Liquid metals for stretchable electronics

TLDR: Liquid metal particles come together to make a stretchable, conductive film

This week, I’m going to tell you about using liquid metal particles to make stretchable films for wearable sensors, batteries, or energy-harvesting devices.

I love liquid metals for a few reasons:

1. My first research experience was using liquid metals to make stretchable wires.

2. What’s more SciFi than liquid metal?

Today’s paper comes from the lab I worked in as an undergrad. They specialize in using liquid metals in creative ways. Professor Dickey is fantastic at giving demos and has a ton of videos on his website that you should check out if you like today’s post.

The Study: Stretchable Liquid Metal Films with High Surface Area and Strain Invariant Resistance

The Authors: Veenasri Vallem, Vidushi Aggarwal, and Michael Dickey from NC State (Go Pack!!!)

Big Takeaways

1. EGaIn is a conductive, low-toxicity liquid metal that can be easily worked with at room temperature

2. Making liquid metal particles from EGaIn increases the surface area, making it a better electrode

3. EGaIn normally has an oxide layer that keeps the particles separated but also reduces their conductivity

4. Treating the particles with acid and a sulfur molecule makes the oxide layer thinner, improving the conductivity while keeping the particles apart

5. Combining the particles with a stretchy tape makes a highly conductive, stretchable electrode useful for stretchable electronics applications

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 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 above). Sulfur reacts with the gallium and makes it so that oxygen has less spots to bond, making the oxide thinner.

Together, the acid removes a lot of the oxide and then the sulfur molecule 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 called VHB. The VHB base 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 surface conductive. A bunch of individual particles doesn’t work.

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.

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.

Neil is a Ph.D. Candidate at Stanford University studying Materials Science and Engineering. He works on combining different forms of drug delivery technologies to make more effective treatments.