Soft gelatin capsules, commonly referred to as soft gels, are a popular pharmaceutical dosage form that encapsulates a liquid or semi-solid substance within a soft, flexible gelatin-based shell. These capsules are widely utilized for the oral administration of a variety of pharmaceuticals and dietary supplements, including fat-soluble vitamins, essential oils, and drugs with poor water solubility. The unique composition of the gelatin shell and its ability to hermetically seal its contents offer distinct advantages in terms of stability, patient compliance, and bioavailability. This document provides an in-depth analysis of the nature of the soft gelatin shell, the characteristics of the encapsulated content, and the various sizes and shapes available for soft gelatin capsules.
1. Nature of the Soft Gelatin Capsules Shell
1.1 Composition of the Shell
The shell of a soft gelatin capsule is a plasticized gelatin film. The key components include:
a) Gelatin (Base Material): Gelatin is a protein derived from collagen, which is extracted from animal connective tissues such as skin and bones. The type and bloom strength (a measure of gelatin’s gelling power) affect the mechanical properties and dissolution of the shell. Typically, a bloom strength of 150–200 is used.
b) Plasticizers: Plasticizers such as glycerin, sorbitol, and polyethylene glycol (PEG) are added to impart flexibility and elasticity to the shell. They reduce the brittleness of gelatin and allow the capsule to maintain its integrity over a range of temperatures and humidity conditions.
- Glycerol (Glycerin): Most commonly used plasticizer.
- Sorbitol: Can be used alone or with glycerin to control shell viscosity and firmness.
- Propylene glycol, PEG: Occasionally used in specific formulations.
The gelatin-to-plasticizer ratio typically ranges between 1:0.4 and 1:1, depending on desired properties.
c) Water: Water is essential for gelatin dissolution and shell formation. Typically, the shell formulation contains 30–40% water before drying. After drying, residual moisture content is about 6–10%.
d) Colorants and Opacifiers: Colorants (FD&C dyes, iron oxides) are added for product identification and brand distinction. Titanium dioxide is often added as an opacifier to protect light-sensitive drugs from photodegradation.
e) Preservatives (optional): Preservatives such as parabens or potassium sorbate may be used to prevent microbial growth during manufacturing and storage.
1.2 Properties of the Shell
- Elasticity and Softness: Due to plasticizers.
- Oxygen Barrier: Protects sensitive ingredients.
- Controlled Permeability: Prevents leakage or evaporation of volatile substances.
- Aesthetic and Smooth Surface: Suitable for oral use and patient compliance.
- Transparency or Opaqueness: Can be adjusted based on formulation needs.
2. Nature of Capsule Content
Soft gelatin capsules are best suited for liquid, semi-solid, or suspension fills that cannot be filled into hard gelatin capsules. The capsule content must be non-aqueous and compatible with gelatin and plasticizers.
2.1 Acceptable Fill Materials
a) Oils and Lipophilic Liquids: These are commonly used for delivering lipophilic drugs and fat-soluble vitamins such as vitamins A, D, E, and K. Examples include vegetable oils like soybean oil, olive oil, and sunflower oil, as well as medium-chain triglycerides (MCTs) and mineral oil. These oils serve as effective carriers for fat-soluble substances.
b) Solutions in Water-Immiscible Solvents: Certain poorly water-soluble drugs require solvents that improve their solubility. In such cases, water-immiscible solvents like polyethylene glycols (PEG 400, 600), propylene glycol esters, and caprylic/capric triglycerides are used. These solvents facilitate the dissolution and stable formulation of the drug within the capsule.
c) Suspensions: When the drug substance is not soluble in the chosen vehicle, it can be incorporated into a suspension. These suspensions are usually prepared using oil or PEG-based vehicles, and to maintain uniform distribution of the active pharmaceutical ingredient (API), surfactants and suspending agents are added.
d) Emulsions: Both oil-in-water (O/W) and water-in-oil (W/O) emulsions can be filled into soft gelatin capsules. However, the aqueous phase in these emulsions should be kept to a minimum to avoid degradation of the gelatin shell, which is sensitive to water.
e) Semi-solids: For thermosensitive drugs, semi-solid bases such as waxes and pastes can be ideal. Materials like beeswax, hydrogenated oils, and polyethylene glycols (PEGs) that are solid at room temperature but melt at body temperature offer a controlled release environment and protect sensitive actives from degradation during storage.
2.2 Incompatible Fill Materials
The following substances must be avoided as they can react with the gelatin shell or compromise integrity:
- Water (>5% in fill)
- Low molecular weight alcohols (ethanol, isopropanol)
- Strong acids or bases
- Aldehydes (e.g., formaldehyde)
- Ketones (e.g., acetone)
- Aromatic solvents (benzene, toluene)
These materials can cause hydrolysis, brittleness, softening, or leakage.
2.3 Formulation Considerations
- pH of the fill should be between 2.5 and 7.5. Below 2.5, gelatin hydrolysis occurs; above 7.5, gelatin cross-linking may prevent dissolution.
- Viscosity should be adjusted for uniform filling and encapsulation.
- Stability testing is essential to ensure no interactions occur between fill and shell.
3. Sizes of Soft Gelatin Capsules
Soft gelatin capsules are manufactured in a variety of shapes and sizes. Unlike hard gelatin capsules which have fixed standard sizes (like 0, 00, 1, etc.), softgels can be custom-designed.
3.1 Common Capsule Shapes
- Oblong (oval-shaped, common for vitamins)
- Round or spherical
- Tube or cylindrical
- Tear drop, fish-shaped (especially for pediatric or animal use)
3.2 Fill Volume Ranges
Fill volume is directly influenced by the size and shape of the capsule. The range typically falls between 100 µL to 1,000 µL.
| Capsule Shape | Typical Volume Range | Common Uses |
| Oval | 100 – 1,000 mg | Vitamins, pain relievers |
| Round | 150 – 400 mg | Single-dose liquids |
| Oblong | 200 – 800 mg | Lipid-based drugs |
| Tube | 100 – 500 mg | Nutraceuticals |
| Twist-off | 300 – 500 mg | Cosmetic serums, pediatric doses |
Some specific size codes used in industry:
| Capsule Size Code | Fill Volume (approx.) |
| 1 | 150 mg |
| 2 | 200 mg |
| 3 | 250 mg |
| 4 | 300 mg |
| 5 | 350 mg |
| 6 | 400 mg |
Note: Volume depends on density of fill material.
4. Manufacturing of Soft Gelatin Capsules
4.1 Rotary Die Process
The most widely used industrial method. Key steps:
- Two gelatin ribbons are formed and brought together between rotating dies.
- The fill is injected through a wedge between the dies.
- Capsules are sealed, cut, and dropped onto a conveyor.
- They are then dried in drying tunnels and stored for further processing.
4.2 Seamless or Bubble Method
- Used for small spherical capsules (pearl-type).
- Fill and gelatin are co-extruded into an immiscible liquid (e.g., paraffin).
- Capsules form and solidify instantly due to temperature or chemical gradient.
5. Advantages of Soft Gelatin Capsules
- Improved Bioavailability: Especially for poorly water-soluble drugs.
- Patient Compliance: Smooth, easy to swallow.
- Hermetic Seal: Prevents contamination and evaporation.
- Customizable Shape and Size: For marketing and branding.
- Protection of Ingredients: From oxygen and light.
- Tamper-evident: Difficult to alter without detection.
6. Disadvantages of Soft Gelatin Capsules
- Higher Cost: More expensive than tablets or hard capsules.
- Moisture Sensitivity: May stick or degrade under humid conditions.
- Not Suitable for Water-Soluble or Alcoholic Fill: Reacts with shell.
- Special Storage Needed: Controlled temperature and humidity.
- Cross-linking: May occur over time, leading to poor dissolution.
7. Stability and Packaging
- Packaging: Blister packs with aluminum foil, HDPE bottles with desiccants.
- Storage Conditions: Typically 15–25°C, 30–50% RH.
- Shelf-life: Typically 1–2 years depending on contents and storage.
8. Applications of Soft Gelatin Capsules
8.1 Pharmaceutical Uses
- NSAIDs (ibuprofen, diclofenac)
- Hormones (progesterone, testosterone)
- Antifungal (itraconazole)
- Antiviral (ritonavir)
- Nutraceuticals (omega-3, coenzyme Q10)
8.2 Nutritional and Cosmetic Uses
- Multivitamins
- Essential fatty acids
- Anti-aging serums (twist-off capsules)
- CBD oils and herbal extracts
9. Regulatory Considerations
Soft gelatin capsules must comply with regulations laid out by:
- US FDA (21 CFR parts 210 and 211)
- European Medicines Agency (EMA)
- Pharmacopeias: USP, BP, IP
- ICH Guidelines: For stability, dissolution, etc.
Soft gelatin capsules represent a technologically advanced, consumer-friendly, and versatile dosage form for delivering a wide variety of active compounds. Their soft, elastic shell is typically composed of gelatin, plasticizers, and water, forming a flexible and protective barrier around a non-aqueous fill. From improving bioavailability to protecting sensitive ingredients, softgels play an essential role in modern pharmaceutical and nutraceutical industries. Proper selection of shell composition, fill material, and capsule size is crucial for product stability, patient compliance, and therapeutic efficacy.