Transdermal Delivery, Explained for People Buying Skincare
Penetration, permeation and absorption are not synonyms. What the words mean, the four ways delivery is actually changed, and how to read the claim.
Transdermal delivery means getting a substance across the skin rather than onto it. In pharmacy the word has a strict meaning. In skincare it is used loosely, and the gap between those two uses is where the confusion lives.
The reason transdermal delivery matters to anyone buying skincare is simple. If the thing your product targets sits below the outer layer of your skin, the product has a delivery problem. Almost nothing on the shelf is sold with an answer to it. This is the vocabulary you need to tell the answers apart from the decoration.
What transdermal delivery means in skincare
Two words get used as though they were the same thing.
Topical means applied to the skin, acting locally. A moisturiser, an acid, a steroid cream: the intent is an effect at or near the site, not somewhere else in the body.
Transdermal means applied to the skin in order to cross it and reach the circulation. A nicotine patch is the standard example. The skin is the route, not the destination.
Every cosmetic is legally topical. When a skincare brand writes transdermal on a carton, it usually wants you to think the product goes deeper. A product genuinely designed to deliver a substance into the bloodstream has a different regulatory life entirely.
Penetration, permeation, absorption: three words, three meanings
These get treated as synonyms in marketing and are not synonyms anywhere else.
| Term | What it means | Example |
|---|---|---|
| Penetration | Entering a layer, without necessarily leaving it | A humectant entering the outer layer and staying there |
| Permeation | Crossing from one layer into the next | A molecule leaving the outer layer and reaching the epidermis below |
| Absorption | Uptake into the circulation | A drug from a patch reaching the bloodstream |
This is why "penetrates deeply" is such a durable phrase. It is technically satisfied by a molecule that gets a few cells into the outer layer and stops, and it is doing none of the work the reader assumes. Permeation is the word that would mean something, and it is the one nobody prints.
What a molecule needs to cross on its own
Passive movement across intact skin is not a decision the product makes. It is physics, and three properties decide most of it.
- Size. The 500 dalton rule is the usual shorthand. It is a widely quoted rule of thumb, not a law: passive movement falls away sharply as molecular weight approaches 500 daltons. Collagen and most large peptides are far above it.
- Solubility. The route across is the lipid matrix between cells, so a molecule needs enough oil solubility to enter it and enough water solubility to leave. Extremes at either end perform badly.
- Charge. An ionised molecule is poorly matched to a lipid environment, and many actives are ionised at the pH of the skin surface.
Add a fourth that is not a property of the molecule: the concentration gradient. Movement runs from more to less, which is why a formula has to hold the active available at the surface rather than let it dry off. The stratum corneum covers the structure doing the refusing, and why your serum is not absorbing covers the routes across in full.
The four ways delivery is actually changed
Every real approach falls into one of four groups. They are not equivalent, and they do not cost the same.
| Approach | What it does | The trade |
|---|---|---|
| Hydration and occlusion | Raises water content of the outer layer, which raises permeability | Mild, and it applies to everything on the skin at once |
| Chemical enhancers | Disorders the lipid matrix so molecules move through it more easily | Disordering a barrier is also how you irritate it |
| Encapsulation and vesicles | Changes how the active is carried and released at the surface | Often improves stability and release more than crossing |
| Physical routes | Makes an actual opening, or drives molecules with energy | Requires a device, and the opening is temporary |
The first three modify the barrier or the formula. Only the fourth stops arguing with the barrier and puts a route through it. That is why microneedles, iontophoresis and sonophoresis are grouped together in the delivery literature, despite feeling nothing alike.
Why "transdermal" on a cosmetic label is a marketing word
In the United States the line between a cosmetic and a drug is drawn by intent. A product intended to affect the structure or function of the body is a drug and is regulated as one. A cosmetic cleanses, beautifies, or alters appearance.
So a brand claiming genuine transdermal delivery of an active is describing a drug and selling a cosmetic. Most of them do not mean it. The word is there to imply depth, and it survives because no regulator polices adjectives as closely as claims.
The honest version of the claim is narrower and more useful: what the mechanism is, what it does to the barrier, and what has and has not been measured. That is the format the science record uses.
What a microchannel changes, and what it does not
Microneedling makes temporary openings through the outer layer. It does not enhance permeation, it bypasses the layer that was preventing it, which is a different kind of answer to the same question.
What it changes: molecules that could never cross passively now have a physical route past the barrier, and molecular size stops being the dominant obstacle it is on intact skin.
What it does not change: the opening is temporary, it does not stay open for your evening routine, and depth still decides which layer the channel reaches. The depth guide covers that, and the at-home safety guide covers doing it without damaging the barrier you depend on.
How long the channel stays open
There is one published human answer worth printing, and it needs its caveats printed with it.
Gupta, Gill, Andrews and Prausnitz, "Kinetics of skin resealing after insertion of microneedles in human subjects", Journal of Controlled Release, 2011, followed ten healthy adults after stainless steel microneedle arrays were applied to the forearm, tracking barrier recovery for 48 hours by electrical impedance. Without a dressing, the barrier resealed within two hours for every needle geometry tested. Under a dressing, closure took between 3 and 40 hours, and geometry mattered a great deal.
Those needles were hundreds of microns wide, many times the 8 µm array on the re:tones MTS, and they were pressed in by thumb rather than driven. re:tones has measured nothing of its own here, does not instruct occlusion, and states no closure time for its own channels. It is the closest published evidence to the question, not a specification for this device.
What the study supports is the shape of the thing: the window is short. That is the reason the ampoule is released at the moment the needle meets skin rather than patted on afterwards, and the reason the cadence is weekly rather than nightly.
How to read a delivery claim
- Which word did they use? Penetrates, permeates, absorbs. Only the second and third are claims about crossing.
- Which layer is named? "Deep into skin" names nothing. Epidermis and dermis are layers.
- What is the mechanism? Hydration, enhancer, carrier, or a physical route. If none of the four is described, the claim is decoration.
- What was measured, and on what? In vitro on excised skin is not the same as a human forearm, and a forearm is not a face.
- How big is the molecule? A large molecule with no delivery mechanism is not crossing, whatever the copy says.
Run those five on any bottle. Most of the category fails at step three, and the failure is not a scandal, it is just the barrier doing its job.
Where this leaves a routine
Half your shelf never needed delivery. Humectants, barrier lipids and acids all work at or near the surface, and they are honest products. The trouble starts with anything aimed lower, which is most of what people spend real money on. Whether serums actually work sorts that out row by row.
Delivery decides whether a mechanism can start. It does not decide how fast the mechanism runs once it has, which is still governed by turnover and by the timelines each mechanism keeps. And nothing here replaces applying the products properly, which is how to layer serums. The ampoule finder matches a concern to a position on the dial.
Frequently asked questions
What is the difference between topical and transdermal?
Topical means applied to the skin for a local effect at or near the site. Transdermal means applied to the skin in order to cross it and reach the circulation, as a nicotine patch does. Cosmetics are topical products. A brand writing transdermal on a cosmetic carton is usually implying depth rather than describing a route.
Can skincare ingredients really reach the dermis?
Some small molecules cross the outer layer and reach the epidermis. Reaching the dermis in meaningful amounts through intact skin is much harder. The molecules most people want down there, collagen and larger peptides, are among the least able to make the trip. That is the delivery gap the category tends not to mention.
Do penetration enhancers work?
They do something, and the something has a cost. Chemical enhancers work by disordering the lipid matrix between cells so molecules move through it more easily. Disordering that matrix is also the mechanism of irritation, so the same property that improves movement raises reactivity. They are used carefully in pharmacy for that reason.
Is microneedling transdermal delivery?
It is a physical delivery route rather than an enhancer. Microneedles create temporary openings through the outer layer, so a molecule that could not cross passively has a path past it. The openings are temporary, so timing matters more than in any other approach, and depth decides which layer is reached.
Does the 500 dalton rule mean nothing above 500 gets in?
No, and it was never stated that strictly. It is a widely quoted rule of thumb: passive movement across intact skin falls away sharply as molecular weight approaches 500 daltons. Exceptions exist, the rule says nothing about how much of a small molecule crosses, and it applies to intact skin rather than to skin with a physical route through it.
How can I tell if a delivery claim is real?
Ask which layer is named, which of the four mechanisms is being used, and what was measured on what tissue. A claim that names no layer, describes no mechanism and cites no measurement is a description of how the product feels. That is not a scandal, but it is not delivery either.
Related reading: the stratum corneum explained, skin cell turnover, and why your serum is not absorbing.