MimSkin® is commercially available
30-Nov-2007
MimSkin® is commercially available for measuring the UVA ratio.


Labsphere Ultraviolet Transmittance Analyser SPF Report Elaboration

Summary:
In vitro sunscreen evaluations:
Instrument:
Labsphere UV-1000 SPF Analyser calibrated weekly using filters which take into account previous problems which were not identified by validations procedures.  Checked against two (2) independent spectrophotometers at the University of Sydney (Dept. Applied Physics, and Dept. Crop Sciences) both with Labsphere integrating spheres.
Parameters measured:
SPF
UVA ratio (UVA cut-off set at 320 nm)
Critical Wavelength
Absorption and Transmission profiles displayed

Substrate:
“Mimskin®”: 1.5 mm x 40 mm quartz glass plate profiled (using technology used in the computer chip industry) with the topography (pattern and groove detph) of human skin derived from casts of human test subject back skin. Prior to application of product for test the MimSkin® is washed in saline to neutralise any accumulated charge.
The countour of this substrate allows for a complete flushing off and removal of material between tests using a combination of solvent washes.
Application:
2 mg / cm2 applied with finger stall lightly (lay-on) and firmly (rub-in) for uniform thickness.  With 15’ dry time.  Blanked against wetted Mimskin without product.
Photodegradation:
Air cooled 400W Metal Halide lamp filtered with 1..0 mm WG 320 filter.  Spectrum closer to sunlight than xenon arc, including UVA, visible and infra red.  Temperature monitored and stable at 25 degrees after 5’. Output intensity mw/cm2 similar to 150W xenon arc at skin surface.  Irradiation times 0.5 in vivo SPF. The spectral irradiance was measured 250-400nm using a computer-controlled McPherson triple grating-monochromater with a prism disperser.  A calibrated tungsten-halogen lamp was used as the reference source.

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The Labsphere instrument (Warsash Scientific) represents a state of the art device for analysing sunscreens in vitro.  It has been successfully developed with a view to overcoming the problems encountered with rival instruments, such as the Optometrics SPF Analyser.


UVA Ratio and Star Category

The above terms refer to “The Boots Star System” of evaluating the UVA protection, or Broad Spectrum capability of a sunscreen.

[There are many proposed methods for evaluating Broad Spectrum capability.  Three methods are detailed in the Australian Standard (AS/NZS 2604:1998).  While these methods have some relative utility, their meaning with respect to skin protection from UVA is unclear.  Validation and inter comparison of these methods has not been done, and together with other proposed methods, like The Boots Star System, they are all under review internationally.  It is noteworthy, that the recently released US FDA Final Monograph on Sunscreens does not include any Broad Spectrum test methods.]

Brian Diffey from the UK invented The Boots Star System based on a well argued rationale that the closer the UV absorption of a sunscreen approximates a neutral density filter, which is to say, absorbs equally at all wavelengths between 290 – 400 then the greater our confidence that the SPF performance is related to an overall reduction in UV reaching the skin, rather than a reduction in the erythemally effective (sunburning) radiation only.  

Contrary to the widely held view that the SPF rating only refers to UVB protection, solar simulators used in sunscreen testing also emit UVA-II (320 – 340 nm) and some UVA-1 (340 – 400 nm).  Therefore the SPF rating already accounts for protection from erythemally (sunburn) effective UV in the UVB and UVA-II regions. (It is noteworthy that the action spectra for non-melanoma skin cancer in humans and hairless mice matches very closely, the erythemal action spectrum.)  Diffey’s concern, shared by others, was that the skin is naturally adapted to a particular spectrum of UV / sunlight, which the application of a sunscreen will change.  We do not fully understand the impact of such changes, especially with respect to melanoma.  The most obvious concern remains that while sunscreen users may be protected from sunburn they may, at the same time, receive a different spectrum of sunlight than that to which the skin is naturally adapted, and in particular, higher doses of UVA.  

While The Boots Star System is not without its own drawbacks (see below), it does address this problem.  Calculations of UV absorbance values at 5 nm intervals allows the further calculation of a UVA Ratio.  This Ratio is that of the total absorption in the UVA to that in the UVB. The closer this ratio approaches 1.0 or unity, the better the approximation to a neutral density filter, and a “flat” profile of absorption/transmission.   Another way of putting this is to say that the closer the ratio approaches unity the better the sunscreen protects only quantitatively, without changing qualitatively the spectrum of sunlight reaching the skin.  



The Star Rating reflects this concept, thus:

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UVA Ratio      Star Category Category Description

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0.0 tp <0.2 too low />
0.2 to <0.4 />
0.4 to <0.6 />
0.6 to <0.8 />
≥ 0.8 **** MAXIMUM

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NB  When interpreting the Boots Star System it is important to understand that the rating refers only to the qualitative aspect of UVB:UVA protection and not to the quantitative.  Thus an SPF 30 product with only three (3) stars may have more (re % absorption/transmission) UVA protection than an SPF 15 product with four (4) stars, but the relative protection in the UVA and UVB regions will be more equal in the SPF 15 product.

As stated above, none of the broad spectrum methods have been compared in a way that enables an unequivocal interpretation of their meaning with respect to each other, or with respect to biological impacts.  A useful approach has been to undertake a battery of UVA tests where the first criterion is that the product must pass them all, and the second that where products are compared they are scored for each type of test.  

Another test performed by the Labsphere SPF Analyser is the Critical Wavelength test which is advanced as an alternative to the Boots Star System.  The APTF recommends that it is considered as an additional, rather than alternative test.  The Critical Wavelength is that wavelength below and including which 90% of the total UV is absorbed.  Designed to answer a rather more simplistic than simple question as to whether a sunscreen product offers any UVA protection, if the critical wavelength falls below 340 nm then no UVA protection is granted.  If the critical wavelength falls between 340 nm and <370 nm, then some uva protection is granted; and if the critical wavelength is> 370 nm then the product is considered to have MORE UVA protection and may be considered BROAD SPECTRUM.

A problem with all the Broad Spectrum tests is that they are in vitro tests. The reproducibility and accuracy (with respect to in vivo human skin testing) of these in vitro tests is problematic and not only depends upon the substrate used (of which there are many alternatives) but also on the operator performing the test, and the manner of application of the test product to the substrate.    Many, if not most sunscreens will also change their characteristics following exposure to UV.  Once again, methods of pre-UV exposure of in vitro test products have been proposed, but the evidence supporting them has not been sufficient to gain universal acceptance.  The APTF undertakes a photodegradation step using a filtered (WG320) metal halide lamp which is air cooled and reproduces the entire spectrum of sunlight.  Products are exposed for a dose equivalent to half their in vivo SPF.

The APTF has endeavoured to address the above problems with in vitro testing in a systematic and scientific way.   The APTF has, and is continuing the development of its own in vitro substrate with characteristics approximating human skin.  Dubbed “Mimskin” the substrate is being developed in collaboration with Departments of Applied Physics and Electron Microscopy, at the University of Sydney.  

It has been found that the method of application of sunscreen to human skin (with the amount, or rate of 2mg/cm2 remaining constant) can affect its performance (SPF).  This is not surprising given the important rheological and thixotropic properties of the emulsion systems in which the “active” UV filters are dispersed.  It is upon these properties that the
final dispersion of actives, and uniformity of sunscreen film depends, and has a crucial impact on the SPF. It is largely as a result of understanding these properties that formulators can design products with very high performance capabilities but with relatively low active concentrations.   In practical terms, the in vitro Labsphere analysis undertaken by the APTF  attempts to examine the behaviour of a sunscreen under two application conditions:  a “lay on” or light application, and a firmer “rub in” method of application with and without photodegradation.

Over two thousand Labsphere analyses have been undertaken using prototype Mimskin substrates. Both Standard Reference Product sunscreens (included in the Australian Standard) with known SPFs and other sunscreen products tested on human skin have been repeatedly tested and compared.   Reproducibility using the Mimskin substrate for UVA ratio evaluation is within two decimal points. 

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Gavin E. Greenoak
Managing and Scientific Directorp>