The skin is the product of a process of adaptation of the human species thanks to which we are able to live on earth. If we did not have skin, we would not have protection against external agents and our internal organs would be exposed. In addition, we would not be able to retain the water that is essential for all the processes of our organism; and without the skin the water would simply evaporate.
And the skin is so impermeable due to a very superficial layer that is the corneous stratum . The corneous stratum is made up of cells that were initially in the deepest part of the skin (keratinocytes) but are changing and becoming more superficial. They lose their nucleus and are arranged in a very particular way, in the form of a barrier. Between them are specialized lipids (fatty acids, ceramides and cholesterol) that are also organized in this barrier.
The skin stays hydrated thanks to the ability to bind and retain water in this layer. This is produced by molecules that are 50% amino acids and 50% salts like lactate and urea. Filaggrin is a protein produced by keratinocytes and degraded into amino acids within corneocytes.
Aged cells simply break off because enzymes break their bonds. If the skin cannot be shed properly, some skin diseases (ichthyosis) occur.
The skin also protects us from the sun by various mechanisms: by the melanin barrier, by the proteins of the stratum corneum, by the antioxidants present in the stratum corneum and by the reflective properties of the skin.
Daily exposure to cleaning with aggressive substances alters proteins and lipids, especially during the colder months, altering this barrier and making it weaker. Which translates into skin with less capacity to retain water and drier (as we see in cold climates or in the winter months).
Moisturizing compounds do not necessarily penetrate the skin, but form a layer above it capable of retaining water and giving the effect of hydration; such as glycerol, present in many cosmetology creams.
Bioactive ingredients must penetrate the skin to perform their function and alter the biology of the skin, such as cosmeceutical substances. Product of engineering, attempts are made to provide these compounds deep into the skin, capable of penetrating the impermeable corneous stratum , such as liposomes, nanoparticles, microspheres, etc. This is easier if we mechanically break this barrier, through mesotherapy, microneedles, lasers, etc., procedures that are part of our Aesthetic Medicine treatments.