https://www.mdpi.com/1424-8220/12/5/6685/htm

This is one of a (the) best papers I've seen.
Typically, strain gauges are made from constantan (micro wires), the resistivity changes are typically in micro ohms.

Using graphite, amorphous carbon, makes so much difference it makes that variation ohms instead of micro ohms. And the materials are exceedingly accessible / easy to obtain, A4 paper and 2B pencils, then maybe copper foils etc, and it can be used to make sensitive weighing balances e.g. with 3d printed or even hand made sensors etc.

With this one could even get away without amplification and use things like a simple microcontroller to measure weights.

 

No mention is made in that article of the effect of humidity on the force sensor performance. I would expect it to be rather significant. The Young's Modulus of soggy paper is very different from that of bone-dry paper and paper can be hygroscopic.

 

humidity can be partially alleviated using PDMS (silicone caulk), epoxy, AC glue etc.
but it is at least useful for a few things, basic force sensors where accuracy isn't the most critical, i.e. detect a push or a pull.
this can normally be achieved in the same way using a tactile button. but this graphite / amorophorous carbon technique makes it possible to make a "pressure sensitive mat", it may even be possible to make it a wired or matrix mat so that it can show a 2d pattern of forces or pressures, e.g. to detect where is pressed.

and "cheap" weighing devices, now you won't need an instrument amp (unlike a real constantan strain gauge) to detect strain changes.
real strain gauges still has its place, they are less sensitive to any other factors, temperatures, humidity etc.

 

Interesting. Very preliminary but it looks like it could have applications in one-time use situations like medical and disposable consumer devices. I suspect making this into a practical device will not be a trivial exercise.

I wish they’d talked about the piezoresistive mechanism. I surmise is is a matter of the density of the carbon particles changing as they are flexed, something like a carbon microphone, but that’s just speculation.

 

this is better, Nickel doped PDMS

this won't have humidity issues

these 'cheap' force sensors, make it possible to make rather 'cheap' robots that can 'touch', e.g. to hold an egg without crushing it.
Actually, real constantan strain gauges isn't expensive as well, just that real constantan strain gauges has resistance variations in microohms.
this requires an instrument amp and tight tuning and tolerances to balance the bridge. That in itself makes it much harder, but real strain gauges are (much) more accurate.

 

material wise, the key is that (semi) conducting material amorphous carbon, graphite etc, that "paper" can possibly be substituted by something else. Then even the conductive material can be changed e.g. in the above nickel, it is likely more stable than amorphous carbon. But that this makes it a 'technique' or even 'technology' to measure strain (hence stress, force, pressure) "cheaply" (e.g. a microcontroller and perhaps with a little amplification).
I think these are offshoots of experiments in "conductive ink", the original aim is being "conductive", then this resistive strain behaviour is the "discovery".

Apparently this behaviour is not limited to constantan
https://en.wikipedia.org/wiki/Gauge_factor
https://en.wikipedia.org/wiki/Piezoresistive_effect