Soft, stretchy, and powerful: liquid metal electronics

The Problem

Liquid metals are exactly what they sound like; metals that are liquid at room temperature.

Thinking of mercury? Me too. Thankfully, there are other liquid metals that won’t kill you that we can use.

One I’m particularly fond of is gallium. When you combine gallium (Ga) with indium (In) at the right concentrations you get an alloy called EGaIn that’s liquid at room temperature.

You can break EGaIn into smaller pieces and get particles that hold their shape due to an outer layer called an oxide. The oxide forms when gallium reacts with oxygen in the air to form GaO.

For many electronic devices or batteries, you want high surface area electrodes to get the best function. EGaIn particles provide this high surface area while being stretchy (because they’re liquid).

However, the oxide layer that lets the particles hold their shape also makes them less conductive.

So, we want conductive small particles for their high surface area but when we make the particles small they stop being conductive. Pretty inconvenient.

The Solution

The authors solved this puzzle by finding a sweet spot where the oxide layer is thick enough to keep the particles from combining but thin enough that they’re conductive. They used chemistry tricks to get rid of some oxide and prevent it from coming back.

Adding an acid (HCl) removes the oxide layer. At the same time, they add a sulfur molecule (called Thiol in the picture above). Sulfur reacts with the gallium and makes it so that the oxide isn’t as thick.

Together, the acid removes a lot of the oxide and then the sulfur stops it from coming back. Perfect! Now they have EGaIn particles that keep their shape but are still conductive.

Next, they took these liquid metal particles and put them onto a stretchy, sticky tape-like material. The stretchy tape combined with the EGaIn particles makes a versatile stretchable electrode.

However, there needs to be a continuous path of connected particles to make the whole device conductive. A bunch of individual particles doesn’t work, they need to connect.

So, the last step to make the finished electrode was to connect the particles. Thankfully, all the authors needed to do was stretch their films and the particles connect.

When stretched enough, the surface oxide breaks and lets the particles combine. Stretching more makes more particles connect, increasing the conductivity.

You can see particles connecting in the images below showing an unstretched film (A, left) and a stretched film (B, right). The red circles show where particles have joined together.

The liquid metal films stretch over 600% of their original length. The further they’re stretched, the more particles connect, and the more conductive the films are. This makes the films useful as stretchable sensors.

The higher conductivity can be used to figure out how far the films are stretched, and therefore how far any device made from the films is stretched. You can imagine this being useful in wearable electronics or soft robotics.

In all, the authors use some tricks to make liquid metal particles hold their shape, be conductive, and make stretchable electrodes that would be useful in many different applications.

See you next week for more science,

Neil