Introduction – Evaluation of the hydrating and elasticizing properties – PRO-YOUTH
Some essential functions for the correct life of the skin are recognized at the epidermis: intense metabolic activity, high cell turnover, regulation of water balance and barrier function. In the last two functions the role played by the stratum corneum is fundamental. The water balance is regulated by the presence of several factors: a) intracellular keratin proteins, b) superficial cutaneous lipids and c) intercellular lipids.
The most active and determining role, due to the ability of the horny body to structure the epidermal water in a stable manner, is the role played by skin lipids, in particular by intercellular lipids. The organization of intercellular lipids in a crystalline liquid state is essential to ensure its functionality and maintain the water level of the epidermis in balance with the environment. The water present in the lipid double layer of the lamellar structure is strictly “bound” and its free evaporation in the environment is prevented (in fact its evaporation rate is about 60 times lower than that of free water at the same temperature). The water content of the horn is in itself a less decisive parameter than the possibility of maintaining the intercellular lipid layers in a liquid-crystalline organization. It is this structure that allows an effective entrapment and water management. The equilibrium liquid crystals / solid crystals is a function of the degree of unsaturation of fatty acids and of the water content. This orderly situation can be altered due to various endogenous and exogenous causes: aging, pathologies, environmental agents, or application of aggressive rinsing products. Factors such as the ambient temperature too low can cause a crystallization of the lipid phase: the organized double lipid layers become brittle and lose fluidity, causing increased evaporation of the water, resulting in skin dryness and decreased barrier function and loss of elasticity.
In these cases, dermo-cosmetic products can be effective and must be used to improve skin hydration and viscoelasticity not only for aesthetic purposes, but also to maintain normal skin conditions (Lachapelle 1996).
Many cosmetic products on the market have moisturizing and elasticizing properties often attributed to the presence of some functional substances, but also to the complex of all the ingredients. In fact, the functional substances, to act in an appropriate manner, must be used in adequate quantities and must be inserted into vehicles able to optimize their penetration of the skin. Low concentrations of active substances are insufficient to ensure a good result, while overdosing can in some cases cause unwanted irritation phenomena or bring high production costs, without thereby achieving better results. The concepts of formulas optimization and scientific verification of the declared activity, in the past a bit empirical in cosmetics, have found valid verification methods in the last decade with a new branch of the cosmetic-dermatological sciences, bioengineering, aimed at research and experimentation of non-invasive methods for measuring skin characteristics. In addition, according to the latest EU legislation, the properties declared by a cosmetic must always be supported by objective evidence that guarantees their truthfulness.
Moisturizers are the most prescribed products in dermatology (Lodén 2005). Objective methods are considered suitable to demonstrate and clarify the mechanisms of action of substances that improve the properties of the skin. Several in vivo studies using modern engineering techniques have been conducted to evaluate the mechanical properties and water content of the epidermis (Jemec 1990, Overgaard Olsen 1993, Pedersen 1999). The tools that were used to assess epidermal hydration are based on conductance measurement (Bettinger 1999), capacitance (Jemec 1990, 1998, 1999, Auriol 1993), and skin impedance (Murray 1996). The tools that have been used to test skin elasticity are based on torsion (Murray 1997) and aspiration methods (Jemec 1990, Overgaard Olsen 1993, Auriol 1993, Murray 1997, Pedersen 1999, Bettinger 1999).
The efficacy test can be evaluated in different ways (Colipa 1997) but tests on human volunteers are preferred. With the introduction of non-invasive instrumental measurement techniques, which apply to human beings, the properties of cosmetic and dermatological products can be evaluated in an objective way allowing to detect subtle changes in morphology and in the function of the skin otherwise not detectable by sensory means .
In medical literature (pubmed) there are several works that demonstrate the moisturizing, protective and elasticizing efficacy of standard cosmetic products or based on plant extracts (Ghadially 1992, Serup 1992, Lodén 1996, Jemec 1999, Dobrev 2000, Lynde 2001, Kapoor 2010, Akhtar 2011).