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:
EVALUATION
OF 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:
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.
Advantages:
·
It
is a simple method.
·
It
is inexpensive method.
Disadvantages:
·
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.
Advantages:
·
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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