Two New, Complimentary Tests for Depilatory Products

Researchers at TRI, Leila Sorrells, and Dr. Xuzi Kang have developed two, complimentary in vitro methods for testing depilatory products' efficacy and skin-damaging effects. These methods are now available to our clients for performance and claims testing studies.

Chemical depilatory products are widely used for hair removal, and work by cleaving the disulphide bonds in hair proteins, causing the hair to break and separate from the skin. These products usually contain reducing agents such as sulphides and thioglycolates, delivered in a high pH (pH > 10) formulation base. Unfortunately, the reducing agents can disrupt both hair and skin structures. Therefore, a balance must be struck between breaking away unwanted hairs and disrupting the stratum corneum, the skin’s outer barrier. Too much skin damage might lead to raised levels of skin irritation and the penetration of harmful substances.

A major challenge for formulators developing these products is screening their efficacy in vitro. Leila and Xuzi, therefore, have developed two, complimentary in vitro test methods to test (1) the hair removal efficacy of these products, and (2) their effects on skin barrier function.

Evaluation of Hair Removal

The efficacy of hair removal can now be tested using a skin-like model with embedded hair fibers. The skin model consists of fully cured, 8 mm thick silicone. Hairs are embedded individually using a punch needle technique with 6.34 mm of exposed hair left showing. The skin models are heated at 40 degrees C for 1 hour to mimic natural body heat before the application of depilatories. After product removal, the number of lost hairs is recorded, including partial losses.

This model allows for the repeated testing of depilatory products without the need for human subjects. It can be customized by skin model size, hair density, hair length, and product application/removal.

Skin-like model with embedded hairs before (left) and during (right) testing.

Validation test data shown below show how hair removal from different products can be measured at different time-points. The use of hair of different diameters also shows how treatment effects can be evaluated for different target consumer groups. The data below show that more hairs are lost after 10 minutes than after 5 minutes, and that hair removal was faster with finer hairs. Hair removal from the two products tested was similar.

Visual representation of hair fiber removal from product 1 (regular) and product 2 (for sensitive skin) 5 minutes after application on both normal and thin fibers. Dark green represents total fiber loss, light green represents partial fiber loss, and gray represents no fiber loss.

Visual representation of hair fiber removal from product 1 (regular) and product 2 (for sensitive skin) 10 minutes after application on both normal and thin fibers. Dark green represents total fiber loss, light green represents partial fiber loss, and gray represents no fiber loss.

Average fiber loss percentages from product 1 vs product 2, 5 minutes and 10 minutes after application on thin and normal fibers for two replicates.

Evaluation of Skin Barrier Function

Skin permeation tests can measure how much different depilatory products can temporarily disrupt the epidermis of our skin. This allows the mildness or harshness of test products to be compared.

Oxybenzone (OXB), a UV filter used in sunscreens, was chosen as the test permeation agent in the new assay. OXB, unlike most UV filters, mimics the structure of our hormones and can penetrate the skin, thus potentially, having harmful effects. Sunscreen formulators have tried different ways to prevent OXB from permeating the skin. However, worryingly, using a depilatory product could cause temporary disruption to our epidermis and increase the permeation of the sunscreen agent.

The Franz cells used for the skin permeation test.

Skin penetration of OXB was measured using glass Franz diffusion cells. Human cadaver skin was pre-treated with depilatory products before mounting to the cells. A commercial sunscreen product containing OXB is applied on the skin surface in the donor chamber. The concentration of OXB is measured at 0, 3, 6, and 24 hours after sunscreen application using HPLC.

Results from validation studies showed that depilatory product 1 (regular) significantly impacts the epidermis while product 2 (for sensitive skin) does not. This method can be used to compare different formulations and determine whether they are friendly to our skin.

Skin penetration of OXB after different depilatory treatments. Treatment 1 = regular depilatory product, Treatment 2 = mild depilatory product, Control = no treatment

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For more information about these and other test methods contact us.