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December 13, 2016

Graphene-enhanced silly putty can measure heartrate and blood pressure

Researchers in AMBER, the Science Foundation Ireland-funded materials science research centre, hosted in Trinity College Dublin, have used graphene to make polysilicone – better known by Crayola’s trademarked name Silly Putty conduct Electricity, and doing so created extremely sensitive sensors. The material is so sensitive that a piece of it pressed against the carotid artery can detect not only the heart rate, but also a persons blood pressure.

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The research was led by Trinity’s Professor of Chemical Physics, Jonathan Coleman, in collaboration with Professor Robert Young of the University of Manchester. According to Trinity College the composite material offers exciting possibilities for applications in new, inexpensive devices and diagnostics in medicine and other sectors. It can be made hundreds of times more sensitive than a traditional strain sensor, something the researchers demonstrated by detecting the footsteps of spiders walking over it. The AMBER team’s findings have been published in Science.

Measuring breathing with ‘G-Putty’

Professor Coleman, Investigator in AMBER and Trinity’s School of Physics worked together with postdoctoral researcher Conor Boland in discovering that the electrical resistance of putty infused with graphene – named ‘G-putty’- was extremely sensitive to the slightest deformation or impact. They mounted the G-putty onto the chest and neck of human subjects and used it to measure breathing, pulse and blood pressure.

The composite material – graphene is basically a sheet of carbon, one atom thick – showed unprecedented sensitivity as a sensor for strain and pressure, being hundreds of times more sensitive than normal sensors. The G-putty also works as a very sensitive impact sensor, able to detect the footsteps of small spiders. The scientists believe that this material will find applications in a range of medical devices.

Unexpected behaviour

According to Coleman the behaviour they found when graphene was added to  the polymer, a cross-linked polysilicone, was unexpected. Silly putty  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.”

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 found with G-putty has not been found in any other composite material, the researchers state. “This unique discovery will open up major possibilities in sensor manufacturing worldwide.”

Graphene flagship research initiative

Professor Mick Morris, Director of AMBER, adds: “Jonathan Coleman and his team in AMBER continue to carry out world-class research and this scientific breakthrough could potentially revolutionise certain aspects of healthcare.” Professor Coleman is a partner in Graphene flagship, a 1 billion euro EU initiative to boost new technologies and innovation during the next 10 years.

Abstract in Science

In the Science journal, Coleman and Bolland write: ‘Despite its widespread use in nanocomposites, the effect of embedding graphene in highly viscoelastic polymer matrices is not well understood. We added graphene to a lightly cross-linked polysilicone, often encountered as Silly Putty, changing its electromechanical properties substantially. The resulting nanocomposites display unusual electromechanical behavior, such as postdeformation temporal relaxation of electrical resistance and nonmonotonic changes in resistivity with strain.

These phenomena are associated with the mobility of the nanosheets in the low-viscosity polymer matrix. By considering both the connectivity and mobility of the nanosheets, we developed a quantitative model that completely describes the electromechanical properties. These nanocomposites are sensitive electromechanical sensors with gauge factors >500 that can measure pulse, blood pressure, and even the impact associated with the footsteps of a small spider.’

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