Soft gel capsules are a widely used oral dosage form for medications and dietary supplements. They consist of a soft, pliable outer shell enclosing a liquid or semi-solid substance. This design allows for efficient encapsulation, providing a convenient method for consumption.
The Outer Shell
The outer shell of a soft gel capsule is made from gelatin, a natural polymer derived from animal collagen. Collagen sources include bovine (cattle), porcine (pig), or piscine (fish) origins. Gelatin constitutes a significant portion of the shell, often 40% to 50% of the total formula.
Plasticizers are incorporated into the gelatin mixture to provide the shell’s flexibility and elasticity. Glycerin and sorbitol are common plasticizers, making up 15% to 30% of the wet shell formulation. These compounds prevent the shell from becoming brittle. Water is also a substantial component of the wet shell formula, contributing 30% to 40% by weight, which is important for proper gelatin structure and viscosity during manufacturing.
While gelatin is the most prevalent material, non-gelatin alternatives are available for dietary preferences or restrictions. Plant-based options include materials such as pullulan, carrageenan, starch, or hydroxypropyl methylcellulose (HPMC). Other additives such as opacifiers, often titanium dioxide, protect light-sensitive contents. Colorants or flavorings may also enhance appearance or palatability.
The Inner Contents
Soft gel capsules encapsulate liquid, semi-solid, or suspension formulations. They are suitable for substances difficult to formulate into traditional tablets or hard capsules. Common contents include oils (e.g., fish oil, vitamin E), fat-soluble vitamins, and certain medications.
The fill material can be a solution where active ingredients are dissolved in a carrier, a dispersion, or a suspension where solid particles are evenly distributed within a liquid. Carrier liquids are often lipophilic, like refined soy oil or medium-chain triglycerides (MCTs), or hydrophilic, such as polyethylene glycols. Encapsulating substances can help maintain stability and improve absorption within the body.
Why Soft Gels Are Used
Soft gel capsules offer several advantages for specific products. They enhance bioavailability, especially for compounds with poor water solubility, improving absorption. The hermetically sealed shell protects sensitive ingredients from light, air, and moisture, improving product stability and shelf life.
The smooth, soft texture of soft gels makes them easier to swallow than hard tablets or capsules, which can improve patient compliance. They are also effective at masking unpleasant tastes or odors of the encapsulated ingredients. Precise and uniform dosing ensures consistent delivery of the active substance.
How Soft Gels Are Manufactured
Soft gel capsules are manufactured using rotary die encapsulation. This method simultaneously forms the capsule shell and fills it with the intended contents. The process begins with the preparation of the gelatin mass, where gelatin, water, and plasticizers are heated and mixed to form a viscous solution. Concurrently, the fill material is prepared, ensuring it is in a liquid or semi-solid state suitable for encapsulation.
During encapsulation, two continuous ribbons of gelatin are fed between rotating dies. As the dies rotate, fill material is injected between the ribbons. The dies then cut and seal the capsules, creating a hermetic enclosure. After encapsulation, the soft gels undergo a drying process to reduce their moisture content, typically to a range of 6% to 13%, which is important for their stability and integrity.