Evaluating the efficacy of leave-on cosmetic compositions to protect skin from pollutants
Disclosed is a method of determining efficacy of a cosmetic composition to inhibit an atmospheric pollutant from contacting skin, comprising the steps of: (i) filling a sample holder of known dimensions with known amount of said composition to form a layer of defined thickness therein, where said sample holder is amenable to fluorescence-based assay; (ii) depositing, on said layer, a known amount of a model fine particulate matter which resembles an atmospheric pollutant at least in size and which comprises a material responsive to fluorescence microscopy which is excitable by a wavelength (λ ex ) and which emits radiation (λ em ) where (λ ex )≠(λ em ); (iii) irradiating said composition and said model fine particulate matter by said λ ex and recording intensity of fluorescence at λ em a plurality of times for a defined period during which said model fine particulate matter interacts with said layer; and, (iv) determining, corrected intensity of fluorescence at λ em by the following equation; corrected Intensity=recorded intensity/average transmittance where said average transmittance is the average of the transmittance of the composition measured at λ ex and λ em determined by Beer-Lambert law by measuring the absorbance at λ ex and λ em , where the corrected intensity is inversely proportional to said efficacy of said composition.
1. A method of determining efficacy of a cosmetic composition to inhibit an atmospheric pollutant from contacting skin, comprising the steps of:
(i) filling a sample holder of known dimensions with known amount of said composition to form a layer of defined thickness therein, where said sample holder is amenable to fluorescence-based assay;
(ii) depositing, on said layer, a known amount of a model fine particulate matter which resembles an atmospheric pollutant at least in size and which comprises a material responsive to fluorescence microscopy which is excitable by a wavelength (λ ex ) and which emits radiation (λ em ) where (λ ex )≠(λ em );
(iii) irradiating said composition and said model fine particulate matter by said A ex and recording intensity of fluorescence at λ em a plurality of times for a defined period during which said model fine particulate matter interacts with said layer; and,
(iv) determining, corrected intensity of fluorescence at λ em by the following equation:
corrected Intensity=recorded intensity/average transmittance
where said average transmittance is the average of the transmittance of the composition measured at λ ex and λ em determined by Beer-Lambert law by measuring the absorbance at λ ex and λ em , where the corrected intensity is inversely proportional to said efficacy of said composition.
2. The method as claimed in claim 1 , wherein said steps are also performed with a reference cosmetic composition, where said efficacy of said composition is the difference between efficacy of said cosmetic composition and said reference cosmetic composition.
3. The method as claimed in claim 1 , wherein said model fine particulate matter resembles PM 2.5 or PM 10 at least in size.
4. The method as claimed in claim 1 , wherein said fluorescence-based assay is fluorescence microscopy or fluorescence spectroscopy.
5. The method as claimed in claim 1 , wherein said defined period is from 1 to 3000 minutes.
6. The method as claimed in claim 1 , wherein slope (k) of a plot of intensity against time in the range of a linear increase is indicative of short-term efficacy of said candidate cosmetic composition and the nature of the plot reaching a plateau is indicative of its longer-term efficacy.
7. The method as claimed in claim 6 , wherein data is collected at intervals of every 2 to 10 minutes starting from T=0.
8. The method as claimed in claim 6 , wherein slope (k) of the linear fit of the data between T start and T end , indicates the penetration speed of said model fine particulate matter into said layer, where T start is the time when fluorescence intensity begins to increase, and T end is the time when fluorescence intensity reaches plateau.
9. The method as claimed in claim 6 , wherein from the slope (k) an extent of block efficacy is calculated using the formula:
Block efficacy=1− k cosmetic composition /k reference cosmetic composition ,
where said block efficacy is indicative of how better the cosmetic composition is as compared to the reference cosmetic composition.
10. The method as claimed in claim 1 , wherein said model fine particulate matter comprises a synthetic polymeric material, a natural polymeric material, a water-insoluble salt, a mineral, a metal, an alloy, glass or a mixture thereof.
11. The method as claimed in claim 10 , wherein said model fine particulate matter is in the form of beads comprising polystyrene and a fluorescent material.
12. The method as claimed in claim 1 , wherein said known amount of said composition is that amount which is sufficient to form, upon drying for 12 hours, a layer of 10 to 1000 μm in the sample holder.
13. The method as claimed in claim 1 , wherein said sample holder is a cuvette or a microplate having plurality of sample wells arranged in a matrix where each well serves as a sample holder.
14. The method as claimed in claim 1 , wherein said sample holder has a length of 0.5 to 2 cm.
15. The method as claimed in claim 11 , wherein said fluorescent material absorbs and emits radiation of wavelength 400 to 800 nm.