How long should a topical stay on the skin to be absorbed?

[harold]

A lot of people have said that as soon as a topical dries out that it is no longer going to diffuse across the skin. I have been looking a lot of places for info on this and it seems to be sketchy at best.
The above rule would imply that a hydroalcoholic solution only has about 5 to 10 minutes to get its point across before it can be washed off. However I did find a study on PubMed with minoxidil which found that after 11 hours there were still significant increases in the amount of topical being absorbed over the 1 hour mark. They discussed a bit about the significance of the alcohol and water evaporating and the PPG solution being left behind. Does PPG stay on the skin in liquid form for that long? I guess that is why it is so horribly greasy.
Other studies looked at the absorption of metals and focussed on the role of sweat and sebum in ionising the metals and allowing them to be absorbed. Is any effect due to a dry residue of minoxidil or any other topical after the vehicle has dried merely due to the small amount of moisture inherent to the skin? Is there likely to be much harm in leaving a hydroalcoholic solution on the scalp for 10 minutes and then washing your hair? Keto seems to do at least something when used as a shampoo for periods of 5 minutes.
I ask because I have long hair, am switching to a few topicals and the combination of these leaves hair looking kinda shitty. Which really defeats the purpose of trying to keep it. If I could wash it after 10 mins and know that most of the good work was done that would be good. If I cant - I want to know that also.
So what do we know about this?
hh

 

[Jacob]

Liposomal/nanosomal type delivery systems allow for time release of the ingredients..as well as absorb better...etc...

 

[Bryan]

A lot of people have said that as soon as a topical dries out that it is no longer going to diffuse across the skin. I have been looking a lot of places for info on this and it seems to be sketchy at best. The above rule would imply that a hydroalcoholic solution only has about 5 to 10 minutes to get its point across before it can be washed off. However I did find a study on PubMed with minoxidil which found that after 11 hours there were still significant increases in the amount of topical being absorbed over the 1 hour mark. They discussed a bit about the significance of the alcohol and water evaporating and the PPG solution being left behind.

There was another excellent pair of studies on that same general theme done by another pair of docs (citations available on request). Their in vitro experiment measured the topical diffusion of minoxidil through skin when it was applied in vehicles consisting of different ratios of ethanol and propylene glycol. The gist of their findings was that the alcohol helps the minoxidil penetrate skin better, but balancing that is the fact that alcohol also evaporates quickly, leaving the minoxidil behind in a useless powdered state. Having lots of PG in the vehicle keeps the minoxidil in a dissolved state and in contact with the skin for much longer periods of time, at the cost of not having as much penetrating power. If I remember correctly, the vehicle which had the best overall results at pushing minoxidil through the skin was the one containing 90% alcohol and 10% propylene glycol (water wasn't included in any of the tested vehicles).

Does PPG stay on the skin in liquid form for that long? I guess that is why it is so horribly greasy.

Yes, although propylene glycol itself will tend to be absorbed into the skin, too. It won't stay there forever, but it'll hang around for quite a while, I believe.

BTW, I did an experiment once with pure propylene glycol which had me momentarily bamboozled: I was interested in just how long it takes for PG to "dry-out" when sitting in the open air (I wasn't sure just how volatile the stuff is, or if it even dries-out AT ALL), so I put a little bit of pure PG into an open container and then put the container onto my digital scale (the scale is accurate to 10 mg), and just sat down and watched it for a spell. You can imagine my surprise when, after a half-hour or so of watching it, the (apparent) mass of the PG on the scale not only hadn't gone DOWN, it was actually slowly going UP!! :dunno:

After thinking about it for a moment, I realized what was happening: it was a warm Spring day here in humid Houston when I was doing that test, and I had the windows open. The very hygroscopic PG was slowly ABSORBING MOISTURE from the air, adding to the mass on the scale, which my sensitive scale was able to detect after just a few minutes! I thought that was pretty cool!

Other studies looked at the absorption of metals and focussed on the role of sweat and sebum in ionising the metals and allowing them to be absorbed. Is any effect due to a dry residue of minoxidil or any other topical after the vehicle has dried merely due to the small amount of moisture inherent to the skin?

I suppose that might also be a small factor...

Is there likely to be much harm in leaving a hydroalcoholic solution on the scalp for 10 minutes and then washing your hair? Keto seems to do at least something when used as a shampoo for periods of 5 minutes.

Yes, but keep in mind that when keto and (probably a better example) NANO are applied as shampoos, they're going in there when the scalp is as thoroughly hydrated as it's ever going to be! It's not the same situation as when you merely apply a minoxidil solution to a DRY scalp, then wait 5 minutes and wash it off.

It's obviously not going to do any HARM if you do it the way you describe, and you'll even get a certain reasonable amount into your hair follicles, mainly because of the nonlinear absorption of topical minoxidil into scalp/skin (it seems to happen much faster at the very beginning, then slows-down considerably over time); but if you're willing to accept that it won't be as efficient at absorbing the minoxidil that you apply, then that's fine.

I ask because I have long hair, am switching to a few topicals and the combination of these leaves hair looking kinda shitty. Which really defeats the purpose of trying to keep it. If I could wash it after 10 mins and know that most of the good work was done that would be good. If I cant - I want to know that also.
So what do we know about this?

I'd be afraid to hazard a guess as to just HOW MUCH of the good work would be done after only 10 minutes, but I would imagine it would be reasonably significant.

 

[harold]

Thanks for the reply Bryan. Given that moisture seems to be such an important factor in the absorption of a topical and thefact that so much of minoxidil and pretty much anything else gets left behind on the scalp after application do you think that a simple rewetting of the scalp with water/alcohol would be almost as effective as a second dose of minoxidil/whatever else later in the day if you are not going to wash the stuff off your scalp?
hh

 

[Bryan]

Yes, I think that's a distinct possibility.

 

[harold]

Thanks for responding Bryan.
hh