Capsules
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Direct answer
Gelatin shells turn an unpleasant, bad-tasting powder into an elegant single swallow, and that single idea explains both capsule families: hard gelatin capsules, two telescoping pieces filled with powder or pellets, and soft gelatin capsules, one-piece flexible shells sealing oils and liquids inside. Hard capsule sizes run from 000, the largest, down to 4 (with 5 rarer), the size number rising as capacity falls — the counter-intuitive fact every MCQ tests. Shells are made by dipping mould pins into warm gelatin solution, and soft capsules are filled and sealed in one continuous rotary die process; storage, in India's climate, is the chapter's practical heart: shells kept too dry turn brittle, too humid turn sticky.
What you must remember
- Hard capsule shell: gelatin, water, colourants and titanium dioxide as opacifier; soft shell adds glycerol or sorbitol as plasticiser in higher proportion.
- Sizes: 000 largest to 4 smallest (5 rarely used); size 0 is the everyday workhorse; as the number rises, capacity falls.
- Shell manufacture: dip polished pins in gelatin sol, dry, strip, trim to length, and join cap and body — the basis of the plate process and industrial rotary dipping machines.
- Filling methods: hand filling by punch method for small scale; semi-automatic and automatic capsule filling machines (auger, dosator or tamping principle) for production, followed by weight variation and content checks.
- Soft gelatin capsules: filled and sealed in one step by the rotary die process, historically introduced by Scherer; fill is a liquid, semisolid or powder in a carrier oil.
- Advantages: taste and odour masking, unit dose accuracy, no compression stress on the drug, hermetic sealing of volatile or moisture-sensitive fills (soft shells), and elegant appearance.
- Limitations: hygroscopic drugs pull water from the shell; highly water-soluble or efflorescent fills crack the shell; aldehydes cross-link gelatin and slow dissolution.
- Storage: dry, cool conditions at controlled humidity; tight packs or blisters with desiccant protect through the Indian monsoon.
- Vegetarian capsules of hypromellose (HPMC) now substitute gelatin for vegetarian and cultural acceptability.
From gelatin sol to sealed shell
Manufacture explains the form's virtues. Warm gelatin solution of controlled viscosity — graded by Bloom strength, higher Bloom meaning a firmer gel — sits in a dipping pan. A battery of polished stainless-steel pins dips, lifts a uniform film on each pin, and passes through drying ovens where the film sets to a shell with 13-16 per cent residual moisture. Stripping, trimming and joining produce empty two-piece capsules ready for filling. The pharmacist or the machine separates cap from body, fills the body by punch method or dosator, rejoins and locks, optionally sealing the joint with a band of gelatin.
Soft capsules differ at every step because the fill and the shell must be married while both are soft. A plasticised gelatin ribbon forms two continuous films; the rotary die machine presses them between rotating dies, injects the fill between them exactly as the dies close, and cuts and seals each capsule in a single revolution. The result is a hermetic one-piece shell — the only practical form for pure oil fills like cod liver oil or concentrated vitamin preparations, and the form that makes liquid doses portable. The quality checks that close the topic are weight variation (or content uniformity for low-dose fills), disintegration, and for soft shells, absence of leakage.
The shelf-side realities
Three practical slips recur. First, the size-numbering inversion: students write "size 4 is the largest" — the number and the capacity move oppositely, and the MCQ is built on exactly that reflex. Second, storage: gelatin shells equilibrate with ambient humidity, and in the Indian monsoon uncontrolled stock fuses, sticks and mildews, while over-dry stock cracks. The professional answer is storage in sealed containers at moderate temperature with controlled humidity, blisters with desiccant where warranted — and never a shop window display where afternoon sun cooks the pack.
Third, the compatibility trap: highly hygroscopic fills draw moisture out of the shell and become lazy to disintegrate; aldehyde-bearing formulations or impurities cross-link the gelatin, forming a slow-dissolving network — the classical reason some capsules fail dissolution on ageing. Viva examiners extend the question to "when would you not choose a capsule": for very high-dose drugs needing compressibility for size? for efflorescent or deliquescent fills, and where cost of a liquid fill in soft gelatin outweighs benefit.
Frequently asked questions
Which capsule size is the largest and which the smallest?
Size 000 is the largest and size 4 (or the rarely used 5) the smallest — as the size number increases, capacity decreases; size 0 is the commonly used size.
How is a soft gelatin capsule manufactured?
By the rotary die process: two continuous plasticised gelatin ribbons pass between rotating dies, the fill is injected as the dies close, and each capsule is simultaneously filled, sealed and cut in one step.
What is the shell of a hard capsule composed of?
Gelatin, water, colouring agents and titanium dioxide opacifier; soft capsule shells additionally contain plasticisers such as glycerol or sorbitol.
Why must capsules be stored protected from humidity?
Gelatin equilibrates with atmospheric moisture — excess humidity softens, sticks and encourages microbial growth, while very dry air embrittles the shell; controlled, sealed storage preserves both shell and fill.
Name two capsule-filling approaches from small to production scale.
Hand filling by the punch method for extemporaneous work, and semi-automatic or automatic machines using dosator or tamping-disk principles for industrial filling.