- Measurement of pH: The pH of various gel formulations can be determined by using digital pH meter. One gram of gel is dissolved in 100ml distilled water and stored for two hours. The measurement of pH of each formulation is done in triplicate and average values are calculated.
- Drug content: In this method, 1g of prepared gel is mixed with 100ml of drug soluble or extractable suitable solvent. Aliquots of different concentrations are prepared by suitable dilutions after filtering the stock solution and the absorbance is measured. Drug content will be calculated using the equation obtained by linear regression analysis of calibration curve.
- Viscosity: Brookfield viscometer is used for the measurement of viscosity of the prepared gel. The gel is rotated at different values of rpm. At each speed, the corresponding dial reading is noted. The viscosity of the gel is obtained by multiplication of the dial reading and the factor given in the Brookfield viscometer catalogues.
- Spreadability: One of the criteria for a gel to meet the ideal quantities is that it should possess good spreadability. It is the term expressed to denote the extent of area to which gel readily spreads on application to skin or affected part. The therapeutic efficacy of a formulation also depends upon its spreading value.Spreadability (S) is expressed in terms of time in seconds taken by two slides to slip off from gel placed in between the slides under the direction of certain load. Lesser the time taken for separation of two slides, better the spreadability. It is calculated by using the following formula: S = ML/T where M = Weight tied to upper slideL = Length of glass slidesT = Time taken to separate the slides
- Extrudability study: The formulations are filled in the collapsible tubes after the gels are set in the container. The extrudability of the formulation is determined in terms of weight in grams required to extrude a 0.5 cm ribbon of gel in 10seconds.
- Skin irritation test: Guinea pigs (400–500 g) of either sex are used for testing of skin irritation. The animals are maintained on standard animal feed and have free access to water. They are kept under standard conditions. Hair is shaved from the back of guinea pigs and an area of 4 cm2 is marked on both the sides; one side would serve as control and the other side as test. Gel is applied (500mg/guinea pig) twice a day for seven days and the site is observed for any sensitivity; the reaction, if any will be graded as 0,1,2, and 3 for no reaction, slight patchy erythema, slight but confluent or moderate but patchy erythema, and severe erythema with or without edema, respectively.
- In vitro diffusion studies: The drug diffusion studies of the prepared gels can be carried out in Franz diffusion cell for studying the drug permeation release of gels through a cellophane membrane or a rat skin or pork skin membrane. Gel sample (0.5g) is taken and the diffusion studies are carried out at 37 ± l°C using phosphate buffer (pH 7.4) as the dissolution medium. At predetermined time intervals, known volume of sample is withdrawn, and each sample is replaced with equal volume of fresh dissolution medium. Then, the samples after suitable dilution are analyzed by analytical methods to determine the drug flux.
- In vivo studies: The pharmacokinetic and pharmacodynamic studies on suitable animal models can be studied after obtaining permission from institutional animal ethical clearance committee.
- Stability: The stability studies are carried out for all gel formulations by freeze–thaw cycling. In this, syneresis is observed by subjecting the product to a temperature of 4°C for one month, at 25°C for one month, and at 40°C for one month. After this, the gel is exposed to ambient room temperature and liquid exudates separation is noted.
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