(Nanowerk Spotlight) Gallium-based liquid metal composites hold promise for stretchable electronics, soft robotics and biointegrated devices that can bend and conform like plastic. Their moldability also presents opportunities for directly printing flexible circuits that repair themselves. But these metastable fluids – which lack rigid crystalline bonds keeping atoms in fixed arrangements – quickly lose imprinted shapes, reverting to formless blobs without a container.
The most common gallium-based liquid metal variant used in research is eutectic gallium indium (EGaIn) – a blend containing 75% gallium and 25% indium, which melts at room temperature. This allows it to remain fluid without external heating while also incorporating indium’s superb conductivity.
Now, researchers at North Carolina State University report a breakthrough – liquid metal composites that spontaneously grow over 400% in volume when exposed to water, while retaining metallic conductivity similar to their starting material. Specifically, patterns expand from an initial resistance of 10-2 ohms to about 0.1 ohms after reactive growth, preserving high conductance comparable to the source composite.
This tunable chemical reactivity turns an enduring barrier into an opportunity. It unlocks new paradigms for directly printing responsive and self-repairing electronics that change shape with time. The findings advance futuristic visions of soft robotics and biointegrated devices that bypass rigidity and brittleness.
Liquid metals like eutectic gallium indium (EGaIn) have long fascinated scientists with their odd combination of metallic conductivity and melting points below room temperature. Unfurled from the vise grip of rigid crystalline bonds, these flowing metals bend like plastic and flow like water. This fluid versatility suggests potential uses as soft, stretchable wires and electrodes that can twist, bend, and conform to any surface or moving parts. Their moldability also presents opportunities for room temperature printing of electronics – sidestepping the intense heat and meticulous environments needed to pattern traditional conductive materials like copper or silicon.
Yet major roadblocks have throttled the promise of liquid metal electronics. Without a container, these metastable liquids quickly lose their imprinted shape and flow into formless blobs. Early attempts at stabilization involved coating droplets with oxidized skin to dampen flow, yielding sub-millimeter conductors too small for practical use. More recent efforts have compounded liquid metals into particle-filled pastes thick enough for direct printing. But printed traces remain stubby, cracking after modest stretching.
Now an interdisciplinary team led by Michael Dickey at North Carolina State University reports an entirely new phenomenon – liquid metal mixtures that grow substantially bigger when ‘watered,’ while retaining useful conductivity. By harnessing known chemical reactions between liquid metals and water, the researchers have turned a hurdle into a breakthrough.
Liquid metal foams (LMFs) respond to moisture by oxidizing and growing as a result of hydrogen evolution within the pores. Procedure for making a dry LMF0 (top) and water-containing LMFs (i.e., LMF1 / LMF10) (bottom). Simply stirring LM in the air produces the foam into which water can be incorporated. (Reprinted with permission by Wiley-VCH Verlag)
The technology builds on earlier advances with liquid metal foams – gallium alloys churned with air bubbles and corralled by stabilizing skins of native gallium oxide. Serendipitously, the team discovered these composites expand dramatically when exposed to even tiny amounts of water. Just 1% water drives foaming and fivefold volume growth within a day. And resulting material remains highly conductive despite being over 85% air.
This occurs because water infiltration promotes oxidation reactions that generate porous gallium oxyhydroxide while freeing hydrogen gas. Normally oxidation just passivates surfaces. But reactions self-propagate throughout this aerated medium, given the abundance of water-accesible gallium interfaces. Gradually accumulating gas exerts internal pressure that expands the foam further – much like bread dough rising from the byproducts of yeast fermentation.
Remarkably, this alien reactivity can be tuned and directed. More initial water begets faster, greater growth before embrittlement sets in. And confining reactive precursors guides expansion like inflating a balloon, enabling shapes to conform to enclosed voids. After growing over 400% in size, final structures retain metallic conductivity similar to their starting paste. But compositions must stay dry, as ongoing moisture exposure eventually consumes electrical properties.
The team put this phenomenon to work printing reactive liquid metal patterns that spontaneously increase in size after deposition. And by constraining expansion in an acrylic channel, they demonstrate a “growing conductor” that automatically fills voids in a circuit. This potentially enables efficient, hands-free electrical bridging without needing complicated print heads or harsh metal deposition environments. It could also reduce the volume of costly liquid metals required.
The gas generation aspect additionally enables pneumatic actuation, which the team harnessed to power a soft gripper. Inflating pouches with evolving hydrogen let the device grasp and lift objects without any rigid components or cumbersome tubing. And avoiding external power and controls yields a remarkably simple construction.
While the grasping strength is currently modest, lead author Febby Krisnadi points out “this demonstration presents exciting opportunities for future development since it is a patterning technique that can 1) be achieved hands-free and without relying on complicated dispensing machinery, 2) reduce the amount of LM required to fill a given volume of channel without compromising on conductance, 3) be done at room temperature, no special environmental conditions needed.”
Though long-term applications demand better encapsulation to maintain dryness and prevent embrittlement, the findings significantly advance liquid metal electronics. The researchers explain the breakthrough represents “a type of 4D printing that combines the automation of printing with spontaneous processes that change the 3D structure in a way that depends on the environment over time.” The rich spectrum of dynamical shape change and actuation enabled by this chemical reactivity stands to unlock new horizons for soft responsive electronics and robotics.
This work has important implications for liquid metal foams since exposure to water (including humidity) can cause dimensional and physical property changes that may or may not be desirable. Here, we seek to better understand and utilize these. By embracing reactivity formerly considered destructive, the researchers have transformed an Achilles’ heel into a strength. Their water-grown liquid metal composites point toward entirely new paradigms for printing self-shaping metallic architectures – bringing us a step closer to someday matching the capabilities of natural tissues.
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