Anyone who was handed a soldering iron and some components in school knows just how fragile electronics can be. All it takes is for a single connection to be damaged and your whole circuit is broken.
But what if circuits could withstand stretching, twisting, and even punctures? That’s what scientists at Virginia Tech have developed: soft electronics that are self-healing, and can even be recycled at the end of the product’s life.
Instead of a rigid circuit board with soldered wires, these circuits are built onto a stretchy material with a layer of tiny, electrically conducting droplets of liquid metal. These droplets are dispersed in an electrical insulator, so each individual droplet is kept separate from its neighbours.
To form a circuit, the ‘wires’ are embossed into the surface, allowing the droplets to connect to each other and conduct an electric current.
“We can then locally break the droplets apart to remake circuits and can even completely dissolve the circuits to break all the connections to recycle the materials, and then start back at the beginning,” said Ravi Tutika, Postdoctoral Fellow at Virginia Tech.
These droplets are what allow the circuit to survive a puncture. If the damage breaks the connection, the droplets simply reconnect around the hole.
The researchers have also shown that the circuit can be stretched to over 10 times its original length and current will still flow.
Assistant Professor Michael Bartlett, who led the research, thinks this technology could be used in wearable devices. “If we can create devices that feel like human skin but still perform the functions in current wearable technologies, we could enable wearable devices that feel more like a second skin,” he said.
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“Additionally, circuitry for emerging soft robots needs to undergo bending, twisting, stretching, and folding which are all possible with the soft circuits we are developing.
“We’re excited about our progress and envision these materials as key components for emerging soft technologies,” he added. “This work gets closer to creating soft circuitry that could survive in a variety of real-world applications.”
Why does graphite conduct electricity?
It’s one of the most commonly used materials on the planet – found in everything from pencils to nuclear reactors – but why does graphite conduct electricity?
Answer: the very reason why metals do. “Metals conduct electricity as they have free electrons that act as charge carriers. Graphite is just the same,” says Dr Dong Liu, physics lecturer at the University of Bristol.
As she points out, graphite is made from carbon atoms, which have four electrons in their outer shells. While three of these form a strong bond with other atoms, one electron is left free (and known as ‘delocalised’) in graphite.
Importantly, the layers in graphite are 'aromatic'. Rather than suggesting they smell nice, this means the atoms in a single layer have alternating single and double bonds. This doesn't only strengthen graphite's structure but allows electrons to move freely along the layers.
“You can think of electricity as like a motorway flow,” adds Liu.
“The free electrons are like cars travelling from one end of the material to the other, carrying charge. In other materials, there may not be free cars to make this journey, meaning they're not conductive."