Your favorite electronic devices all rely on the useful properties of metals—namely, the ability to conduct electricity and to transmit heat. Building on the success of technologies like smartwatches and smartglasses, the future of wearable electronics will include e-textiles and other flexible devices (Fig. 1). But while traditional smartphones and desktop computers use heat sinks—blocks of metal attached to heat-generating components—to transmit heat to the environment and keep them at a safe operating temperature, no one wants a rigid block of aluminum sticking out of their shirt! To make devices that are bendable and stretchable, scientists must develop new materials that are soft and "skin-like," closely matching the mechanical properties of biological tissue, but still electrically and thermally conductive like traditional metals.

The solution to this conundrum is . . . liquid metal! While most metals have high melting points and are liquid only at temperatures above 1000 degrees Fahrenheit, a few are are liquid even at room temperature (Fig. 2). The most familiar of these are mercury and gallium. When gallium is mixed with another metal, indium, its melting point is even lower. A mixture of 75% gallium and 25% indium is an appealing metal to use in consumer devices because it is liquid at room temperature (no heating required) and non-toxic (unlike mercury). Some devices are already using liquid metals to improve performance, but they must contend with a critical problem. Liquids lack structural integrity—i.e., they flow!

The Sony PlayStation 5 and Asus ROG laptops, for example, include liquid metal components, but they require an insulating sheet to confine the liquid metal. A new method gets around the problem of liquid flow by dispersing droplets of liquid metal throughout a silicone rubber, creating a composite material that bends like rubber but with ten times greater thermal conductivity. Stretching the metal-infused rubber unlocks its full potential: when stretched, the spherical droplets of liquid metal morph into a needle-like shapes, enhancing the conduction of heat in the direction of the stretch. In its stretched form, the material conducts heat up to 50 times better than the original rubber, and nearly as well as rigid metals like stainless steel!

But can we make needle-shaped liquid metal droplets without having to stretch a rubber sheet? YES! Recently, researchers developed a way to control the metal droplet shape by adjusting the height and speed of a 3D printer nozzle. The 'ink'  consists of spherical metal droplets dispersed in silicone. When the printing nozzle is set farther from the print bed and moved slowly, the liquid metal droplets in the ink maintain their original, spherical shape. But when the nozzle is brought very close to the print bed—less than 0.1 mm, the thickness of a piece of paper—and moved at a faster speed, the liquid metal is physically transformed at the printing nozzle from spherical to elongated, needle-like droplets (Fig. 3).

Using this method, we can selectively control the shape and orientation of the liquid metal droplets to create patterns in the composite. Fig. 4 shows a 3D-printed film containing mostly spherical droplets, but with an N-shaped region containing needle-like droplets. When the film is heated, the greater thermal conductivity of the needle-like droplets allows the N-shaped region to release heat more easily, so it stays cooler than the surrounding material.

And this 3D printing method has yet another trick up its sleeve! In Fig. 4, the film conducts heat, but since the liquid metal droplets are not connected to each other, there is no path for electricity to flow. We can create electrically conductive regions in the film, however, by rapidly tapping the nozzle on the needle-shaped droplets after printing. This tapping motion forms liquid metal connections, similar to smearing the ink from a marker or ball-point pen. (In fact, scientists have used a method very much like this to "draw" liquid metal circuits—see the post "Using Liquid Metals to Draw Functional Circuits" by Michael Dickey!) By continuously tapping the nozzle along a particular path, we create a trace of liquid metal which acts like a wire, transforming the isolated metal droplets into a circuit. Fig. 5 shows an LED being powered through an electrically-conductive liquid metal pathway formed by this 3D printing method.

Now imagine using this method to create much more complex circuits! Microelectronic components and sensors could be integrated with the liquid metal wiring to create smart skins, or wearable electronics for fitness applications and medical monitoring. In addition to being soft and stretchable, circuits made of liquid metal composite materials also have the potential to self-heal when damaged. The future is flexible . . . can YOU connect the dots?