Electric eels inspire University of Cambridge researchers to create stretchable ‘jelly batteries’
Inspired by electric eels, University of Cambridge researchers have developed soft, stretchable ‘jelly batteries’.
They could be used for wearable devices or soft robotics - or even implanted in the brain to deliver drugs or treat conditions such as epilepsy.
Electric eels stun their prey with modified muscle cells called electrocytes.
The jelly-like materials developed by the researchers have a similar layered structure, described as being like sticky Lego, that makes them capable of delivering an electric current.
Incredibly, the self-healing jelly batteries can stretch to more than 10 times their original length without affecting their conductivity.
The results, reported in Science Advances, represent the first time that such stretchability and conductivity has been combined in a single material.
“It’s difficult to design a material that is both highly stretchable and highly conductive, since those two properties are normally at odds with one another,” said first author Stephen O’Neill, from Cambridge’s Yusuf Hamied Department of Chemistry. “Typically, conductivity decreases when a material is stretched.”
The jelly batteries are made from hydrogels, which are 3D networks of polymers containing more than 60 per cent water, and held together by reversible on/off interactions that control mechanical properties.
Since these properties can be precisely controlled and mimic the characteristics of human tissue, hydrogels are ideal candidates for soft robotics and bioelectronics - but for that to work it is essential for them to be both conductive and stretchy.
“Normally, hydrogels are made of polymers that have a neutral charge, but if we charge them, they can become conductive,” said co-author Dr Jade McCune, also from the Department of Chemistry. “And by changing the salt component of each gel, we can make them sticky and squish them together in multiple layers, so we can build up a larger energy potential.”
While conventional electronics use rigid metallic materials with electrons as charge carriers, the jelly batteries use ions to carry charge, as electric eels do.
The hydrogels stick strongly to one another because of reversible bonds that form between the layers using barrel-shaped molecules called cucurbiturils - like molecular handcuffs
The strong adhesion enables the jelly batteries to be stretched, without the layers coming apart or any loss of conductivity.
It means they could be used in future in biomedical implants, since they are soft and mould to human tissue.
“We can customise the mechanical properties of the hydrogels so they match human tissue,” said Professor Oren Scherman, director of the Melville Laboratory for Polymer Synthesis, who led the research in collaboration with Prof George Malliaras from the Department of Engineering. “Since they contain no rigid components such as metal, a hydrogel implant would be much less likely to be rejected by the body or cause the build-up of scar tissue.”
The hydrogels can withstand being squashed without permanently losing their original shape and can self-heal when damaged.
Future experiments will test the hydrogels in living organisms to assess their suitability for a medical applications.
The research was funded by the European Research Council and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI). Oren Scherman is a Fellow of Jesus College, Cambridge.
