suspensions Flashcards
Define a dispersed system. What are the phases?
A dispersed system is a system containing two phases with a boundary between these components.
The dispersed phase is the internal phase and the continuous phase is the dispersion medium.
Define a suspension.
A suspension is a coarse dispersion - it contains a solid in liquid dispersion where particles are above colloidal size.
Describe an orally delivered suspension and give an example.
Orally delivered suspensions deliver lipophilic drugs in an aqueous vehicle, and is used for for geriatric and paediatric use. Taste making is achievable as the active is not dissolved and the drug is protected from hydrolysis.
Some of the active is actually dissolved but the solubility in the medium is quickly reached and most of it is suspended.
An example is paracetamol suspension.
Describe a topically applied suspension and give an example.
A topically applied suspensions allows drug release to be slowed down, as formulation needs to dissolve in order to act.
An example is zinc cream, where zinc oxide powder is suspended in an emulsion base.
Describe parenteral suspension delivery.
This can be used via IM and SC, but not IV (blocks veins). Hydrophilic drug in a hydrophobic base is possible, and vice versa.
These are typically utilised for sustained drug release, where is it controlled by drug dissolution and formulation viscosity.
What are the limitations of suspensions? Briefly explain.
- stability: coarse dispersions will sediment over time, where caking is the formation of non-redispersible sediment.
- dose precision: homogeneity is not guaranteed
- risk of microbial contamination if the medium is aqueous
- bulky nature of product: bottles are harder to carry around than tablet skillets.
Describe aspects considered in the choice of active and medium.
The chemical form of an active will impact release behaviour. Salts and polymorphs can be chosen based on desired dissolution behaviour.
Aqueous dispersion mediums are usually used for oral delivery, and oily mediums for parenteral.
What are wetting agents? Why are they used? Give examples.
Wetting agents are stabilising agents used to reduce interfacial tension between the active and dispersing medium. This results in less drive for solid/solid interactions.
Wetting agents can be surfactants, solvents or hydrophilic colloids.
Describe the relevance of stoke’s law in suspensions.
Dilute pharmaceutical suspensions conform roughly to stoke’s law, where particle size is proportional to sedimentation velocity.
More concentrated suspensions carry a greater incidence of particle collisions.
This law is restricted to larger particles, where smaller ones follow brownian motion.
How do smaller particles behave in suspensions? what are the consequences of this? Compare this to larger particle size.
Smaller particle size means that there is a slower rate of sedimentation, resulting in better physical stability. It also means there is less mechanical irritation.
- particles greater than 5mcm may cause irritation after injection, or feel gritty during instillation. Particles can also block needles, but can control dissolution rate; where larger particles release drug slower.
How do suspensions ideally behave?
- sediment slowly, having higher viscosity.
- easily re-dispersed when sediment is produced
- homogenous mixture for a sufficient period
- easily transferable from the container to application.
What are ideal rheological properties of suspensions? Why?
High viscosity at low shear rates so that particles remain suspended or settle slowly upon storage.
Moderate shear rates so that viscosity falls sufficiently to allow suspension to be poured (pseudoplasticity).
Original viscosity should be regained after use in a relatively short time to maintain adequate physical stability (thixotropy).
What is the sedimentation volume ratio (F)?
This is the degree of sedimentation, where;
F = Vsed/Vtotal
Vsed = volume of sediment
Vtot= volume of original suspension
The higher the F, the more sedimentation there is.
Describe a flocculated suspension
In a flocculated suspension, loos sediments are held together by weak vdW forces, which are easily re-dispersable. In a controlled flocculated system, these sediments will have a degree of seperation where the dispersing medium is between them. This results in a higher F. Faster sedimentation occurs and supernatant is clear.
Describe a deflocculated system
In a deflocculated system, the rate of sedimentation is lower. This allows for more uniform dosing, but caking can occur. Supernatant is cloudy.
