Various types of capsules, with shells made of different materials, are available. When choosing a capsule type, formulators should consider factors such as the shell’s barrier to water and oxygen, reactivity, and the material it is made of.
Traditional capsules are made from gelatin, which is a widely produced animal-based product. More recently HPMC (hydroxyprolymethyl cellulose) and Pullulan have become successful alternatives and are available commercially for both pharmaceutical and nutraceutical products.
Classification of Capsules
Hard gelatin capsules
The gelatin used in the manufacture of most common capsules is obtained from collagenous material by hydrolysis. Gelatin is a natural, safe, non-allergenic, clean, and economical ingredient. The two-piece hard gelatin capsule is available in a range of sizes; from largest to smallest, these sizes are 000, 00, 0, 1, 2, 3, 4, 5.
Hard gelatin capsule is a kind of capsule mainly composed of gelatin and purified water. Some colored hard gelatin capsules also contain a small amount of Titanium dioxide and colorants.
Soft Gelatin Capsule:
The soft gelatin capsule (SGC) was invented by Mothes, a French pharmacist, in 1833. It is One piece, hermetically sealed of gelatin shell containing, liquid, a suspension or a semisolid.
The composition of soft gelatin capsules is similar to that of hard gelatin capsules, except that the different moisture proportion & Plasticizers. Usually, plasticizers will be added to keep capsules’ elasticity and stability, including Glycerol, Sorbitol etc. It may contain preservative to prevent the fungal growth.
HPMC Capsule is mainly composed of HPMC and purified water. HPMC is a kind of cellulose obtained by the hydrolysis of plants and is made by etherification.
HPMC capsules are stable at low humidity levels, have low moisture content (3–8%), and low static charge. These natural HPMC capsules are suitable for highly reactive molecules (because they have no cross-linking reactions). Compared to hard gelatin, HPMC is more suitable for moisture-sensitive products, hygroscopic products, and for low relative-humidity applications.
Because HPMC is derived from plants, it is a vegetarian capsule (herbal product).
Modified Release Capsule
Both hard or soft gel capsules can be chemically modified to alter the release of the active ingredient(s). If the drug is water-soluble and in a hard capsule and a fast release is desired, the excipients should be hydrophilic and neutral whereas for slow release of water soluble drugs the excipients will slow the release. Rapid release from capsules can also be obtained by piercing the outer film with small holes or incorporating a small quantity of citric acid and sodium bicarbonate to assist in opening the capsule by evolution of carbon dioxide.
A small concentration (up to 1%) of sodium lauryl sulfate may be added to the gel of a soft capsule to enhance penetration of water and speed dissolution. If slow release from a soft capsule is required, polymer or alginates may be added.
Enteric Coated Capsules
Enteric Coated Capsules do not belong to a certain type of capsules. They are made by applying an enteric coating to other common capsules. Enteric Coated Capsules are mainly made to meet the needs of releasing medication in the intestine.
Its general principle works like that, with the coating process, the enteric capsule only can be dissolved by the alkaline liquid in the intestinal tract.
Starch capsules are made from potato starch. Their dissolution is pH independent, and they are suitable for enteric coating. The moisture content of starch capsules ranges between 12–14% w/w, with more than 30% being tightly bound.
These vegetarian capsules are made from tapioca, which is naturally fermented into pullulan. They provide a high barrier to oxygen.
Pullulan is a water-soluble mucopolysaccharide, a mature food additive
Polyvinl acetate (PVA) capsules
Capsules made from PVA can be used for filling insoluble drugs dissolved in polyethylene glycol (PEG) 400. PEG 400 when used as single vehicle is not compatible with other hard capsules. The oxygen permeability of PVA is low, resulting in a high barrier to oxygen.
Liquid-filled hard capsules
Two-piece hard capsules made of either gelatin or HPMC can be used for filling and band sealing non-aqueous liquid, paste, suspension, hot melts, and other vehicles that melt up to 70 °C and flow easily. LFHC can also be filled with tablets, pellets, or other capsules as combination fill. LFHC can be used for moisture-sensitive drugs. These can be a cost-effective alternative to some soft gelatin capsule products and can also enhance bioavailability and improve product stability. Liquid encapsulation technology helps overcome many problems associated with the use of softgel capsules including high cost, waste, cross-contamination, migration of the drug into the capsule shell, and issues with low bioavailability. Liquid-filled and semi-solid capsules by their nature are resistant to crushing and powdering and therefore provide a good basis for developing an abuse-resistant formulation. These capsules can also be enteric coated. HPMC hard capsules do not become brittle when they lose water.
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