116 ANTIPERSPIRANTS
has a large impact on quality. The industry uses a range of drying technologies to produce plant extracts including spray, drum, tray, oven, microwave, and freeze- drying. In general, higher drying temperatures and longer exposure to heat increase oxidisation. Colour and odour of the dry extract largely depends on the extraction process, plant species, and plant part. However, brown to black extracts are an indicator of oxidation, especially if light coloured batches were previously produced. Excipients like starches or minerals may be added to dry extracts to reduce caking, but a high concentration of excipient can limit the amount of extract that can be added to formulas, just like liquid extract carriers. Also, we have observed a reduction in phytochemical yield (unpublished data) in the presence of certain ‘inert’ excipients, possibly due to irreversible adsorption or other chemical interactions.22
One significant hurdle of
using dry extracts is difficult dissolution. Experimental trial and error is required to find the optimal solvents, heat, and agitation for dissolution.
Antiperspirant formulation: stability and colour
Many cosmetic chemists are aware of the unique challenge posed by natural products in cosmetic formulations. High batch temperatures must be avoided to reduce phytochemical degradation. When using waxes that require high melting temperatures, the extracts could be incorporated at the end of the batch, after the temperature has dropped. Even after extracts or pure phytochemicals are fully dissolved and incorporated into the formula, they can precipitate once the batch cools or after a longer period of time. They can cause an emulsion to split over time or lose viscosity. They can react with packaging and frequently change the formula colour from light to brown. Although stability problems can be anticipated, there is not always an easy way to avoid the problem. A rigorous experimental process is usually required to find a formula that optimises both phytochemical and formula stability. Since most plant extracts are coloured, a pure white formula may not be possible without sacrificing clinical efficacy or formula sensory quality. Many consumers are wiling to adjust their expectations for formula aesthetics when purchasing a natural formula, as evidenced by the numerous green, yellow, beige, and other colours of skin creams on the market.
Efficacy testing As already mentioned, since antiperspirants are intended to reduce sweat production in the underarms, several
PERSONAL CARE EUROPE
Figure 4: Cloves (Syzygium aromaticum), a commonly adulterated fragrant spice crop used as a traditional antiperspirant ingredient (Photograph by Jorge Barrios, 2007).
clinical study approaches are available, including the similar protocols recommended by the FDA (USA) and Clearcast (UK). These protocols are gravimetric methods as they quantify the actual amount of eccrine sweat produced in the underarms during specific time intervals and under controlled conditions. In summary, after a washout period of at least 17 days in which no antiperspirant is used, baseline sweat collections are obtained from the panel lists in a controlled environment hot room, and those panellists that qualify are then treated with the test product under one axilla for typically three or four consecutive days. The other axilla serves as an untreated control, or a placebo can be used as the control product. Hot room sweat challenge is then repeated 24 hours after the last product application. The pre and post sweat collection data are then assessed statistically using the Wilcoxon signed rank test to determine sweat reduction effectiveness. These protocols establish that the formulation meets the statutory definition of an antiperspirant. Other, more sophisticated testing
options are also possible depending on how much claim support is desired or considered realistic for the product. For example, longer duration of efficacy, physical activity resistance, psychological stress protection, extreme heat resistance, night sweats, etc.
There is virtually no reliable animal or
in vitro test method for antiperspirant efficacy testing, which is a major hurdle to research and development. Therefore, the standard approach is in vivo testing performed on the body site where the product is intended to be applied. The gravimetric method is the most widely used option, particularly when quantitative sweat collections are used to measure sweat reductions. In general, the variability of antiperspirant clinical study results is increasing, and the reasons are not fully understood at present. Based on our knowledge and testing of natural extracts, they will exhibit less efficacy than the aluminum-based salts. So it is important to ensure that their wetness reduction performance can be measured with statistical confidence. Also, assessing small amounts of sweat at weak excretion rates is difficult, and requires experimental skill. Therefore, the following is recommended in order to design and conduct a successful clinical test of natural extracts: l Make sure that the baseline sweating values in both armpits are similar and not skewed. A minimum of 100 mgs per armpit is often used, and it is recommended that the spread between the lowest and highest sweaters is at least 600 mgs.
l Panellists should not be enrolled if they have a history of being pro-perspirers, i.e., sweating more after product treatment.
April 2020
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