Friday, March 01, 2013

Giving the Skin Off My Back

For very personal reasons, this picture on the right instantly captured my interest like few others ever have. The personal reasons are that I once looked like this, or something close to it. Let me explain.

On more than one occasion in my career, I've worked on developing medical adhesives. The difficulty in developing them is that they end up on human skin, which is very, very different than other "standardized" surfaces such as the various metals or plastics available, something I've discussed in more detail in the past. Just one important difference is that the peel tests measure the force not only of removing the adhesive from the skin but also the force required to deform the skin. Initially you test the adhesives on those artificial surfaces, but ultimately you need to move to human skin. That means you need a volunteer, and I certainly wasn't going to expose someone to something that I wouldn't test on myself. So it was off with the shirt to expose my rippling six-pack abs, bulging python arms no...the pasty white skin on my back. The back was the preferred substrate for testing since it was large and relatively flat (the measure peel strength of an adhesive will vary with the peel angle, so a flat surface is important for maintaining a constant peel angle). Someone would stick the pieces of tape to my back in parallel lines on both sides of my spine, with the tapes running perpendicular to the spine. As you would expect during development, there were multiple iterations in the formulations. Some weren't sticky enough while others were more than a bit too aggressive, which means that I have literally given "the skin off my back" in order to please clients. (How many other chemists can say that?) [*]

The picture and the research were an embarrassing wake-up call to me as I never knew skin was mechanically anisotropic in the planar direction. Skin in under anisotropic tension are caused by the orientation of the underlying collagen fibers. This anisotropy could have a large impact on measured adhesion values if the principal directions varied inconsistently with the direction of the tape. So now you may see why that picture was so captivating. The researchers were not only measuring the tensile strength of the skin, but also at different orientations. The underlying dashed lines in the picture are Langer lines, which are believed to indicate lines of tension in the skin. Langer originally discovered them indirectly 150 years ago but cutting circular holes in the skin and noticing that they deformed into ellipses. The point of this latest research was to pull ASTM dogbones to directly measure the mechanical anisotropy and see if in fact the Langer lines are correct. Not surprisingly, the researchers found that in fact the tensile properties of the skin are consistent with the Langer lines. And fortunately the lines run perpendicular to the spine, meaning that all that back-testing data is just fine. Wheeew.

[*] Some of the adhesives also ended up on EKG electrodes, which meant that I would be wearing some of those buggers on my chest. My wife used to joke that if I ever had a heart attack, the EMT's would be shocked to cut open my shirt only to find that I was "pre-wired" and ready to go.

1 comment:

AB said...

Interesting article. Came across this blog for the first time and found it really interesting. I am a polymer engineering student from India and hope to regularly interact with you.