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eye on moisture several times its own weight. This makes it useful as an anti-caking agent to counter the effects of mois- ture that powders and granulated products can adsorb from the envi- ronment during production, packag- ing, and storage. That is, it prevents moisture from causing products to cake, lump, and bridge, which im - pedes flow and can clog equipment. In fact, because silica is odorless, taste less, non-toxic, and mostly inert, it is one of the most common anti- caking agents used to provide storage stability to powders. It is also useful as a moisture scav- enger when formulating water-labile drugs or moisture-sensitive formula- tions. For example, the addition of even a small amount of silica can help maintain tablet hardness and disintegration when a formulation's ingredients have an affinity for mois- ture. Use levels depend on many fac- tors, such as the powder's hygroscop- icity, particle size, and packaging to name a few. Typical use levels range from 0.25 to 2.0 percent. Many types of silica can scavenge moisture, in - cluding fumed silica, precipitated sil- ica, calcium silicate, magnesium alu- minum silicate, magnesium silicate, and magnesium trisilicate. Glidant One of the most common uses of silica is as a glidant to improve the flow properties of a powder. Silicas have no lubricating properties but, like lubricants, coat particles through shear forces produced during mix- ing/blending. This causes agglomer- ated silica to break down into very fine silica particles and coat the larger particles in the blend. This coating reduces inter-particulate friction and static charge, thereby improving the flow properties of powders. Because silica has extremely fine particles and a low bulk density, it is difficult to handle. It's best to pre-blend it with other ingredients rather than screen- ing it into the blend. If your particular active pharmaceutical ingredient (API) has poor flow properties, pre- blend it with silica before adding the remaining ingredients. Improving powder flow has sev- eral benefits in the production of tablets and capsules. First, the move- ment of powder from the hopper into the tablet press or encapsulator is more uniform. Indeed, one of the more common ways to fix rat-holing and funnel flow in a hopper is to add a small amount of silica to the pow- der blend. Second, silica improves powder flow into the die cavity of a tablet press or dosing disc of an encapsulator, improving weight and content uniformity. In general, the flow of powders can benefit from the addition of only a small amount of silica, typically in a range of 0.1 to 2.0 percent; 0.5 percent is commonly used. The amount needed depends mostly on the surface area of the other particles in the blend, i.e., the larger the surface area, the more gli- dant required. Adding too much sil- ica, however, can decrease the bulk density of the blend, leading to poor flow and compaction properties. In general, when silica is used as a glidant, it is added at the beginning of the blending process, but be sure you understand the process because conditions can strongly influence Tablets & Capsules April 2015 37 John A. McCarty McCarty Pharma Consultants excipients This edition of the column is the final installment in a three-part series about using silicon dioxide (silica) as a pharma- ceutical excipient. In it, John McCarty discusses silica's numerous pharmaceutical applications and functions. The columns in the previous two issues discussed the various forms, types, sources, and grades of silica, as well as the chemical and physical properties, reactivity, regulatory sta- tus, pharmacology, and toxicology. This edition discusses the applica- tions and functions of silica in the preparation of tablets and capsules. Applications Some of the more common func- tions of silica in the preparation of oral solid dosage forms include: • Anti-caking agent (moisture absorbance) • Powder flow agent (glidant) • Carrier for liquid ingredients, including flavors • Tabletting aid • Film coating ingredient • Solubility enhancer • Spray drying aid • Disintegrant • Agent to prepare moisture- activated dry granulations (MADGs). The types of silica and how they are used in these applications are dis- cussed below. Anti-caking agent Silica's hydrophilic nature and large surface area enable it to scavenge