When we wash our face with facial cleanser, our skin will start to feel tight. This feeling tends to go away after applying your favorite moisturizer. This sensation in our skin may seem subjective, but researchers at Stanford University recently revealed the mechanism behind these sensations. Their findings, published today (September 26) in the Proceedings of the National Academy of Sciences (PNAS Nexus), show how mechanical changes in the outer surface of the skin are translated into sensations and provide a quantitative method to determine how people perceive their skin after applying moisturizer or cleanser.
"This work gives us new insights into how products affect the physical properties of skin, not just skin health but also how the skin feels. This is a major advance," said Reinhold Dauskardt, the Ruth G. and William K. Bowes Professor in the Department of Materials Science and Engineering at Stanford University. "This provides a completely new understanding of how to design these formulations."
The skin is the largest organ of the human body and is constantly exposed to the surrounding environment. The outermost layer of skin - the stratum corneum - acts as a barrier, blocking harmful chemicals and bacteria and retaining moisture. When we use harsh cleansers, they remove some of the lipids that retain moisture, causing the stratum corneum to shrink. A good moisturizer will increase the moisture content of the cuticle, causing it to swell.
Dauskardt and his colleagues predicted that the mechanical force generated by this contraction or expansion would travel through the skin, reach mechanoreceptors (sensory receptors that convert mechanical force into nerve signals) under the epidermis, and then send signals to the brain that we understand as the feeling of tight skin.
To test their theory, the researchers studied the effects of nine different moisturizing formulas and six different cleansers on donor skin samples from three areas of the body - cheeks, forehead and abdomen. They measured changes in the stratum corneum in the lab and then fed this information into a complex model of human skin to predict what signals the mechanoreceptors would send out.
The researchers were able to rank the different formulas based on how subjects described how their skin felt. The predictions from their analysis matched almost exactly what people reported for each formulation in human trials. Collaborators at L'Oréal Research and Innovation recruited 2,000 women in France to evaluate nine moisturizers and 700 women in China to evaluate six cleansers. Participants ranked the skin tightness they felt after using the formulas they were given.
"We compared the predictions to what the subjects told us, and the results were absolutely consistent. In other words, what we predicted was exactly what they told us, and that's absolutely a remarkable correlation with very high statistical significance."
Understanding and predicting how people will feel after using skin care products can help cosmetics companies improve formulas before asking people to test them. With such a detailed model of how mechanical stress is transmitted through the skin layers, Dauskardt said, these methods have the potential to be used for more than just assessing skin tightness.
"It provides a framework for new product development," Dauskardt said. "If you do anything to the outer layer of the skin that causes it to change its strain state and stress state, then we can tell you how that information is conveyed and how consumers will interpret and report that information."
Dowscutt also hopes to apply this new understanding to the development of wearable devices. For example, if we knew how the brain interprets small changes in skin tension, we might be able to exploit this mechanism to send intentional signals. Just as braille readers translate sensations at their fingertips into words, devices that create tiny mechanical changes in our skin may be able to convey messages.
"What we've done is reveal how mechanical information is transmitted from the outer layer of the stratum corneum down to neurons lower down in the skin," Dauskardt said. "Now, can we communicate through human skin? Can we use our understanding of these mechanisms to create a device that provides information to others in a non-verbal, non-visual way? This is one of the areas that we are very interested in."