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Bhatt Preeti *, Gnanaranjan. G , kothiyal Preeti
Department of pharmaceutics,
Shri Guru Ram Rai Institute Of Pharmacy,
Dehradun-248001, India

The motto of writing this review on additives in pharmaceutical gel was to compile the recent literature with special focus on rational approach to topical formulation and basic components used in gel for topical drug delivery systems. Topical drug delivery systems involve the introduction of a drug to the surface of the body, in a formulation which can be absorbed. Topical gels are intended for skin application or to certain mucosal surfaces for local action or percutaneous penetration of medicament or for their emollient or protective action as they show better potential as a vehicle to administered drug topically in comparison to ointment. Additives are inactive ingredients used for structuring dosage form. Selection of topical vehicles depends on various dermatological factors and pharmaceutical factors. Dermatological factors are absorption penetration, skin condition, compatibility, emollient properties. Pharmaceutical factors are stability, solvent properties, emulsifying property.  The choice of suitable exepient for a formulation development is made on the basis of the drug delivery requirements and the particular need to impart sufficient emolliency or other quasi-medicinal qualities in the formulation.

Reference Id: PHARMATUTOR-ART-1988



Anatomy and physiology
The skin acts as a boundary which separates the external environment from the internal organs. The skin provides physical protection of internal organs and acts as a sensory organ. It controls body temperature and water loss, and functions as a regulatory barrier which controls the movement of substances into and out of the body. The skin has gained increasing favour as a target site for drug delivery as it avoids problems associated with oral drug administration, namely pH and to some extent, enzyme driven drug degradation and hepatic first-pass metabolism.

There are numerous diseases which affect different regions of the skin. Any drug used will be required to reach the site of the disease in order to exert its pharmacological activity. Unless it is for a local effect on the surface only, the drug must either pass through the stratum corneum(SC) or go through hair follicles or sweat glands to reach its target site. Once in the skin, a lipid-soluble drug will tend to accumulate in lipophilic regions while more water-soluble drugs will tend to enter the blood capillaries and are removed from the skin [1].

Skin is composed of three primary layers-
. Epidermis
. Dermis
. Hypodermis

An important function of the epidermis is the generation of the stratum corneum. The epidermis is avascular and is sustained by nutrients it receives by diffusion from the underlying dermal capillaries through the basement membrane. The epidermal layer forms as a result of the death of basal cells via a specialised differentiative process. Epidermis further divided into the following2 :-
1. Stratum corneum
2. Stratum lucidum
3. Stratum granulosum
4. Stratum spinosum
5. Stratum Basale

Cells are formed through mitosis at the basale layer. The cytoplasm is released and the protein keratin is inserted. They eventually reach the corneum and slough off (Desquamation). This process is called keratinization and takes place within about 30 days. This keratinized layer of skin is responsible for keeping water in the body and keeping the harmful chemicals and pathogens out.

A gel is an intermediate state of matter possessing property of a solid and a liquid, termed as viscoelasticity. It is a two-component, cross linked three-dimensional network consisting of structural materials interspersed by an adequate but proportionally large amount of liquid to form an infinite rigid network structure which immobilizes the liquid continuous phase within4. The structural materials that form the gel network can be composed of inorganic particles or organic macromolecules, primarily polymers 5.

Properties of gels
Gels should posses the following properties7 :-
1. Ideally, the gelling agent for pharmaceutical or cosmetic use should be inert, safe, and should not react with other formulation components.

2. It should possess suitable anti-microbial to prevent from microbial attack.

3. The gelling agent included in the preparation should produce a reasonable solid-like nature during storage that can be easily broken when subjected to shear forces generated by shaking the bottle, squeezing the tube, or during topical application.

4. The ophthalmic gel should be sterile.

5. The topical gel should not be tacky.

Polymers are used to give the structural network, which is essential for the preparation of gels. Gel forming polymers are classified in table no.1.

Gelling agent
Gelling agent are hydrocolloids substance which gives thixotrophic consistency to the gel. Gelling agents are organic in nature. Gelling agents are also known as solidifiers or stabilizer and thickening agent. Gelling agents are more soluble in cold water than hot water. Gelling agents like methylcellulose and polaxamers have better solubility in cold water while bentonite, gelatin and sodium carboxymethylcellulose are more water soluble in hot water. Gelling agents are used in concentration of 0.5 up to 10% depending on the agent most gelling agents require 24-48 hours to completely hydrate and reach maximum viscosity and clarity. It is easier to add the active drug before the gel is formed if the drug does not interfere with the gel formation. The viscosity of the gelling agents in the gelling layer be within range of about 1000 cps to about 100,000 cps. Various hydrocolloids used as gelling agents are:

1. Natural polymer

a. Proteins

i) Gelatin:-

  • Gelatin is a common natural polymer ( water soluble polymer) or protein which is normally produced by denaturing collagen [8].
  • In addition, gelatin can be easy to manipulate due to its isoelectric point that allows it to change from negative to positive charge in an appropriate physiological environment or during the fabrication, a property that has found it being very attractive to many pharmaceutical researchers[10] .
  • Gelatin is one of the natural polymers used as support material for gene delivery, cell culture, and more recently tissue engineering. Gelatin-based systems have the ability to control release of bioactive agents such as drugs, protein, and dual growth factors[11, 9, 12] .
  • However, some setbacks have been identified, and they are said be associated with the use of gelatin-based systems in pharmaceutical applications. These setbacks include poor mechanical strength and ineffectiveness in the management of infected sites[13].

ii) Collagen:-
Collagen is a major natural protein component in mammals that is fabricated from glycine-proline-(hydroxy) proline repeats to form a triple helix molecular structure[14]. Till now nineteen types of collagen molecules have been isolated, characterized, and reported in both medical and pharmaceutical applications[13-15]. collagen gels are one of the first natural polymers to be used as a promising matrix for drug delivery and tissue engineering[16].

b. Polysaccharides
Natural polysaccharides are extensively used for the development of solid dosage forms. These polymers of monosaccharide’s (sugars) are inexpensive and available in a variety of structures with a variety of properties. They are highly stable, safe, non-toxic, and hydrophilic and gel forming in nature. Pectin’s, starch and amylase are a few polysaccharides commonly used in controlled release dosage forms. Non-starch, linear polysaccharides remain intact in the physiological environment of the stomach and the small intestine, but are degraded by the bacterial inhabit-ants of the human colon which make them potentially useful in targeted delivery systems to the colon[17].

i) Pectin:-
Pectins are non-starch, linear polysaccharides extracted from the plant cell walls[18]. In the food industry, folic acid incorporated microcapsules were prepared using alginate and combinations of alginate and pectin polymers so as to improve stability of folic acid. The blended alginate and pectin polymer matrix increased the folic acid encapsulation efficiency and reduced leakage from the capsules as compared to those made with alginate alone, they showed higher folic acid retention after freeze drying and storage[19].

Natural complex polysaccharide variable in its rheological property and its microbiological quality. It is the sap of legumes of several species of genus Astralagus plant. It is viscous, odourless, and tasteless of which 5% concentration is used in medicated gels.  It must be pre-wetted with ethanol or glycerin before dispersion in water. As amucilage or paste it used as topical treatment for burns. It is acidic in nature and has high molecular weight 840, 0000 and yields glucoronic acid and arabinose when hydrolysed. It acts as demulscent and suspending agent. Complex glucoarabinose polysaccharides isolated from a related asian species, which stimulate the production of T-cells and antibody producing plasma cells. The different types of flakes of tragacanth gum are shown in figure no.3.



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