INTRODUCTION
Understanding the hydration of facial skin poses a challenge in part because the skin is exposed to environmental humidity, sunlight, and pollution, which can impact water loss.1 Traditionally, skin hydration is presented as data (charts, numbers); but for both consumers and healthcare professionals these data may be difficult to grasp intuitively. In addition, hydration assessed at a single location may under- or over-estimate hydration in other sites.1, 2 Further, facial skin has increased water loss compared with other body sites, along with lower levels of ceramides and natural moisturizing factors.1 Heatmapping is a technique to visualize skin hydration in real-life conditions.1,2 Facial mapping of multiple sites of skin capacitance helps visualize the moisture within the skin in 2-dimensional color maps that are then superimposed onto digital images.2
The primary function of the epidermis is to create a permeability barrier around the body to protect against the external environment.3 The stratum corneum (SC), the outermost layer of the epidermis, largely controls skin barrier function.4 When skin is healthy, the SC provides an efficient transport barrier while being soft, strong, and pliable enough to respond to physical strain and stress.4 Water content is one of the major factors that control SC flexibility and skin hydration (the water content in cells) occurs primarily via the uptake of water by corneocytes.4, 5 Skin cells are embedded in a lipid-rich intercellular matrix, which confers waterproof properties.5 The interplay of these factors maintains barrier function and control of transepidermal water loss (TEWL) even in a dry environment.4,5 Poor skin hydration leads to dry, itchy, uncomfortable skin and even diseases such as eczema.5 Well-hydrated skin is supple and protects against skin cracking or fissuring.6
Cleansing skin removes dirt, oil, and bacteria from the skin surface and washes away dead cells.7 While cleansing alone does not maintain skin hydration, it is an important aspect of skin health. Gentle cleansers that do not compromise the skin barrier or result in compensatory sebum production or other undesired effects should be recommended to all dermatology patients.7 Moisturizing reduces the evaporation of water from the skin, minimizing TEWL.5 Hydration of the skin results in a smooth and soft texture as well as a younger seeming complexion.8,9 This study was designed to assess the moisturizing effect of a gentle skin cleanser (GSC) and moisturizing lotion (ML) with the hydrating ingredients glycerin, panthenol, and niacinamide.
The primary function of the epidermis is to create a permeability barrier around the body to protect against the external environment.3 The stratum corneum (SC), the outermost layer of the epidermis, largely controls skin barrier function.4 When skin is healthy, the SC provides an efficient transport barrier while being soft, strong, and pliable enough to respond to physical strain and stress.4 Water content is one of the major factors that control SC flexibility and skin hydration (the water content in cells) occurs primarily via the uptake of water by corneocytes.4, 5 Skin cells are embedded in a lipid-rich intercellular matrix, which confers waterproof properties.5 The interplay of these factors maintains barrier function and control of transepidermal water loss (TEWL) even in a dry environment.4,5 Poor skin hydration leads to dry, itchy, uncomfortable skin and even diseases such as eczema.5 Well-hydrated skin is supple and protects against skin cracking or fissuring.6
Cleansing skin removes dirt, oil, and bacteria from the skin surface and washes away dead cells.7 While cleansing alone does not maintain skin hydration, it is an important aspect of skin health. Gentle cleansers that do not compromise the skin barrier or result in compensatory sebum production or other undesired effects should be recommended to all dermatology patients.7 Moisturizing reduces the evaporation of water from the skin, minimizing TEWL.5 Hydration of the skin results in a smooth and soft texture as well as a younger seeming complexion.8,9 This study was designed to assess the moisturizing effect of a gentle skin cleanser (GSC) and moisturizing lotion (ML) with the hydrating ingredients glycerin, panthenol, and niacinamide.
MATERIALS AND METHODS
This was a half-face, intra-individual open-label study conducted on healthy volunteers to visualize the level of skin hydration via heatmap. Subjects applied a gentle skin cleanser (GSC) and moisturizing lotion (ML) to the whole face (both Cetaphil, Galderma, Dallas, TX) and assessments were performed on half of the face. GSC was administered in a single application that was then wiped off the face. Moisturizing lotion was applied at least once daily for one week. Facial digital images were captured by VISIA-CR (Canfield Scientific, Inc., NJ, USA) Hydration measurements were made by using a Corneometer (Courage + Khazaka electronic GmbH, Germany) at 30 pre-defined points on half of the face, at baseline, and at 30 minutes post-application; an additional assessment at one week was made for ML (Figure 1).1,2
Heatmaps were generated using the PyCharm algorithm, a Python programming software (JetBrains, Prague, Czech Republic), to interpolate hydration values to colors that were then superimposed onto the subject's facial image. Adobe Illustrator and Photoshop (Adobe Inc, CA, USA) were used to blend colors into a gradient heatmap. A rainbow color scale was applied with green-blue hues associated with the
degree of hydration (relatively high hydration) and yellow-red hues associated with the degree of dryness (relatively low moisture content).
RESULTS
A total of 5 subjects completed the cleanser (GSC) assessments, and 5 subjects completed the 30-minute evaluation for the lotion (ML) with 4 completing the week 1 assessment. Subject demographics are presented in Table 1. There was a visible shift in skin hydration post-GSC application from values approximately in the 12-42 AU (arbitrary unit) range to 30-60 AU at 30 minutes (Figure 2). 
Similarly, the moisture maps demonstrate a shift in hydration from baseline to 30 minutes that continued to increase through week 1 of ML use (Figure 3). The visible change was appreciated in both individual hydration results and results averaged from the 5 participants.
DISCUSSION
Two-dimensional heatmaps are a useful tool to visualize skin hydration of the entire face. Changes in hydration before and after product application were demonstrated for GSC at 30 minutes after cleansing and for ML at 30 minutes and after 7 days of moisturizer use. This indicates a rapid and sustained hydration-boosting effect via clear color shifts. Not only is a
