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 Suspensions:
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.
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.
Controlled Release: Suspensions can be formulated to provide slow and sustained drug release, which is helpful in cases where controlled delivery is desired.
Taste Masking: Insoluble drugs can be suspended, masking their unpleasant taste better than in solution form.
Flexibility in Dosing: Suspensions allow for easy dose adjustments by measuring out different volumes.
Disadvantages of Suspensions
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.
Poor Dose Uniformity: Inadequate shaking may result in non-uniform dosing.
Bulky and Inconvenient: Suspensions are often bulkier than tablets or capsules, making them harder to store and transport.
Stability Issues: Suspensions are physically unstable and prone to caking, making resuspension difficult.
Classifications of Suspensions:
Suspensions can be classified based on several factors:
1) Based on dispersion medium:
A. Oral suspensions: For ingestion (e.g., antacids, antibiotics).
B. Topical suspensions: Applied to skin (e.g., calamine lotion).
C. Parenteral suspensions: Injected into the body (e.g., depot injections).
D. Ophthalmic suspensions: For application in the eyes.
2) Based on particle size:
A. Coarse suspensions: Particle size > 1 μm.
B. Colloidal suspensions: Particle size < 1 μm
3) Based on method of preparation:
A. Flocculated Suspensions: Particles form loose aggregates or flocs.
B. Deflocculated Suspensions: Particles remain as individual entities and do not form aggregates.
4) Based on Concentration of Solid Content:
A. Dilute suspensions: Solid content < 2%.
B. Concentrated suspensions: Solid content > 2%.
Preparation of Suspensions
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.
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.