World’s first liquid transistor could usher in a new era of liquid computers

For decades now transistors, those tiny electrical switches that process signals and data, and that are the brain power behind every electronic device, from laptops and smartphones to the thermostat in your fish tank, have been shrinking in size, and getting more powerful and pervasive. However, as we increasingly try to build the first generation of soft machines that have the look and feel of soft natural organisms it’s becoming increasingly clear that we need to look beyond today’s rigid materials and transistors and find something new.

Now, two researchers, mechanical engineers Carmel Majidi and James Wissman from the Soft Machines Lab at Carnegie Mellon University who’ve been doing precisely this have announced that they’ve made a breakthrough, and they announced the results earlier this week.

Rather than making circuits from rigid metals like copper or silver the pair mixed Indium and Gallium together to create a special metal alloy that’s liquid at room temperature, and the result was a non-toxic electronic device that can be embedded into rubber to make circuits that are as soft and elastic as human skin.

But that’s not all, and that’s just the start of the story. After their breakthrough they were lucky enough to team up with another researcher, Michael Dickey at North Carolina State University, and it’s then that they discovered that not only is their new liquid metal electronic device a great fit for squishy machines, robots and fabrics, but that it could also be used to make the world’s first liquid metal switches, switches, yes, you’ve guessed it, that are better known to you and I as transistors. And it could open up a whole new era in computing.

The new liquid transistors work like a conventional transistor except in this case they are opening and closing the connection between two liquid metal droplets. When a voltage is applied in one direction, the droplets move towards each other and coalesce to form a metallic bridge for conducting electricity. When voltage is applied in a different direction, the droplets spontaneously break apart and turn the switch to open, and by quickly alternating between an open and closed and open switch state with only a small amount of voltage, the researchers were able to mimic the properties of a conventional transistor, something that is all made possible because they were able to exploit a phenomenon called “Capillary instability.”

"We see capillary instabilities all the time," says Majidi, "if you turn on a tap at home and the flow rate is really low, sometimes you'll see this transition from a steady stream to individual droplets. That's called a Rayleigh instability."

The researchers had to find a way to induce this instability in the liquid metal so it could seamlessly transition from one droplet to two, and after performing a series of tests on droplets within a Sodium Hydroxide bath they realised that the instability was driven by the coupling between an applied voltage and an electro-chemical reaction. This coupling caused a gradient in the droplet's surface oxidation, which then resulted in a gradient in the droplet's surface tension, which finally caused the liquid flow to separate into two droplets, and voila, the first liquid metal “computer” was born.

"We have these two droplets that are analogous to source and drain electrodes in a field-effect transistor, and we can use this shape programmable effect to open and close the circuit," said Majidi, "you could eventually use this effect to create these physically reconfigurable circuits."

Needless to say the applications for this type of new programmable matter are endless, whether it’s in a metallic shape shifting robot or something more mundane, like a military drone or a piece of sportswear, and if materials can be programmed to change shape, which they can thanks to some recent breakthroughs, then they can also change their function depending, on their configuration, or even reconfigure themselves to bypass damage in extreme environments, a capability I’ve already seen elsewhere in the field of self-healing electronics and self-healing materials. And it goes without saying that these two fields, as well as a couple of others, are all complimentary to each other.

"It could be on a structure that's undergoing some very large physical deformations, like a flying robot that mimics the properties of a bird," says Majidi, "when it spreads its wings, you want the circuitry on the wings to also deform and reconfigure so that they remain operational or support some new kind of electrical functionality."

Another massive area of opportunity though includes the ability to build the world’s first liquid computers. For example, think of miniature liquid computers that could interface with biological material to monitor disease in the body or restore brain function to a stroke survivor, wearables to the extreme, and imagine search and rescue robots that can self-assemble new parts when damaged to name just a few of millions of new applications.

Although this might sound like science fiction, one day Liquid Computing could be as commonplace as today’s smartphones, and that’s what makes this new discovery so exciting. The teams work was published in the journal Advanced Science.