meets diverse clinical requirements. As tablet coating is a process driven by technology, it relies on advancements in coating techniques, equipment used for the coating process, evaluation of coated tablets, and coated material used. Although different techniques were employed for coating purposes, which may be based on the use of solvents or solvent-free, each of the methods used has its advantages and disadvantages, and the techniques need continued modification too. During the process of film coating, several inter-and intra-batch uniformity of coated material on the tablets is considered a critical point that ensures the worth of the final product, particularly for those drugs that contain an active medicament in the coating layer. Meanwhile, computational modeling and experimental evaluation were actively used to predict the impact of the operational parameters on the final product quality and optimize the variables in tablet coating. The efforts produced by computational modeling or experimental evaluation not only save cost in optimizing the coating process but also saves time. This review delivers a brief review on film coating in solid dosage form, which includes tablets, with a focus on the polymers and processes used in the coating. At the end, some pharmaceutical applications were also discussed.
Around 1500 BCE, the first reference to the term pill as a solid dosage form came into existence. The first source of pills in ancient Egypt was recorded to be written on papyruses. The pills were made from bread dough, grease, and honey. Pills were made of simple hand-using ingredients like spices or plant powders. In ancient Greece, medicines were termed katapotia [1]. Roman scholars termed Pills as pilula (little ball). In medieval times, pills were coated using slippery substances obtained from plants. By 1800, gelatin capsules were invented. William Brockedon made a machine that can formulate lozenges and pills with the help of pressure on suitable dies [1]. This device compresses the powder without using adhesive into tablets. Professor Brockedon, 1844 in England, developed the first compressed tablets. These tablets were hard, and no reference was found concerning their disintegration time and solubility. In 1871, Messrs Newbery had purchased Professor Brockedon’s business.
The Brockedon method of tablet compression was used by Philadelphian Jacob Dunton to formulate tablets of different formulations, including quinine [2]. In 1872, two brothers, Mr. Henry Bower and John Wyeth built an advanced machine that was not only more advanced than the previous one but also reduced the cost of producing tablets. In 1878, Dr. Robert R. Fuller from New York, for the first time, suggested the concept of loading these molds with medicated milk sugar. Mr. Fraser, in 1883, started to fabricate molded tablets in a completely new concept that we use today. From the start of the 1940s to the 1990s, synthetic and semisynthetic polymers were used for enteric coating. Dextroamphetamine sulfate was the first manufactured by Kline, Smith, and French as sustained release products using the Spansule method [2,3].
Drug carriers were used to incorporate nutraceutical, cosmeceutical, and pharmaceutical formulations. As per the report published in 2015, it was estimated that $178 million were spent on drug delivery systems, which could be increased to $310 billion by the year 2025.
By definition, the carrier means a system capable of incorporating a specific quantity of medicinal agents to increase their efficiency, selectivity, and bioavailability. The system’s efficiency depends on how much the system could bear the protective barrier. The release of the medicinal agents from the carrier system depends upon the rate and shape of kinetics, the viscosity of the media, and the drug release profile. Lipid-based dosage forms, including microspheres and microcapsules, tend to avoid drug leakage after its administration.
One of the basic advantages of incorporating the drug into the carrier system is to protect the drug during its overall stay within the body, starting from its point of administration until it reaches a specific site of action. Dosage forms were designed according to their usage, specificity, and stimulus-based. Nanosystems were extensively employed to incorporate active constituents, including nanospheres, nanocapsules, niosomes, liposomes, and dendrimers.
With artificial variation in drug release profile, drug release at a specific site could be achieved. The use of the carrier system has successfully bypassed the traditional drug delivery system, which has strong gastrointestinal consequences (gastric irritant drugs). Another reason for using a drug carrier system is to prevent the drugs from biodegradation and increase the bioavailability of specific drugs in the target tissue [4].
By 1992, with the advancements in research, a new carrier system made of mesoporous material gained much attention. The system was further utilized for large pore volume, having increased surface area and a well-organized structure. For the first time in 1998, Zhao and their co-workers synthesized Santa Barbara Amorphous (SBA-15), mesoporous silica with a hexagonal arrangement with high thermal stability, pore size, and surface area. Co-condensation and direct synthesis were the two most commonly used methods for functionalization. Much recently, these mesoporous particles gained more attention as nanoparticles due to their small size ranging from 0.6–1 nm. NSAIDs are muco-irritant and were loaded (as an adsorption phenomenon), especially in SBA-15, due to their nanosized. Once SBA-15 [5] was treated with hydrophobic octadecyl chains using the Surface functionalization technique, the release of the drug from the delivery system becomes quicker than SBA-1 [6].
Drugs of Biopharmaceutical classification system II (BCS-II) that followed the problem of low solubility, but high permeability was resolved using techniques including solid dispersion techniques, use of the drug in amorphous form, and complexation techniques. With an increase in solubility, the bioavailability of the longer half-life drugs also increased, which results in the accumulation of drugs leading to drug toxicity. Mesoporous silica materials (MSM) and other mesoporous particles exhibit hexagonal structure (MCM-41) having an increased surface area used as a reservoir for drug release. HMS and MCM-41 both were used as drug carriers for drugs that exhibit low solubility [7,8].
The concept of coating was initiated in ancient times. To begin with, Rhazes used psyllium seeds mucilage to unmask the taste of pills. Subsequently, it was reported that Avicenna coated the pills with silver and gold. In those days, various materials were used for coating purposes. Talc was introduced by White, known as a pearl coating and used to coat pills. In 1838, Garot introduced the coating of pills by using gelatin. Poisoned tablets were coated with wax to prevent unintentional poisoning. In earlier times, working individuals at pharmacies coated tablets for extemporaneous compounding only; later, this practice started in the pharmaceutical industry, and production was started on a large scale. In 1842, the first sugar-coated (SC) pill was imported from France to the United States (US). In 1856, a Pharmacist in Philadelphia pharmacist, indigenously manufactured coated pills [9]. By 1950, SC was considered the technique for coating purposes, and much work was done on that particular coating technique [10,11,12,13]. Firstly, coating in pharmaceutical industries was done on the pans, and the same technique was used for coating candies. The technique was further modified and evolved by the Middle Ages. Currently, pharmaceutical industries are well developed and organized using a wide variety of coating processes, including coating pans. The pans are of broad designs and range constructed of stainless steel. The coated material was then dried using heated air. Extra moisture and dust from the pans were removed using an air extraction system. By the twentieth century, conventional pharmaceutical processes of pan coating were employed for SC. Until the start of the 1950s, SC was the dominant type of coating in the pharmaceutical industry. Meanwhile, a new type of coating was introduced at that time, called film coating. Film coating (FC) gained significant interest because of its significant improvements in the possible limitations of SC processes. SC requires more operator skill, and it is a long operating process that kills time [14,15,16]. FC reduces the time, and less skilled operators, were required for that coating process [17]. Dissolving suitable polymers in volatile solvents could reduce the products’ coating duration from days to a few hours [9]. Moreover, FC improves the stability of dosage forms by protecting them from temperature, light, and humidity. FC also improves aesthetic properties and masks the unpleasant odor and taste [18]. Different types of drug release can be obtained by film coating, ranging from conventional to modified release, including enteric-coated, extended, and delayed-release [12].
The tablet is one of the most popular and extensively used solid dosage forms. Tablets are the compressed dosage form that may or may not contains the active ingredient. They differ in size, shape, and weight depending on the mode of administration and active ingredients used. Of all the dosage forms, approximately 70% of the medications were administered in tablets [15]. Tablets have some advantages over other dosage forms, such as feasibility, precise dose, and patient compliance, because they are produced on a large scale [19].
Patents appeared to be a source of innovation and a critical asset for any company. Patents provide detailed information about presenting, analyzing, and searching for scientific information. The search for patents has drastically changed from manual search to online search due to inventions in the field of computer science [20]. Some of the patents regarding FC are represented in .
The coating is defined as a procedure in which the desired dosage form may be a granule or tablet coated with an outer dry film to obtain specific objectives such as masking taste or protecting against environmental conditions. The coating material may be composed of coloring materials, flavorants, gums, resins, waxes, plasticizers, and a polyhydric alcohol. In the modern era, polymers and polysaccharides were principally used as coating materials along with other excipients like plasticizers and pigments. Many precautions must be considered during the coating process to make the coating durable and steady. According to the International Council for Harmonisation (ICH) guidelines, organic solvents are avoided in the formulation of pharmaceutical dosage forms due to their safety issues [21]. Tablets that are susceptible to degradation by moisture or oxidation must be coated by using the FC technique. This technique could increase its shelf life, mask its bitter taste, and make a smoother covering, which makes swallowing easier. Chitosan and other mucoadhesive polymers were also used for coating tablets to adhere these tablets to mucous membranes and achieve sustained drug release in localized areas [22]. In recent times, coating of the dosage form by using biopolymers has been extensively studied [23]. Active pharmaceutical ingredients (APIs), which are sensitive to light, can be protected by coating with opacifying agents. Similarly, enteric-coated tablets reach the intestine after an extended time and possibly help maintain the efficacy level of acid labile APIs [24].
Common forms of tablet coating are FC and SC. The coating helps maintain the physical and chemical integrity of the active ingredient; meanwhile, it also controls the drug release as it is controlled or continues to be released at a specific target site. Additionally, the coating was used to enhance the elegance of the pharmaceuticals, and the sophistication of appearance was enhanced by printing or making them with attractive colors [25].
Coating provides stability to the tablets in handling and prevents them from sticking together. The coating also improves the mechanical strength of the dosage form, causes the dosage form smoother and more suitable for swallowing purposes. Pharmaceutical companies could print their marks, symbols, or abbreviations on the tablets and mask a disagreeable color or odor of the tablets. The release of the active ingredient can even be controlled with the help of coatings. Coated dosage forms could be site-specific. The coating prevents acid-sensitive drugs from having a negative impact on the intestine. The drug release rate in the gastrointestinal tract (GIT) could be controlled by controlling the dissolution rate of the tablet [26].
The drawbacks of FC are represented in [27].
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