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DrX Whiz Niraj
Semisolid Dosage Forms: Complete Guide
Master the formulation of Ointments, Creams, Pastes & Gels. Detailed notes on Mechanisms of Dermal Penetration, Excipients, and Evaluation tests.
Semisolid dosage forms are topical formulations intended for application to the skin or mucous membranes (eyes, nose, vagina, rectum) for local or systemic effects. They possess intermediate properties between solid and liquid dosage forms.
Ointments
Greasy, highly viscous preparations containing dissolved/dispersed drugs in an oleaginous base.
Creams
Viscous Emulsions (O/W or W/O type). O/W creams are non-greasy and water-washable (vanishing creams).
Pastes
Very stiff, containing a high percentage of solid powders (20-50%). Do not melt easily at body temp.
Gels
Transparent/translucent semi-solids formed by a 3D network of polymers (like Carbopol) trapping a liquid.
For a topically applied drug to act systemically or deep in the tissue, it must penetrate the Stratum Corneum (the tough, dead, outermost layer of the epidermis). There are three primary routes of penetration:
Factors Influencing Penetration:
- Physicochemical properties of Drug: Drugs with low molecular weight (< 500 Da) and adequate lipid/water partition coefficient (Amphiphilic) penetrate best.
- Physiological Factors: Skin hydration (hydrated skin absorbs 5x faster), skin thickness (soles are thickest, scrotum is thinnest), age, and broken skin.
- Vehicle Effect: The base used (e.g., Oleaginous vs Emulsion) dictates how fast the drug is released. Permeation enhancers (like DMSO, Urea, Propylene Glycol) disrupt the lipid layer to speed up penetration.
The vehicle in which the drug is dispersed is called the Base. Selecting the right base is critical for drug release.
| Type of Base | Characteristics | Examples |
|---|---|---|
| 1. Oleaginous (Hydrocarbon) Bases | Highly greasy, immiscible with water, difficult to wash off. Highly occlusive (prevents moisture loss). | Soft Paraffin (Vaseline), Hard Paraffin, Liquid Paraffin. |
| 2. Absorption Bases | Can absorb water to form W/O emulsions. Less occlusive than hydrocarbons. | Wool Fat (Lanolin), Beeswax. |
| 3. Emulsion (Water-Removable) Bases | O/W emulsions. Non-greasy, easily washed with water. Better drug release. | Vanishing Creams, Hydrophilic Ointment USP. |
| 4. Water-Soluble Bases | Contain no fats/oils. Completely soluble in water. Non-occlusive. | Macrogols (Polyethylene Glycols / PEGs). |
1. Trituration Method
Used when the drug is a solid powder insoluble in the base, and the base is soft at room temperature.
- The finely powdered drug is mixed with a small amount of base or liquid (Levigating agent like Mineral oil) on an ointment slab using a spatula to form a smooth paste.
- The remaining base is added gradually (Geometric dilution).
2. Fusion Method
Used when the base contains solid ingredients of different melting points (e.g., Waxes, Stearyl alcohol).
- Ingredients are melted together in decreasing order of their melting points over a water bath.
- The drug is added to the melted mass, stirred continuously, and allowed to cool slowly to avoid crystallization.
To ensure the quality, safety, and efficacy of semi-solids, the following tests are performed:
- Physical Appearance & Homogeneity: Visual inspection for color, odor, and grittiness.
- pH Measurement: Tested using a pH meter to ensure it matches skin pH (5.5) to avoid irritation.
- Rheological Properties (Viscosity): Measured using a Brookfield (Rotational) Viscometer to ensure proper consistency.
- In-Vitro Drug Release: Franz Diffusion Cell is used to determine how fast the drug is released from the base through a synthetic membrane.
Spreadability vs Extrudability:
- Spreadability: It denotes the extent of area to which the formulation readily spreads on application to skin. Good spreadability means less friction and easy application. Evaluated by pulling two glass slides apart (Formula:
S = M.L / t). - Extrudability: The force required to extrude (squeeze out) the material from a collapsible tube. Important for patient compliance.