Physiology & Anatomy of Skin to understand Absorption and Penetration of Skin Care Products

Before we talk about how products enter the skin, understanding the physiology and anatomy of your skin will help you understand how our body works and how using different products has an effect on the function of the skin. 

The skin is a constantly renewing, dynamic organ that acts as a protective barrier for the body. Being the largest organ of the body, it consists of three layers: the epidermis, the dermis and the subcutaneous (or subcutis) layer. These three layers also have their own numerous layers each of which is made up of several sub-layers. Skin appendages – such as follicles and sebaceous and sweat glands – also play various roles in its overall function.

The Epidermis

The epidermis is the outermost layer of the skin and protects us from toxins and bacteria as well as fluid loss. To do all of this effectively, this skin layer actually contains four thinner sub­ layers of cells

1. The stratum corneum is the top layer and consists of dead cells (corneocytes), which are regularly shed. This layer varies in thickness depending on where it is on the body, which is why the palms of your hand, the heel of your foot and other areas that require additional protection in order to grip objects or avoid injury or abrasion, are thicker than, say, your eyelids.
2. The next layer, the stratum granulosum, contains cells with visible granules (hence the name) that die and move to the surface. Keratin is formed here and is extremely insoluble, which, in turn, helps waterproof the skin.
3. The stratum spinosum is where the synthesis of keratin takes place.This area is made up of cells that bind other cells together – like glue for your skin cells.
4. The stratum basale is the bottom skin layer and it houses the stem cells, which divide to produce new skin cells. Those are then pushed up to the skin’s surface.

The Dermis

The majority of the dermis is made up of collagen and elastin. The cells that create both are known as fibroblasts. This skin layer is divided into two parts – the papillary dermis and the retinal dermis.

The papillary dermis regulates temperature and has a network of capillaries that are used to supply nutrients to select layers of the epidermis. The reticular dermis contains:

•  Collagen fibres – give support to the tissues

•  Elastin fibres – responsible for flexibility

•  Glycosaminoglycans – polysaccharides – large molecules that are able to absorb water

Sweat glands live in the dermis. They emit sweat through your pores, which serves to both cool the body and clear out toxins.In addition, the dermis contains hair follicles and sebaceous glands, which produce the oil that softens and smoothes skin or, if overactive, make it slick, oily and a great environment for spots and pimples.

The Subcutaneous Layer (Hypodermis)

This innermost fatty layer of skin stores energy while padding and  insulating the muscles, bones and body organs. The subcutaneous layer is so deep that active ingredients from  your skin care products cannot reach it, but it does affect the uppermost layers of skin as it contains yet more capillaries, nerve endings, the roots of hair follicles and  the deepest sebaceous glands.

Penetration vs Absorption of skin care products

Penetration and absorption are two terms that may be mistakenly misused. Penetration is when a substance enters the deeper layers of the skin. The sebaceous gland coats the surface of the stratum corneum with a waxy substance called sebum. It consists of triglycerides, wax esters, squalene and metabolites of adipocytes (fat-producing cells). This helps to waterproof your skin, which also makes it harder for hydrophilic molecules to penetrate your skin. Emulsifiers and solvents are typically penetrated. Emulsifiers are surfactants that contain a hydrophobic and a hydrophilic segment on the molecule, allowing compatibility between the polar and non-polar ingredients. 

Whereas, absorption is a route of a substance entering the body (bloodstream) via skin. Products that are absorbed are more effective, however, due to them entering the bloodstream, we need to be careful what we allow to enter into our bodies.

Absorption of a substance depends on the following factors:

• Concentration
• Molecular weight of the molecule
• Duration of contact
• Solubility 
• Physical condition of the skin

Product must pass through 1 of 3 routes in order to be absorbed:

1. Epidermis
2. Glands
3. Hair follicles 

The stratum corneum is the outermost layer of the epidermis and the rate-limiting barrier in absorption of an agent. 

In order for a product to be absorbed, the chemical needs to be small enough to pass through 1 of the 3 routes. Lipid-soluble chemicals make it through the layer and into the circulation faster. Some skin care products will use for example, ethanol, as an additive to increase the absorption rate of the chemicals. This is because it is able to break down the skin’s natural barriers and pulls apart chemicals into individual constituents, making them small enough to absorb. Silicone additives for example, dimethicone, sit at the top of the skin resulting in unabling absorption. 

Conclusion

When purchasing a product you need to remember two things: is it going to penetrate or absorb? Due to the possibility of the product entering the bloodstream, we need to be careful what we allow to enter our bodies.