1. GELS

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. CREAMS

8. 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.

9. 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.

10. 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.

11. creams
An o/w cream is non-occlusive because it does not deposit a continuous film of water-impervious 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.

12. Pastes

13. 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.

14. 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.

15. 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.

16. 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.

17. 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%.

18. 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.

19. 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.