Graphene Flagship researchers from Trinity College Dublin in collaboration with the National Graphene Institute (NGI) at The University of Manchester, have used graphene to make a polysilicone polymer, known commonly as the novelty children’s material Silly Putty®, conduct electricity. Using this conductive polymer they found that they were about to create sensitive electromechanical sensors.
The team’s findings have been published in the journal Science*.
This research, led by Professor Jonathan Coleman, Trinity College Dublin, in collaboration with Professor Robert Young of The University of Manchester, potentially offers exciting possibilities for applications in new, inexpensive devices and diagnostics in healthcare and other sectors.
Professor Coleman, Investigator in AMBER and Trinity’s School of Physics along with postdoctoral researcher Conor Boland (both seen in the image below), discovered that the electrical resistance of putty infused with graphene (‘G-putty’) was extremely sensitive to the slightest deformation or impact, having a gauge factor >500. They mounted the G-putty onto the chest and neck of human subjects and used it to measure breathing, pulse and even blood pressure. It showed unprecedented sensitivity as a sensor for strain and pressure, hundreds of times more sensitive than current sensors. The G-putty also works as a very sensitive impact sensor, able to detect the footsteps of small spiders.
Professor Coleman said: “What we are excited about is the unexpected behaviour we found when we added graphene to the polymer, a cross-linked polysilicone. This material is well known as the children’s toy Silly Putty. It is different from familiar materials in that it flows like a viscous liquid when deformed slowly but bounces like an elastic solid when thrown against a surface. When we added the graphene to the silly putty, it caused it to conduct electricity, but in a very unusual way. The electrical resistance of the G-putty was very sensitive to deformation with the resistance increasing sharply on even the slightest strain or impact. Unusually, the resistance slowly returned close to its original value as the putty self-healed over time.”
He continued, “While a common application has been to add graphene to plastics in order to improve the electrical, mechanical, thermal or barrier properties, the resultant composites have generally performed as expected without any great surprises. The behaviour we found with G-putty has not been found in any other composite material. This unique discovery will open up major possibilities in sensor manufacturing worldwide.”
In their paper the team show that following the addition of graphene to the polymer it not only became conductive (reaching approximately 0.1 S/m at approximately 15 volume % of graphene) but importantly, it retained its viscoelasticity characteristics. The graphene sheets are able to respond to polymeric deformation in a time dependant manor, forming networks that break and reform during mechanical deformation. This changes the conductivity of the polymeric material, enabling it to sense by deformation.
Following the initial development work at Trinity College Dublin scientists at the NGI at The University of Manchester analysed the structure of the material and were able to develop a mathematical model of the deformation of the material which explains the effect of its structure upon its mechanical and electrical properties.
Robert Young, Professor of Polymer Science and Technology at the NGI said: “The endless list of potential applications of graphene, never ceases to amaze me. We have now developed a new high-performance sensing material, ‘G-putty’, that can monitor deformation, pressure and impact at a level of sensitivity that is so precise that it allows even the footsteps of small spiders to be monitored.
“It will have many future applications in sensors, particularly in the field of healthcare. The collaboration has been undertaken under the umbrella of the European Graphene Flagship, in which Trinity College Dublin and The University of Manchester both play a prominent role. It is an excellent example of what is being achieved in the Flagship programme.”
Learn more: G-Putty sensors – an unexpected breakthrough
For the first time researchers succeeded to place a layer of graphene on top of a stable fatty lipid monolayer. Surrounded by a protective shell of lipids graphene could enter the body and function as a versatile sensor.
The results are the first step towards such a shell, and have been published in the journal Nanoscale on 28 September 2016.
In contrast to previous work, the researchers observed a stable structure when placing graphene on a single layer of lipids. A patent has been submitted for these findings. PhD candidate Lia Lima and co-workers made this discovery under the supervision of chemist Grégory Schneider.
Graphene is a surface material that consists of a single layer of carbon atoms. It is extremely thin, strong and flexible. In addition, graphene is wanted in the technological world for its effective conduction of electricity. The applications of graphene vary widely. ‘Graphene is particularly sensitive and can respond to its environment in the body’, says Schneider. Therefore, future applications for the body are for example biosensors and systems that allocate the right spot for performing diagnosis.
Bonding with graphene
To make graphene suitable for these applications, hard inorganic materials are often used as a support. However, these hard materials are not ideal for the use of graphene in the body. For this reason scientists are looking for soft, organic molecules to bind with graphene, in this case lipids.
Lipids on graphene
Lipids are fats that can be found in the protective layer of a cell – the cell membrane. This membrane consists of a double layer of lipids. When graphene could be placed between these two layers, it could travel through the body freely. ‘A method that is already used with cancer medicines,’ explains Schneider. ‘We made a single layer of lipids in the lab and transferred graphene on top: a first step towards mimicking the cell membrane.’
In their research the scientists discovered that a layer of lipids provides good support to graphene. The researchers used infrared measurements to prove the stability of the lipid layer. They also found that the lipids improve the electrical conduction of graphene. This effect of lipids is promising for future applications. Improvements of electrical conduction make it possible to measure the electrical signals of graphene in the body. These signals tell something about the environment of graphene, like the acidity or the presence of certain proteins.
Eventually graphene could travel through the body when it is stabilised by lipids. ‘However, we still have a long way to go’, says Schneider. ‘The next step is to place a lipid layer on both sides of graphene, like a sandwich.’
Learn more: Travelling through the body with graphene