Evaluation of Creams

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Chapter: Textbook of Cosmetic Formulation : Creams

Due to the use of number of additives, it is necessary to evaluate the effectiveness of the skin products. Evaluation is carried out by two methods. They are:



Due to the use of number of additives, it is necessary to evaluate the effectiveness of the skin products. Evaluation is carried out by two methods. They are:



1.           In-vitro methods


2.           In-vivo methods.



1.           In-vitro Methods: Tests are carried out to know the performance of the products. These tests also help in evaluating, new product concepts. Various instruments have been developed by the investigators to know the effect of temperature and humidity on the skin. Since, the softness of skin is directly related to the water content present in it. The effects of temperature and humidity on skin are studied by observing the changes in the mechanical properties of the stratum corneum. The instruments help in evaluating moisturizing capacity of the products and screening of raw materials used in the formulation.



Various techniques or instruments involved in in-vitro method are:


(a)         Tensile strength tester


(b)          Hargen's Gas Bearing Electro dynamometer (GBE)


(c)          Occlusive potential of ingredients.


(d)          Gravimetric analytical method.


(e)          Thermal analytical methods.


(f)           Electrical methods.



(a)           Tensile Strength Tester:


This method is useful for determining the tensile property of the excised stratum corneum of the skin. It provides information on the water content present in stratum corneum and also acts as a screening device for moisturizing ingredients. The stress or strain characteristics of stratum corneum obtained from various sources can be studied by using this instrument (i.e., tensile strength tester), and it also helps in knowing the effects on stratum corneum passed through various treatments.



(b)          Hargen's Gas Bearing Electro Dynamometer (CBE):


This instrument is helpful in determining and monitoring the viscoelastic behavior of the skin. It also helps in determining the effects on the skin by passing it through various treatments. It is used both as in-vitro and in-vivo test.



Disadvantages: The instrument lacks sensitivity sometimes.



(c)            Occlusive Potential of Ingredients:


The occlusive potential of raw materials or ingredients used in the formulation of skin cream, are determined by knowing the water diffusion rate. Membranes used in this method can be stratum corneum of neonatal rat or artificial membrane.



(d)          Gravimetric Analytical Method:


This method is helpful in establishing relationship between water content present in stratum corneum and relative humidity. This is done by suspending hits of callus (undifferentiated mass of cells) in different dilutions of sulfuric acid. Then the weight of the sample (i.e., callus) is determined by using sensitive electro balance. This weight of the sample is taken after it reaches an equilibrium state (i.e., one week). After this the water content is determined by subtracting dry weight of the tissue and weight of the sample which has attained equilibrium state, (i.e.. equilibrium value).



Water Content (Stratum Corneum) = Dry Weight of the Tissue- Equilibrium value.



This method is also useful in determining sorption and desorption phenomena which takes place in test stratum corneum after passing through various treatments.





·                               It is a simple method.


·                               It is inexpensive method.





·                               It is a time consuming method.


·                               It requires lot of labour efforts.



(e)                       Thermal Analytical Methods:


Various thermal analytical methods like Differential Scanning Calorimetry (DSC), Thermo-Mechanical Analysis (TMA) and Thermo-Gravimetric Analysis (TGA) are used. They are used in order to provide information about the effect of temperature which causes changes in the stratum corneum. These methods also provide information on physical properties and components of stratum corneum, but are not popular in determining the moisturizing efficacy.



(f)             Electrical Methods:


Various electrical properties such as capacitance, impedance and dielectric constant are measured by electrical methods which provide information about the variations in the water content present in the stratum corneum of the skin. One such method is four-point micro electrode method. This method helps in measuring the resistivity (resistance power) of the excised stratum corneum. It also helps in measuring electrolyte levels and water binding capacity of stratum corneum. This method is considered to be more sensitive and reliable than another electrical method except for measuring moisturizing efficacy.



Advantages Of In-Vitro Method:


·                               It provides data which is less variable.


·                               Environment can be easily controlled by this method.


·                               Large number of products are easily and rapidly evaluated or assessed.



Disadvantage Of In-Vitro Method:


Simulated and artificial environment which is not close to the real condition.



2.           In Vivo Methods:


In-vivo methods are helpful in providing information on hydration or moisturization process of the skin. Various methods are:


(a)                       Transpirometry


(b)                      Scanning electron microscopy (SEM)


(c)                       Optical microscopy and macro photography.


(d)                      Skit friction


(e)                       Sensitivity tests.



(a)          Transpirometry: This method helps in measuring Trans Epidermal Water Loss (TEWL) of the skin which helps in providing information on moisturizing potential. In this method, skin surface of the fore arm is used, in this surface, a collection chamber is attached through which nitrogen or stream of air of known relative humidity is introduced. The water vapours leave the surface of the skin and enter into the collection chamber. Then the gas present in the chamber carries water vapour to suitable detection devices like dew point, hygrometry, thermal conductivity or gas chromatography. This method is useful in detecting three sources of water i.e., eccrine sweat transepidermal water loss and stratum corneum water and also detects the water supplied by cosmetic products.



Note: Detection of eccrine sweat is troublesome due to its volume and sporadic nature .The excessive loss of eccrine sweat can be prevented by either conditioning the test in a cold temperature i.e.,. 20° C or by giving anticholinergic which help in avoiding excessive sweating.



(b)                       Scanning Electron Microscopy (SEM): Skin replicas are used in this method to know the effects of topical preparations on the skin conditions i.e., dry and rough skin (good substrate). Polyethylene beads are melted on the surface in order to get impression of skin on the silicon rubber. This rubber is then metalized to prevent charging and observed under the microscope. This method provides surface architectures which bears no resemblance to artificiality and hence effects are easily determined.



(c)            Optical Microscopy and Macro photography: with the help of low magnification photography, stereomicroscopic tests, biopsies of skin surfaces and microphotographs, the changes in the dry rough skin are observed before and after application of moisturizers. They also provide information on moisturizing potential preparations.



(d)           Skin Friction: Damp (slightly wet) skin has high friction surface compared to wet and dry skin. Investigation of friction surface shows the relation between the effect of hydration on stratum corneum and process of moisturization. Frictional properties are also related to elastic nature of skin and helps in evaluating the performance of the product.



(e)          Sensitivity tests: these tests are performed in order to measure the irritancy, sensitization potential and phototoxicity of the skin.



(i)           21 Day (or 3 Weeks) Cumulative Irritancy Patch test: In this test, the test material is applied daily on the same site i.e., fore arms of 24 subjects under the occlusive tapes. Then score are recorded daily. This test is carried out for 21 days or until irritation produced on the fore arm. This irritation is noted as maximum score. The core ranges from 0-4, where '0' score indicates no visible reaction on typical erythema (redness of the skin dale to dilation and congestion) of capillaries) and '4' score indicates erythema with edema and vesicular erosion (erosion of vesicles). This test can also be carried out with fewer subjects and less application of test material.



(ii)          Draize-shelanski repeat-insult Patch Test: This test is carried out on 100 individuals to measure the extent of sensitization and irritation caused by the product to the skin. The test material is repeatedly applied on the same site under occlusion for 10 alternate days. After a gap of 7 days, test material is again applied to a new site only for 24 hours. The scores are recorded after the removal of occlusive tape. Then the score is again recorded after 24 hours. The score ranges from 0-4, where '0' score indicates no visible reaction on erythema and '4' score indicates erythema with edema and vesicular erosion.



(iii)          Kligman “Maximization" Test: This test is used to measure sensitizing potential of the product, when it comes in contact with the skin. The test material is applied on the site by using an occlusive tape for a period of 48 hours. Then the site is treated with sodium lauryl sulfate solution on each exposure under occlusion. After a gap of 10 days, the test material is again applied on a new site under occlusion for a period of 48 hours, which is then treated with solution of sodium lauryl sulfate.





·                               The test consumes less time.


·                               The test materials are applied on fewer subjects only.

·                               Sodium lauryl sulfate solution is used as it helps in detecting weaker allergens easily and rapidly.


Sensitivity tests are also suitable for detecting weak irritants and contact sensitizers. If the tests give positive results then the product should not be immediately discarded or considered unsafe. The actual risk arises if the product is used for longer time or the product concentration is more or on the condition of the skin.


Example: Benzoyl peroxide is a potent sensitizer which is used in Draizeshelanski and Kligman maximization test. But, it still produces low sensitization in case of patients suffering from acne.


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