Washing the body is a daily activity that impacts the stratum corneum (SC). And as skin care concepts develop, so does the understanding of the mechanics and impact of washing on the upper layers of skin. For example, while the SC is both a physical and chemical barrier that acts as a substrate for the attachment of bacteria, the process of cell shedding counteracts this attachment to help prevent infection.
As another example, according to Anthony Rawlings, Ph.D., a prominent expert in skin biology, the SC was initially thought to be structured like a brick wall but it is now understood to be more like a continuous poly-proteinaceous membrane structure. The “bricks” of this wall are the corneocytes, which compose much of the SC. These are tightly interconnected by corneodesmosomes in the majority of SC layers and by tight junction molecules in the lower layers.
Rather than rigid-like bricks, corneocytes are more reminiscent of sponges formed from keratin that contain absorbent, natural and moisturizing factor molecules. These molecules hydrate extensively and are interspersed between a continuous, mostly highly ordered lamellar (hexagonal and orthorhombically packed) lipid phase. This thermodynamically unstable yet kinetically stable tissue matures from its lower to its upper layers. This facilitates its intrinsic hygroscopic properties and the process of desquamation.1
Washing the skin removes the acid mantle and gently facilitates desquamation. First described more than nine decades ago, the acid mantle was identified as researchers began to investigate the significance of skin pH and microbiome. At that time, the acidic skin surface was thought to serve a protective mechanism against invading microorganisms. Today, the acid mantle descriptor still refers to its protective quality, though the understanding of its acidic character has gradually changed to include the orchestration of epidermal differentiation and corneocyte shedding.2
In relation, regular desquamation is essential for skin health. A delay in desquamation leads to the accumulation of corneocytes on the surface of the SC, resulting in dry and flaky skin on the body or rough skin on the face. In contrast, premature desquamation also can occur in a state of sub-clinical inflammation and barrier abnormality, which can be induced by surfactants or the environment. In some cases, such as eczematous or photo-damaged facial skin, a thinner SC becomes apparent.3 Importantly, washing the skin, i.e., sensory envelope, not only gently facilitates desquamation, but also can mentally refresh the human body
Cleansing and Post-Wash Recovery
Gentle cleansing is essential to maintaining the optimal physiological function of skin, and cleanser technologies have improved to mildly cleanse while imparting moisturizing benefits. Harsh surfactants can compromise the skin and cause damage to SC proteins and lipids, leading to barrier damage and clinical after-wash tightness, dryness, irritation and itch. Thus, for skin benefits, cleansers must first minimize surfactant damage and additionally deposit, under in-use washing conditions, beneficial occlusive, humectant or emollient ingredients. These actives improve skin hydration as well as align the mechanical and visual properties of the skin.
All surfactants tend to interact with lipids but their interactions with proteins can vary significantly depending upon the nature of their functional head group. The least irritating cleansers contain surfactants that interact minimally with both skin lipids and proteins, and are adjusted to a pH to minimize SC swelling and lipid rigidity. As such, the mildest cleansers have a neutral or acidic pH—close to normal pH of the SC, and are enriched with ingredients that ameliorate surfactant damage and replenish skin lipids.4
In relation, the irritation potential of cleansers can be assessed by corneosurfametry using cyanoacrylate strips and staining. However, it is important to note that the industry strives to more precisely visualize irritant effects on deeper layer damage after product application.5
To know an ingredient’s irritation potential, it also is crucial to understand the kinetics of skin recovery. As an example, research has examined the facial skin of Asian women across all skin types and reported the results of various parameters measured post-cleansing. Skin hydration, sebum and trans-epidermal water loss (TEWL) were significantly reduced 20 min after cleansing, compared with the baseline. Interestingly, while skin hydration values recovered after 40 min, sebum and TEWL returned to pre-wash values after 120 min. This highlighted that each skin parameter has a specific restoration time point.6
- Rawlings, A., Ph.D. (10, Mar 2020). Secret Life of Skin: Information and Inspiration on the Skin Microbiome. Microbes: Fighting for space on a fragile interface. Https://thesecretlifeofskin.com/2020/03/10/microbes-fighting-for-space-on-a-fragile-interface/
- Surber, C., Humbert, P., Abels, C. and Maibach, H. (2018). The acid mantle: A myth or an essential part of the skin health. Curr Probl Dermatol 54 1-10. Https://www.ncbi.nlm.nih.gov/pubmed/30125885
- Rawlings, A.V.and Voegeli, R. (Feb 2013). Stratum corneum proteases and dry skin conditions. Cell Tissue Res 351(2) 217-35.
- Ananthapadmanabhan, K.P., Moore, D.J., Subramanyan, K., et al. (2004). Cleansing without compromise: The impact of cleansers on the ski nbarrier and the technology of mild cleansing. Dermatol Ther17 Suppl 1 16-25.
- Liu, M., Mollica, L., Regan, J., et al. (Apr 2016). Modified corneosurfametry as a new accelerated high-throughput ex vivo methodology for predicting cleanser effects towards human skin. Int J Cosmet Sci 38(2) 178-86.
- Eo, J., Seo, Y.K., Baek, J.H., et al. (May 2016). Facial skin physiology recovery kinetics during 180 min post-washing with a cleanser. Skin Res Technol 22(2) 148-51.