Definition of Suspension:
The suspension is a biphasic liquid dosage form, which contains two phases. One is the disperse phase and another is the continuous phase. The disperse phase is that phase that uniformly dispersed into the continuous phase.
Examples of Suspension:
- Milk of magnesia
- Carboxymethyl Cellulose
Pharmaceutical Applications of suspensions:
- Some drugs are insoluble or partially soluble, there is the application of suspension. Example- Prednisolone.
- Some of the drugs are very bitter or unpleasant taste. Suspension helps to mask the unpleasant taste of the drug. Example- Chloramphenicol palmitate suspension.
- Suspension prevents degradation or improves the stability of the drug. Example- Oxytetracycline suspension.
- Parenteral suspensions are also applicable in order to control the rate of drug absorption, Example- penicillin procaine.
- In the cosmetic industry, there are huge applications of suspension. Example- Lotions contain insoluble solid particles to spread a thin coating of medicament on the skin. Like- calamine lotion, etc.
Suspending agents:
Suspending agents are those substances, which help active pharmaceutical ingredients to stay suspended into the suspension.
Examples: Methylcellulose, Carboxymethylcellulose (CMC), Hydroxypropylmethylcellulose (HPMC), Sodium alginate, etc.
Classification of Suspensions:
1. Classification of suspension depending on the route of administration:
- Oral suspension: Those suspensions which administered through the oral route. Example: Paracetamol suspension, Aluminum Hydroxide, and Magnesium Hydroxide Suspension.
- Parenteral suspension: Those suspensions administered through the intravenous and intramuscular routes. Example: Sodium benzylpenicillin suspension.
- Ophthalmic suspension: This type of suspension is used for the eyes. The particle size should very fine, non-irritant, sterile, and isotonic.
- Topical suspension: Those suspensions for topical or external uses. Example: Calamine lotion.
2. Based on Proportion of Solid Particles:
- Dilute suspension: It contains 2 to10% solid in weight per volume(w/v). Example: Cortisone acetate, Prednisolone acetate, etc.
- Concentrated suspension: It contains 50% solid in weight per volume(w/v). Example: Zinc oxide suspension, etc.
3. Based on Size of Dispersed Particles:
- Molecular Dispersion: In this type of suspension particle size is less than 1 nm.
- Colloidal Dispersion: In this type of suspension particle size is between 0.1-0.2 µm.
- Coarse Dispersion: In this type of suspension particle size is greater than 0.2 µm.
Ideal Properties of a Suspension:
- The particle sedimentation should be slow.
- Re-dispersion of sedimented particles should be rapid on shaking of the container.
- Cake formation of sedimented particles is not allowed.
- The dispersed particle size should be small and uniform in size.
- It should be physically and chemically stable.
Advantages of a Suspension:
- A suspension is a dosage form that can improve the chemical stability of certain drugs. Like Procaine penicillin G.
- Suspension can improve the taste of various unpleasant/bitter tastes of the drug by masking them. Example Chloramphenicol
- The drug in suspension exhibits a higher rate of bioavailability than other dosage forms.
- Duration and onset of action can be controlled. For example Protamine Zinc-Insulin suspension.
Disadvantages of a Suspension:
- Sedimentation of the particle is one of the disadvantages of this dosage form.
- The formulation is quite difficult than other dosage forms.
- 100% uniform and accurate dose are not possible.
Methods For Formulation of Suspension:
1. Precipitation method of suspension
- Organic solvent precipitation
- Precipitation by pH
- Double decomposition
2. Dispersion Method
3. Controlled flocculation
4. Structured vehicle
Additives or excipients used in the preparation of suspension:
- Suspending agents: Already discussed.
- Wetting Agents: Hydrophilic materials are wetted by water but hydrophobic materials are not wetted by water, it's wetted by non-polar liquids.
- Surfactants: Example- Polysorbate 80
- Hydrophilic colloids: Example- acacia, tragacanth, alginates, guar gum.
- Solvents: Example- alcohol, glycerin, polyethylene glycol, and polypropylene glycol.
- Buffers: Example- buffers are salts of weak acids such as carbonates, citrates, gluconates, phosphate
- Preservatives: Preservatives are those ingredients used for the preservation or for the protection of the formulation from the attack of microorganisms. Examples- Propylene glycol, Benzalkonium chloride, and Benzoic acid.
- Flavoring Agents: Acacia, Ginger, Sarsaparilla syrup, Anise oil, Glucose, Spearmint oil.
- Coloring Agents: Colouring agents are used in the suspension to improve the acceptance of consumers. Examples- Brilliant blue, Indigo carmine(blue), Tartrazine (yellow), Titanium dioxide (white), Amaranth (red),
- Sweetening Agents: Sweetening agents are used to overcome the unpleasant taste of the formulation. Examples- Xylose, ribose, glucose, and mannose.
- Humectants: It absorbs moisture to prevent the degradation of active pharmaceutical ingredients by moisture. Examples- Propylene glycol, and glycerol.
- Antioxidants: Examples- Ascorbic acid, erythorbic acid, glycerol, cytosine, acetylcysteine, etc.
Evaluation of the stability of suspension:
1. Sedimentation Method
2. Rheological Method
3. Electrokinetic Method
4. Micromeritic Method
