Most suspending agents perform two functions. Besides acting as a suspending agent they also imparts viscosity to the solution. Suspending agents form film around particle and decrease interparticle attraction. Suspending agents also act as thickening agents. They increase in viscosity of the solution, which is necessary to prevent sedimentation of the suspended particles as per Stoke’s’s law.
A good suspension should have well developed thixotropy. At rest the solution is sufficient viscous to prevent sedimentation and thus aggregation or caking of the particles. When agitation is applied the viscosity is reduced and provide good flow characteristic from the mouth of bottle.

List of Suspending Agents
• Alginates
• Methylcellulose
• Hydroxyethylcellulose
• Carboxymethylcellulose
• Sodium Carboxymethylcellulose
• Microcrystalline cellulose
• Acacia
• Tragacanth
• Xanthan gum
• Bentonite
• Carbomer
• Carageenan
• Powdered cellulose
• Gelatin

The selection of amount of suspending agent is dependent on the presence of other suspending agent, presence or absence of other ingredients which have an ability to act as a suspending agent or which contributes viscosity to the medium.

The stability of the suspensions depends on the types of suspending agents rather than the physical properties of the drugs. the physical stability of suspension was mainly dependent on the type of suspending agent rather than the physical characteristics of the drug.

Stability pH range and concentrations of most commonly used suspending agents for suspension

Suspending agents Stability pH range Concentrations used as suspending agent
Sodium alginate 4-101-5 %
Methylcellulose 3-111-2 %
Hydroxyethylcellulose 2-121-2 %
Hydroxypropylcellulose 6-81-2 %
Hydroxypropylmethylcellulose3-111-2 %
CMC 7-9        1-2 %
Na-CMC 5-100.1-5 %
Microcrystalline cellulose 1-110.6-1.5 %
Tragacanth 4-81-5 %
Xanthangum 3-120.05-0.5 %
Bentonite >60.5-5.0 %
Carageenan 6-100.5-1 %
Guar gum 4-10.5                            1-5 %
Colloidal silicon dioxide 0-7.52-4 %

Characteristics of Most Commonly Used Suspending agent for suspension 

Alginates 
  • Alginate salts have about same suspending action to that of Tragacanth.
  • Alginate solution looses its viscosity when heated above 60 ºC. due to depolymerization.
  • Fresh solution has highest viscosity, after which viscosity gradually decreases and acquires constant value after 24 hrs.
  • Maximum viscosity is observed at a pH range of 5-9.
  • Due to significant thickening effect, alginate is used at lower concentration to avoid problem of viscosity. High viscosity suspensions are not readily pourable.
  • 1 % solution of low viscosity grade of alginate has viscosity of 4-10 mPas at 20 ºC.
  • Chemically alginates are polymers composed of mannuronic acid and glucuronic acid monomers. The ratio of mannuronic acid to glucuronic acid determines the raft-forming properties. High ratio (e.g. 70 % glucuronic acid) forms the strongest raft.
  • The concentration of alginate is optimized by raft-forming ability of the suspension in order to avoid pourability problem by too much increase in viscosity of suspension. In practice, alginate is used at concentration less than 10 % w/w, particularly at 5 % w/w.
Methylcellulose
  • Methylcellulose is available in several viscosity grades, difference in methylation and polymer chain length.
  • Methylcellulose is more soluble in cold water than hot water. Adding Methylcellulose in hot water and cooling it with constant stirring gives clear or opalescent viscous solution.
  • Methylcellulose is stable at pH range of 3-11.
  • As methylcellulose is non-ionic, it is compatible with many ionic adjuvants.
  • On heating to 50 ºC, solution of Methylcellulose is converted to gel form and on cooling, it is again converted to solution form
Hydroxyethylcellulose
  • Hydroxyethylcellulose (HEC) having somewhat similar characteristics to Methylcellulose.
  • In HEC hydroxyethyl group is attached to cellulose chain. Unlike methylcellulose, HEC is soluble in both hot and cold water and do not form gel on heating.
Carboxymethylcellulose (CMC)
  • Carboxymethylcellulose is available at different viscosity grades : Low, medium and high
  • The choice of proper grade of CMC is dependent on the viscosity and stability of the suspension.
  • In case of HV-CMC, the viscosity significantly decreases when temperature rises to 40 ºC from 25 ºC. This may become a product stability concern. Therefore to improve viscosity and stability of suspension MV-CMC is widely accepted.
Sodium Carboxymethylcellulose (NaCMC)
  • It is available in various viscosity grades, difference in viscosity dependent on extent on polymerization.
  • It is soluble in both hot and cold water and stable over a pH range of 5-10.
  • As it is anionic, it is incompatible with polyvalent cations.
  • Sterilization of either powder of mucilage form decreases viscosity.
  • It is used at concentration up to 1 %.  
Microcrystalline Cellulose (Avicel)
  • It is not soluble in water, but it readily disperses in water to give thixotropic gels. It is used in combination with Na-CMC, MC or HPMC, because they facilitate dispersion of MCC.
  • MCC coprocessed with CMC together with titanium dioxide (opacifying agent) can be used for thixotropic pharmaceutical gels.
  • MCC: alginate complex compositions are excellent suspending agents for water insoluble or slightly soluble API. The advantages of MCC: alginate complex compositions are that they provide excellent stability. Further suspensions prepared with them are redispersible with small amount of agitation and maintain viscosity even under high shear environment.
  • Commonly, Na-CMC is used as the coprecipitate in MCC. Na CMC normally comprised in the range of 8 to 9 % w/w of the total mixture. These mixtures are available from FMC under trademark; Avicel RTM CL – 611, Avicel RTM RC – 581, Avicel RTM RC – 591. Avicel RC- 591 is most commonly used. It contains about 8.3 to 13.8 % w/w of Na CMC and other part is MCC.
Acacia
  • Acacia is not a good thickening agent, but widely used in extemporaneous suspension formulation.
  • For dense powder acacia alone is not capable of providing suspending action, therefore it is mixed with Tragacanth, starch and sucrose which is commonly known as Compound Tragacanth Powder BP.
Tragacanth
  • Tragacanth solution is viscous in nature, it provides thixotrophy to the solution.
  • It is a better thickening agent than acacia.
  • The maximum viscosity of the solution of Tragacanth is achieved after several days, because several days to hydrate completely.
Xanthan Gum
  • Xanthan gum may be incorporated at a concentration of 0.05 to 0.5 % w/w depending on the particular API :
  1. Antacid suspension : Xanthan concentration is 0.08 to 0.12 % w/w.
  2. ibuprofen and acetaminophen suspension : Xanthan concentration is 0.1 to 0.3 % w/w.

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