PARIS—Playing in their lab, student Conor Boland and supervisor Prof. Jonathan Coleman doing research in AMBER at Trinity College Dublin made a chance discovery by mixing viscoelastic compounds with graphene.
"The idea came from playing in the lab. My student, Conor Boland, thought it would be interesting to mix graphene with the x-linked polymer that makes up silly putty. It was! It turned out that the very soft matrix resulted in quite interesting properties. This started a 30 month research program" told Prof. Coleman in a telephone interview with EE Times Europe.
The resulting paper "Sensitive electromechanical sensors using viscoelastic graphene-polymer nanocomposites" published in the Science journal details how mixing graphene nanosheets to highly viscoelastic polysilicone matrices (also commercialized as children's play material silly putty) gave rise to unusual electromechanical properties that could be exploited to manufacture ultra-sensitive strain sensors.
Playing in their lab, student Conor Boland and supervisor Prof. Jonathan Coleman doing research in AMBER at Trinity College Dublin made a chance discovery by mixing viscoelastic compounds with graphene.
Credit: AMBER, Trinity College Dublin.
What the researchers have shown with their homemade "G-putty" as they call it, is that the dense uniform and isotropic network of graphene nanosheets formed within the low viscosity cross-linked polymer not only drastically increased the electrical conductivity of the compound (reaching about 0.1 S/m at a 15 volume %) but also remained highly mobile and compliant.
This high mobility of the graphene nanosheets means that as a small tensile strain step is applied to the G-putty, a sharp increase in electrical resistance is observed before decaying slowly as the network of graphene nanosheets slowly relaxes, reforming connections and giving a resistance decrease. The researchers were able to develop a quantitative model to describe accurately that post-deformation temporal relaxation of the electrical resistance and the non-monotonic changes in resistivity with strain.
"This network relaxation can be thought of as a self-healing process. Such filler mobility is unprecedented in nanocomposites at room temperature. However, it also represents plasticity, meaning deformations are not fully reversible" the researchers wrote in their paper.
By monitoring their electrical resistance, the G-putty performed as a very sensitive electromechanical sensor with a gauge factor over 500, able to measure pulse and blood pressure (when pressed against the carotid artery) or acting as an impact sensor. In the case of impact on a thin sheet of G-putty, the resistance waveforms show a rapid jump on impact followed by a power law decay. The sensor was able to discriminate the individual footsteps of a small spider which happened to be wandering in the lab.
(A) breathing, and (B) pulse. The inset in (B) shows a single period of the pulse-waveform, with the characteristic dicrotic notch indicated. (C) Fractional resistance change associated with a spider (cellar spider, bottom inset) walking across a thin circular sheet of G-putty about 2mm thick. (Top inset) Magnified response showing individual footsteps. Click here to enlarge image.
Prof. Jonathan Coleman is with his graphene-putty
alongside his son Oisin, trying out the novelty
children's material silly putty.
Credit: AMBER, Trinity College Dublin.
"In a practical device, the putty would have to be mounted in some sort of wristband with appropriate encapsulation. This should be straightforward" told us Coleman.
"Due to the viscoelasticity nature of the composite, the dynamic sensitivity will depend on frequency. This makes the sensor particularly suitable for dynamic sensing at a well-defined frequency such as pulse or breathing" he added.
So would these properties limit the G-putty's use as a strain sensor to relatively fast events?
"It's probably better for fast (higher frequency) sensing as then the elastic part dominates and the viscous response is minimised. However, we have not done high frequency testing yet" concluded Coleman.
Visit the AMBER centre at http://ambercentre.ie/
—Julien Happich is editor in chief of EE Times Europe.