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PASTES AND GELS

A Pharmaceutical Sciences Assignment

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TABLE OF CONTENTS

1. Introduction to Semisolid Dosage Forms
2. Pastes — Definition, Classification, Formulation, Preparation, Uses
3. Gels — Definition, Classification, Formulation, Preparation, Uses
4. Differences Between Pastes and Gels
5. Summary
6. References

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1. INTRODUCTION TO SEMISOLID DOSAGE FORMS

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2. PASTES

2.1 Definition

"Pastes are stiff preparations intended for application to the skin. They differ from ointments in that they contain a large proportion of finely powdered material." — British Pharmacopoeia (BP)

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2.2 Classification of Pastes

• Fatty base (soft paraffin, hard paraffin, lard) + high powder content
• Examples: Zinc Oxide Paste (Lassar's Paste), Compound Zinc Paste

• Water or glycerin as vehicle
• Used in dentistry and on mucous membranes
• Examples: Triamcinolone Dental Paste, Carmellose Gelatin Paste (Orabase)

• Contain active pharmaceutical ingredients (APIs)
• Examples: Dithranol Paste, Salicylic Acid Paste

• Protective/emollient only; no active drug
• Example: Plain Zinc Oxide Paste

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2.3 Formulation Components (Ingredients) of Pastes

• Zinc oxide — protective, astringent, mild antiseptic
• Starch — absorbent, soothing
• Calamine — cooling, antipruritic
• Kaolin — absorbent
• Titanium dioxide — photoprotective

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2.4 Preparation Methods of Pastes

Method 1: Fusion / Melting Method (Fatty Pastes)

1. Melt higher-melting base components (hard paraffin, beeswax) on a water bath.
2. Add lower-melting components (soft paraffin, liquid paraffin) to the melt.
3. Finely powder medicaments; pass through 150 µm sieve.
4. Gradually incorporate powders into the melted base with continuous stirring.
5. Stir throughout cooling to prevent powder settling.
6. Fill into airtight containers.

Key Point: Continuous stirring during cooling is essential for uniform powder dispersion.

Method 2: Trituration / Levigation Method

1. Levigate powders with a small amount of liquid paraffin or glycerin to a smooth paste.
2. Incorporate the remaining base in small portions, mixing thoroughly each time.
3. Final product should be smooth and free of gritty particles.
4. Spatula-and-slab method can be used for small-scale preparation.

Method 3: Machine Mixing (Industrial Scale)

• Triple-roller mills or planetary mixers
• Ensures fine, homogeneous powder-base dispersion

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2.5 Uses and Applications of Pastes

• Protective barrier (nappy rash, skin excoriation)
• Absorptive for exuding/weeping wounds
• Astringent (zinc oxide contracts tissues)
• Anti-inflammatory (corticosteroid pastes)
• Keratolytic (salicylic acid softens/removes hyperkeratotic skin)
• Dental adhesive and therapeutic applications

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3. GELS

3.1 Definition

"Gels are semisolid systems consisting of either small inorganic particles or large organic molecules interpenetrated by a liquid." — USP

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3.2 Classification of Gels

• Topical (skin), Ophthalmic, Nasal, Vaginal, Rectal, Oral mucosal

• Elastic/reversible gels (agar, gelatin) — regain structure after distortion
• Rigid/irreversible gels (silica gel) — do not revert once broken

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3.3 Formulation Components (Ingredients) of Gels

Gelling Agents (most critical component)

Other Excipients

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3.4 Preparation Methods of Gels

Method 1: Dispersion & Neutralization Method (Carbomer Gels) — Most Common

1. Disperse carbomer powder slowly onto the surface of purified water (do not add in bulk). Allow hydration for 30–60 minutes. The dispersion is acidic (pH ~3–4).
2. Dissolve API, preservatives, and humectants in a small portion of water separately.
3. Add the drug solution to the carbomer dispersion; mix gently.
4. Neutralize dropwise with triethanolamine (TEA) with slow stirring until a clear, viscous gel forms (target pH 6–7).
5. Adjust volume with purified water. Package in collapsible tubes or pump dispensers.

Key Points:

• Sprinkle carbomer gradually onto water; never add water to carbomer
• Avoid high-shear mixing — breaks polymer chains and reduces viscosity
• Degree of neutralization controls viscosity and clarity

Method 2: Heat-and-Cool Method (HPMC / Methylcellulose / Gelatin Gels)

1. Heat one-third of purified water to 80–90°C.
2. Disperse gelling agent (e.g., HPMC) in hot water with stirring.
3. Add the remaining cold water (ice-cold) to the hot dispersion while stirring — cools and gels the mixture.
4. Dissolve API in a suitable solvent and incorporate into the cooled gel base.
5. Adjust pH, add excipients, fill into containers.

Key Point: HPMC and methylcellulose are reverse-thermal polymers — they gel on cooling, not heating.

Method 3: Direct Dissolution Method (Tragacanth / CMC / Alginate Gels)

• Gelling agent dissolved directly in water at room temperature
• Drug and excipients added and mixed uniformly
• No neutralization or special temperature control needed

Method 4: Industrial Manufacturing

• Jacketed vessels with planetary or high-shear mixers
• Vacuum mixing to remove entrapped air
• In-line quality checks: pH, viscosity, clarity, drug content uniformity
• Aseptic processing for ophthalmic gels (sterile, preservative-free)

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3.5 Uses and Applications of Gels

• Anti-inflammatory — topical NSAID gels
• Antimicrobial — antibiotic gels for acne, vaginal infections
• Local anaesthetic — urethral catheterization, dental procedures
• Ophthalmic — prolonged corneal contact time
• Hormonal transdermal delivery — testosterone, estrogen gels
• Wound management — hydrogels maintain moist healing environment
• Antacid — aluminium/magnesium hydroxide gels

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4. DIFFERENCES BETWEEN PASTES AND GELS

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5. SUMMARY

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6. REFERENCES

1. Aulton, M.E. & Taylor, K.M.G. (2021). Aulton's Pharmaceutics: The Design and Manufacture of Medicines (6th ed.). Elsevier.
2. Lachman, L., Lieberman, H.A. & Kanig, J.L. (2019). The Theory and Practice of Industrial Pharmacy (4th ed.). CBS Publishers.
3. Banker, G.S. & Rhodes, C.T. (2002). Modern Pharmaceutics (4th ed.). Marcel Dekker.
4. Allen, L.V., Popovich, N.G. & Ansel, H.C. (2014). Ansel's Pharmaceutical Dosage Forms and Drug Delivery Systems (10th ed.). Lippincott Williams & Wilkins.
5. British Pharmacopoeia 2023. MHRA, London.
6. United States Pharmacopeia (USP 46–NF 41). USP Convention.
7. Rowe, R.C., Sheskey, P.J. & Quinn, M.E. (2022). Handbook of Pharmaceutical Excipients (9th ed.). Pharmaceutical Press.
8. Sinko, P.J. (2017). Martin's Physical Pharmacy and Pharmaceutical Sciences (7th ed.). Lippincott Williams & Wilkins.

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• Pastes — 4 classification types, 7 ingredient categories with examples, 3 preparation methods (fusion, levigation, machine mixing), clinical uses table
• Gels — 4 classification systems, 10 excipient categories including gelling agents table with concentrations, 4 preparation methods with step-by-step procedures and key points, clinical uses table
• Comparison table — 16 parameters comparing both dosage forms side by side
• All sections include pharmacopoeia definitions (BP, USP), named examples, and referenced standard textbooks