Soft gelatin capsules, also known as softgels or soft elastic capsules, are hermetically sealed one-piece capsules containing a liquid or a semisolid fill without a bubble of air or gas. They are made from a more flexible, gelatin film plasticized by the addition of glycerine, sorbitol, or a similar polyol.
As with hard gelatin capsules, soft gelatin capsules are predominantly administered orally. Some can be formulated and manufactured to produce a number of different drug delivery systems such as
Schematic diagrams illustrating different shapes of soft gelatin capsules
Soft gelatin capsules have grown in popularity in recent years because they enable administration of liquids in a solid dosage form with a bioavailability advantage over other commonly used solid dosage forms (e.g., tablets). They are available in a variety of sizes, shapes, and colours that may be specific to the manufacturer.
In this article, we will discuss the following:
Contents
Soft gelatin capsules may be selected as the preferred formulation approach for one or more of the following reasons:
Presenting a drug substance to the gastrointestinal tract in a solubilized form overcomes the processes of disintegration and dissolution that are required from solid dosage forms (such as tablets and hard gelatin capsules) before the active pharmaceutical ingredient (API) is available for absorption. This has been utilized to improve the bioavailability of drugs with a range of solubilities.
Also, the use of certain lipid vehicles can be associated with increased oral bioavailability and reduced intra- and inter- patient variability by modification of gastrointestinal digestive processes.
Presentation of drug substances in a solution form can overcome the challenges of achieving dosage unit homogeneity compared to other solid oral dosage forms. Because soft gelatin capsules are filled with liquids or suspensions, excellent content uniformity can be achieved with even the most potent of drugs. The accuracy of the filling mechanism enables the dose to be filled to a tolerance of ± 1% for solutions and ± 3% for pastes.
When the API is an oily liquid, has a melting point lower than about 75 °C or proves difficult to compress, liquid filling of softgels (with or without other diluents) can provide a successful approach to presenting it in a solid oral dosage form.
Most of the unit operations involved in the manufacture of tablets and hard gelatin capsules can generate a significant quantity of airborne powders. This can be a cause of concern for the manufacture of highly potent or cytotoxic compounds because of safety considerations for the operator and environment.
In the case of soft gelatin capsules, the powder handling is restricted to drug dissolution or dispersion in a liquid medium. Therefore, soft gelatin capsules provide greater operator safety during manufacturing. In addition, as the drug formulation is hermetically sealed in a shell, the exposure to the medication is minimized during dispensing as well as use.
Preparations of liquid-filled soft gelatin capsules have proven beneficial to oxidative degradable drugs. Varying the level of glycerol in the shell formulation will alter the permeability of the shell to oxygen. The filling process can be performed under nitrogen, so by appropriate selection of shell composition, this technology can provide excellent protection for oxygen-sensitive drugs.
Drugs available as soft gelatin capsules can offer patient benefit, such as ingestion without water and portability a good example of such drugs are cough and cold medicines.
The major components of soft gelatin capsule shell are gelatin, plasticizer and water. Besides these three components, soft gelatin capsule shell may contain other ingredients such as colourants and/ or opacifiers for visual appeal and/or reducing the penetration of light for the encapsulation of photosensitive drug substances.
Flavours and sweeteners may be added to improve palatability. Preservatives e.g., potassium sorbate, and methyl, ethyl, and propyl hydroxybenzoate are added to prevent the growth of bacteria and mould in the gelatin solution during storage.
Acid-resistant polymers when present in the capsule shell formulation are used to impart enteric release characteristics. They can also be used to formulate chewable soft gelatin capsules e.g., ChildLife’s Pure DHA chewable 250 mg softgel capsule.
| Component | Function | Typical content (% w/w) |
| Gelatin | Polymeric base | 66.3 |
| Glycerine | Plasticizer | 33.0 |
| Methylparaben + propylparaben (80/20 ratio) | Preservative | 0.1 |
| Colour | Colourant | 0.1 |
| Titanium dioxide | Opacifier | 0.5 |
| Water | Solvent/process aid | q.s. (0.7–1.3 × of gelatin) |
Typical composition of a soft gelatin capsule shell
Softgels are prepared to contain a variety of liquid, paste, and dry fills. Liquids that may be encapsulated into soft gelatin capsules include the following:
Note: Liquids that can easily migrate through the capsule shell are not suitable for soft gelatin capsules. These materials include water above 5% and low molecular weight water-soluble and volatile organic compounds such as alcohols, ketones, acids, amines, and esters.
Soft gelatin capsules (softgels) are manufactured using the following methods
Read more on manufacture of softgel capsules
Manufacturers should keep the followings in mind when producing soft gelatin capsules
The contents of soft gelatin capsules vary from solids, solid in liquid, solution or suspension, a combination of miscible liquid, or a simple liquid formulation. Fill materials are limited to
those that do not interact with the gelatin shell and/or other components of the shell.
The moisture content of soft gelatin capsules plasticized with glycerol is considerably higher than that of hard gelatin capsules. Therefore, to ensure chemical stability of the drug, moisture-sensitive drugs should not be formulated in soft gelatin capsules.
This is another important consideration in the development of soft gelatin capsules. During encapsulation, fill materials may be heated up to 35°C. Hence, highly thermolabile drugs may not be encapsulated in soft gelatin capsules.
Extreme acidic and basic pH should be avoided because a pH below 2.5 would tend to hydrolyse gelatin (leading to leakage). Also, fill materials with pH greater than 9 and aldehydes has a tanning effect on the gelatin. Tanning process involves crosslinking of gelatin, which results in hardening of the shell. The shell becomes insoluble in water and resistant to digestion by gastrointestinal enzymes: trypsin and chymotrypsin.
One of the most important aspects of soft gelatin capsule formulation is to ensure that there is minimum interaction or migration between the liquid fill matrix and the softgel shell. Drug substances can migrate from an oily vehicle into the shell, and this has been related to their water solubility and the partition coefficient between water and the nonpolar solvent. The possible migration of a drug into the shell must be considered in the packaging of topical products in soft gelatin tube-like capsules, as this could affect drug concentration in the ointment, as applied. For other products, such as oral capsules or suppository capsules, both the shell and the contents must be considered in judging drug content when migration occurs.
Quality control and inspection for soft capsules are almost the same as for other solid dosage forms and must follow good manufacturing practices. To read more on quality control of soft gelatin capsules click the link: Quality Control of Soft Gelatin Capsules
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