Tests of Cleansing Efficiency

A personal cleanser’s ability to clean the skin is dependent on a number of factors including its (surfactant) composition, its in-use concentration, the application time and method, the soil load, and the surface characteristics of the particular skin being cleaned. The past several decades saw a change in how personal cleansers are viewed, the focus shifting from their role as skin cleansing aids to their role as agents with a potential to damage skin (6). Thus, while numerous publications describing methods to assess and compare personal cleansers’ skin compatibility appeared in this time frame, in-use cleansing performance was largely ignored. However, this question deserves consideration given the greatly expanded range of personal cleansing products now available, both in terms of forms and ingredients.

Weber described a method to assess cleansing that employed a device designed to wash forearm skin in a controlled manner (7). A colored model soil was applied to forearm skin of normal subjects and three subject groups with psoriasis, atopic dermatitis, or non – lesional skin disease. Four cleansing bars ranging from full soap to synthetic detergent (syndet) were tested on each subject group. The amount of color on skin was measured photometrically before and after cleansing. Weber found differences in cleansing, not only between the cleaner types but also between subject populations. Skin cleansing was in all cases best with the syndet bar, poorest with the soap. The measured cleansing response was greatest in psoriatics, which could reflect soil removal by detergency and the mechanical removal of stained psoriatic plaques by the washing process. Cleansing was poorest in atopics, which the author attributed to higher skin dryness (roughness) and greater adherence of the model soil.

Schrader and Rohr also used a device to assess personal cleansers’ skin cleansing ability under controlled conditions (8). Their device was designed for use on the forearm, with a dual-chamber arrangement for simultaneous testing of two products. Agitators with felt inserts rested on the forearm surface at a controlled pressure and moved in a back-and – forth motion to effect washing. A mixture comprising oleaginous materials (including lanolin, petrolatum, and mineral oil) and lipid – and water-soluble dyes was used as a model soil. The published study compared soap-based and syndet-based liquid cleansers at 2% and 8% concentrations. Water and a 2% solution of sodium lauryl sulfate (SLS) were used as controls. The color (L – value) on skin before and after “washing” was measured with a chromameter. This work showed greater cleansing efficiency for the soap-based cleanser.

These authors conducted a separate experiment to assess the skin roughening effect of the test cleansers. Subjects used the test solutions for forearm washing over a two-week period. Skin roughness was assessed using silicon replicas taken at baseline and study end and analyzed by laser profilometry. The 2% solution of the soap-based cleanser produced greater roughening than did the 2% solution of the syndet-based cleanser. Changing the concentration of soap-based cleanser from 2% to 8% did not increase skin roughness. However, skin roughening for the syndet-based cleanser showed a concentration effect and at the higher concentration skin roughening was comparable to that produced by the soap – based cleanser. This illustrates the concentration-dependence of cleanser effects on skin and, since an 8% concentration is representative of cleansers’ concentration on the skin during actual use (9), the importance of understanding test conditions when judging how a cleanser will affect skin. This is particularly important when attempting to predict cleansers’ in-use skin effects.

Wolf and Friedman used a modification of Schrader’s method to assess the skin cleansing effect of soaps (10). An oleaginous mixture (petrolatum, lanolin, mineral oil) was again used as a model soil but in this case it was applied to the dorsum of the hand. The soiled hand was immersed for five minutes in a beaker filled with a stirred, 1% solution of the test cleanser maintained at 37°C. Sebumeter® readings made before and 30 minutes after immersion were used to determine the amount of soil removed. The authors report that this method is a convenient and economical alternative to the method of Schrader that can reliably and reproducibly measure and discriminate the skin cleansing ability of different products. A study comparing a syndet to a mild cleanser containing “25% hydrating soothing cream” showed that the latter product removed less of the model soil from the skin, i. e., it was a poorer cleanser. The authors conclude that for a product to function as an effective cleanser it must also dry the skin to a certain degree.

Imokawa used a model soil consisting of a mixture of triolein, cholesterol, squalene, palmitic acid, and Sudan Black dye (11). This mixture was applied to six glass slides, which were placed into a beaker containing 40°C surfactant solution and stirred at

1300 rpm for 10 or 30 minutes. Cleansing efficiency was judged by spectrophoto- metrically or gravimetrically measuring the amount of soil removed from the slides.

Lockhart and Lazer presented work that examined the impact of various physical conditions on cleansing (12). Charcoal applied to the dorsum of the hand served as a model soil. Four “wash” conditions were examined: simple soaking and placing the hand in a whirlpool, a simulated shower, or in an ultrasonic bath. Water temperature was maintained between 32°C and 38°C in all cases. Cleansing efficacy was judged by measuring color at the charcoal-stained area with a chromameter before and after washing. The results showed that the conditions ranked, in order of increasing cleansing effectiveness, soaking < whirlpool < shower < ultrasonic bath. While this study did not involve a cleansing agent or oleaginous soil, it demonstrates the potential for physical conditions and mechanical action to influence removal of a simple soil from the skin’s surface. Personal cleansers are used under a range of conditions and with a variety of implements, and these factors will affect overall cleansing efficacy.

The above methods all used a device in an attempt to reduce variability associated with the washing process. Other authors describe cleansing efficacy methods that more closely approximate in-use conditions. Sauermann et al. used mineral oil containing 0.1% anthracene as a model soil (13). The material was applied to the lower inner forearm, and the site was washed in a regular manner for 30 seconds with warm (32°C) water and then gently blotted dry. Cleansing efficacy was calculated based on fluorescence measured at the site before and after washing. These authors reported greater cleansing efficacy for soap bars than for syndet bars.

Puvvada et al. describe a method using makeup materials (e. g., lipstick or mascara) as model soils (14). Washing involved rubbing a (wetted) bar on the skin for one minute, rinsing with 35°C water for 30 seconds, and then patting dry. Cleansing efficacy was estimated from the difference in chromameter measurements taken before and after washing. While this method employs model soils that represent everyday cleansing needs, the wash conditions are exaggerated beyond expected use. Mills et al. also described a method using makeup (opaque camouflage cream) as a model soil (15). The makeup was applied to nine test sites on the ventral forearms, and then a technician washed each site in a controlled manner with a pad lathered with one of the test cleansers. The sites were rinsed to remove all traces of lather then rank-ordered based on the level of cleansing. Of the cleanser types tested, a bar soap product was ranked among those with the poorest cleansing efficacy, followed by a liquid soap marketed for sensitive skin. Cleansing products based on sugar surfactants (polyhydroxy fatty acid amides) were ranked as having the best cleansing efficacy. These products were also found to have the best skin compatibility in a chamber scarification test (16).

We also assess cleansing efficacy using a makeup removal model. Subjects are screened on the basis of skin tone (chromameter L – value); only subjects with sufficiently light skin are enrolled to assure good contrast with the model soil. A dark, oil-based makeup is applied to application areas marked on the volar forearms, and 30 minutes later the color at each site is measured again. Then a bar or liquid cleanser lather is generated, and a technician washes a randomly assigned site with lathered fingers for 10 seconds; the site is rinsed for 10 seconds with warm water and gently patted dry. A water-only wash is commonly included as a control. Thirty minutes after rinsing the color at the site is measured again. The color difference (DE) is calculated from the pre – and post-wash Lab values as an indicator of cleansing efficacy. This method is useful for assessing the relative cleansing efficacy of a variety of personal cleanser types. For example, the makeup removal method was used to compare cleansing efficacy of traditional soap bars

and a syndet bar. The cleansing efficacy for the soap bars was significantly better than that for the syndet bar under this method (Fig. 1).

Liquid personal cleanser forms are becoming increasingly popular, and some of these cleansers incorporate benefit agents such as petrolatum that deposit on skin during use. This product performance model seems inconsistent with a cleanser’s purpose, i. e., how can products that are designed to deposit material onto the skin function effectively as cleansers? One strategy involves employing technology that takes advantage of varying conditions that exist at different stages of the wash process. The benefit agent remains suspended in the lather during cleansing but upon rinsing this lather becomes dilute and the emulsion suspending the benefit agent “breaks,” depositing the benefit agent onto the skin. To demonstrate that this type of cleanser can effectively remove soil, we used the makeup removal test to assess the cleansing efficacy of two marketed liquid hand cleansers and three prototype liquid hand cleansers containing different levels of petrolatum (Fig. 2). The petrolatum-depositing products showed significantly better cleansing efficacy than the marketed cleansers under this model, and the results suggest that cleansing efficacy improved with increasing petrolatum level. Since the model soil is an oil-based makeup product this could reflect a “like dissolves like” phenomenon, which should translate to good removal of lipophilic soils from the skin in actual use. There are other examples of using lipophilic materials to aid soil removal. In ancient times the Romans applied oil to their skin during the cleansing process (17), and lipid-based washing products are again being promoted for use by patients with sensitive skin and atopic dermatitis (18,19).

Cleansing efficacy is important but for a product like a hand wash, which is used multiple times each day for washing, good skin compatibility is also necessary. We conducted a controlled application pilot study simulating in-use exposure to assess this parameter for the petrolatum-depositing hand wash (25% petrolatum). Healthy adult females were enrolled in a hand washing study comprising a seven-day pre-treatment period and a five-day treatment period. Subjects were provided with a regular liquid hand cleanser for hand washing and a syndet-based bar to use for showering. They were instructed not to apply cleansing products to the dorsal part of their hands and to avoid any activity that required hand immersion in a surfactant solution, e. g., washing dishes. Moisturizer use was prohibited. Ten subjects who exhibited a sufficient level of hand dryness entered the treatment phase. Treatment was conducted as a paired comparison.

A technician washed one randomly assigned hand with the petrolatum-depositing hand wash product for 10 seconds following a prescribed procedure; the other hand was wet, rinsed, and patted dry. There were five wash visits each day, spaced by 30 minutes, with the washing procedure conducted four times in succession at each visit. Thus, subjects’ hands were washed a total of 20 times each day. Hand condition was evaluated visually (20) and instrumentally (CM-825) at baseline, before washing, and two hours after the final wash each day. Subjects acclimatized for 30 minutes in a controlled environment room before each evaluation.

Expert visual evaluation showed little difference in erythema production between the hand wash and control (Fig. 3). In fact, the hand wash generally produced somewhat less erythema than the control. In addition, the hand wash produced marked dryness improvement compared to control at the post-wash evaluations, and there was progressive improvement in dryness observed at the pre-wash evaluations over the course of treatment (Fig. 3). Trends in the skin capacitance measurements, which provide an indirect assessment of stratum corneum hydration, paralleled the expert dryness scores. These results demonstrate that this petrolatum-depositing hand wash shows good skin compatibility and can actually improve dry skin condition, even under exaggerated exposure conditions.