Compared to hard gelatin, HPMC is more suitable for moisture-sensitive products, hygroscopic products, and for low relative-humidity applications. HPMC capsules are suitable for highly reactive molecules (because they have no cross-linking reactions). These natural capsules are available from size 00 to 4. HPMC capsules are stable at low humidity levels, have low moisture content (3–8%), and low static charge. The two-piece hard gelatin capsule is available in a range of sizes from largest to smallest, these sizes are 000, 00el, 00, 0xel, 0el+, 0el, 0, 1el, 1, 2, 3, 4, 5. Gelatin is a natural, safe, non-allergenic, clean, and economical ingredient. The gelatin used in the manufacture of most common capsules is obtained from collagenous material by hydrolysis. In such cases, primary packaging material such as aluminum strip packing, moisture barrier blister foil (e.g., Aclar), or bottle packs should be used. If stored at high humidity, the capsules become flaccid. Gelatin capsules do not protect hygroscopic materials from atmospheric water vapor because moisture can diffuse through the gelatin wall. Storage of hard-gelatin capsules at very low humidity can cause them to turn brittle. The shell of hard gelatin capsules contain 13–16% water. Fill weights in the smallest capsules might be 39 mg, for example the largest may weigh 1425 mg. Fill weights increase with the size of the capsule as well as with the bulk density of the filled material, which can range from 0.3–1.5 g/cc. The larges size (000) is mainly used in veterinary practice. Liquid-filled capsules include hard capsules (gelatin or HPMC) and softgel gelatin capsules. Dry-filled capsules include mainly hard gelatin and hard hydroxypropyl methylcellulose or hypromellose (HPMC) capsules. The most widely used capsules can be classified as shown in Figure 1 and discussed in the following sections. When choosing a capsule type, formulators should consider factors such as the shell’s barrier to water and oxygen, reactivity, and the material it is made of. Various types of capsules, with shells made of different materials, are available. Some drugs are available only as capsules or tablets, and some are available as both. Oral-solid dosage (OSD) drugs can be formulated in tablet or capsule form.
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