Image: A prototype flexible sensing element filled with graphene emulsion (Photo courtesy of the University of Sussex).
Functional liquid structures could soon be used to develop wearable health technologies, such as baby sleep suits.
Under development by researchers at the University of Sussex (Brighton, United Kingdom) and the University of Brighton (United Kingdom), the functional liquid structures are created by emulsification of graphene (or other two-dimensional nanomaterials) in water and oil to create functional macroscopic assemblies. Due to the liquid-phase exfoliation of the graphene, the liquid structures exhibit electrical conductivity by inter-particle tunneling. Graphene liquid technology is so sensitive, that when a channel or tube holding the liquid is stretched, even by a small amount, the conductivity of the liquid changes.
The technology opens the way for a range of wearable, flexible, liquid sensors, such as a strain sensing application. With a large gauge factor of around 40--the highest reported in a liquid--the sensors could be integrated into 'fitness tracker'-like bands, or even embedded within the fabric of a sensor vest in order to measure respiration rates and movement. The unobtrusive sensors could also be used for anyone with life-threatening conditions such as sleep apnea. In addition, because graphene is cheap to produce, the new breakthrough should be affordable. The study was published on January 9, 2018, in Nanoscale.
“What we've done is similar to how you might make a salad dressing; by shaking together water and oil, you make tiny droplets of one liquid floating in the other because the two don't mix,” said lead author Matthew Large, PhD, of the University of Sussex. “Normally, the droplets would all collect together and the liquids separate over time, like the droplets in a lava lamp. We've resolved this by putting graphene in. The graphene, which is an atom thick, sits at the surface of the droplets and stops them from coalescing.”
“What's quite exciting about this new type of conductive liquid is how sensitive it is to being stretched. When the graphene particles are assembled around the liquid droplets electrons can hop from one particle to the next; this is why the whole liquid is conductive,” concluded Dr. Large. “When we stretch our sensors we squeeze and deform the droplets; this moves the graphene particles further apart and makes it much harder for the electrons to hop across the system. The sensitivity of this new kind of strain sensor is actually much higher than a lot of existing technologies, and it is the most sensitive liquid-based device ever reported by quite a significant margin.”
Graphene is a monolayer atomic-scale honeycomb lattice of carbon atoms which combines the greatest mechanical strength ever measured in any material (natural or artificial) with very light weight and high elasticity. It also has unique optical and photothermal properties that allow it to release energy in the form of heat in response to light input, and very high electrical conductivity. Andre Geim and Kostya Novoselov of the University of Manchester (United Kingdom) were awarded the Nobel Prize in Physics in 2010 for its development.
University of Sussex
University of Brighton