Suspension is a preparation containing the solid ingredients in the form of subtle and not soluble, dispersed in a liquid carrier. The main aim in the preparation of suspension is to keep the suspended particles dispersed and for this number of components which belongs to the vehicle itself.

Structured Vehicle

For the need of a stable suspension, the term Structured vehicle is most important for formulation view and stability criteria. The main disadvantage of suspension dosage form that limits its use in the routine practice is its stability during storage for a long time. The structured vehicle is the vehicle in which viscosity of the preparation under the static condition of very low shear on storage approaches infinity. The vehicle behaves like a ‘false body’, which is able to maintain the particles suspended which is more or less stable.

Structured vehicle concept is applicable only to deflocculated suspensions, where hard solid cake forms due to settling of solid particles and they must be redispersed easily and uniformly at the time of administration. Flocculated suspension settled floccules get easily redispersed on shaking that is why structured vehicle concept is not applicable.

Particle size

The particle size play a key role in the formulation of suspension, particles must be of small size. This decreases rate of sedimentation. Particle size for the preparation for parenteral and ophthalmic use, is better less than 5microns. More than 25microns cause blockage of the needle. Spherical shape is best for suspension formulation. And there will be the crystal growth on standing for certain period of time. This is because as the temperature decreases, the solubility decreases and leads to formation of crystallization. Its better to take powder having less variation in their size. Because the small particles have more solubility and so their size decreases continuously, whereas the size of larger particles will keep on increasing.

Wetting agents

Wetting agents are surfactants. They function by reducing the contact angle and interfacial tension between the dispersed phase and dispersed medium. A liquid phase containing suitable wetting agent helps in passing of liquid through the powder when it is added and hence it displaces the air around the particles. Non-ionic surfactants functioning as wetting agents should posses HLB value between 7 to 10. Eg., Sodium lauryl sulphate – anionic surfactant; Sorbitan esters- Non-ionic surfactant.

Flocculating agents

Flocculating agents is Non-ionic wetting agents decreases the interference tension between the particles and liquid medium so that deflocculation is achieved. The ionic surfactant can produce both flocculated and deflocculated suspension. It is based on the charge on particle. Electrolytes generally reduce the zeta potential and increase in flocculation. Ionic and non-ionic surfactants can cause flocculation. Various polymers will form network and assist in flocculation.

In addition agents to increase viscosity and protective colloids which form a hydration layer on the surface of dispersed phase. They function as protective colloid in low concentration and at more than 0.1% concentration, they increase the viscosity of dispersion medium.

Rheological considerations of suspensions

The viscosity of the medium is to be considered, because the settling of the particles depends on visocity. Next comes, when the suspension is changed from one container to another or during shaking, the variation in the flow properties should be considered. In case of suspensions intended for external use, the spreading capability should be considered. An agent that becomes a gel during storage conditions and when we shake, looses its viscosity is best.