Interesting about the generic.

Brand Lidoderm is all I use. I haven't needed a refill yet.

Lidoderm's adhesive is aqueous based. If you leave one out like I did by accident (a used one), outside where it can absorb dew (I took one off my knee on vacation once and left one on the dock by accident)...
it will swell up and get viscous(jelly-like) .... I think there is something like a gel that swells up on the patch. Sort of like those Soil Moist bits for gardening that swell up when exposed to water.

I remember when Lidoderms were experimental, I read somewhere that getting a good adhesive base was a big challenge and difficult during development.

We used to get endless calls from the nursing homes from the nurses about proper placement and improving adhesion in the long term care patients.

This RXlist link gives the inactive ingredients:
from http://www.rxlist.com/lidoderm-drug.htm

I've bolded the major ingredients that would react to moisture in that list. Polyvinyl alcohol is in some artificial tears products too.

I don't have access to the ingredients in the generic at this time however, to compare

This link shows the aqueous nature of the adhesive layer patent from 1998:
http://www.drugs.com/availability/generic-lidoderm.html

It is important to keep any cut portions of a whole patch in a waterproof sealed ziploc with its backing still on it and intact. I use the original envelope folded down, and placed then in an airtight (air squeezed out) ziploc to prevent the gel backing layer from drying out on any cut portions that I don't use immediately. I use this part up first next application before opening another whole patch.