Coarse Dispersion: Suspension Advantages and Disadvantages

• Suspension:

Pharmaceutical suspension is a heterogeneous biphasic liquid dosage form of finely divided solid medicament particles ranging from 0.5 to 5 microns dispersed in a liquid or semisolid vehicle with the help of suspending agents. The coarse suspension contains solid drug particles ranging from 10 to 1000 micrometers.

Advantages

• 1. Stability: Drugs are unstable in solution. Their insoluble salt is dispensed in suspension form. Penicillin G is soluble in water and an unstable drug in solution. Procaine penicillin G is insoluble in water and stable in suspension form

• 2. Suitability: Medicaments insoluble in water are dispensed in suspension form. Solvents other than water are not preferred for oral dosage form.

• 3. Taste Masking: The unpleasant taste of drugs can be masked in suspension form by adding suitable flavoring agents and sweetening agents. Chloramphenicol is bitter in taste and soluble in water. Its bitter taste is masked by dispensing it as Chloramphenicol palmitate in suspension form.

• 4. Immediate action: Suspension offers a faster onset of action than a solid dosage form.

• 5. Prolonged Release: Insoluble drugs are absorbed slowly into the body system. It remains in the body over a longer period prolonging their duration of action. Insoluble drugs can be formulated as controlled-release suspensions or prolonged-release suspensions. This increases the therapeutic efficacy of the drug and reduces the frequency of administration.

• 6. Bioavailability: The bioavailability of the drugs in suspension is always higher than a drug in the tablet or capsule.

• 7. Flexible Dosing: The dose of drugs can be easily adjusted by changing the volume administered.

• 8. Easy to administer: Suspension is easier to swallow than a tablet or capsule.

• 9. Versatility in administration: Suspension can be formulated to administer through routes like oral, topical, and parenteral.

Disadvantages

• 1. Sedimentation: Suspended particles tend to sediment. This lead to false alarm about the suitability of the product and non-uniform dosing. It is a must to shake well before use. This may be inconvenient to patients.

• 2. Palatability: Flavouring agents, sweetening agents, etc are used to mask the unpleasant taste of medicaments. But they cannot mask the completely unpleasant taste and grittiness of the medicaments. Thus, it is not suitable, especially for pediatric and geriatric patients.

• 3. Patient Compliance: Need to shake well before use, unpleasant taste, and gritty texture are inconvenient for the patient. Patients may forget to shake bottles before use.

• 4. Difficulty in absorption: Drugs in suspension form are absorbed slowly. It is not suitable if rapid absorption of the drug is required. The rate of drug absorption from suspension is slower than from solution form.

• 5. Physical Instability: Aggregation, flocculation, and coalescence of suspended particles result in changes in the appearance, consistency, and efficacy of the product.

• 6. Dosage Uniformity: Maintaining dosage uniformity in suspension is challenging especially if there is variation in particle sizes. Therefore, potent drugs are not dispensed in suspension dosage form.

• 7. Risk of Clogging: Suspension as injection may clog the syringe needle. This will complicate the drug administration through injection.

• 8. Limited shelf-life: Temperature, light, and exposure to air promote oxidation and hydrolysis of the medicaments. They lead to limited shelf life compared to tablets, capsules, etc.

• 9. Microbial Growth: Suspending agents are prone to microbial growth. They are liquid dosage form that also promotes microbial growth. They require proper attention and preservatives to protect the product from microbial attacks.

• 10. Bulkiness: Suspensions are liquid dosage forms. They are relatively bulky than tablets and capsules. They require special care during Handling and transportation.

• Characteristics/Qualities of Good/Ideal Suspension

Suspension contains insoluble solids that are prone to sediment during storage. A well-formulated suspension has the following qualities.

• 1. Homogeneity: Dispersed solid particles should be evenly distributed in the liquid phase. This ensures uniform dosing and efficacy.

• 2. Particle Size Distribution: The particle size of dispersed solid should be under a narrow range. Uniform particle size improves the physical stability of suspension, bioavailability, and patient tolerance.

• 3. Physical Stability: The dispersed solid particles should be of a suitable size that does not settle readily to form a cake at the bottom of the container.

• 4. Chemical Stability: Ingredients of suspension should remain stable throughout its shelf life period.

• 5. Compatibility: All ingredients of suspension should be compatible with each other.

• 6. Microbial Stability: The formulation should resist microbial contamination. It should contain a suitable approved preservative.

• 7. Redespersibilty: Suspension should be redispersed readily on moderate shaking. The sediment of solid particles should form a hard cake.

• 8. Reconstitution: Some suspensions are supplied as dry powder. It should be easy to reconstitute after the addition of a suitable diluent.

• 9. Viscosity: Appropriate viscosity of suspension should allow easy pouring of suspension from the container, ease of administration, slow down sedimentation, and accurate dosing.

• 10. Palatability: Suspension for oral administration should be palatable, especially if patients are pediatric or geriatric. It should contain an approved flavoring agent, sweetening agent, and coloring agent to improve taste and acceptance.

• 11. Smooth and Spreadable: Suspension for topical application should be free from gritty particles and easily spreadable at the site of application.

• 12. Biological availability: The formulation should ensure optimal release and absorption of medicaments from the site of application.

Interfacial Properties of Suspended Particles

The boundary between suspended solid particles (dispersed phase) and liquid phase (dispersion medium) in suspension is called an interface. Interfacial properties play an important role in the behavior and stability of the suspension.

Solid particle size is reduced to 5 micrometers to form a stable suspension. These particles exhibit Brownian motion that improves suspension stability. The size reduction of solid particles increases the surface area of solid particles. This also leads to an increase in surface energy on the solid particle surface that leads to the formation of thermodynamically unstable suspension. The solid particles spontaneously interact to form agglomerate to become thermodynamically stable. But suspension shall be unstable.

The following interfacial properties of solid particles play an important role in the stability of the suspension.

• 1. Interfacial tension is the force acting at the interface between solid particle and liquid phase. It influences the wetting of the solid particles in the liquid phase. Proper wetting ensures uniform dispersion of solid particles and prevents solid particles from agglomeration or settling. A decrease in interfacial tension increases the wetting of the solid particles.

• 2. Contact Angle: The angle formed between the liquid phase and solid particle is called contact angle. A lower contact angle usually indicates better wetting behavior and dispersibility of the solid particles in the liquid phase.

• 3. Adsorption: The adherence of molecules at the interface (between the solid particles and the liquid phase) is called adsorption. Surfactants or stabilizing agents are used in the suspension. Their molecules adhere at the interface and form a protective layer around solid particles. This prevents or slows down the aggregation and settling of solid particles in the suspension.

• 4. Stabilizers and Surfactants: They are added to suspension to modify interfacial properties. They reduce the interfacial tension, improve solid particle wetting, and form a protective layer to prevent aggregation, coagulation, or settling in suspension.

• 5. Zeta Potential: Zeta potential is the electric potential at the interface. It is a measure of electrostatic repulsion or attraction between solid particles in the suspension. A higher zeta potential indicates greater repulsion between solid particles in the suspension. This leads to enhanced suspension stability.

• 6. Electrostatic Stabilizer: Electrostatic forces play a significant role in stabilizing suspensions. Like charges on particles create repulsion and prevent solid particles from coming together. This prevents solid particle aggregation and maintains suspension homogeneity.

• 7. Steric Stabilization: Polymers or large molecules are used are used for steric stabilization. They are adsorbed on the particle surface and create a barrier to stop particle-particle interactions. This helps in preventing agglomeration and settling of the solid particles.

• 8. Rheological Properties: Interfacial properties also affect the rheological behavior of the suspension. The interactions between solid particles and the liquid medium affect the viscosity and flow characteristics of the suspension. This impacts the suspension’s ease of administration.

Alok Bains