Hey Beth, well I think the same rules apply. Your skin is a barrier, evolved to keep most things out, and to keep your parts inside. So it’s really very impermeable to most things. The top layers are made of flattened, dead cell skeletons, with a super tough and insoluble protein (keratin), which is surrounded by a bunch of fats. People describe this as “brick and mortar” if that helps to visualise it. Below that, you have live cells which are linked to each other by proteins that kinda suture them together (tight junctions). Below that you have a basement membrane, which is a layer of protein. In order to reach the dermis (where collagen, elastin etc is), you need to get through all of those layers.
Things which can pass well through the skin tend to be small and fat soluble - so things like ceramides, steroid-based molecules (corticosteroids, oestrogen, testosterone etc). Something like a peptide is relatively large, water soluble, so the penetration through the skin isn’t going to be great. Ceramides are a type of lipid, so they will integrate into the skin (mostly the epidermis) and improve barrier function. Vitamin C has very poor penetration, which I tested for myself in the lab using mouse skin. Some formulations might have a better chance because they mix it with lipids etc.
That said, all of that applies to a sort of model “normal” skin. In reality, skin on your forearm, lip, eyelid, palm etc are quite different and will allow things through at different rates. You also have appendages, like sweat glands, hair follicles etc which some molecules can enter. So I’m not making any sort of blanket statement like “a peptide can never cross the skin”, or “this will never work”, but I am a little sceptical unless there’s some supporting evidence. Also, consider that from a regulatory point of view, if something cross the epidermis and enters the dermis, it has access to the blood stream - that makes it a transdermal drug, not a cosmetic. (Think fentanyl patch, hormone replacement gels etc - they are all transdermal drugs).
Also, I am making an assumption that something needs to cross the barrier in order to “work”. But if we take something like hyaluronic acid, that’s a pretty big molecule which won’t diffuse easily across the epidermis. However, it does get stuck in the epidermal layers and it soaks up water, producing a hydrating and plumping effect. So cosmetically it does work. So maybe exosomes have some similar effects.
Lastly, something like microneedling or nanoneedles totally changes the game because you’re punching holes right through that epidermal barrier.
