Film Coated Tablet

A film coated tablet is a type of oral medication where the tablet’s surface is covered with a thin layer of a polymer or similar material. This coating serves several purposes, including masking the taste of the medication, protecting the tablet from moisture, and making it easier to swallow.

Types of film coating

The common way to classify the film coating is by the effect of the applied coating on drug release characteristics. The film coating can be:

  1. Immediate release film coating (nonfunctional coatings): These coatings are typically readily soluble in water, so they do not significantly affect the biopharmaceutical properties of the drug.
  2. Modified release film coatings (functional coatings): These may be categorised as extended-release coatings or delayed-release coatings.

Delayed release coatings, also known as gastro-resistant coatings, are designed to be soluble in high pH values, typically above 5-6. This value exists in the intestine and is intended to either protect the drug from the stomach or prevent the release of the drug in the stomach.

Description of the film-coating process

Film coating involves the application of a liquid, polymer-based formulation to the surface of the tablets. Additionally, specialized equipment is typically used to take advantage of the fast coating and high degree of automation. Moreover, it is possible to use conventional panning equipment.

The coating liquid, whether a solution or suspension, contains a polymer along with other ingredients such as pigments and plasticizers, all in a suitable liquid medium. This solution is sprayed onto a rotating or fluidized mass of tablets. Drying conditions are employed in the process to remove the solvent, leaving a thin coating around each tablet core (Fig. 1).

Film coated tablet production
Fig. 1: illustration of the production of film-coating tablets. (It takes from Remington Essentials of Pharmaceutics)

Film Coating raw materials

The major components in any film coating formulation consist essentially of a polymer, plasticizer, colorant, and solvent.

Types of film-coating polymers:

Polymers for immediate-release coatings:

  • Hydroxypropyl methylcellulose (HPMC)

Is the most widely used of cellulosic polymers. The polymer is prepared by reacting alkali-treated cellulose first with methyl chloride to introduce methoxy groups and then with propylene oxide to introduce propylene glycol ether groups. It is readily soluble in aqueous media and
coatings that are relatively easy to apply.

  • Vinyl derivatives

Polyvinylpyrrolidone is the most commonly used vinyl polymer in pharmaceutical applications, but its tackiness limits its use in film-coating formulations. Copovidone, a copolymer of vinylpyrrolidone and vinyl acetate, is preferred for its better film-forming properties. Another valuable vinyl polymer is poly(vinyl alcohol) (PVA), derived from poly(vinyl acetate) hydrolysis, which can produce film coatings with appropriate mechanical properties and strong adherence to pharmaceutical tablets.

  • Aminoalkyl methacrylate copolymers

These acrylic copolymers dissolve easily in aqueous solutions with low pH, making them crucial for coating dosage forms where effective taste masking is essential. Typically, these polymers are applied as solutions in organic solvents, although special forms can also be used to create aqueous polymer dispersions.

Plasticizer

Plasticizer is added to the formulation to enhance the flexibility of the coating, reduce the risk of film cracking, and potentially enhance adhesion to the substrate. The plasticizer must be highly compatible with the polymer and remain in the film permanently to maintain consistent coating properties during storage.

Examples of typical plasticizers include glycerin, propylene glycol, polyethylene glycols, triacetin, acetylated monoglycerides, citrate esters (e.g., triethyl citrate), or phthalate esters (e.g., diethyl phthalate).

Colorants

Colorants are typically added to enhance product appearance, aid in product identification, and improve the stability of coated products. They can be categorized as water-soluble dyes or insoluble pigments. Water-soluble dyes are not suitable for organic solvent-based film coating due to their lack of solubility in the solvent system. Therefore, pigments, particularly aluminum lakes and inorganic pigments like titanium dioxide and iron oxides, are preferred for coloring film-coating systems. Using insoluble pigments in aqueous formulations is preferred because they:

  1. Act as bulking agents to increase the solids content in the coating dispersion without significantly increasing viscosity.
  2. Are more light stable.
  3. Are generally opaque, aiding in the stability of photolabile APIs like nifedipine.
  4. Additionally, decreasing coating permeability to moisture can potentially improve product stability.

Solvent

Film coating relies on various types of solvents, including alcohols (like methanol, ethanol, and isopropyl alcohol), ketones (such as acetone), esters (like ethyl acetate), chlorinated hydrocarbons (such as dichloromethane), and water.These solvents are crucial for applying the coating to the substrate’s surface. Additionally, effective interaction between the solvent and polymer is essential for achieving optimal film properties as the polymer solution dries and forms a solid film coating.The initial interaction between solvent and polymer leads to maximum polymer chain extension, resulting in films with high cohesive strength and excellent mechanical properties. Moreover, film-coating formulations vary widely based on the materials used. They typically consist of 5–25% (w/w) coating solids in the vehicle, with higher concentrations preferred for aqueous formulations. These formulations usually contain 60–70% polymer, 6–7% plasticizer, and 20–30% pigment.

Reference:

  • Aulton, M. (2018). Aulton’s pharmaceutics, the design and manufacture of medicines. Edinburgh. : Elsevier
  • Felton. L. (2013). Remington Essentials of Pharmaceutics. London. UK: Pharmaceutical Press.