MAURICIO GUZMÁN-ALONSO¹, TANIA M. CORTAZÁR²
1. Innovation and Development Centre, Belcorp, Tocancipá, Colombia
2. National University of Colombia. Science Faculty. Department of Chemistry, Bogotá, Colombia

Abstract

Proper hydration is absolutely critical to obtaining healthy skin. Hydration is the most basic consumer expectation toward a skin care product and is therefore, the most common and functional benefit promised by the cosmetics industry. Exists a plethora of endogenous and exogenous ingredients, and moisturizing formulations that contribute to the maintenance of water in the skin, covering various mechanisms and interactions that influence skin functionality and structure. Currently it is known in detail the skin composition and some of the interactions of its molecules involved in maintaining the hydration and water flow. This review will describe some aspects of skin hydration, molecules and formulations involved in the modulation of water content at different skin depths, and briefly will present methodologies used in the evaluation and information that they provide about the complex process of the skin hydrationand moisturizer influence.

INTRODUCTION
Skin hydration is defined as the water content of the epidermis and the dermis (1). The inner milieu of our body consists of about 70% water (2). Approximately 20% of water present in the body is accumulated in the skin, with 60–70% of this amount being accumulated in the dermis (3).  Water is important for the structure and mechanical properties of many proteins and their mutual interactions (4). Functionally, the amount of water in the skin can be divided into free and bound water. In healthy skin, most of the water is bound to macromolecules. The ability of the skin to hold water is primarily related to the stratum corneum (SC), which plays the role of barrier to water loss (5). The retention of water in the SC is  dependent mainly on the presence of natural hygroscopic agents within the corneocytes and the SC intercellular lipids ordely arranged to form a barrier to transepidermal water loss (5). The glycosaminoglycan polymer hyaluronan (HA, hyaluronic acid) provides a scaffold on which sulfated proteoglycans and matrix proteins are organized. These supramolecular structures are able to entrap water and ions to provid ...