Transdermal Creams are nothing new, but if you don’t believe active ingredients can pass through your skin and into your body? Really? That’s fine; here hold this handful of mercury for me, and take a shower in Agent Orange, try this pain relieving liniment, Vicks on your chest or add some crushed garlic to the soles of your feet. See if you can smell it and taste it on your breath.
Transdermal technology has been around for many years. We understand that pharmaceuticals and toxins can pass through your skin into your body. However, many people are skeptical about healthy or natural compounds passing through the skin. How can it possibly access subcutaneous tissues and microcirculation?
Transdermal delivery is basically using creams, serums, patches, and lotions applied directly to the skin. This is done in order to deliver active ingredients into the peripheral circulation or to a specific distal site.
It means you can bypass the pitfalls associated with oral delivery such as bad taste or large serving sizes of capsules and powders. The other benefit is avoiding gut upset and other intolerances.
Transdermal delivery also means that you can bypass the digestive process and first pass metabolism by the liver that can reduce the bioavailability. Often, ingested ingredients are affected by breaking it down and eliminating the actives before they get into the systemic circulation.
Direct delivery to the target site can also overcome poor circulation to problem areas. Stubborn subcutaneous fat; sites of pain, recurrent injury, wounds or infections; weak or injured muscles, tendons and nerves and distal locations with poor circulation. These can all benefit from applying the therapeutic ingredients directly on the area that you want to treat.
The Skin as a Barrier
Transdermal absorption of active ingredients is mostly limited by the skin’s outermost layer called the stratum corneum. This layer which is 10 to 20 µm thick (Fig. 1). is our barrier between us and the outside world.
The stratum corneum resembles a brick wall under a microscope; with non-living skin cells making up the bricks and oily watery layers like mortar. Chemicals with a small enough particle size, the right amount of oil and water solubility can penetrate this barrier by winding between the bricks through the mortar.
Natural and synthetic chemicals with molecular weights of 100 to 800 and adequate fat and water solubility can permeate through the stratum corneum bricks and mortar. It’s an efficient way to get them in and inside your body. Phytoecdysteroids have a molecular weight of approximately 480, forskolin 410, progesterone 314, testosterone 288, chrysin 255, arginine 210, ibuprofen 206 and caffeine 195.
Underneath the stratum corneum barrier is the viable epidermis, which measures 50 to 100 µm and does not have a blood supply. But, below this layer is the dermis, which is 1–2 mm thick and contains a rich blood supply. This is in the form of a bed of capillaries that allows for systemic absorption of the active ingredients.
The dermis is a largely fibrous layer that provides skin’s mechanical support, as well as the skin’s blood supply and anchoring for the sweat gland and hair follicles. The sweat gland and hair follicles provide another transport pathway to deliver active ingredients past the stratum corneum to the microcirculation of the dermis layer and then into the local fatty layers and systemic circulation.
Fig. 1 Histological structure of mammalian skin. (a) Skin structure. (Image of H&E stained porcine skin provided courtesy of Samantha Andrews, Georgia Institute of Technology). (b) Stratum corneum structure. Drug penetration across the stratum corneum is limited primarily by the lipids organized in bilayer structures (L) that fill the intercellular spaces between corneocytes (C). (Cryo-scanning electron micrograph provided courtesy of Joke Bouwstra, Leiden University).
Penetration enhancers can increase skin permeability and provide an added driving force for transport by increasing drug partitioning into the skin.
The concept can be quite simple. There are pathways between the skin cells, via hair follicles, via sweat glands that molecules of a small enough size are capable of passing through. For example chemicals with molecular weights of 100 to 800 and adequate solubility can permeate your skin. Phytoecdysteroids have a molecular weight of approximately 480, forskolin 410, progesterone 314, testosterone 288 and ibuprofen 206, arginine 210, and caffeine 195. So you can see how close the molecule size is between natural and pharmaceutical compounds.