Why Topicals Work — Skin Anatomy and Absorption Routes

Topical products either sit on the skin, soak into the skin, or cross the skin into the body. Knowing which one you are aiming for, and which one you will actually get, is the difference between a thoughtful formulator and one who follows recipes.

The three layers in five sentences The epidermis is the outermost layer, 0.05-1.5 mm thick, made of cells that gradually flatten and die as they migrate from the basal layer to the surface. The stratum corneum — the outermost few cell layers of the epidermis — is the barrier; it is what keeps water from leaving your body and what keeps most foreign substances from entering. The dermis is the layer below, where blood vessels, hair follicles, sebaceous glands, sweat glands, and nerve endings live; this is where systemic absorption begins. Below that is the subcutaneous fat layer. Topical products mostly act on or just below the stratum corneum; some — especially with the right vehicles — penetrate to the dermis; very few reach systemic circulation in clinically meaningful amounts.

The barrier function and what gets through The stratum corneum is sometimes described as "bricks and mortar" — flat dead cells (corneocytes, the bricks) held in a matrix of lipids (the mortar). Anything that passes through has to either dissolve into the lipid matrix, slip between the bricks, or use one of the appendages (hair follicles, sweat ducts) as a shortcut. This sorts molecules into three rough categories. Small lipophilic molecules (essential-oil constituents, plant resins, vitamin A derivatives) cross relatively easily. Small hydrophilic molecules (water itself, glycerin, urea at low concentrations) cross less easily but do cross. Large molecules of either type (most plant polysaccharides, proteins, mucilages) sit on the surface and rarely cross at all.

What this means for plant medicine Several practical conclusions follow.

First, the active medicine in most topical herbal preparations is not the polysaccharide or mucilage fraction (which sits on top) but the small lipid-soluble fraction (which crosses the barrier). This is why infused oils — which preferentially extract the lipid-soluble fraction — are the dominant topical preparation in herbal practice. A water-based plant preparation applied topically wets the skin pleasantly and may calm surface inflammation, but it does not cross the barrier the way an oil-based preparation does.

Second, products that occlude the surface (thick salves, balms with high beeswax content) work partly by trapping moisture in the skin and partly by holding the medicine in contact with the surface long enough for it to gradually cross. A salve is in part a delivery vehicle and in part a barrier.

Third, the skin is not the same everywhere. The skin over the eyelid is 0.5 mm thick; the skin on the palm of the hand is 1.5 mm thick. Forearm skin absorbs much more readily than skin on the back. Genital skin absorbs more than almost any other site. Damaged or inflamed skin absorbs more than intact skin. Formulating for a face product is different from formulating for a hand-and-foot balm.

Hair follicles and the shortcut route Hair follicles are tiny channels that bypass the stratum corneum entirely and lead down into the dermis. Their density varies enormously across the body — high on the scalp and face, low on the trunk. Lipophilic actives delivered with a vehicle that moves along the follicle (most plant-infused oils do) effectively skip the barrier. This is why scalp oils for hair-and-scalp conditions work better than the surface area would suggest, and why face-care formulators pay more attention to follicular delivery than body-care formulators.

Where systemic effects come from A small fraction of any topical preparation crosses into systemic circulation. For lipophilic, low-molecular-weight constituents — terpenoids in essential oils, certain alkaloids in concentrated infusions — the dose is small but real. This is why some oils that are safe in normal cooking-quantity ingestion are still cautioned-against during pregnancy when applied topically over large areas (clary sage, rosemary, fennel). The dose math matters. A drop of essential oil in a tablespoon of carrier, applied to a 2-inch area on the arm, delivers approximately the same constituent load as a milliliter of a strong tea — small but not zero.

What to carry forward For the rest of the course, every formulation choice should be examined through the question "what is this product trying to do — sit on the surface, soak in, or cross the barrier — and have I built it to do that?" An occlusive barrier asks for more wax. A penetrating moisturizer asks for less wax and a more skin-affinity carrier oil. A surface anti-inflammatory may not need the deepest penetration. Next lesson, we dig into carrier oils — the most consequential single choice in any topical formulation.