Pharmaceutical Manufacturing Handbook: Production and

GELS 297
Ethylcellulose Ethylcellulose is a synthetic polymer made of β - anhydroglucose
units connected by acetyl linkages. It is obtained by ethylating alkaline cellulose
solution with chloroethane. Ethylcellulose is available as a free - fl owing powder
which is tasteless and white in color. Although it is insoluble in water, it is incorporated
into topical preparations due to its viscosity - enhancing properties. Ethanol or
a mixture of ethanol and toluene (2 : 8) is used as a solvent. A decrease in the ratio
of alcohol increases the viscosity. The viscosity of the dispersion is increased by
increasing the concentration of ethylcellulose or by using a high - molecular - weight
material. As ethylcellulose is prone to photo - oxidation at higher temperature, and
gels are prepared and preserved at room temperature and dispensed in airtight
containers [28] .
Gelatin Gelatin is a protein obtained by acid or alkali hydrolysis of animal tissues
that contain large amounts of collagen. Based on the method of manufacture, it is
named type A or type B gelatin. Type A is obtained by partial acid hydrolysis and
type B is obtained by partial alkaline hydrolysis. They differ in their pH, density,
and isoelectric point. Gelatin is available as yellow - colored powder or granules. It
swells in water and improves the viscosity of dispersions. Different molecular weights
and particle size grades are commercially available. Gels can be prepared by dissolving
gelatin in hot water and cooling to 35 ° C. Temperature greatly infl uences the
viscosity and stability of gelatin dispersions. It transforms to a gel at temperatures
above 40 ° C and undergoes depolymerization above 50 ° C. The viscosity of gelatin
gel is also affected by microbes [29] .
Guar Gum Guar gum is a high - molecular - weight polysaccharide obtained from
the endosperms of guar plant. It mainly contains d - galactan and d - mannan. It is
available as powder which is odorless and white to yellowish white in color. It readily
disperses in water and forms viscous gels. The viscosity of gel is infl uenced by the
particle size of material, pH of the dispersion, rate of agitation, swelling time, and
temperature. Viscosity reduces on long - time heating. Maximum viscosity can be
achieved within 2 – 4 h. Gels are stable at pH between 7 and 9 and show liquifi cation
below pH 7. Addition of antimicrobial preservatives improves the microbial stability
of guar gum gels. Rheological properties of these gels can be modifi ed by incorporating
other plant hydrocolloids such as tragacanth and xanthan gum [30] .
Hydroxyethyl Cellulose ( HEC) HEC is a partially substituted poly(hydroxyethyl)
ether of cellulose. It is obtained by treating alkali cellulose with ethylene oxide. HEC
is available as a powder and appears light tan to white in color. Different viscosity
grades of HEC are commercially available which differ in their molecular weights.
Clear gels are prepared by dissolving HEC in hot or cold water. Dispersions can be
prepared quickly by altering the stirring rate of dispersion, temperature, and pH.
Slow stirring at room temperature during the initial stages favors wetting. Increasing
the temperature at this stage increases the rate of dispersion. Although HEC dispersions
are stable over a wide pH range, maintaining basic pH improves the dispersion.
The preservation temperature, formulation pH, and microbial attack infl uence the
rheological properties of HEC dispersions. Viscosity reduces at higher temperature,
but reverts to the original value on returning to room temperature. Lower
and higher pH of the vehicle usually results in hydrolysis or oxidation of HEC,