Soft battery can stretch by 1500%
Researchers in the UK have developed a soft, stretchable battery that could be used for wearable devices or soft robotics or even implanted in the brain.
The researchers from the University of Cambridge used layered hydrogels for the self-healing soft battery that can stretch to fifteen times its original length without reducing the conductivity. This is the first time that such stretchability and conductivity has been combined in a single material, says Stephen O’Neill the Department of Chemistry, although several research teams around the world are working on stretchable and printable battery technology.
Hydrogels are 3D networks of polymers that contain over 60% water. The polymers are held together by reversible on/off interactions that control the mechanical properties and it is this ability to precisely control mechanical properties and mimic the characteristics of human tissue makes hydrogels ideal candidates for soft robotics and bioelectronics. However, they need to be both conductive and stretchable for battery and medical electronics.
“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 O’Neill. “Typically, conductivity decreases when a material is stretched.”
The formation of a stable interface enables access to a fully stretchable all-hydrogel power source, which can be stretched up to physiologically relevant strains (10 to 50%) while maintaining a stable voltage output of 115 to 125mV and 248mV for two cells in parallel with 2uA of current, say the researchers.
“Normally, hydrogels are made of polymers that have a neutral charge, but if we charge them, they can become conductive,” said 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.”
The hydrogels stick 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 at 0.1S/cm.
“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 the softness, the hydrogels in the stretchable battery were also surprisingly tough and could withstand being squashed without permanently losing the original shape, and can self-heal when damaged.
Stephen J.K. O’Neill et al. ‘Highly Stretchable Dynamic Hydrogels for Soft Multilayer Electronics.’ Science Advances (2024). DOI: 10.1126/sciadv.adn5142
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