# In Vivo Assessment of Water Content, Trans-Epidermial Water Loss and Thickness in Human Facial Skin
## 2 Related Research  
Mapping skin in terms of its properties has a long history and has received increasing attention in the last two decades. This in part reflects the evolution of the measuring devices, some of which have become available only in recent times [10, 11, 12]. Research about regional differences of the skin thickness dates back to the early 1970s with the works of [PERSON] et al. [13] and has generated a number of relevant studies since [4,14,15,16, 17]. Capacity, TEWL and level of sebum in relation to anatomical site, sex and age were investigated by [PERSON] et al. [17]. Later on, a comprehensive mapping of the human body (23 sites) in terms of hydration was carried out by [PERSON] et al. [7]. In [5] [PERSON] et al. investigated variations of sebum content, hydration, TEWL, elasticity, erythema and melanin index as a function of body region (eight sites), gender and age. Of late, differences in hydration and TEWL between five body regions (cheek, forearm, mid-calf, lower back and abdomen) were analysed by [PERSON] et al. [18], and [PERSON] et al. [19] mapped skin pH, TEWL and hydration in patients with atopic dermatitis.  
The facial area has also elicited much research attention due to its obvious implications in cosmetics, beauty treatments and reconstructive surgery. Differences in the biophysical properties have been reported by a number of authors: [20,21] are good introductory reviews on the subject, albeit unfortunately not entirely up-to-date. In [22] [PERSON] et al. tested ten face sites for significant inter-area differences in TEWL. [PERSON] et al. [23] compared cheeks and forehead regions in terms of capacitance, sebum level, temperature, TEWL, colour and relief. [PERSON] et al. [24] investigated distributional patterns of TEWL, temperature and sebum level on the foreheads, cheeks and chins of female subjects. [PERSON] and [PERSON] [25] studied variations of TEWL, conductance, size of corneocytes, amount of surface lipids, pH, blood flow and surface temperature on five facial sitesaround the period region. In a similar study, [PERSON] and [PERSON] [26] evaluated skin blood flow, TEWL, water content, temperature, pH and sebum content on nine locations of the face. A comparison among seven forehead/cheek biophysical parameters (hydration, sebum level, pH, melanin and erythema index, elasticity and TEWL) was carried out by [PERSON] et al. [27] on Southeast Asian female subjects. [PERSON] et al. [28] measured eight skin parameters (skin gloss, water content, TEWL, melanin level, individual typology angle, erythema, sebum level and elasticity) on the foreheads, cheeks and temples of \(n=442\) Caucasian female subjects, which makes it the study with the largest population to date. More recently, [PERSON] et al. [29] took things a step further by providing continuous, three-dimensional facial maps of hydration, sebum level, barrier function and pH in four ethnic groups (Asians, Black Africans, Caucasians and Indians). Skin thickness has also attracted significant research interest due to its central role in cosmetic and reconstruction surgery. For instance, in [30] the authors studied variations in facial skin thickness using an ultrasound scanner, whereas in [31; 32] topographic maps of the skin thickness were obtained via post-mortem biopsy.