Gels are transparent semisolid preparations meant for external application to the skin or mucous membrane. They consist of either a suspension of small inorganic particles or large organic molecules in a liquid vehicle that appears jelly-like when a gelling agent is added.
These are organic hydrocolloids or hydrophilic inorganic
substances. They contain Tragacanth, Sodium Alginate, Pectin, Starch, Gelatin,
Cellulose Derivatives, Carbomer, and
Poly Vinyl Alcohol Clays. Numerous gelling agents vary in gelling
ability.
Clear gels are microemulsions in which the diameter of the
dispersed phase globules is in the range of 10 to 60 nm. These emulsions are
thermodynamically stable. Microemulsions are transparent as the globule
diameter of the dispersed phase is less than the wavelength of light.
Microemulsions can be distinguished from other types of gels
by the vibrations or 'ringing' that
occurs when the emulsion is subjected to impact.
The procedure of Clear Gel
• Gel is prepared by mixing a suitable thickening agent with
an aqueous vehicle.
• The drug is dissolved in an aqueous vehicle and the
thickening is added by triturating in a mortar.
• The trituration is carried out until a homogenous
preparation is formed.
Ingredient: Quantity for 50 gm
Sodium Carboxymethyl cellulose (thickening agent) - 2.5
gm
Glycerol (binding agent) - 1.5 gm
Preservative - 0.05 gm
Color - 0.0005 gm
Purified water (vehicle)
To make - 50 gm
Method: Dissolved methylparaben in water by heating.
Add SCMC to glycerine and stir thoroughly. Add this mixture to the aqueous
vehicle and stir in a uniform, homogenous preparation is formed. Add the dye
color and stir. Transfer to a suitable container.
Evaluation of Gel
The various evaluation parameters involved in the assessment
of the properties of the gels are as follows:
1. Yield Value: It measures the force required
to extrude the material from the deformable bottle tube. It can be determined
by using an instrument called a Penetrometer. A penetrometer consists of a metal needle that pierces through the system. The distance of penetration of the needle is measured,
from which the yield value may be calculated.
2. Spreadability: The Spreadability test is performed
to determine the extent of gels' spreadability based on their rheological
properties.
3. Stability: This test, known as the shipping test, is performed to determine the extent of stability of gels at varying
temperatures, which the product may experience while exporting to other
countries.