GELS
GELS
• Gels are semisolid systems consisting of
dispersions of small or large molecules in an
aqueous liquid vehicle rendered jellylike by
the addition of a gelling agent.
• In a typical polar gel, a natural or synthetic
polymer builds a three-dimensional matrix
throughout a hydrophilic liquid.
GELS
• Among the gelling agents used are:
• synthetic polymers (Carbopol), such as carbomer
934;
• cellulose derivatives, semisynthetic materials such as
methylcellulose, hydroxyethylcellulose,
hydroxypropylmethylcellulose and
carboxymethylcellulose
• natural gums, such as tragacanth, carrageenan,
pectin, agar and alginic acid.
• Carbomers are high-molecular-weight water-soluble polymers
of acrylic acid cross-linked with allyl ethers of sucrose and/or
pentaerythritol.
• Their viscosity depends on their polymeric composition.
• The NF contains monographs for six such polymers,
carbomers 910, 934, 934P, 940, 941, and 1342.
• They are used as gelling agents at concentrations of 0.5% to
2.0% in water.
GELS
• Single-phase gels are gels in which the macromolecules
are uniformly distributed throughout a liquid with no
apparent boundaries between the dispersed
macromolecules and the liquid.
• A gel mass consisting of floccules of small distinct
particles is termed a two-phase system, often referred to
as a magma.
GELS
• Milk of magnesia (or magnesia magma), which consists
of a gelatinous precipitate of magnesium hydroxide, is
such a system.
• Gels may thicken on standing, forming a thixotrope, and
must be shaken before use to liquefy the gel and enable
pouring.
CREAMS
creams
• Pharmaceutical creams are semisolid
preparations containing one or more medicinal
agents dissolved or dispersed in either a W/O
emulsion or an oil-in-water emulsion or in
another type of water-washable base.
• Oil-in-water emulsions are most useful as water
washable bases, whereas water-in-oil emulsions
are emollient and cleansing.
creams
• Creams find primary application in topical skin products
and in products used rectally and vaginally.
• Patients often prefer a w/o cream to an ointment
because the cream spreads more readily, is less greasy,
and the evaporating water soothes the inflamed tissue.
• Pharmaceutical manufacturers frequently manufacture
topical preparations of a drug in both cream and
ointment bases to satisfy the preference of the patient
and physician.
creams
• O/w creams ('vanishing‘ creams):
• containing large percentages of water and stearic acid or
other oleaginous components.
• rub into the skin;
• After application, the continuous phase evaporates,
leaving behind a thin residue film of the stearic acid or
other oleaginous component
• This increases the concentration of a water soluble drug
in the adhering film.
• The concentration gradient for drug across the stratum
corneum therefore increases, promoting percutaneous
absorption.
creams
• An o/w cream is non-occlusive because it does
not deposit a continuous film of waterimpervious liquid.
• However, such a cream can deposit lipids and
other moisturizers on and into the stratum
corneum and so restore the tissue's hydration
ability, i.e. the preparation has emollient
properties.
Pastes
• Pastes are ointments containing as much as 50%
powder dispersed in a fatty base and therefore
are stiffer.
• They are less greasy than ointments because the
powder absorbs some of the fluid hydrocarbons.
• Pastes lay down a thick, unbroken, relatively
impermeable film.
Pastes
• Pastes can be prepared in the same manner as
ointments, by direct mixing or the use of heat to
soften the base prior to incorporating the solids,
which have been comminuted and sieved.
• when a levigating agent is to be used to render
the powdered component smooth, a portion of
the base is often used rather than a liquid, which
would soften the paste.
Pastes
• Because of the stiffness of pastes, they remain
in place after application and are effectively
employed to absorb serous secretions.
• Because of their stiffness and impenetrability,
pastes are not suited for application to hairy
parts of the body.
MISCELLANEOUS SEMISOLID
PREPARATIONS
• PLASTERS
• Plasters are solid or semisolid adhesive masses spread
on a backing of paper, fabric, or plastic.
• The adhesive material is a rubber base or a synthetic
resin.
• Plasters are applied to the skin to provide prolonged
contact at the site.
• Unmedicated plasters provide protection or
mechanical support at the site of application.
PLASTERS
• Medicated plasters provide effects at the site of
application.
• They may be cut to size to conform to the surface to be
covered.
• Among the few plasters in use today is salicylic acid
plaster used on the toes for the removal of corns.
• The horny layers of skin are removed by the keratolytic
action of salicylic acid.
• The concentration of salicylic acid used in commercial
corn plasters ranges from 10% to 40%.
GLYCEROGELATINS
• Glycerogelatins are plastic masses containing
gelatin (15%), glycerin (40%), water (35%), and an
added medicinal substance (10%), such as zinc
oxide.
• They are prepared by first softening the gelatin in
the water for about 10 minutes, heating on a
steam bath until the gelatin is dissolved, adding
the medicinal substance mixed with the glycerin,
and allowing the mixture to cool with stirring
until congealed.
GLYCEROGELATINS
• Glycerogelatins are applied to the skin for the long
term.
• They are melted before application, cooled to slightly
above body temperature, and applied to the affected
area with a fine brush.
• Following application, the glycerogelatin hardens, is
usually covered with a bandage, and is allowed to
remain in place for weeks.
• The most recent official glycerogelatin was zinc gelatin,
used in the treatment of varicose ulcers.