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2.Semi-synthetic polymers

a. Cellulose  derivatives
Cellulose is the most abundant naturally occurring biopolymer [20,21]. It consists of long chains of anhydro-D-glucopyranose units (AGU) with each cellulose molecule having three hydroxyl groups per AGU, with the exception of the terminal ends.

Cellulose is insoluble in water and most common solvents [21] ; In spite of its poor solubility characteristics, cellulose is used in a wide range of applications including composites, netting, upholstery, coatings, packing, paper, etc.

Chemical modification of cellulose is performed to improve process ability and to produce cellulose derivatives (cellulosics) which can be tailored for specific industrial applications [22]. Large scale commercial cellulose ethers include carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxyethylcellulose (HEC), hydroxypropyl methyl cellulose (HPMC), hydroxylpropyl cellulose(HFC), ethyl hydroxyethyl cellulose (EHEC), and methyl hydroxyethyl cellulose(MHEC).

i).Hydroxyethyl cellulose :- It is ether like methyl cellulose where the substituent group is hydroxyl ethyl and It forms an occlusive dressing when lightly applied to the skin and allow to dry.

ii). Methyl cellulose:-It is non-ionic and stable over a wide spectrum of pH. It is soluble in cold water but insoluble in hot water. Compatible with water, alcohol (70%), and propylene glycol (50%). Marketed methyl cellulose is Methocel HG and Methocel MC.

iii). Carboxymethyl cellulose :- It isGenerally used as the sodium salt known as carmellose sodium.  It contains carboxy methyl group (- CH2 COOH) and makes thicker gels but less tolerance than hydroxyl propyl methylcellulose.  Maximum stability at pH 7-9.

iv). Hydroxypropyl cellulose :- It Makes thinner gels with high tolerance for added drugs and saltsHydrates and swells in water or hydroalcoholic solution.

v). Hydroxypropyl methyl cellulose :-It Makes thicker gels but lower tolerance for positively charge ions.Disperse in cool water. It is a Good gelling agent for time released formulation.

3. Synthetic polymers

a. Carbomer

Carbomer polymers are polymers of acrylic acid cross-linked with polyalkenyl ethers or divinyl glycol. They are produced from primary polymer particles of about 0.2 to 6.0 micron average diameter. The flocculated agglomerates cannot be broken into the ultimate particles when produced. Each particle can be viewed as a network structure of polymer chains interconnected via cross-linking23.

Carbopol polymers are offered as fluffy, white, dry powders (100% effective).
The carboxyl groups provided by the acrylic acid backbone of the polymer are
responsible for many of the product benefits. Carbopol polymers have an average
equivalent weight of 76 per carboxyl group24. The general structure
can be illustrated with fig. No.4.

Poloxamers (Pluronics)

  • Poloxamers are copolymer of polyoxy ethylene and polyoxy propylene.
  • They form thermoreversible gels in concentration ranging from 15 to 50%.
  • Poloxamers are white, waxy granules that form clear liquid when dispersed in cold water.
  • “PLO” gel formed by combining pluronic F-127 with lecithin an isopropyl palmitate.
  • Pluronic F-127 is a grade of poloxamer.

It is less soluble in cold water.  It used at 2.5% concentration to prepare gel. Borax often adds in PVA solution. It is formed from ploymerised vinyl acetate.

4. Inorganic substances

is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite28 . Montmorillonite is superfine flaky aluminosilicate, where due to lattice nestehiometrical cations exchange surplusnegative charge appears and compensates cation-exchange located in interlayer area. High waterproofing is conditioned on this. In case of water tempering, water penetrates in interlayer area of montmorillonite, hydrates it's surface and cation-exchange, so that provides swelling of the mineral. During further water dilution bentonite forms firm viscous suspension with thixotropic properties. Montmorillonite has high cation-exchange and adsorption properties.

Owing to this properties bentonite has found wide application as viscous gelling agent and filtration diminishing agent in drilling muds production for well-boring, as binding agent in foundry sands and iron ore pellets and also as hydraulic and adsorption material.

Humectant and cosolvants in gel
Humectant is a substance that absorbs or helps another substance retain moisture, as glycerol. It is a hygroscopic substance or often a molecule with several hydrophilic groups, most often hydroxyl groups, but amines and carboxyl groups, sometimes esterifies, can be encountered as well.

Some of the examples are as follows:-

  • Glycerine, propylene glvcol (E 1520) and giyceryl triacetate (EI5I8).
  • Others can be polyols like sorbitol (E420), xylitol and maltitoi (E965).
  • Or polymeric polyols like polvdextrose (El 200)
  • Or natural extracts like quillaia (E999), or lactic acid or urea.

Lithium Chloride is an excellent humectant.

Bases and medicaments sensitive to heavy metals are sometimes protected by chelating agent, such as E.D.T.A.(Ethlylene diamine tetra acetic acid).

Additives are inactive ingredients in dosages form. Additives are non drug component for structuring dosage form. Selection of topical vehicles depends on various dermatological factors and pharmaceutical factors. Natural polymers have proven their potential for the development of new, advanced and efficient drug delivery system. Gel is most widely used semisolid topical dosage form using various gelling agents such as tragacanth, pectin, gelatin, cellulose derivatives, carbopols, polaxamers, alginates in gel formation. Humectant is a hygroscopic substance. It is used in almost all topical dosages forms except topical powder. Its function is retaining moisture by absorb water from air. Preservatives act as stabilizer in topical dosages forms. Paraben, benzoic acid, phenol, benzalkonium chloride, actamer is used as preservative in topical dosages forms.

Figure no.1: Transverse Section of Human Skin [3]

Figure no.2: gelatin powder

figure no.3: tragacanth gum flakes

Figure no.4: general structure of carbopol

Table no.1: Gel forming polymers7





Natural polymer

a. Proteins

b. Polysaccharides

a.i. Gelatin ii. Collagen

b.i. Alginic acid ii. Agar

iii. Tragacanth iv.Sodium or Potassium carrageenan

 v. Pectin vi. Gellum Gum

 vii. Xanthin viii. Cassia tora

ix. Guar Gum


2. Semisynthetic polymers

a. Cellulose derivatives

i. Hydroxyethyl cellulose

ii. Methylcellulose

iii. Hydroxypropyl methyl cellulose

 iv. Hydroxypropyl cellulose

 v. Carboxymethyl cellulose


3. Synthetic polymers

a. Carbomer

b. Poloxamer

c. Polyvinyl alcohol

d. Polyacrylamide

e. Polyethylene and its co-polymers

a.i. Carbopol -941 ii. Carbopol -940 iii. Carbopol -934


4. Inorganic substances

a. Bentonite

b. Aluminium hydroxide


5. Surfactants

a. Brij-96

b. Cetostearyl alcohol



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