Nowadays, consumers are a lot more concerned about what their skincare products contain and how these ingredients can actually penetrate through the skin layer.
In this post, I will give you an overview of skin structure so you can have a basic understanding.
First, let take a look at the skin diagram below.
Skin is composed of three layers:
Epidermis: The outermost layer of skin contains five sublayers.
From superficial to deep, these layers are stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.
The epidermis is mainly made of keratinocyte, which stores a common protein, keratin. Keratin forms barrier protection for your skin. Another component is melanocytes or melanin which is skin pigment.
Let’s have a detailed discussion about the stratum corneum.
Stratum corneum is the first layer that any molecules or microorganisms have to encounter when trying to penetrate our skin. It consists of corneocytes (when keratinocytes move to the upper layer, they lose their nucleus and become corneocytes) which is what we usually call dead skin. Corneocytes are surrounded by cornified cell envelopes (covalently bound lipid) while corneodesmosomes hold each corneocyte together.
Stratum corneum also contains natural moisturizing factors (NMF’s), which are primarily composed of free amino acids and their derivatives. Their function is to maintain skin hydration. This stratum has low permeability due to the lipid matrix and densely cross-linked layer of proteins. For this reason, the hydrophilic molecules in our cosmetic products (facial moisturizer, lotion…) cannot penetrate our skin easily without the help of some enhancers such as surfactant and occlusive. I will discuss more of these ingredients later.
Dermis: The middle layer consists of an extensive vascular network, which is where all the skin activity takes place. It provides the skin’s nutrition (elastin and collagen), immune response, and thermal regulation. When the active ingredient in a cosmetic product can actually penetrate the epidermis and reach the dermis layer, creating biological changes in our body; it is not considered a cosmetic product but a drug.
Hypodermis (subcutaneous): The fatty layer, which conserves our body’s heat and protects the inner organs.
So how do the molecules in skincare products actually penetrate the skin?
The molecules can pass through the lipid membrane or between the corneocytes in the stratum corneum. Because of the structure of the lipid membrane, molecules have to be lipophilic (oil-loving) to get through this lipid layer. On the other hand, hydrophilic molecules (water-loving) find it hard to go through. The epidermis does a great job of protecting us from harmful microorganisms, which also blocks our skin’s absorption and penetration. The acid pH of our skin is an example. Another route that molecules can penetrate the skin is through the hair follicle. This is also an important pathway for OTC drugs (like acne treatment).
To sum up, we can say that ingredients in our skincare products have to encounter such complicated barriers to actually penetrate into our skin and do their work.
So, this is my very first post, and I’ll try to keep up my writing, thanks for reading.
Reference:
Cell strata of skin (Benson, H., Robert, M., et al. 2019)
New arrangement of proteins and lipids in the stratum corneum cornified envelope (O. Lopez, et al. 2007)
