You have asked a technical question which is very interesting.
The term "mil" as referencing adhesive thickness does not refer to
millimeters but rather thousands of an inch. Typical pressure
sensitive adhesive thickness is in the range of 0.75 mils (.00075") to
2.0 mils (.002").
Transdermal patches made by Smith & Nephew are described as:
"....IontoPatch features hypoallergenic adhesive that creates a
strong, yet comfortable, bond to the skin through the full range of
motion. The IontoPatch is single-use and disposable."
For skin use, these adhesive coatings are in the lower end of the
range of coating thickness. I know this from having been in the
pressure sensitive tape business for about 30 years having started out
as a development chemist through general management.
Whereas cloth tapes were used for holding everything to the skin when
I first started out (about 1.5 to 2.0 mil adhesive thickness), we now
see perforated film tapes with .75 to 1.25 mils of adhesive used to
provide strong and secure bonds.
3M talks about a transdermal system where the drug is formulated into
the adhesive for maximum effectiveness.
If you click on the PDF document (transdermal glossary) shown on the
web site shown above, you will get definitions of terms for
transdermal products including a definition of of the caliper (another
word for thickness) as I have described it above.
Another PDF document available from 3M's web site explains their
system of having the drug be formulated into the adhesive layer.
Since the 3M system is quite unique, as you would expect from 3M, I
called their technical department to get confirmation of the
adhesive/drug layer thickness. When they call me, I'll post the
information as a clarification to this answer.
Patches similar to those used for electrocardiogram leads utilize a
donut approach to drug delivery. The drug is placed in the center of
the donut-like patch while the adhesive holding it patch on surrounds
the drug. This approach is illustrated in the following web site from
ALZA which is part of Johnson & Johnson:
This field of transdermal drug delivery is a growing
market...but...the basic technology regarding sticking something to
skin is quite old with natural rubber adhesives used at first, then
synthetic rubber adhesives, then acrylic adhesives...starting out with
solvent dispersed adhesives, then water based adhesives, to hot melt
adhesives, to technology that uses electron beam curing instead of
heat. A fascinating subject.
As soon as I receive a reply from 3M, I'll post the
information....but....I am 98% sure the answer will be in the range of
0.75mil to 1.25mil and will eat my hat if it is anything else.
Thank you for asking this question. If I can provide clarification to
the information provided in this answer, please don't hesitate to ask.