3D Printing of Smart Electronic Clothing with Liquid Metal Microgels (with Video)

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March 10, 2022

(News from Nanowerk) 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: “Use of liquid metals in nanotechnology”). Hg has a low melting point of -38.8°C, but its potential hazards preclude it for many applications. Ga has a melting point of 29.8°C and is considered to have low toxicity, making it suitable for many applications.

As we have already pointed out, networks of liquid metals provide an ideal platform for scalable electronics.

In recent years, gallium-based liquid metals, as fluid flexible materials with superb metallic conductivity, have attracted enormous attention and been used to manufacture flexible electronic components, including electronic skins, retrieval devices of energy, wearable health monitoring equipment, 3D circuits and nanotips. , and more.

However, limited by its own enormous surface tension and high fluidity, liquid metal tends to aggregate into intermittent low surface energy spheres rather than forming the desired continuous patterns during processing, making patterning difficult. Direct pure liquid metal into desired circuits with high resolution. . To complicate the problem, many common substrate materials, such as paper and fabrics, are weakly adhesive to liquid metal, making its pattern available for very few substrates to which it can easily adhere.

To solve this problem, the researchers propose a practical and inexpensive strategy to prepare a printable and recyclable liquid metal microgel (LMM) ink by encapsulating liquid metal microdroplets in alginate microgel shells to directly print flexible electronic components on various substrates.

They report their findings in Applied materials and ACS interfaces (“Liquid Metal Microgels for Three-Dimensional Printing of Smart Electronic Garments”).

Liquid-metal-microgel (LMM) ink direct-to-ink preparation and process. (A) Schematic diagram of the LMM ink preparation procedure with mechanical agitation. (B) Process of formation of the LM-alginate core-shell structure by mechanical shearing. (C) Mechanism of rheological modification of alginate during extrusion printing. (Reproduced with permission from the American Chemical Society) (click image to enlarge)

As shown above, the researchers made their new printable LMM ink by mechanically agitating the mixture of liquid metal and aqueous sodium alginate solution and using the crosslinking reaction of Ga3+ and alginate chains to obtain liquid metal droplets wrapped in gallium alginate microgel shells.

As the authors point out, due to the presence of the microgel shells, the LMM ink composed of liquid metal droplets has excellent printability and adhesion to the substrate compared to pure liquid metal.

Although the printed circuits are not initially conductive, the conductivity of the circuits can be activated by microdeformation (less than 5%) because the networks of dehydrated alginate in the circuit are almost inextensible and break easily when subjected to stress. small deformation.

Characterization of the surface morphology and principle of activation of the microcircuit printed with liquid metal ink Characterization of the surface morphology and principle of activation of the printed microcircuit with LMM ink. (A) Microcircuit activation process: (I) initial microcircuit, (II) microcircuit with small deformation and (III) activated microcircuit. Scale bars: 10mm. (B) SEM images of the non-activated microcircuit. Scale bars: 500 (I) and 100 µm (II). (C) SEM image of the activated microcircuit. Scale bar: 100 µm. (Reproduced with permission from the American Chemical Society)

Additionally, freezing and pressing can also activate circuit boards with LMM ink, allowing LMM ink to be applied under extreme working conditions, such as frozen switches in space.

According to the team, an activated LMM circuit exhibits excellent electrical properties such as good conductivity, significant resistance response to stress with low hysteresis and high durability to non-planar forces, which are important for electronics. flexible. Additionally, LMM ink can also be used to print flexible heating filaments for portable thermal management due to its superior Joule heating performance.

To demonstrate the capabilities of their LMM ink, the researchers fabricated smart electronic clothing by directly printing functionalized flexible electronic components onto commercial clothing.

They fabricated a near-field communication (NFC) tag on a commercial T-shirt-based LMM reel, which can communicate with NFC-enabled 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 web page (shown in the video below).

The authors are confident that due to the benefits of LMM ink and 3D printing, their work will greatly facilitate the low-cost, standardized realization of smart electronic wearables with health monitoring and human-computer interaction.

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