Formulation of suspensions - Pharmaceutics - I B. Pharma 1st Semester

Formulation of suspensions

Learning Objectives

• At the end of this lecture, student will be able to

• Explain the formulation of suspensions

• List out methods of dispensing suspensions

• Give examples of suspensions

Formulation of Suspensions

1. Medicament/Drug

2. Flocculating agents

3. Deflocculating agents

4. Suspending agents

5. Thickening agents

6. Wetting agents

7. Preservatives

8. Organoleptic additives

1. Flocculating agents

- induce flocculation

E.g

a) Electrolytes: sodium salts of acetates, phosphates and citrates.

b) Surfactants: ionic and non-ionic

c) Polymers – starch, alginates, cellulose derivatives, Tragacanth, carbomers and silicates

2. Deflocculating agents

- induce deflocculation

E.g Ionic surfactants

Ionic surfactants as flocculating and deflocculating agents

3. Suspending agents

- Reduce the rate of sedimentation of the dispersed particles.

- Increasing the viscosity of the external phase

E.g.

i) Tragacanth BP (internal or external suspensions)

ii) Compound Tragacanth Powder BP (containing: 15% Tragacanth BP, 20% Acacia BP, 20% Starch BP and 45% Sucrose BP)

iii) Bentonite BP (external suspensions)

4. Thickening agents

-  Increase the viscosity of the continuous phase

- Particles remain suspended for a sufficiently long time

- Easy to withdraw a uniform dose

- Viscosity of the preparation –easy pouring from the container and transferring to the site of application

E.g.

1. Natural polysaccharides-   Acacia, Tragacanth, Alginates

2. Semi synthetic (cellulose derivatives)- Methyl cellulose, SCMC

3. Inorganic agents – Bentonite, Magnesium aluminium silicate (Veegum)

4.  Synthetic compounds – Carbomers, Colloidal silicon dioxide (Aerosil)

5.   Wetting agents

- Reduce the interfacial tension between the solid particles and the liquid medium

- Increases the affinity of the particles towards the surrounding medium

- Helps in penetration of liquid into the particles

- This produces a good suspension. E.g. Spans and Tweens, Saponins

6. Preservatives

- To protect the suspension against bacterial growth

- Should be effective against a wide range of microorganisms

- Should be chemically & physically compatible with the other ingredients

E.g. Benzoic acid, Parabens (methyl and propyl paraben)

7. Organoleptic additives

- Flavouring agents: for a distinct flavour

E.g. Vanilla flavour, banana flavour, strawberry.

- Sweetening agents: to provide sweetness

E.g. Sucrose, Saccharin sodium, aspartame

- Colouring agents: to increase aesthetic appeal

E.g. Amaranth, tartrazine

- Perfumes: to increase patient acceptability

E.g. Rose water, Lavender oil.

Methods of Dispensing Suspensions

1) Suspensions containing Diffusible solids

2) Suspensions containing Indiffusible solids

3) Suspensions containing precipitate forming liquids

4) Suspensions containing poorly wettable solids

5) Suspensions produced by chemical reaction

1) Suspensions containing Diffusible solids

Preparation:

a) The drug is finely powdered with the other solid ingredients.

b) 3/4th of the vehicle is added to the powder mixture and triturated to form a smooth cream.

c) More vehicle is added.

d) The suspension is filtered to remove any foreign particles.

e) Any other liquid ingredients if present, is added and the suspension is made upto volume.

f) The suspension is transferred to a bottle & shaken well.

E.g.      Rx        Light Kaolin

Light magnesium carbonate Sodium bicarbonate Peppermint water

Labeling: SHAKE WELL BEFORE USE

2) Suspensions containing Indiffusible solids

• They contain a suspending agent / thickening agent

• Delay the sedimentation rate & prevent the formation of aggregates.

• The thickening agents used are:

i) Compound Tragacanth powder: mixture of powdered tragacanth, acacia, starch & sucrose. It is used when the vehicle is not water or chloroform water.

ii) Tragacanth mucilage: mixture of tragacanth powder, 90% ethanol & chloroform water.

•It is used only when the vehicle is water or chloroform water

• If it is added to preparations containing medicinally active vehicle, it may replace the medicinally active vehicle, thereby decreasing its activity.

3) Suspensions containing precipitate forming liquids

• Liquid preparations containing resinous matter when mixed with water show precipitation of resin

• It sticks to the sides of the bottle & will not diffuse on shaking.

E.g. Compound benzoin tincture, Tolu tincture

• To prevent this, suspending agents like Compound Tragacanth powder or tragacanth mucilage is added.

4) Suspensions containing poorly wettable solids

• Substances like sulphur & hydrocortisone - are insoluble in water & poorly wetted by it

• In simple aqueous dispersions, it is difficult to disperse clumps

• The foam produced on shaking is slow to subside since it is stabilized by the film of unwettable solid at the liquid/air interface.

•To ensure wetting, the interfacial energy between the solid & liquid must be reduced.

Wetting is measured by determining the contact angle of a liquid or solid

High contact angle = poor wetting                 Low contact angle = good wetting

• Wetting is achieved by adding a suitable wetting agent

• This is adsorbed at the solid liquid interface

• The affinity of the particles for the surrounding medium is thus increased

E.g. Sulphur lotion – Quillaia tincture is used as the wetting agent. The saponins in quillaia extract have been used to suspend sulphur in lotion.

5) Suspensions produced by chemical reaction

• The insoluble active constituent of a lotion is produced by a chemical reaction

• Finer precipitate -   if dilute solutions of the reactants are mixed

• Hence   the   reacting   substance   should   be   dissolved separately in approx. half volumes of the vehicle and the 2 parts mixed.

• The precipitate is diffusible & no suspending agent is required.

E.g 1. Magnesium Hydroxide mixture

2. Zinc Sulphide lotion B.P.C (used to treat acne & scabies)

Rx        Sulphurated potash

            Zinc sulphate

            Concentrated camphor water

            Water

• Sulphurated potash is a mixture of potassium polysulphides & other sulphur containing compounds

• Should be freshly prepared as its solubility decreases on storage

• If sulphurated potash is added to the zinc sulphate: the ppt. is diffusible

• If zinc sulphate is added to sulphurated potash: the ppt. is indiffusible.

• Protection from light is necessary to reduce oxidation of the Sulphide to sulphite or sulphate.

Summary

1. Formulation of Suspensions:

- Medicament/Drug: Insoluble drug

- Flocculating agents: Induces flocculation

- Deflocculating agents: Induces deflocculation

- Suspending agents: Delay sedimentation rate

- Thickening agents: Increases viscosity to delay sedimentation rate

- Wetting agents: Helps in penetration of liquid into the particles

- Preservatives: Prevent microbial contamination

- Organoleptic additives: Imparts sweetness, increases palatibility and aesthetic appeal

2. Methods of dispensing

- Diffusible suspensions

- Indiffusible suspensions

- Suspensions containing precipitate forming liquids

- Suspensions containing poorly wettable solids

- Suspensions produced by chemical reaction