Gelatin is a translucent proteinaceous substance that is colorless, practically tasteless, and brittle when dry but elastic when mixed with a precise amount of liquid. In the presence of water, it produces a semisolid colloid gel with a temperature-dependent gel–sol transformation and viscoelastic flow. Crystallites (microscopic crystals generated during the chilling step of capsule shell manufacturing) sustain the three-dimensional gel network structure and are responsible for gelatin solution streaming birefringence.
The cap and body of a hard gelatin capsule are made up of two parts. The body is slightly smaller than the cap's diameter and fits inside the cap. They are manufactured unfilled and then filled in a different process. The body of the capsule is filled with the medicament, then the cap is inserted over the body during the capsule filling unit operation.
To suit the production and use requirements of hard gelatin capsules, the forms and interlocking arrangement of the body and cap have changed.
Hard gelatin capsules are frequently used to create unusually difficult medicinal compounds. A drug candidate with a low melting point or that is liquid at room temperature, for example, has poor tablet manufacturability, especially if the dose is high. A liquid or semisolid-filled hard gelatin capsule can be used to encapsulate such a substance.
Furthermore, when manufactured as a tablet, very low-dose medications can face content homogeneity issues. When these medications are encapsulated as a solution in a liquid or semisolid matrix in a hard gelatin capsule, their distribution can be considerably improved.
In general, hard gelatin capsules require fewer formulation components and lay less emphasis on the powder qualities of the formulation. They can also provide formulation versatility by allowing for the filling of a variety of systems, including as powders, granules, pellets, and tiny tablets. Hard gelatin capsules can also be used to blind participants in clinical trials.
The intrinsic high moisture content requirement of gelatin causes the disadvantages of hard gelatin capsules. Highly soluble salts of pharmaceuticals, such as iodides, bromides, and chlorides, are not usually put in hard gelatin capsules because they might pull moisture from the shell, making it brittle. The shell can also become brittle when stored in low humidity conditions, such as when using desiccant in packing. Furthermore, gelatin is susceptible to cross-linking in the presence of very low (parts per million) formaldehyde concentrations, which can be found in pharmaceutical excipients such polyethylene glycol (PEG).
According to a capsule manufacturer, hard gelatin capsules have a simpler, less time-consuming, and less expensive production procedure than soft gelatin capsules. This is because the creation of gelatin ribbons during the encapsulation process is required for soft gelatin capsules, whereas hard gelatin capsules use pre-manufactured capsule shells. After the medication formulation is encapsulated, there is no need for a curing or moisture-loss stage in the hard gelatin capsule manufacturing process.
Hard gelatin capsules have less residual water in their shells (10–16 percent w/w) than soft gelatin capsules (30 percent w/w). This can directly affect the stability of the encapsulated formulation by increasing the rate of deterioration via chemical degradation (e.g., hydrolysis) of water-sensitive chemicals or plasticization of the reaction medium with water. Furthermore, the high oxygen absorption rate of soft gelatin capsule shells can contribute to the oxidation of sensitive medications.