Advances in tablet coating
As the functional domain of the coating widened, so increased the newer technologies imparting niche over the existing ones, say S K Thakral and Dr Vipin Bihari Gupta
Tablet coating is one of the oldest pharmaceutical processes; perhaps, in early times the basic purpose of coating had been elegance or organoleptic enhancement, now there are many more reasons (Table 1) to it. As this process maintained its status more of an art than a science, it consistently kept offering enough avenues for the pharmaceutical researcher to work upon and result into financially rewarding patents (Table 2).
Traditionally, major techniques for applying coating have been sugar coating, film coating and compression coating. Sugar coating is a cumbersome and time-consuming process; still it is widely used by many pharmaceutical companies. Film coating is a highly economical and time saving process, however, this uses organic solvents, which causes many problems including flammability hazards, toxicity hazards, and concerns over environmental pollution. Compression coating, which is similar to the preparation of multiple layer tablets having an inner core and an outer shell is a process of making a second tablet around the first. It’s mechanically a bit too complex an operation and calls for high initial investment on equipment.
As the functional domain of the coating widened, so increased the newer technologies imparting niche over the existing ones. Each of these emerging technology has something special in it and offers some specific advantages for typical case applications.
The promising ones are as follows:
Dip coating: In this, cores to be coated are a held in a suitable device eg: baskets, dipped into coating solution and then dried taking care to prevent adherence to one another. For obtaining more perfect or heavier coats the dipping and drying steps may be repeated several times one after another. Several dipping arrangements are obtainable, amongst them the sophisticated devices comprise tiny suction tubes, which hold the individual tablets apart until drying is accomplished, before proceeding to coat additional tablets or begin recoating cycles.
Perhaps, number of dipping techniques have been proposed, they lack speed, versatility, coat uniformity and reliability of spray coating, hence haven’t so far been widely accepted. None-the-less it remains a fertile area for research.
Electrostatic coating: Electrostatic coating is employed for applying films of electro-conductive materials. In this, an ionic charge is imparted to the core and an opposite charge to the coating material. This technology ensures thin, continuous and electronically perfected film to the surface.
Laminate coating: Laminated coating provides multiple layers for incorporation of medicament; for example (1) Repeat-action tablet, here a portion of the drug is kept in outer lamella or coating; (2) Enteric tablet, here one drug could be made available for gastric absorption while another for release in intestine; and (3) Buccal-swallow tablet, this could first be administered sublingually, and upon a signal, such as release of flavour from the inner core, the same may be swallowed as a normal peroral tablet.
Vacuum film coating: This employs a specially designed baffled pan, which is water-jacketed and could be sealed to achieve vacuum. Tablets are placed in the sealed pan, the vacuum is applied and the coating material is introduced through airless hydraulic spray system. Since the pan is completely sealed, organic solvents could be effectively used with minimal environmental or safety concern.
One step aqueous enteric coating or latex coating: Aqueous enteric coating systems are currently one of the most widely employed systems as they offer substantial advantages over solvent systems, particularly with regard to environmental and toxicological concerns. Earlier these were merely advancements of traditional solvent systems and required separate addition of plasticizers, detackifiers, pigments and other process aids involving multiple time-consuming steps.
In addition, many of these systems were provided as liquid dispersions, hence, problematic in handling, transportation and storage. Now, two fully formulated, aqueous enteric coating systems are commercially available in dry dispersible powders form and these use one-step process and are simple to apply. The first ’Sureteric System’ is based on poly vinyl acetate phthalate (PVAP), and the second ’Acryl-Eze System’ is based on methacrylic acid copolymer type C (Eudragit L100-55). Typically, the coating is done in the 48 inches side-vented pan equipped with four spray gums. The dispersions are delivered to the spray guns through individual tubes fed from peristaltic pump.
Sustained-release coating: This is modified film coating, which is designed to extend drug release over a period of time and it eliminates the need of multiple dosage regimens for those drugs that require constant blood level over a long period of time.
Materials that have been found suitable for producing sustained release coatings include:
- Mixtures of waxes, shellac and zein-polymers that remain intact until the pH of gastrointestinal contents becomes less acidic;
Ethylcellulose, which remains intact throughout the gastro intestinal tract, however, it does permit water to permeate the film, dissolve the drug, and again diffuse out;
Acrylic resins, which behave similarly to ethyl cellulose as a diffusion-controlled drug-release coating material; cellulose acetate; and silicone elastomers.
Similar to enteric coatings many synthetic polymers suitable for sustained release film-coating applications have been prepared in aqueous dispersions form (often called latexes or pseudolatexes). These are commercially available and facilitate use of aqueous film coating technology.
Supercell technology: Supercell coating technology is an invention of Niro Pharma, which uses a small modular design where tablets are coated in batches ranging from 30 to 40 grams, and which is amenable to linearly scale up to the production capacities.
In this, typically tablets are coated with coating spray in the same direction as the drying gas, hence, resulting in a more efficient process.
This technology provides unique air distribution and the tablets moves quickly and predictably through the spray zone, receiving only a small amount of coating per pass, and therefore, achieving higher coating accuracy.
The process time is short, seconds or minutes are required as opposed to hours required in other techniques, and therefore, it is gentler on the tablets. Henceforth, it allows for coating of friable tablets, as well as, flat or highly oblong tablet shapes and even extremely hygroscopic cores.
The accuracy of deposition is high, therefore, if required the active ingredients could be layered onto tablets, and uniform layers of taste masking or modified release coatings could also be applied.
Soflet technology: This is an invention of Banner Pharmacaps and it employs application of gelatin layers over the table cores. Specially designed machines form two ribbons in which tablets are sandwiched and then cut-sealed by thermal compression. The tablets may have two different colours at two different sides. This technology is claimed to providing better swallowbility in addition to other features like oxygen barrier capabilities of the gelatin.
BioProgress, a new entrant in coating technology, has come out with a similar technology where gelatin is replaced by
The writers are with Department of Pharmaceutics, L M College of Science & Technology, Jodhpur