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Types of Capsules

Nov. 16, 2022

In the manufacture of pharmaceuticals, encapsulation refers to a range of dosage forms—techniques used to enclose medicines—in a relatively stable shell known as a capsule, allowing them to, for example, be taken orally or used as suppositories. The two main types of capsules are:

 

Hard Capsules

Hard capsules are rigid cylindrical shells, usually consisting of two parts - the main body and the cap - which are filled with dry or liquid ingredients and then specially sealed to prevent leakage. These are one of the oldest forms of drug capsules.

 

Soft Capsules

Soft capsules (also known as soft gels) are conjoined capsules, i.e. made in one go. Although the manufacturing process for softgels is more complex, this method allows manufacturers to produce capsules with liquid or semi-liquid fillings in different shapes and sizes.


Some key considerations for hard and soft gelatin capsules

The manufacture of hard and soft capsules is a complex process that requires precise calculations. Once the capsules have been manufactured, their performance may also vary. Let's take a look at some of the key technical considerations for hard and soft capsules.

 

Active Pharmaceutical Ingredient (API) / Dissolution

This is the rate at which the capsule dissolves in the stomach, allowing the active ingredient to be absorbed by the body. For example, gelatine has one of the best (if not the best) API dissolution rates, allowing the API to be absorbed quickly by the body. This is one of the most important considerations for pharmaceutical manufacturers.

 

The dissolution process normally proceeds smoothly in the body. However, exposure to inappropriate heat or humidity conditions, or the presence of certain aldehydes, can lead to cross-linking of the capsule shell. Due to the low humidity, HPMC does not suffer from cross-linking problems. Gelatins used to be susceptible to this phenomenon, but new generations of gelatins such as StabiCaps have been developed that can significantly reduce and mitigate the cross-linking problems that can occur.

 

Types of Capsules

 Gelatin Capsules

Cost-effectiveness

Gelatine is the most cost-effective compared to alternative excipients. First-generation HPMC hard capsules cost approximately four times as much to make as gelatine, while HPMC capsules cost approximately three times as much to manufacture. This of course has an impact on the costs charged to the healthcare system, consumers, and taxpayers.

 

Processability

Gelatine has a high level of processability, which means that it tolerates mechanical processing well and therefore does not require additional ingredients or machinery. HPMC is less processable: first generation HPMC requires an auxiliary gelling agent, while second-generation HPMC does not require a gelling agent, but does require machining.

 

Mechanical resistance

The capsule must be strong enough to cope with mechanical stress. Any weakness can lead to defects. Compared to HPMC, gelatine is the most resistant. When it comes to soft capsules, gelatine and modified starch are comparable in strength.

 

Types of Capsules

Gelatin Capsules

Oxygen permeability

Some APIs have complex properties that make them sensitive to oxidation. For these formulations, capsules are the best choice as they offer the best protection. Due to its low oxygen permeability, gelatine is the best excipient when dealing with oxygen-sensitive APIs. The alternative HPMC has a high oxygen permeation potential, which means that other ingredients (e.g. antioxidants) must be used, which increases the overall cost. Therefore, whenever possible, manufacturers will always choose gelatine capsules when considering oxygen sensitivity.

 

Water permeability

Another consideration is water permeation. Depending on the type of capsule, vapor can have a different effect. HPMC has a lower water content than gelatine. However, it is inconclusive as to whether HPMC really performs better than gelatine in terms of water permeability.


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