What a Topical Can Actually Reach

The stratum corneum is about fifteen microns of dead, tightly packed cells in a lipid matrix, and it evolved to keep the outside outside. Almost everything interesting about topical skincare follows from that.

A molecule crosses it if it is small, if it is fat-soluble enough to move through the lipids, and if it is stable long enough to get there. Miss any of those and the ingredient sits on the surface, whatever the packaging says about it.

The rough rule

Under about 500 daltons, a molecule has a realistic chance of penetrating intact skin. Above it, the odds fall away quickly.

That number does a lot of sorting. Retinoic acid is 300 daltons and gets through. Ascorbic acid is 176 and gets through, if it has not oxidized first. Hyaluronic acid as sold in most serums is hundreds of thousands of daltons and does not; it sits on the surface and holds water there, which is a real and useful effect and a completely different one from the claim on the box.

Growth factors are proteins, typically 6,000 to 30,000 daltons. Whatever happens when you apply them, it is not what happens when a fibroblast encounters one.

What clears the bar

Retinoids. The strongest-evidenced topical in dermatology, with randomized, vehicle-controlled, histologically confirmed data going back to the 1980s. They increase epidermal thickness and dermal collagen, and they improve fine lines and mottled pigmentation over months. The reason people conclude they do not work is that the irritation peaks in the first six to twelve weeks and most people stop inside that window. Two nights a week at a low strength, building up, is the difference between a treatment and a tube in a drawer.

Sunscreen. The only intervention here with a randomized trial measuring visible aging as its endpoint over years. Daily use, not beach use.

Ascorbic acid, when the formulation is still active. The chemistry is the whole story: it oxidizes on contact with air, light and water, and an oxidized serum does nothing. Most of the variation in results between vitamin C products is a packaging and stability story rather than an ingredient one.

Azelaic acid, niacinamide, alpha hydroxy acids. Modest, well tolerated, genuinely useful for tone and texture. Nobody markets them aggressively because nobody owns them.

What is being sold ahead of its evidence

Peptides. Short chains, so size is not the obstacle, and some do have signaling activity in the laboratory. The trials in human faces are small, short and mostly funded by the ingredient supplier. Reasonable to use, not reasonable to pay a premium for over a retinoid.

Growth factors and exosomes. Interesting biology with a delivery problem that the marketing does not mention. The most plausible use for either is on skin that has just been resurfaced, where the barrier is temporarily open and the mechanism is closest to the biology. That is also where almost no controlled data exists yet.

PDRN and polynucleotides. The current arrival, mostly from Korean brands, usually derived from salmon DNA. Injected, there is a small body of evidence for skin quality. In a cream, the molecule is very large, and the claim rests on delivery technology that varies enormously between products and is rarely described. Watch this one; it may earn a better grade. It has not yet.

The two questions worth asking about any product

What is the active, and at what concentration? A formula that lists twenty-four actives has meaningful amounts of none of them. Ingredient lists are ordered by concentration down to one percent, and everything below that is in whatever order the brand prefers, which is where the interesting-sounding molecules usually sit.

What is the evidence, and who paid for it? Almost every study in this category is manufacturer-sponsored, which does not make it wrong and does mean independent replication is the thing to look for. A product citing a study on its ingredient, run by the ingredient supplier, in twenty people, over eight weeks, has told you very little.

What this cannot do at all

Nothing applied to the surface addresses volume loss, tissue descent, or a line etched by years of muscle movement. A cream works on the top two layers of a structure whose problems are mostly underneath them, and no delivery technology changes that.

That is not a reason to skip topicals. It is a reason to expect the right thing from them: better skin quality, better tone, slower accumulation of damage, and a face that photographs better in ordinary light. Those are worth having, and they are what a good routine buys.