Suspensions

Question 1

What is a suspension?

Answer 1

suspensions are dispersions in which insoluble drug particles (solid) are suspended in liquid vehicle (external phase)

disperse phase - insoluble solids
continuous phase - liquid medium

Question 2

Why are suspensions used?

Answer 2

used for insoluble or poorly soluble drug
- volume of liquid required to dissolve the drug is too large

used due to the instability of some drugs in water

• preparation immediately before use
• limited shelf-life

Question 3

What are the advantages of suspensions?

Answer 3

mask the taste of therapeutic agents
patients who have difficulty swallowing solid dosage forms
provide controlled drug delivery (intramuscular injections)

Question 4

What are the characterises of a stable suspension?

Answer 4

good wettability

• depends on the contact angle
• required wetting agents

sedimentation velocity

• stoke’s law
• particle size = smaller is better
• viscosity of continuous phase (vehicle) = higher is better

sediment re-dispersibility

• depends on flocculated and deflocculated systems
• require flocculating agents

Question 5

How does powder wettability affect suspension stability?

Answer 5

determined by measuring the contact angle = between the water droplets and the powder

smaller contact angle is required = hydrophilic

• good spreadability
• easy dispersing

interfacial tension

• between liquid and solid
• must be reduced as it pulls sphere inward creating a more spherical shape
• require a less spherical shape

surface tension

• liquid and gas= must be reduced to create a less spherical shape
• solid and gas = not affected, pushes surface outwards

Question 6

What are examples of wetting agents?

Answer 6

hydrophilic polymers
- cellulose
- acacia
increases viscosity of the continuous phase

co-solvents can be used to enhance solid wettabilty

Question 7

How can rate of sedimentation be controlled? Why must it be controlled?

Answer 7

stoke’s law

• reduce particle size = less irritation, ease of administration, no coalesce
• increase viscosity of the continuous phase (vehicle) = prevents/slows movement of disperse phase, hydrophilic polymers, polysaccharides
• reduce density difference

must remain stable to get consistent uniform doses
- prevent overdose and underdose

Question 8

How can sediment re-dispersibilty affect stability of suspension? What system is required?

Answer 8

aggregated clusters that form as the suspension sits must be re-dispersed to get a uniform dosage

deflocculated

• low sedimentation
• low volume
• sediment contains few voids
• sediment is difficult to redisperse

flocculated

• high sedimentation
• high volume
• sediment contains many voids
• sediment can be re-dispersed
• no caking

Question 9

What are the possible interactions for control flocculation?

Answer 9

no interaction = slow individual sedimentation

repulsion > attraction = slow individual sedimentation, compact sediment

attraction > repulsion = irreversible interaction, coagulation

balanced attractive-repulsive forces - loose interaction, loos floccules/aggregates produced , re-dispersible sediment

Question 10

What are flocculating agents?

Answer 10

electrolytes
surfactants
polymers
- starch