A soft gel capsule, or softgel, is a sealed, single-dose oral delivery system containing a liquid or semi-solid material. This hermetically sealed structure protects pharmaceutical compounds or dietary supplements sensitive to light or oxygen. The softgel format is commonly used to deliver ingredients like fat-soluble vitamins, oils, or certain medications. It masks unpleasant tastes and odors while offering a smooth, easily swallowed, flexible shell containing a precise dose.
The Primary Components of the Shell
The outer shell of a soft gel capsule is a blend of ingredients designed to be elastic, durable, and readily digestible. The primary structural component is the gelling agent, most often gelatin. Gelatin is a protein derived from animal collagen, typically sourced from bovine, porcine, or fish materials. This protein forms a reversible, thermo-sensitive gel structure when mixed with water.
Gelatin alone results in a brittle shell, so plasticizers are incorporated to add flexibility and elasticity. Common plasticizers include glycerin, sorbitol, or a combination of both, constituting 15 to 30 percent of the wet shell formula. These substances interact with the gelatin network to reduce rigidity, making the final capsule pliable enough for manufacturing and consumption. The amount of plasticizer is carefully controlled depending on the desired shell strength.
Water acts as the initial solvent for dissolving the gelatin and plasticizer during gel mass preparation, making up 30 to 40 percent of the wet shell formula. Most water is removed during drying, but a small percentage remains in the finished shell to maintain flexibility. The shell formulation also includes secondary additives for functional purposes.
Opacifiers, such as titanium dioxide, are added to create an opaque shell necessary to shield light-sensitive fill ingredients from photodegradation. Colorants and flavorings are also incorporated for product identification and to improve the swallowing experience. For those seeking non-animal options, alternative gelling agents like carrageenan, modified starches, and cellulose derivatives are now used to create vegetarian soft gels, fulfilling the same functional requirements as gelatin.
The Material Contained Within the Capsule
The fill material contained within a soft gel capsule is typically non-aqueous liquids, suspensions, or pastes. This dosage form is well-suited for lipophilic (oil-soluble) substances, such as omega-3 fatty acids, fish oils, and fat-soluble vitamins like D and E. These oily liquids are often the active ingredient or are used as a carrier vehicle to dissolve the active compound, enhancing stability and bioavailability.
Suspensions and pastes are semi-solid fill materials where the active substance is dispersed throughout a non-aqueous carrier, such as polyethylene glycol (PEG) or a waxy matrix. This method is used when the active ingredient does not easily dissolve in the liquid carrier, ensuring uniform distribution in a thick, flowable medium. The viscosity of the fill material is critical: it must be low enough for accurate injection during encapsulation but high enough to remain stable within the sealed shell.
Soft gels are generally incompatible with highly aqueous solutions; formulations containing more than 10 percent water are avoided. Since the gelatin shell is hydrophilic, high-water content fill causes water to migrate into the shell. This migration acts as a super-plasticizer, which compromises the shell’s structural integrity over time, potentially causing leakage or brittleness. Therefore, soft gel technology is optimized for anhydrous or non-aqueous fills, which preserve the shell’s consistency and product stability.
How Soft Gels Are Constructed
Manufacturing begins with preparing the shell material, known as the gel mass. This involves combining the gelling agent, plasticizers, and water in large, jacketed tanks, where the mixture is heated until the gelling agent is completely dissolved into a viscous, molten solution. This hot, liquid gel mass is then transferred to the encapsulation machine, maintaining its elevated temperature to prevent premature gelling.
The primary method for forming capsules is the rotary die process. The hot gel mass is cast onto cooling drums to form two continuous ribbons of gel film. These ribbons are then fed between a pair of synchronized, rotating die rolls that contain pockets shaped like the final capsule.
As the two ribbons converge between the dies, a heated wedge assembly precisely injects the measured dose of the liquid or semi-solid fill material into the pocket. The pressure from the injection causes the ribbons to expand into the die pockets. Simultaneously, the die rolls cut the capsule shape and use mechanical pressure and heat to hermetically seal the two halves of the shell around the fill.
The newly formed, still-moist capsules are subjected to a drying process, often involving tumbling dryers and tray drying in controlled rooms. This multi-stage drying removes excess moisture from the shell, reducing its water content to roughly 10 percent. This level is necessary for the shell to reach its final, stable mechanical strength. Drying time can vary significantly, sometimes lasting several days, and is crucial for finalizing the shell structure and ensuring long-term product stability.