(Nanowerk News) The term ‘liquid metal’ refers to metals with melting points near or below room temperature. Mercury (Hg) and gallium (Ga) are the two most recognized elemental liquid metals (read more here: “Using liquid metals in nanotechnology“). Hg has a low melting point of -38.8 °C, but its potential hazards rule it out for many applications. Ga has a melting point of 29.8 °C and is considered to have low toxicity, which makes it suitable for many applications.
In recent years, gallium-based liquid metals, as fluid soft materials with superb metallic conductivity, have drawn tremendous attention and have been employed to manufacture flexible electronics, including electronic skin, energy-harvesting devices, wearable health monitoring equipment, 3D circuits and nanotips, and more.
However, limited by its own huge surface tension and strong fluidity, liquid metal tends to aggregate into intermittent low-surface-energy spheres rather than forming desired continuous patterns during processing, making it challenging to directly pattern pure liquid metal into desired circuits with high resolution. Compounding the problem, many common substrate materials such as paper and fabrics, are weakly adhesive for liquid metal, making its patterning available to very few substrates to which it can easily adhere.
Addressing this issue, researcher propose a convenient and low-cost strategy to prepare a printable and recyclable liquid-metal-microgel (LMM) ink by encapsulating liquid metal microdroplets into alginate microgel shells for directly printing flexible electronics on various substrates.
Preparation and the direct ink writing process of the liquid-metal-microgel (LMM) ink. (A) Schematic diagram of the procedure for preparing the LMM ink with mechanical stirring. (B) Formation process of the LM-alginate core−shell structure by mechanical shearing. (C) Mechanism of rheological modification of alginate during extrusion printing. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
As illustrated above, the researchers fabricated their novel printable LMM ink by mechanically stirring the mixture of liquid metal and sodium alginate aqueous solution and utilizing the cross-linking reaction of Ga3+ and alginate chains to obtain liquid metal droplets wrapped in gallium alginate microgel shells.
As the authors point out, owing to the presence of the microgel shells, the LMM ink composed of liquid metal droplets exhibits excellent printability and adhesion to the substrate compared with pure liquid metal.
Although the printed circuits are not initially conductive, the conductivity of the circuits can be activated by microstrain (less than 5%) because the dehydrated alginate networks in the circuit are almost unstretchable and easily broken when subjected to a small deformation.
Surface morphology characterization and the activation principle of the microcircuit printed with the LMM ink. (A) Microcircuit activation process: (I) initial microcircuit, (II) microcircuit with a small strain, and (III) activated microcircuit. Scale bars: 10 mm. (B) SEM images of the unactivated microcircuit. Scale bars: 500 (I) and 100 µm (II). (C) SEM image of the activated microcircuit. Scale bar: 100 µm. (Reprinted with permission by American Chemical Society)
Additionally, freezing and pressing can also activate circuits printed with the LMM ink, which makes the LMM ink have the potential to be applied in extreme working conditions, such as freezing-on switches in outer space.
According to the team, an activated LMM circuit exhibits excellent electrical properties such as good conductivity, significant resistance response to strain with small hysteresis, and great durability to nonplanar forces, which are important for flexible electronics. Furthermore, the LMM ink can also be employed to print flexible heating filaments for wearable thermal management due to its superior Joule heating performance.
To demonstrate the capabilities of their LMM ink, the researchers fabricated smart electronic clothes by directly printing functionalized flexible electronics on commercial clothes.
They fabricated a near-field communication (NFC) tag on a commercial T-shirt-based LMM coil, which can communicate with NFC compatible hardware, such as a smartphone. In this demonstration, approaching the NFC tag, the smartphone will automatically execute the instruction written in the chip, which in this case was to open a specific webpage (shown in the video below).
The authors are confident that due to the advantages of the LMM ink and 3D printing, their work will significantly facilitate low-cost and standardized realization of smart electronic clothes with health monitoring and human− computer interaction.
