Formulating Considerations
We are often asked to comment on our experience with the measurement of SPF and the expected vis measured SPF variability, so here is a brief overview.
Overview
During the In vivo determination of SPF, expected or nominated SPF can be under or over-estimated significantly due to a number of factors, which are listed below.
Higher SPF does not directly correlate with increased content of actives. Conversely, proportionate decrease of active percentages does not decrease SPF in a linear relationship. High SPF is primarily achieved through a combination of the contribution of each active plus what can be substantial synergy of combinations of actives.
In vivo SPF determination is a measurement of the performance of a formulation, not just the contribution of individual actives. A large number of factors can impact on the experimentally determined compared with the theoretical or expected SPF number. Whilst some test protocols require that the expected SPF be included in the test report, the only valid reason for providing this to the test facility is in order to set a target range for initial exposures onto test subjects. Our clients tend to be conservative in setting of this number, as the result of over-enthusiastic estimation can be burns to the backs of the test volunteers.
Synergy from combinations of actives
Whilst individual sunscreen actives each contribute to SPF, substantial synergistic effects can be obtained with well selected combinations of active. This is particularly so when inorganic actives are included. This effect is positive in terms of increasing the in vivo SPF result. As inorganic actives reflect and scatter UV light, as well as absorbing it, it is not easy to predict this impact in combinations with organics.
Synergy from the base
If the excipients include other inorganics, such as pigments, then the effect is also to increase SPF. Again, the inorganics will interact with the UV light and act as “pseudo-actives”.
Film Thickness
The in vitro SPF test is conducted in a protocol which requires drying down of a film of the test product which has been applied at a controlled rate (2 mg/sqcm). Test products can vary in the content of volatile component (water, ethanol. volatile silicones etc) so that the final film thickness which is tested can vary from 20 microns for volatiles free formulations, such as sticks and ointments, down to 4 or 5 microns for alcohol based sprays.
Discontinuity of Film
Formulations which do not apply evenly, or include film formers which do not dry down to an even film will produce SPF values which are lower than expected or bring about wide variability in SPF results between test subjects.
Insolubility of Actives
Whilst the completed formulation can show full solubility of actives, this may not be the case once the film is dried down. In this case, SPF will be lower than expected. This formulation fault can usually be detected by microscopic checking.
Photostability
Whilst the majority of actives are UV stable for the period of the SPF test, some are well known to degrade over this period of intensified UV light flux. The SPF test takes this into account. However, inclusion of a stabiliser may be a more cost effective option for enhancement of SPF yield.
Changes during transport or storage
Variability in SPF can occur if the sample has been subjected to temperature variability. This effect may or may not be reversible. A replacement sample might be needed.
Dispersion of Inorganic Actives
Where TiO2 and ZnO are incorporated into the formulation, even and energetic dispersion of these actives will help to significantly increase and maximise SPF, as well as providing consistent SPF results. Where aggregation occurs, SPF is likely to be inconsistent or low.
Physical Stability
Formulations that are not physically stable will generally not produce consistent SPF. Even though “shake the bottle” is actioned before each testing, this does not always achieve an even remix of the sample. Additionally, spread will not be even.
