Density - resistance - tactile

I'm an artist with medium-low experience with electronics. I'm working on an interactive project where I'd like to measure the density or resistance of someone's skin. Their mushiness if you will. I initially thought I could work with flex sensors with a bit of math distance-flex-speed but I find them unreliable and would need more precision. Now I'm researching range finders, photocells with and LED (in sliding tubes), linear encoders, incremental capacitive transducers (like in digital callipers).
I was wondering if anyone could point to something they've seen in this area or suggestions.
Thank you for your time

Based on my wife's experience as a medical technologist, the only way to do what you desire is ti ACTUALLY penetrate the skin. Perhaps you need a different project.

Maybe expand more on what you are doing with the data and we might think of alternatives. My first thought would have been flex sensors or resistive pressure mats.

The resistance highly depends on the size of the surface being in contact with the probe.
Every such probing needs 2 points of contact and the distance between them.
The humidity of the skin also plays a roll.

The data is only used comparatively. My first thought was flex sensors also but I dont find them accurate enough. The equivalent action irl would be poking someone with your finger. There are touch sensors but they are limited for the application and more for the user touching it than the other way around.
Thanks for your thoughts
Screen Shot 2022-03-09 at 3.17.14 PM

True. Although I'm looking for accuracy for the depth, actual skin resistance is not important, as in the data isn't for scientific use. I should add that it could also be on clothing not skin only.
Thank you

lol, no I wont be cutting people up and it isn't literal 'skin only' but rather body density in one area. Like the Pilsbury dough boy when he gets poked hou hou. How soft is he? Ideally it would be similar to a digital calliper that would be spring loaded. You'd get a precise .00 mm reading.
Measurement at the beginning of touch for a distance of 5mm (never mind speed). On me the calliper might read 0.2mm because i'm 'soft' and someone that's hard would read 5mm
Thank you

Can you point to a medical device that does something similar to what your project will do?

Use a linear actuator and monitor current.
Or put a small FSR on the tip of a linear actuator.

Neither will be very precise, but they may be good enough for an art project.

None that's i've seen but I'll do some research in that area. Always difficult finding the proper terms to find what you want.

Monitoring current is an interesting idea. The FSR are not well suited for poking and wouldn't give out accurate enough ..or consistent enough data.. Even though it is for 'art' it is something I take quite seriously.

Thank you

Tekscan makes a 0.3" diameter FSR. I imagine that wouldn't be too hard to put it on the tip of a linear actuator or other poking device. Can you specify your accuracy, precision, repeatability, and other technical requirements? Are these good enough?

(Tekscan's specs for their A101)

Thank you for the research. I'll order some and try. I have a couple of items on their way to test out

A short length of silicon tubing fused into a balloon on the end of a small air pressure sensor would be very accurate and repeatable. What size are you aiming for?

So far, I think the FSR idea is the best one. They come in very small sizes and can be repeatable: I have seen them used in medical therapy applications.

Yes I like the pressure sensor idea but haven't looked into it. The contact area would be approximately 1"x1". I'm still looking into linear encoders right now. I think my biggest concern is 1.repeatability 2.accuracy. The distance would only be 1/2"
Thank you

How do you think you will use a "linear encoder"?

I thought if it was weighed or spring loaded.
The movement that's poking (call it Z) would continue moving when the encoder starts a reading (starts poking the person) an extra -10 mm.
The linear encoder reading should approximate a density. So a mushy person's reading for the 10mm travel might be 2mm while a a knuckle might read 9.5mm

This... not clear to me.

I think what you are trying to say is:

  1. Use a spring or a weight to drive something into the body (but not so hard as to inflict pain :slight_smile: )

  2. Use an encoder to measure how far the thing travels. The travel distance will be larger for a "mushy" person than a "firm" person. Say the travel distance is "X".

  3. Also say the largest possible amount of travel, in jello-bodied blob, is "Y".

  4. Set the "density" Z equal to Y minus X, so Z increases with increasing firmness. For example, Mr. Jello will have Z = zero and Ms. Rock-hard Abs will have Z = 10.

That could work. Of course, the devil is in the details.

Yes it is.
My first goal is to find an accurate way to measure displacement. Second will be to fine tune a method of resistance (spring/weight).
I love the digital calliper incremental capacitive transducer method and accuracy. I'd rather find a simpler and less costly method even if it would mean fabricating 'simple' sliders . Of course fabricating isn't simple. This came up in research.


The simplest, cheapest way to fabricate a precision displacement measuring device is to use a potentiometer. Convert linear motion to rotary motion and put a good quality pot at the rotating axis. This has been used for decades in anything from massive machine tools weighing many tons that required positioning to below 0.001", to tiny hand held devices.

It's only recently that digital methods became cheaper.