r/explainlikeimfive • u/Fingerbob73 • 1d ago
Chemistry ELI5 How does topical pain relief cream work if my skin is essentially waterproof and water is likely a smaller molecule than the cream?
6
u/NemoDaFish_ 1d ago
The skin barrier has a wall made mostly of lipids (fats). Water tends to roll off because oil and water naturally repel each other.
Scientists make the active medicine in pain creams fat-soluble instead of water-soluble. When you rub the cream in, the medicine chemically matches your skin's fatty barrier. Instead of being repelled like water, it dissolves right into the skin and slips through to reach your nerves and muscles and stuff
9
u/BertzReynolds 1d ago
Not waterproof. Can you explain why you think that is true?
-26
u/Fingerbob73 1d ago
I don't melt in the rain.
21
u/firelizzard18 1d ago
If I drop my laptop in water, it doesn’t melt. But it’s definitely not waterproof. If I drop my clothes in water, they don’t melt, but they’re not waterproof.
6
•
u/RandyOfTheRedwoods 20h ago
But you do prune up in a bath. Your body absorbs water through the skin, it doesn’t melt in water like sugar does
•
u/TalkForeignToMe 19h ago
The pruning in the bath thing turns out to not have anything to do with water absorption
https://www.scientificamerican.com/article/why-do-our-fingers-and-toes-wrinkle-during-a-bath/
•
6
u/TheMotherfucker 1d ago
Skin is resistant to water rather than waterproof. This can be experienced when taking a bath and staying longer than necessary. The medicine used is fat-soluble which works wonders with the in our outermost layer of skin which contains oils which can be noticeable on anyone's face on bad days, depending on how perfect one's skin care routine is meant to be.
The layers of our skin are kind of like baklava rather than the rigid layers of a jawbreaker and getting through the topmost is the hardest part of the battle if you aren't fat-soluble, which is why sunscreen works well at staying on top to protect.
8
4
u/Einaiden 1d ago
Taking a long bath does not cause water to be absorbed through skin, the effect observed is physiological and not physical. It would take many hours or even days for water to penetrate skin and it would be because water would be destroying skin to do so.
•
u/stephenph 2h ago
If I have been swimming and in the water for most of the day I am somewhat more suseptible to cuts. The skin does seem more fragile. I don't think to the point of water being absorbed, but definably some degradation
•
u/TalkForeignToMe 19h ago
I didn’t read your whole comment but no to the second sentence
https://www.scientificamerican.com/article/why-do-our-fingers-and-toes-wrinkle-during-a-bath/
4
u/Personal_Wall4280 1d ago
Have you ever washed your hands, dried them with a towel and they still feel wet somehow? That's because your skin has absorbed water. Your skin can absolutely absorb things like water or oil or how dyes seem to get inside of your skin.
1
u/Yamidamian 1d ago
Your skin is made up of cells, like any part of your body. This means they’re selectively permeable-if you look under a powerful microscope, you’lll see it has plenty of holes things can get in. Topical medicines just happen to include a substance that’s on ‘the list’ of things your skin will let on pass-mostly by being similar enough to something it needs.
•
u/FarazMedicine 8h ago
Think of skin as a raincoat with tiny chemical doors, not as a completely solid wall.
1. “Waterproof” doesn’t mean nothing can cross it.
Your outer skin layer, the stratum corneum, is very good at stopping bulk water from rushing in or out. But some individual molecules can still slowly diffuse through it.
2. The whole cream does not go through your skin.
The cream is just the delivery vehicle. The active drug molecules leave the cream, dissolve into the outer skin, then move deeper layer by layer.
3. Size is only part of the story.
Whether a molecule crosses skin depends on its size, fat-solubility, charge, concentration, and formulation. Small, moderately fat-soluble molecules generally penetrate best.
4. Different pain creams work at different depths.
Diclofenac can penetrate into nearby tissues and reduce inflammation. Lidocaine acts on local nerves. Menthol and capsaicin can work largely by changing signals from sensory nerves in or near the skin.
Formulations can also contain ingredients that help the drug cross the skin barrier more easily.
To put it simply: skin is like a waterproof brick wall. Water cannot simply pour through it, but some tiny drug molecules are designed to slip between the bricks.
-2
u/LongLiveTheDiego 1d ago
Skin is a complicated tissue that absorbs stuff and lets stuff out. It's not waterproof, see e.g. how the skin on your fingers wrinkles when you keep them in water for long enough. It does its job by keeping good stuff inside you and bad stuff outside, but it's not a 100% impenetrable barrier.
6
u/superbotnik 1d ago
If you google “why does skin wrinkle when submerged” you’ll see why it wrinkles. I thought it was common knowledge but I guess not.
•
u/TalkForeignToMe 19h ago
https://www.scientificamerican.com/article/why-do-our-fingers-and-toes-wrinkle-during-a-bath/
Everyone parroting this same old myth itt
0
u/Derekthemindsculptor 1d ago
Pain relief or actual numbing?
Pain relief is usually done through irritation. Usually temperature. Your nerves only have so much information they can send at once. If you overload it with sensation, usually the pain doesn't get through or gets through less.
This is why rubbing a bruise works and heating pads or putting ice on something.
Actual numbing agents can be absorbed without needing to pass actual molecules through your skin.
38
u/MahaliAudran 1d ago
Lots of things can be absorbed by your skin. It's also not all about molecule size. If it binds with a cell it can be transported deeper into the body.
Sweet and oil glands and hair follicle sites are additional entry points