Electric eels stun their prey with modified muscle cells called electrocytes. Like electrocytes, the jelly-like materials developed by Cambridge University researchers have a layered structure that makes them capable of delivering an electric current.
The so-called jelly batteries can stretch to over ten times their original length without affecting their conductivity – the first time that such stretchability and conductivity has been combined in a single material, the team reported. Their results are detailed in Science Advances.
The ‘jelly batteries’ are made from hydrogels, which are polymer networks that contain water. The polymers are held together by reversible on/off interactions that control the jelly’s mechanical properties.
The ability to precisely control mechanical properties and mimic the characteristics of human tissue makes hydrogels suitable for soft robotics and bioelectronics; but they need to be conductive and stretchy for such applications.
In a statement, first author Stephen O’Neill, from Cambridge’s Yusuf Hamied Department of Chemistry, said: “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. Typically, conductivity decreases when a material is stretched.”
“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.”
Conventional electronics use rigid metallic materials with electrons as charge carriers, while the jelly batteries use ions to carry charge, like electric eels.
The hydrogels adhere strongly to each other because of reversible bonds that can form between the different layers, using barrel-shaped molecules called cucurbiturils. The strong adhesion between layers allows for the jelly batteries to be stretched, without the layers coming apart and without any loss of conductivity.
The properties of the jelly batteries make them promising for future use 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 Professor 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.”
In addition to their softness, the hydrogels can withstand being squashed without permanently losing their original shape, and can self-heal when damaged.
The researchers are planning future experiments to test the hydrogels in living organisms to assess their suitability for medical applications.
The research was funded by the European Research Council and the Engineering and Physical Sciences Research Council (EPSRC).
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