11. BIPHASIC DOSAGE FORMS

Biphasic Dosage Forms

Emulsions:

An emulsion is a liquid preparation containing two immiscible liquids, where one is dispersed as globules (dispersed/internal phase) in the other (continuous/external phase). Droplet size ranges from 0.1 to 100 µm. Emulsions are thermodynamically unstable and require emulsifying agents for stabilization.

Emulsions protect drugs susceptible to hydrolysis and oxidation and provide prolonged medication action. For example, oil-in-water (o/w) emulsions of ephedrine have a more prolonged effect on nasal mucosa than oily solutions.

Advantages of Emulsions:

  • Mask unpleasant taste and odor of drugs (e.g., castor oil, cod-liver oil).
  • Protect drugs prone to oxidation or hydrolysis.
  • Useful in external preparations like creams, lotions, and aerosols.
  • Intravenous stable emulsions can administer fats, carbs, and vitamins to patients unable to take orally.
  • Improve absorption of oils when taken internally.
  • Used as radio-opaque agents in X-rays.

Disadvantages:

  • Pharmaceutical emulsions are thermodynamically unstable and may be difficult to manufacture.
  • Storage conditions affect stability, with risk of microbial contamination and container breakage.
  • Accurate dosage may be challenging to achieve.

Types of Emulsions:

  • Oil in Water (o/w): Oil droplets dispersed in water; preferred for internal use.
  • Water in Oil (w/o): Water droplets dispersed in oil; mainly used externally.

Classification Based on Emulsifying Agent:

  1. Natural gums (e.g., gum acacia, tragacanth).
  2. Gum substitutes (e.g., cellulose derivatives).
  3. Soaps and saponins.
  4. Natural waxes (e.g., wool fat, beeswax).
  5. Synthetic waxes (e.g., emulsifying wax).
  6. Others (e.g., pectin, egg yolk, gelatin, lecithins).

Classification Based on Route of Administration:

  1. Oral emulsions – usually o/w type for medicinal oils.
  2. External emulsions – both o/w and w/o.
  3. Parenteral emulsions – for fat-soluble vitamins and hormones.
  4. Rectal emulsions – used as enemas for bowel evacuation.

Tests for Emulsion Type:

  • Dilution Test: Water cracks w/o emulsions; oil cracks o/w emulsions.
  • Conductivity Test: Electrical conductivity present in o/w, absent in w/o.
  • Dye Test: Water-soluble dye dissolves in aqueous phase (o/w); oil-soluble dye dissolves in oil phase (w/o).
  • Fluorescent Test: UV light fluorescence differs for oil and water phases.
  • Filter Paper Test: O/w spreads rapidly; w/o migrates slowly.
  • Cobalt Chloride Test: Paper changes color with o/w emulsions.

Pharmaceutical Applications:

  • Improve palatability.
  • Deliver water-insoluble liquids effectively.
  • Facilitate absorption of oils.
  • Parenteral delivery of oil-soluble drugs.
  • External cosmetic and therapeutic uses.

Formulation:

Emulsifying agents reduce interfacial tension and stabilize emulsion. No single agent is ideal; sometimes mixtures are used. The Hydrophile-Lipophile Balance (HLB) method guides selection.

Preservation and Stability:

  • Clean equipment and quality ingredients.
  • Proper oil-water ratios and pH maintenance.
  • Use suitable preservatives and antioxidants.
  • Store in cool, protected conditions to prevent cracking and creaming.

Suspensions:

Suspensions are biphasic liquid dosage forms with finely divided solid particles dispersed in liquid or semisolid vehicles. They are administered orally, parenterally, or externally.

Qualities of a Good Suspension:

  • Slow settling and easy redispersion.
  • Pourability and chemical inertness.
  • No caking, no large particles.

Classification of Suspensions:

  1. Oral suspensions
  2. Parenteral suspensions
  3. Ophthalmic suspensions
  4. External suspensions

Oral Suspensions:

Flavored and sweetened for palatability, often dry powders reconstituted before use.

Advantages: Easy to swallow, palatable, suitable for bulky insoluble powders.

Disadvantages: Shake before dose, less accurate dosing, storage instability.

Other Suspension Types:

Parenteral suspensions must be sterile with appropriate particle size. Ophthalmic suspensions are rarely used and require very fine particles and isotonicity. External suspensions should avoid grittiness and ensure ease of application.

Detailed Notes:

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