Non-volatile, miscible liquids
Non-volatile, miscible liquids may be mixed with the molten fat in the evaporating basin. Alternatively, if a pre-prepared base is used, then incorporate as for vol- atile or immiscible liquids.
Volatile or immiscible liquids
Volatile or immiscible liquids (e.g. coal tar solutions) should be triturated with the ointment on the ointment tile. A very small amount of the ointment 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 ointment 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.
See Ointments video for a demonstration of the incorporation of a liquid into an ointment.
Pastes
Pastes are semi-solid preparations for external use. They consist of finely powdered medicaments com- bined with White Soft Paraffin BP or Liquid Paraffin BP or with a non-greasy base made from glycerol, mucilages or soaps. Pastes contain a high proportion of powdered ingredients and therefore are normally very stiff. Because pastes are stiff they do not spread easily and therefore this localises drug delivery. This is particularly important if the ingredient to be applied to the skin is corrosive, such as dithranol, coal tar or salicylic acid. It is easier to apply a paste to a discrete skin area such as a particular lesion or plaque, and thereby not compromising the integrity of healthy skin.
Pastes are also useful for absorbing harmful chemicals, such as the ammonia that is released by bacterial action on urine, and so are often used in nappy products. Also because of their high powder content, they are often used to absorb wound exudates.
Because pastes are so thick they can form an unbroken layer over the skin which is opaque and can act as a sun filter. This makes them suitable for use by skiers as they prevent excessive dehydration of the skin (wind burn) in addition to sun blocking.
The principal use of pastes traditionally was as an antiseptic, protective or soothing dressing. Often before application the paste was applied to lint and applied as a dressing.
Example 10.1Compound Aluminium Paste BPC (also known as Baltimore Paste) (BPC 1973, page 767) Formula: Aluminium Powder BP 200 g Zinc Oxide BP 400 g Liquid Paraffin BP 400 g Method:
1 Sieve the Aluminium Powder BP and the Zinc Oxide BP.
2 Mix the powders using the ‘doubling-up’ technique.
3 Mix the combined powders with the Liquid Paraffin BP, stirring until a smooth product is formed.
This was what was originally used to pro- tect the skin and prevent maceration around colostomies and ileostomies.
Example 10.2 Zinc and Coal Tar Paste BP (also known as White's Tar Paste) (BP 1988, page 868)
Formula:
Zinc Oxide BP (finely sifted) 60 g Coal Tar BP 60 g Emulsifying Wax BP 50 g Starch BP 380 g Yellow Soft Paraffin BP 450 g
Method:
1 Melt the Yellow Soft Paraffin BP and Emulsifying Wax BP at the lowest possible temperature.
2 Mix well and stir until just setting. 3 Mix the powders using the ‘doubling-up’
technique.
4 Levigate the semi-molten base with the powders on a warmed tile.
5 Finally incorporate the Coal Tar BP. The emulsifying wax is added to help with the dispersal of the coal tar through the prod- uct. The method outlined is designed to reduce the amount of heat to which the coal tar is exposed as the constituents of coal tar tend to precipitate out quickly on heating.
Example 10.3 Zinc Gelatin BPC (also known as Unna's Paste) (BPC 1968, page 1097)
Formula:
Zinc Oxide BP (finely sifted) 150 g Gelatin BP 150 g Glycerol BP 350 g
Water 350 g (or sufficient quantity)
Method:
1 Heat the water to boiling.
2 Remove from the heat and add the gelatin.
3 Stir gently until dissolved.
4 Add the glycerol which has previously been heated to 100C (no higher). 5 Stir gently to prevent incorporation of
air bubbles until solution is complete. 6 Maintain the base at 100C for 1 hour to
remove any contaminant microorganisms.
7 Adjust the base to weight by evaporation or adding hot water as required. 8 Sift the Zinc Oxide BP and add in small
amounts of molten base.
9 Continue stirring until the preparation is viscous enough to support the powder but is still pourable.
10 Pour into a shallow tray and allow to set. This is usually cut up into cubes which are weighed out and dispensed. The preparation is designed to be re-melted before application as a dressing for varicose ulcers. The gel reforms on cooling and the zinc oxide acts as an absorbent and mild astringent.
Re-melting is achieved by standing the con- tainer of Unna’s Paste in hot water. By cutting the paste into cubes this reduces the tendency of the zinc oxide to sediment out. Usually this is dispensed in portions of appropriate size intended for single use by the patient or nurse.
Gels
Pharmaceutical gels are often simple-phase, transpar- ent semi-solid systems that are being increasingly used as pharmaceutical topical formulations. The liquid phase of the gel may be retained within a three- dimensional polymer matrix. Drugs can be suspended in the matrix or dissolved in the liquid phase.
The advantages of gels are that:
* they are stable over long periods of time
* they have a good appearance
* they are suitable vehicles for applying medicaments to skin and mucous membranes, giving high rates of release of the medicament and rapid absorption.
Gels are usually translucent or transparent and have a number of uses:
* anaesthetic gels
* coal tar gels for use in treatment of psoriasis or eczema
* lubricant gels
* spermicidal gels.