Semi-solid preparations

Fig. 11.5 Plates for the assessment of bacteriostatic effect of semi-solid preparations A, cup-plate B, ditch-plate.

Softened water is often used for washing containers before filling with liquid or semi-solid preparations and for cooling systems. Unless precautions are taken, the microbial count in a cooling system or jacketed vessel will rise rapidly and if faults develop in the cooling plates or vessel wall, contamination of the product may occur. 

Issues relating to chemical incompatibility and instability may be less significant for solid dosage forms compared with liquid and semi-solid preparations. 

Jellies are transparent or translucent, non-greasy medicated semi-solid preparation used externally, sometime containing local anaesthetic agent also e.g. Lignocaine jelly. 

Liniments are liquid, semi-liquid and some-times semi-solid preparation used externally on the skin. Liniments are counter-irritant and stimulating type and are massaged or rubbed into the skin, and must not be applied to the broken skin e.g. liniment turpentine. 

Pastes are semi-solid preparation for external application that differ from similar products (i.e. ointment) in containing a high proportion of finely powdered medicaments. 

Percentage weight in weight (% w/w) is the number of grams of drug in 100 g of final product. This term is encountered most often in solid dosage preparations such as powders, and semi-solid preparations such as creams and ointments, e.g. 1 % salicylic acid ointment. 

Eye ointments are sterile semi-solid preparations intended for application to the conjunctiva. They are attractive because of their increased contact time and better bioavailability compared to solutions. They can be very useful for night-time application, However, they are not always well accepted by patients because upon application they often cause blurred vision. A wide variety of ointments are available commercially. 

Occlusion of semi-solid preparations may be useful if systemic absorption is desired. A careful risk-benefit evaluation is then required. Transdermal patches are a more elegant alternative. Being virtually painless and easy to apply, if they achieve controlled delivery, they provide a formulation of choice for prolonged-release systemic uptake. They would have to be adapted to skin maturation, formulated with adhesives with low allergenic potential and be available in different strengths. Of course, the same inter- and intra-patient variability in relation to sites of administration and skin condition occurs in children as in adults. 

Ophthalmic ointments are sterile, homogeneons, semi-solid preparations intended for application to the conjunctiva or the eyelids. 

European Directorate for the Quality of Medicines (EDQM) (2014) Monographs Liquid preparations for cutaneous application. Powders for cutaneous application. Semi-solid preparations for cutaneous application. Patches, transdermal. European Pharmacopeia, 8th edn. Council of Europe, Strasbourg... 

Both the dose and the contact time (exposnre) with the skin are chosen to mimic intended use conditions. The amount of the formnlation to be applied is between 2 and 5 mg/cm for solids and semi-solid preparations, and up to 10 pl/cm for liquids. The volume of formulation used should be enough to spread the sample homogeneously over the skin surface. This strongly depends on the viscosity of the formulation. 

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