A detailed note on suspension and types

1. 1/17/2024
Assistant Professor
Faculty of Pharmacy
The University of Lahore

2. Suspensions
Definition:
A suspension: is a disperse system in which one substance (the
disperse phase) is distributed in the form of fine particles
throughout another (the continuous phase) phase in which it is
insoluble (i.e. at least 2 phases).
Suspensoid:
Particles that are finely distributed throughout a continuous phase
are called suspensoids or dispersed phase of suspension
1/17/2024

3. Classification of Suspensions
Classification of suspensions::
Suspensions could be classified in a number of ways
1. According to the particle size of the dispersed phase
 Coarse suspension: which is a dispersion of particles with a mean
diameter ranging from 10 µm-50 µm .
 Colloidal suspension: is a dispersion of particles with a mean diameter
less than 1 µm.
 Fine suspension: suspensions with particle size of 1-10 µm
1/17/2024

4. Classification of suspensions
2. On the basis of use
On the basis of use suspensions could be divided into three subtypes
 Oral suspensions:
These are used orally. e.g. Brufen, Fixitil, Focin, Velosef, Cefspan
 Topical suspensions:
They are meant for application on to skin
Examples: Calamine lotion, zinc oxide suspension
 Parenteral suspensions:
They are intended for Intramuscular, Subcutaneous injection
Examples: Vaccines, Penicillin G, Tetanus toxoid, Insulin
3. On the Basis of geometry of dispersed phase
 Un flocculated suspensions
 Flocculated suspensions
1/17/2024

5. Pharmaceutical application of suspensions
Why suspensions?
Against solid dosage forms:
• If patient has a difficulty of swallowing solid dosage forms then there is a
need for oral liquid dosage form.
• Faster rate of dissolution and oral absorption than solid dosage forms, yet
slower than solutions.
• Bulky insoluble powders as kaolin are better formulated as suspensions so
that they are easier to take.
1/17/2024

6. Pharmaceutical application of suspensions
Why suspensions?
Against solutions:
• Drugs that have very low aqueous solubility are usefully formulated as
suspensions.
• Drugs that have an unpleasant taste in their soluble forms (e.g.,
chloramphenicol (soluble) vs. chloramphenicol palmitate (insoluble )).
• Prolongation of effect (e.g. I.M and S.C. suspensions).
• Stability and instability issues:
• Drugs that are unstable in solution form are formulated as suspensions for
example oxytetracycline hydrochloride is soluble in water but hydrolyzed
rapidly therefore it is formulated as suspension of oxytetracyline calcium.
1/17/2024

7. Properties of a good pharmaceutical suspension
• The dispersed phase of the suspension should not settle to the bottom readily.
• The dispersed phase of the suspension should be easily redispersed on gentle
shaking even if the suspension is stored for a longer period of time
• The suspension should be easily pourable.
• Particles of the dispersed phase should be of uniform size.
• Suspensoid in the suspension should be uniformly distributed through out the
dispersion medium so that required dose could be dispensed accurately
1/17/2024

8. There are three most commonly faced issues in suspensions which include
1. Sedimentation.
2. Wetting issues.
3. Caking
1/17/2024 BA-FP-JU-C
Formulation defects of suspensions

9. Sedimentation
Definition:
The downward movement of dispersed phase of suspension is called sedimentation.
The rate of sedimentation is governed by different factors governed by Stokes law
The Stokes' equation is given below
• Where v= velocity of sedimentation of a spherical particle of radius r,
• d1 = Density of dispersed phase particles
• d2 = density of the continuous phase and viscosity µ
• Where, g is the acceleration due to gravity.
1/17/2024

10. According to Stokes law equation sedimentation can be prevented
by
1. Decreasing the particle size of dispersed phase
2. Decreasing the difference in the densities of dispersed phase
and continuous phase
3. Increasing the viscosity of dispersion medium
Uncontrolled sedimentation may lead to caking in the suspension
1/17/2024 BA-FP-JU-C
Prevention of sedimentation

11. Caking
Definition
Caking refers to aggregation of dispersed phase of suspension in
a way that individual particles loose their integrity.
 Caking is an irreversible process
 Once cake is formed, suspension is destroyed. We
cannot dispense the desired dose from a caked
suspension.
 Factors that affect the sedimentation also control
caking
1/17/2024 BA-FP-JU-C

12. The solid powders to be dispersed in aqueous vehicle could be divided into two types :
 Wetable solids:
These are insoluble solids that are light and easily wetted by water. They mix readily with water, and
stay dispersed long enough for an adequate dose to be measured. After settling they redisperse easily.
Examples include magnesium trisilicate, light magnesium carbonate, bismuth carbonate and light kaolin.
 Non wettable solids:
These are the solids that are not easily wetted, and some particles may form large porous clumps in the
liquid, whereas others may remain on the surface. These solids will not remain evenly distributed in the
vehicle long enough for an adequate dose to be measured. They cannot be redispersed easily. Examples
for internal use includes aspirin, phenobarbital and chalk and for external use calamine, hydrocortisone,
su1phur and zinc oxide.
1/17/2024
Wetting

13. Reason of wetting issues and its solution
 Due to high interfacial tension between solids and the aqueous phase, air may
get entrapped in the cracks and pores present in the solids due to which water
cannot penetrate into these pores and solid is not wetted.
How to overcome this issue
1. This issue can be addressed by addition of surface active agents that will
reduce interfacial tension between water and solid surface allowing water to
penetrate into the pores.
Intermediate HLB surfactants such as tweens and spans are used for internal
preparations. Sodium lauryl sulphate is used in external preparations.
2. Pre-wetting with Solvents such as ethanol, glycerol and the glycols also
facilitate wetting.
1/17/2024

14. Flocculated suspensions
 In order to prevent caking in the suspensions, flocculated suspensions are intentionally prepared
What are floccules
Floccules are the aggregates of solid particles that are intentionally created
Floccules are loose aggregates and solid particles in floccules do not loose their integrity
On the basis of presence and absence of floccules, suspensions could be divided into two types
• Deflocculated suspension: the dispersed solid particles remain separate and settle slowly.
However, the sediment that eventually forms is hard to redisperse and is described as a 'cake' or
clay.
• Flocculated suspension: Individual particles aggregate into clumps or floccules in suspension.
Because these flocs are larger than individual particles, sedimentation is more rapid, but the
sediment is loose and easily redispersible.
8-5-20

15. How flocculated suspensions are prepared
Flocculated suspensions can be prepared by number of ways
1. Using clays such as bentonite
In case where clays cannot be used as suspending agents such as in case of
parenteral suspensions some other ways are adopted for generation of
floccules. These methods are
2. Adjustment of pH
3. Use of electrolytes
4. Using surface active agents
1/17/2024 BA-FP-JU-C

16. Suspending agents
• Suspending agents increase the viscosity of the continuous
phase, thereby slowing down sedimentation.
• Most agents can form thixotropic gels which are semisolid on
standing, but flow readily after shaking.
• Care must be taken when selecting a suspending agent for oral
preparations.
• Suspending agents can be divided into five broad categories:
natural polysaccharides, semi-synthetic polysaccharides, clays,
synthetic thickeners and miscellaneous compounds.
1/17/2024 BA-FP-JU-C

17. Suspending agents
Natural polysaccharides
• The main problem with these agents is their natural variability
between batches and microbial contamination.
• These materials should not be used externally as they leave a sticky
feel on the skin.
• They include tragacanth, acacia gum, starch, agar, guar gum,
carrageenan and sodium alginate.
• Tragacanth:
 Is a widely used suspending agent and is less viscous at pH 4-7.5.
 As a rule: 0.2g tragacanth powder is added per 100 mL suspension or
2g compound tragacanth powder per 100 mL suspension. Compound
Tragacanth Powder BP 1980 contains tragacanth, acacia, starch and
sucrose and so is easier to use.
 Tragacanth powder requires to be dispersed with the insoluble
powders before water is added to prevent clumping .
1/17/2024 BA-FP-JU-C

18. Suspending agents
Semi-synthetic polysaccharides
• These are derived from the naturally occurring polysaccharide
cellulose.
• Examples include
 Methylcellulose (Cologel ®, Celacol®)
 Hydroxyethylcellulose (Natrosol 250®)
 Sodium carboxymethylcellulose (Carmellose sodium®)
 Microcrystalline cellulose (Avicel®).
1/17/2024 BA-FP-JU-C

19. Suspending agents
Clays
• These are naturally occurring inorganic
materials which are mainly hydrated silicates.
• Examples include bentonite and magnesium
aluminium silicate (Veegum®).
1/17/2024 BA-FP-JU-C

20. Suspending agents
Synthetic thickeners:
• These were introduced to overcome the variable quality of
natural products.
• Examples include:
 Carbomer (Carboxyvinyl polymer, Carbopol®),
 Colloidal silicon dioxide (Aerosil®, Cab-o-sil®)
 Polyvinyl alcohol (PVA).
Miscellaneous thickeners:
• Gelatin used as a suspending agent and a viscosity increasing
agent
1/17/2024 BA-FP-JU-C

21. Preservation of a suspension
• Water is the most common source of microbial contamination.
• Also the naturally occurring additives such as acacia and
tragacanth may be sources of microbes and spores.
• Preservative action may be diminished because of adsorption of
the preservative onto solid particles of drug, or interaction with
suspending agents.
• Useful preservatives in extemporaneous preparations include
chloroform water, benzoic acid and hydroxybenzoates.
1/17/2024

22. Formulation of suspension
1. Control particle size. On a small scale. this can be
done using a mortar and pestle. to grind down
ingredients to a fine powder.
2. Use a thickening agent to increase viscosity of
vehicle by using suspending or viscosity-increasing
agents.
3. Use a wetting agent.
1/17/2024 BA-FP-JU-C

23. Containers for suspension
• Suspensions should be packed in amber bottles,
plain for internal use and ribbed for external use.
• There should be adequate air space above the
liquid to allow shaking and ease of pouring.
• A 5 mL medicine spoon or oral syringe should be
given when the suspension is for oral use.
1/17/2024 BA-FP-JU-C

24. Special label and advice for suspension
• The most important additional label for suspensions is
'Shake well before use',
• Store in a cool place. Stability of suspensions may
be adversely affected by both extremes and variations
of temperature.
• Some suspensions. such as those made from
reconstituting dry powders, may need to be stored in a
refrigerator.
• Extemporaneously prepared and reconstituted are
required to be recently or freshly prepared, with a 1-4-
week expiry date.
1/17/2024 BA-FP-JU-C

25. 200ml Calamine Lotion BP
Master formula 200ml
Calamine 15g 30g
Zinc oxide 5g 10g
Bentonite 3g 6g
Sodium citrate 500mg 1g
Liquified phenol 0.5ml 1ml
Glycerol 5ml 10ml
Water to 100ml to 200ml
1/17/2024 BA-FP-JU-C