They are viscous semisolid emulsion system intended for application to the skin i.e. for external use. Have opaque appearance compared with the translucent ointments. Consistency and rheological characters depend on whether the cream is w/o or o/w. Creams are of two types, aqueous creams and oily creams. In case of aqueous creams the emulsions are oil-in-water type (o/w) and in case of oily creams emulsions are of water-in-oil type (w/o).
Composed of small droplets of oil dispersed in a continuous aqueous phase. Are more comfortable and cosmetically acceptable as they are less greasy and more easily washed off using water.
Composed of small droplets of water dispersed in a continuous oily phase. Are more difficult to handle but many drugs which are incorporated into creams are hydrophobic and will be released more readily from a w/o cream than an o/w cream. W/O are also more moisturizing as they provide an oily barrier which reduces water loss from the stratum corneum, the outermost layer of the skin.
Due to the presence of water soluble bases they can be easily removed from the skin. The aqueous creams have a tendency to grow bacterial and mold growth; therefore a preservative must be added in their formulation. E.g. cetomacrogol cream, cetrimide cream, hydrocortisone cream, zinc cream BPC. Properly designed W/O creams are elegant drug delivery system, pleasing in both appearance and feel after application. O/W creams are non-greasy and are rinsable (easily removed by washing).
• As with other types of emulsion, hygiene is extremely important and all surfaces, spatulas and other equipment must be thoroughly cleaned with industrial methylated spirits (IMS). IMS is better than freshly boiled and cooled purified water as it will quickly evaporate, leaving no residue.
• Always make an excess as it is never possible to transfer the entire cream into the final container.
• Determine which of the ingredients are soluble in/ miscible with the aqueous phase and which with the oily phase. Dissolve the water-soluble ingredients in the aqueous phase.
• Melt the fatty bases in an evaporating dish over a water bath at the lowest possible temperature. Start with the base having the highest melting point. These should then be cooled to 60 °C (overheating can denature the emulsifying agent and the stability of the product can be lost).
• Substances that are soluble/miscible with the oily phase should then be stirred into the melt.
• The temperature of the aqueous phase should then be adjusted to 60 °C.
• The disperse phase should then be added to the continuous phase at the same temperature. Hence, for an oil-in-water (o/w) product add oil to water. For a water-in-oil (w/o) product add water to oil.
• Stir the resulting emulsion without incorporating air, until the product sets. Do not hasten cooling as this produces a poor product.
Incorporation of solids into a cream base
If the cream base has been prepared from first principles, the solid can be incorporated into the cream as it cools. Alternatively, if using a pre-prepared base, soluble and insoluble solids may be incorporated using the method employed for insoluble solids.
• Soluble solids should be added to the molten cream at the lowest possible temperature and the mixture stirred until cold.
• Insoluble solids should be incorporated using an ointment tile and spatula. If there is more than one powder to be added these should be triturated together in a mortar using the ‘doubling-up’ technique prior to transfer to an ointment tile.
For coarse powders a minimum quantity of cream should be placed in the centre of the tile and used to levigate the powders. A considerable lateral shearing force should be applied to avoid a gritty product. The powder/ fatty base mixture may then either be returned to the evaporating basin with the remaining cream and stirred until cold or the remaining cream in the evaporating basin may be allowed to cool and triturated with the powder/cream mixture on the tile.
Fine powders may be triturated into the otherwise finished cream on an ointment tile. Small amounts of powder should be added to an equal amount of cream (i.e. using the ‘doublingup’ technique). These should be well triturated.
Incorporation of liquids into a cream base
• Non-volatile, miscible liquids may be mixed with the molten cream in the evaporating basin. Alternatively, if a pre-prepared base is used, then incorporate as for volatile or immiscible liquids.
• Volatile or immiscible liquids (e.g. coal tar solutions) should be triturated with the cream on the ointment tile. A very small amount of the cream should be placed on the tile and a ‘well’ made in the centre. Traditionally, small quantities of liquid should be gently folded in to avoid splashing.
An alternative method is to spread a small amount of the cream on the tile and then ‘score’ it with a spatula. Then add small quantities of the liquid and fold into the base gently. If using coal tar or other volatile ingredients, these should not be weighed until immediately before use and the beaker in which it has been weighed should be covered with a watch glass to prevent evaporation.