Suspensions

Definition of Suspensions

• A suspension is a heterogeneous mixture in which finely divided solid particles are dispersed in a liquid medium.
• The particles in a suspension are larger than those in a solution and do not dissolve in the liquid.
• Over time, these solid particles settle out due to gravity, making suspensions distinct from solutions and colloidal dispersions.
• Examples: Antacid suspensions, Calamine lotion, and some antibiotic suspensions.

Advantages of Suspension

1. Increased Stability: Certain drugs that are unstable in solution form can be stabilized in a suspension, as the solid form of the drug is generally more stable than in solution.
2. Ease of Administration: Suspensions allow the administration of poorly soluble drugs in a liquid form, making them easier to swallow for pediatric or geriatric patients.
3. Controlled Release: Suspension can be formulated to provide slow and sustained drug release, which is helpful in cases where controlled delivery is desired.
4. Taste Masking: Insoluble drugs can be suspended, masking their unpleasant taste better than in solution form.
5. Flexibility in Dosing: Suspension allow for easy dose adjustments by measuring out different volumes.

Disadvantages of Suspension

1. Sedimentation: Over time, particles tend to settle out of the suspension, leading to uneven distribution of the active ingredient if not shaken well before use.
2. Poor Dose Uniformity: Inadequate shaking may result in non-uniform dosing.
3. Bulky and Inconvenient: Suspensions are often bulkier than tablets or capsules, making them harder to store and transport.
4. Stability Issues: Suspensions are physically unstable and prone to caking, making resuspension difficult.

Classifications of Suspension

Suspension can be classified based on several factors:

1. Based on Route of Administration:

   1. Oral suspensions: For ingestion (e.g., antacids, antibiotics).
   2. Topical suspensions: Applied to skin (e.g., calamine lotion).
   3. Parenteral suspensions: Injected into the body (e.g., depot injections).
   4. Ophthalmic suspensions: For application in the eyes.

2. Based on Particle Size:

   1. Coarse suspension: Particle size > 1 μm.
   2. Colloidal suspension: Particle size < 1 μm.

3. Based on Electrokinetic Properties:

   1. Flocculated Suspension: Particles form loose aggregates or flocs.
   2. Deflocculated Suspension: Particles remain as individual entities and do not form aggregates.

4. Based on Concentration of Solid Content:

   1. Dilute suspensions: Solid content < 2
   2. Concentrated suspensions: Solid content > 2

Preparation of Suspension

The preparation of suspensions involves several critical steps to ensure a homogenous and stable formulation:

1. Wetting of Particles:

   • The solid drug particles must first be wetted to prevent clumping or floating.
   • Wetting agents like glycerin, ethanol, or surfactants (e.g., polysorbates) can be used.

2. Dispersion of Solid Particles:

   • The wetted solid is dispersed uniformly in the dispersion medium (aqueous or non-aqueous).
   • High-speed mixers or colloid mills may be employed for this process.
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3. Stabilization of the Suspension:

   • Stabilizing agents such as viscosity enhancers (e.g., methylcellulose, arbopol) are added to reduce the settling rate of particles.
   • Surfactants can also be added to prevent flocculation or caking.

4. Homogenization:

   • The suspension is passed through a homogenizer to reduce particle size and ensure uniform distribution.

5. Packaging:

   • The suspension is then packaged in appropriate containers, ensuring stability and protection from light and air.
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