60 TESTING
Another aspect of patch testing which is sometimes overlooked is that as well as looking at just the redness, which is the primary endpoint, we can also look at other consumer relevant issues such as spots, papules, scaling and itching that a product might cause the skin. So we get a wealth of information from this, and these are the sorts of things which consumers often report in the market where they have problems with a particular formulation. There are lots of different ways of doing patch testing. So for this research, we tried to standardise as far as possible on the protocol with four applications of the product under occlusion and we examined the skin over five days, including the day after the product was last applied to the skin. There are some variations, depending on the product. We compared clinical scoring to erythema and skin temperature measurements as well.
Results and discussion
The first experiment was to take well-known surfactants of varying recorded irritancy potential and see how the two systems compared in vitro versus in vivo to make sure that we had a legitimate scientific method that we could apply. Figure 1 shows the initial in vitro evaluation of the test using the well-known surfactants. These are SLS, SLES, CAPB and also a novel surfactant. Novel surfactants, including biosurfactants, are really coming to the fore now in the market and many of them are marketed as particularly mild and they are more expensive than their traditional counterparts. So we think an important application of this work is looking at how mild those novel surfactants actually are. On the vertical axis is the percentage of viability of the skin cells in the skin model.
33.0 32.5 32.0 31.5 31.0 30.5 30.0 29.5 29.0
*
A B C D E Control
Rank order of irritancy E (SLS 70%)
C (SLES/CAPB blend 3) A (SLES/CAPB blend 1) B (SLES/CAPB blend 2) D (SLES/CAPB blend 4) Control (E45 Cream)
Cumulative irritation scores
12 4 3 0 0 0
Figure 5: SLES/CAPB blends. Relationship of clinical scoring to skin temperature (day 5).
This is a measure of damage to the skin cells over time. Over time, as the profile decreases, this denotes there is actually cell damage occurring and they are less viable. We calculate from these graphs what time
ET50 determination of TA1-4 120 100 80 60 40 20 0 1 Time (h) Figure 6: Mild surfactant formulations. PERSONAL CARE EUROPE February 2020 10 100
Rank order of irritancy using linear extrapolation and logic equation
ET50 1.65
D > C > A 8.33
B 8.57
A (“mild” formulation 1) B (“mild” formulation 2) C (“mild” formulation 3) D (“standard” formulation)
8.94
that was taken to reduce the viability to 50% compared with the untreated control. This the ET50 value. Here we were able to identify a rank order of irritation using a linear extrapolation model and have shown
Percentage of viability relative to untreated control
Skin temperature (C)
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