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SUSPENSION

DEFINNITION

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A pharmaceutical
suspension is a coarse dispersion in which the insoluble particles, generally
greater than 1µm in diameter, are dispersed in a liquid medium, usually
aqueous.

INTRODUCTION

Suspensions are
heterogeneous systems consisting of two phases. The continuous or external
phase is generally a liquid or a semisolid, and the dispersed or the internal
phase is made up of the particulate matter that is essentially insoluble in,
but dispersed throughout the continuous phase; the insoluble matter may be
intended for physiologic absorption or for internal or external coating
functions.

An aqueous suspension is
a useful formulation system for administering an insoluble or poorly soluble
drug. The large surface area of dispersed drug ensures a high availability for
dissolution and hence absorption. Aqueous suspensions may also be used for
parenteral application of dermatological materials to the skin. Suspensions are
used similarly in veterinary practice and a closely allied field is that of
pest control. Pesticides are frequently presented as suspensions for use as
fungicides, insecticides, ascaricides and herbicides.

CHARECTERISTICS
OF AN IDEAL SUSPENSION

An acceptable suspension
possesses certain desirable qualities among which are the following:

1.     
The suspended particles should not settle
too rapidly.

2.     
The particles which do settle down to the
bottom of the container must not form a hard mass.

3.     
The settled particles must be readily
dispersed into a uniform mixture when the container is shaken.

4.     
The suspension must not be too viscous to
pour freely from the orifice of the bottle or to flow through a syringe needle.

PHYSICAL
STABILITY OF SUSPENSIONS

The physical stability of
a pharmaceutical suspension may be defined as, the condition in which they
remain uniformly distributed throughout the dispersion. Since the ideal
situation is seldom realized, it is appropriate to add that if the particles do
settle they should be easily resuspended by a moderate amount of agitation.

TYPES
OF SUSPENSIONS ON THE BASIS OF ROUTE OF ADMINISTRATION

1.     
Oral suspensions: antibiotic preparations,
antacids and radiopaque suspensions.

2.     
Topical suspensions: calamine lotion USP.

3.     
Parenteral suspensions: insoluble forms of
penicillins, antibiotics.

FORMULATION
CONSIDERATIONS

A pharmaceutical
suspension consists of following formulation components:

1.     
Wetting agents

2.     
Deflocculants or dispersing agents

3.     
Flocculating agents

4.     
Suspending agents

5.     
Protective colloids

6.     
Formulation adjuvants

METHODS
OF SUSPENSION FORMATION

There are various
techniques used for the preparation and formulation of the pharmaceutical
suspensions, namely;

1.     
Precipitation methods

a.       Organic
solvent precipitation

b.      Precipitation
by Ph

c.       Double
decomposition

2.     
Dispersion methods

3.     
Preparative techniques

EVALUATION
OF SUSPENSION STABILITY

Techniques for the
evaluation of the heterogeneous systems are generally complex, some of the
methods applicable for the stability testing of the suspensions are as follows:

1.     
Sedimentation volume

2.     
Degree of flocculation

3.     
Re-dispersibiltiy

4.     
Rheological methods

5.     
Electro-kinetic techniques

 

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