The process is predictable, reliable and repeatable, costs only a few pence, and the resulting sensors are durable and comparable in performance with commercial sensors, researcher Dr Alan Dalton told Electronics Weekly.
Medical uses are expected – bands around wrist, chest and neck have been used to measure pulse and breathing, and detect speech, respectively.
“You can run a fast Fourier transform on the waveform to calculate the frequency associated with pulse or breathing,” said Dalton.
Treated bands remain pliable and still stretch to 1,100% of original length – while sensing up to 800%, and have been used to sense vibration above 160Hz and strain rates up to 6,000%/s (at 60Hz, strain is final/initial length).
Connection to the band can be as simple as silver paint.
Getting the band to become a strain-dependent resistor requires inserting enough graphene platelets to maintain conductive paths even when the band is stretched.
How do you do it?
Rubber bands are made from cross-linked natural latex.
The team opens pores in the latex by swelling the bands in toluene.
To diffuse them in, graphene platelets (200nm-1µm across) are dispersed in a mixture of water and the solvent NMP (n-methylpyrrolidone).
“We can control the infusion to control weight fraction. Once there is enough in, they are conductive,” said Dalton. “Graphene is very uniformly distributed. This is a very simple cheap process.”
Although the bands can be calibrated in a known system, their beauty is the simplicity with which movement is converted to changing resistance.
The length/resistance curve is fairly linear, and detailed study of this curve, plus measuring the effect of different amounts of graphene in the latex, suggests changes are due to physical contact between flakes, said Dalton.
This prediction is based on percolation theory – a statistical technique that can be used to predict when there will be long-range physical contact between items dispersed throughout a volume.
“If you take an empty cube, and start to fill it with [free-floating conductive] rods or sheets, at certain point conduction goes from almost zero to some non-zero value,” said Dalton. “Looking at the electronic properties as a function of mechanical properties, the bands have typical percolation behaviour.”
Temperature dependency has not yet been studied, although Dalton expects it to be linked to thermal expansion of rubber, and irrelevant to foreseen applications.
These applications include many medical uses, including some that were previously too expensive to contemplate, or in developing countries.
“Until now, no such sensor has been produced that can be easily made. It could really help remote healthcare,” said Dalton.
The is described in the journal ACS Nano.
Professor Jonathan Coleman from the Trinity College team suggests lightweight sensor suits could be constructed to remotely monitor feeding, breathing, pulse and joint movement in premature babies and other vulnerable patients.