10 Chapter 1582

The hard-shell capsule is a commonly used dosage form, and it has been estimated that approximately 60 billion capsule shells are used annually for pharmaceutical products (Podczeck, 2004). The product essentially consists of a hard shell, usually gelatin based, that is almost always filled with a particulate solid. Among the advantages claimed for the hard-shell capsule compared with tablets are enhanced bioavailability owing to the highly porous nature of the fill, less-demanding requirements for powder flow and the ability to fill formulations into a capsule shell that are not compressible to the extent needed for tablet manufacture. However, Jones (2001) has pointed out the fallacy of the belief that ‘powder filled capsules are a very simple product that does not need much skill to prepare’. The requirements of formulations to be filled into hard shells can be quite complex, and understanding the filling process has been greatly facilitated by fitting appropriate instrumentation to the filling equipment. There are several similarities between instrumentation for a tablet press and that of capsulefilling equipment. Tablet making and capsule shell filling involve compressing a particulate mass and so in both types of instrumentation, the parameters of most interest are force (pressure) and distance, both almost invariably recorded as a function of time. The transducers that have been used to measure these in tablet presses – various types of strain gauge and displacement transducer – have been used in capsule-filling equipment, and an instrumentation system for capsule-filling equipment requires the same components as for an instrumented tablet press, namely a power supply, suitably calibrated transducers, amplification circuitry and devices for recording and manipulating the amplified outputs of the transducers. Many of the approaches that have been used in the instrumentation of tablet presses have also been used to instrument capsulefilling equipment, and the use of simulators has been particularly successful. However, there are important differences. The forces used in the compression of the contents of hard-shell capsules are much lower – of the order of tens of newtons – whereas in tablet manufacture, forces of tens of kilonewtons are needed. Hence a sensitive measuring system that is capable of distinguishing a signal of this magnitude from background noise is required to study capsule filling. Earlier chapters in this book have stressed the importance of the correct siting of transducers and this is just as important with capsule-filling machinery. Indeed there is a particular problem in this case. To give a stable and meaningful output, the transducer must be attached to a massive and secure component of the equipment. Suitable sites are readily available in a tablet press but are not so available in capsule-filling equipment. The uses made of instrumented capsule-filling machines parallel those of instrumented tablet presses: the effect of compression force on plug properties such as physical strength, dispersion and dissolution have been studied, together with the lubricant requirements of the formulation. 10