Gels are colloids (aggregates of fine particles, as described above, dispersed in a continuous medium) in which the liquid medium has become viscous enough to behave more or less as a solid. Contraction of a gel, causing separation of liquid from it, is called syneresis.

1. Gels
Md. Saiful Islam
BPharm, MPharm (PCP)
North South University
Fb Group: Pharmacy Universe

2. Gelling agents
A number of synthetic macromolecules, cellulose
derivatives and natural gums are used as gelling agents.
A few of the more common ones are discussed below:
Alginic acid:
- Obtained from seaweed and prepared product is
tasteless, odorless and white to yellowish-white colored
fibrous powder
- 1-5% is used as thickening agent in gels
- It swells in water to about 200-300 times its own weight
without dissolving and can be dispersed upon vigorous
stirring for 30 minutes
- Addition of Ca-salt causes cross linking with increased
viscosity

3. Bentonite:
- It is a colloidal hydrated aluminum silicate
- It forms a thixotropic gel, bentonite, at the concentration
above 4%
Carbomer:
- Carbomer resins are high-molecular-weight, cross-linked
acrylic acid- based polymers modified with C10-C30 alkyl
acrylates
- They are fluffy white dry powders with large bulk density
- Acceptable viscosity and clarity obtained at pH 4.5 to 5

4. - The viscosity ranges of different carbomer resins varies
from 0 - 80,000 cps
- Carbomer 910 is very low concentrations when low
viscosity is desired
- Carbomer 934 is highly effective in thick formulations
such as viscous gels
- Carbomer 934P is intended for oral and mucosal contact
applications and is the most widely used in the
pharmaceutical industry
- Carbomer 940 forms sparkling clear water or
hydroalcoholic gels

5. Carboxymethylcellulose (CMC):
- 4-6% of medium viscosity CMC is used to produce gels
- Glycerin may be added to prevent drying
- Maximum stable at pH 7 to 9
- Precipitation of CMC can occur below pH 2
- It is incompatible with ethanol
Colloidal silicon-di-oxide:
- It can absorb large amount of water without liquefying
- The viscosity is largely independent of temperature but
sensitive to change in pH
- It can form gel when combined with 1-dodecanol and
n-dodecane

6. Carboxymethylcellulose (CMC) sodium:
- Soluble in water at all temperature
- Na CMC is first dispersed in cold water with high shear,
then the particles are hydrated and swell to sticky gel
grains agglomerating into lumps
- After well dispersion, the solution is heated with
moderate shear to about 60ºC for fastest dissolution
- These dispersions are sensitive to pH changes because
of carboxylate group
- The viscosity of the product falls markedly below pH 5 or
above pH 10

7. Gelatin:
- Gelatin is dispersed in hot water and cooled to form gels
- An alternative way to make gel is to moisten the gelatin
with 3-5 parts of an organic liquid ( ethyl alcohol or
propylene glycol ), then hot water added and cooled
Magnesium aluminum silicate (Veegum):
- In concentration of about 10% it forms a firm thixotropic
gel
- It is best used above pH 3.5
- The material is inert but has a few incompatibilities, may
bind some drugs and limit their bioavailabilities

8. Methylcellulose:
- A long chain substituted cellulose that can be used to
form gels in concentrations up to about 5%
- It hydrates slowly in hot water
- The powder is dispersed with high shear in about one-
third of the required amount of water at 80º-90ºC
- After the fine dispersion of powder, the rest of the water
is added, cooled and moderately stirred to cause prompt
dissolution
- Maximum clarity, fullest hydration and highest viscosity
will be obtained if the gel is cooled to 0-10ºC for about
an hour
- A preservative should be added

9. Plastibase: (jelene)
- A mixture of 5% low-molecular-weight polyethylene and
95% mineral oil made at 90ºC
- When cooled below 90ºC, the polymer precipitates and
causes gelation
Polyvinyl alcohol: (PVA)
- 2.5% concentration is used in the preparation of various
jellies
- It dries rapidly when applied to the skin
- For best results, PVA is first dispersed in cold water,
then hot water applied

10. Povidone:
- At higher molecular weights, about 10% povidone can be
used to prepare gels
- It is compatible in solution with a wide range of inorganic
salts, natural and synthetic resins and other chemicals
- It is also used to increase the solubility of a number of
poorly soluble drugs
Sodium alginate:
- 10% concentration can be used to produce gels
- Aqueous preparations are most stable at pH 4 to 10
- Preservative should be used in sodium alginate gel for
external use

11. Tragacanth gum:
- Powdered tragacanth gum tends to form lumps when
added to water. So, aqueous dispersions are prepared by
adding the powder to vigorously stirred water
- Before mixing with water, it can be wetted with ethanol or
glycerin or propylene glycol
- Tragacanth gum gel is most stable at pH 4 to 8
- Preservatives must be added to these gels
- The gels may be sterilized by autoclaving

12. Poloxamer or Pluronic:
- Made from selected forms of polyoxyethylene-
polyoxypropylene copolymers in concentrations ranging
from 15-50%
- Poloxamers generally are waxy white free-flowing
granules that are practically odorless and tasteless
- Aqueous solutions are stable in presence of acids,
alkalis and metal ions
- Different types of poloxamers are available, among
which Pluronic F-127 has low toxicity, good solubilizing
capacity and optical properties and a good medium for
topical drug delivery systems