How To Measure In Vivo UVA and UVB Blocking Sunscreens and Cosmetics on Human Skin

Transcription

1 How To Measure In Vivo UVA and UVB Blocking Sunscreens and Cosmetics on Human Skin Jeffrey L. Taylor, Ph.D. Jillian F. Dlugos HUMAN HEALTH ENVIRONMENTAL HEALTH 2015 PerkinElmer

2 Skin Related Spectral Regions This is a plot of the important spectral wavelength regions for sunscreen analysis. The visible region from 800 nm to 400 nm (in green) are the wavelengths of light we can see with the human eye. The Ultra- Violet region is separated into UVA (yellow from nm), UVB (red nm), and UVC (gray below 290 nm) The wavelengths from 400 nm to 250 nm are the business end of sunscreens and SPF rated cosmetics. UVA rays account for up to 95 percent of the UV radiation reaching the Earth s surface. Although they are less intense than UVB, the UVA rays are 30 to 50 times more abundant. UVB wavelengths, the chief cause of skin reddening and sunburn, tends to damage the skin s more superficial epidermal layers. It plays a key role in the development of skin cancer. Most UVC rays are absorbed by the ozone layer and do not reach the earth. 2

3 The Rayleigh Scattering Equation Depicted here is the generalized form of the Rayleigh light scattering equation for particle scattering intensity. There are numerous factors that contribute to scattering intensity; the distance to the particle, the scattering angle, and the refractive index of the particle. However, we will be primarily concerned with the two factors of particle size and the wavelength of incident light. The wavelength of incident light (shaded green in the equation above) relates to the scattering intensity by the inverse 4 th power of the wavelength. Nanoparticle scatter is highly dependent on incident light wavelength. The shorter wavelengths (ultra violet or blue light) are scattered much more intensely than longer wavelengths (red light). This is why the sky appears blue, because of intensely scattered blue light from the gas molecules of the atmosphere. The particle diameter is also important (shaded red) where the scatter intensity is related to a direct proportion of the 6 th power. In addition, large particle (larger than the incident wavelength of light) scatter is not wavelength dependent. Wavelength component Particle diameter component Where: R = distance to the particle Θ = scattering angle n = refractive index of particle d = diameter of particle λ = wavelength of incident light 3

4 The 150mm Integrating Sphere Samples that are solids, powders, gels, or creams can be measured on a UV/Visible spectrophotomer equipped with a 150 mm ASTM standard integrating sphere. The optical diagram of a typical double beam integrating sphere can be seen below. The reference and sample beams are both measured in the sphere. For all measurements the reference bean strikes a Spectralon target. Scatter transmission samples are placed in front of the sphere; whereas, diffuse reflectance samples are placed on the opposite side port. The 150 mm integrating sphere fitted into the detector compartment area of the instrument. The scatter transmission port can be seen in the center of the picture on the left hand side of the sphere. The diffuse reflectance port is under the blue cover on the right of the sphere. 4

5 The Two Types Of Integrating Sphere Measurements Lets consider how a sunscreen sample can be measured. Such sample are typically prepared as a film on a quartz plate substrate. If the sample is placed in front of the sphere at the scatter transmission port, the transmitted and forward scattered light is collected. This is depicted in the picture at bottom left in blue and on the left in the gray area. If the same sample is placed on the opposite side of the sphere at the diffuse reflectance port, the diffusely reflected and backscattered light is collected. This is shown in the picture at top left in red and on the right in the grey area. Between these two measurements all of the scattered light should be collected and measured. Note that in both scatter transmission and diffuse reflectance measurements, light can be lost to absorbance of the sample as well. This is shown in the pink areas in the diagrams on the left. 5

6 The Spectral Properties Of Absorbance Component Sunscreens A scatter transmission spectrum displays the wavelengths of sunlight that pass through the sunscreen layer to reach the skin. Shown at right are four chemically similar sunscreens, with increasing SPF values, that use light absorbing molecules to accomplish UV blocking. Note that in the visible region that there is a somewhat uniform light blockage which is due to large particle scattering of the product s substrate emulsion. Below 400 nm is where the active ingredients start to absorb UV radiation. The higher the SPF value, the greater the active ingredient absorbance, the better the skin protection. Note how the 100 SPF sunscreen blocks almost 95% of all UV radiation below 400 nm. 6

7 The Spectral Properties Of An Absorbance Vs. Nanoparticle Scattering Component Sunscreens Another class of UV blocking agents are nanoparticles, usually of titanium dioxide or zinc oxide. These nanomaterials can be employed as active ingredients either by themselves or in combination with organic absorbing agents. The nanoparticles physically backscatter the UV radiation away from the skin, and because of their nano size, the scattering of short wavelength UV is much more intense than the longer wavelength visible light. This spectral plot displays a scatter transmission comparison of absorbing and scattering sunscreen agents of identical SPF values. Note the clear spectra differences throughout the UV and visible regions. The nanoparticle sunscreen displays a distinct curvature starting in the long wavelength visible which becomes much more pronounced in the UV region. 7

8 Yes, But What s The SPF Value On A Person???? The SPF values reported on sunscreen and cosmetic labels are obtained by scatter transmission spectra of the material on a quartz plate; however, how representative is that In Vitro measurement with respect to what is happening on human skin. The spectra of SPF agents can be measured, In Vivo, on human skin through the used of integrating sphere diffuse reflection spectroscopy. Stratum Corneum Human skin consists of three sub layers; the stratum corneum is a thin layer of dead cells employed for protection, the epidermis, and the dermis. Reflection spectroscopy penetrates and contains chemical information from all three of these skin sub layers. The epidermis is made up of only cells; however, the bottom of the epidermis contains specialized melanin containing cells called melanocytes which are instrumental in ethnic skin pigmentation and tanning. 8

9 The Spectroscopy Of Ethnic Human Skin The reflectance of skin is dominated by the chemistry of the dermis and is varied for the different human ethnic skin types. Changing amounts of hemoglobin and melanin lead to drastically different spectral shapes. The major epidermal chemistry originates from proteins (the amino acids tryptophan, phenylalanine, and tyrosine), DNA, melanin, and urocanic acid. The dermis contains nerves, blood capillaries, as well as cells. Additional dermal chemical constituents include oxygenated hemoglobin, deoxygenated hemoglobin, and bilirubin. Albino For transparent skin types, such as albino, the incoming light penetrates deeply into the dermal tissue. In the mid-visible region of the spectra the twin peak structure (540 nm and 580 nm) of oxygenated hemoglobin is clearly seen. The oxygen free form of hemoglobin, with a single peak at 560 nm, is buried under the oxygenated hemoglobin peaks. A broad combined oxygenated and deoxygenate hemoglobin peak is found at 425 nm. A small peak for bilirubin appears at 540 nm. All of the compounds are present because of the generous blood supply of the dermal tissue layer. For darker skin types epidermal melanin shields the dermal chemistry from penetrating light. 9 In %R spectra, absorbance peaks are minima Caucasian Hispanic African American

10 Comparison Nanoparticle And Organic Sun Blocking Formulations On Palm Let us now consider the spectroscopy of various UV sun blocking agents on human skin. Is there actually a change in palm skin diffuse reflectance as a function of sunscreen application? Note on the plot at right that the untreated palm (black line) has the highest percent reflectance value, the three sunscreens are considerably lower in the UVA/UVB region (box). One can see that the different sunscreens have very different wavelength dependent profiles; however the percent reflectance scale is not the best format to evaluate this data. Conversion to optical density should help. 10

11 Conversion Of Reflectance To Optical Density The Log(1/R) is the reflectance equivalent of optical density and gives a much better view of the spectral details for skin-sunscreen reflectance. The boxed UVA/UVB region now displays this better separation. When applied to skin the Bull Frog SPF 50 and the lower Sport SPF 30 appear to have very similar blocking profiles in the UVA/UVB region. Both demonstrate markedly higher optical densities than the untreated palm. Of further interest, the nanoparticle sunscreen appears to not be as good a blocker, on the skin, as its similar SPF value organic absorbing counterpart Sport sunscreen. The aggregation of nanoparticles into larger, less efficient, UV scattering agents could be a reason for this diminished performance. 11

12 Reflectance Spectra Of Low SPF Tanning Oils On Palm So how sensitive is this In Vivo diffuse reflectance technique utilizing human shin as a substrate? In order to find out we measured very low SPF sun tanning oils. With these type of agents it is even possible to obtain the oil without any active sun blocking agents. What is interesting to note here is that all the oils have a lower percent reflectance than the untreated palm. It would appear that even the simple oil substrate, without any active ingredients, still offers some minor protection. There also does not appear to be much of a linear correlation with increasing SPF values in the UVA/UVB spectral region (box). 12

13 Conversion Of Reflectance To Optical Density Again, conversion to optical density expands the differences and makes them easier to see. In the UVA/UVB region (boxed), all of the oils have a higher optical density than the untreated palm. Even the oil lacking any active sun blocking ingredients offered some minimal protection. Indeed, there was only a modest difference between the SPF 4 and the SPF 0 oil. What is notable is that even these small differences at low SPF values are measurable on skin spectra. These types of In Vivo skin measurements are important in understanding the efficacy of sun block formulations under normal product usage on human skin. 13

14 What Can Sunscreen Skin Spectroscopy Do For You? 1. Evaluation of true SPF values when applied to skin (and not a quartz plate) 2. Measurement of optimal dosage amounts and application techniques 3. Product R & D formulation methodologies 4. Quality Control evaluation methodologies 5. Aging of applied sunscreen under different environmental conditions 6. Comparisons of different functional sun block active ingredients (scattering nanoparticles versus absorbing organic agents) 14

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