The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices ...that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e‐skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self‐healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.
Liquid metals offer an unrivaled combination of conductivity and deformability for stretchable and soft electronics. Alloys of gallium with low toxicity form surface oxides that allow them to be patterned into wires, interconnects, electrodes, and antennas. Liquid metals can also be used as active components in memory devices, capacitors, sensors, diodes, and shape‐reconfigurable electronics.
Gallium and several of its alloys are liquid metals at or near room temperature. Gallium has low toxicity, essentially no vapor pressure, and a low viscosity. Despite these desirable properties, ...applications calling for liquid metal often use toxic mercury because gallium forms a thin oxide layer on its surface. The oxide interferes with electrochemical measurements, alters the physicochemical properties of the surface, and changes the fluid dynamic behavior of the metal in a way that has, until recently, been considered a nuisance. Here, we show that this solid oxide “skin” enables many new applications for liquid metals including soft electrodes and sensors, functional microcomponents for microfluidic devices, self-healing circuits, shape-reconfigurable conductors, and stretchable antennas, wires, and interconnects.
This review summarizes progress toward programming two-dimensional (2D) polymer sheets which respond to a variety of external stimuli to form three-dimensional (3D) shapes or topographical features ...on macroscopically planar sheets. Shape programming strategically adds value or function to 2D sheets, films, or coatings that can be created inexpensively. 2D substrates are common form factors that are compatible with ordinary 2D patterning techniques (i.e., inkjet, photolithography, roll-to-roll printing) and may be stored, packed, and shipped efficiently. Polymer materials are attractive due to their flexibility, light weight, low price, and compatibility with high throughput processing. This review highlights strategies for triggering shape change in planar polymeric materials. The strategies are divided into four broad categories: (1) 2D substrates with latent topography “programmed” using conventional microfabrication, (2) 2D substrates that form topography due to imposed or self-generated stress, (3) 2D substrates that form 3D shapes by out-of-plane bending, and (4) 2D substrates that use “hinges” to achieve out-of-plane folding. The review highlights all strategies while focusing primarily on last two approaches.
This work discusses the attributes, fabrication methods, and applications of gallium‐based liquid metal particles. Gallium‐based liquid metals combine metallic and fluidic properties at room ...temperature. Unlike mercury, which is toxic and has a finite vapor pressure, gallium possesses low toxicity and effectively zero vapor pressure at room temperature, which makes it amenable to many applications. A variety of fabrication methods produce liquid metal particles with variable sizes, ranging from nm to mm (which is the upper limit set by the capillary length). The liquid nature of gallium enables fabrication methods—such as microfluidics and sonication—that are not possible with solid materials. Gallium‐based liquid metal particles possess several notable attributes, including a metal–metal oxide (liquid–solid) core–shell structure as well as the ability to self‐heal, merge, and change shape. They also have unusual phase behavior that depends on the size of the particles. The particles have no known commercial applications, but they show promise for drug delivery, soft electronics, microfluidics, catalysis, batteries, energy harvesting, and composites. Existing challenges and future opportunities are discussed herein.
This review discusses gallium‐based liquid metal particles. Gallium‐based liquid metals possess several unique properties including the combination of metallic and fluidic properties at room temperature. Liquid particles can be produced through a wide range of techniques that are not possible with solid metals. Such particles show promise for drug delivery, soft electronics, microfluidics, catalysis, composites, and energy harvesting.
The emergence of soft machines and electronics creates new opportunities to engineer robotic systems that are mechanically compliant, deformable, and safe for physical interaction with the human ...body. Progress, however, depends on new classes of soft multifunctional materials that can operate outside of a hard exterior and withstand the same real-world conditions that human skin and other soft biological materials are typically subjected to. As with their natural counterparts, these materials must be capable of self-repair and healing when damaged to maintain the longevity of the host system and prevent sudden or permanent failure. Here, we provide a perspective on current trends and future opportunities in self-healing soft systems that enhance the durability, mechanical robustness, and longevity of soft-matter machines and electronics.Progress in soft machines and electronics depends on new classes of soft multifunctional materials that can self-repair and heal when damaged so that they can survive the same real-world conditions that human skin and other soft biological materials are typically subjected too. Here, we provide a perspective on current trends and future opportunities in self-healing soft systems that enhance the durability, mechanical robustness, and longevity of soft-matter machines and electronics.
Ionogels are compelling materials for technological devices due to their excellent ionic conductivity, thermal and electrochemical stability, and non-volatility. However, most existing ionogels ...suffer from low strength and toughness. Here, we report a simple one-step method to achieve ultra-tough and stretchable ionogels by randomly copolymerizing two common monomers with distinct solubility of the corresponding polymers in an ionic liquid. Copolymerization of acrylamide and acrylic acid in 1-ethyl-3-methylimidazolium ethyl sulfate results in a macroscopically homogeneous covalent network with in situ phase separation: a polymer-rich phase with hydrogen bonds that dissipate energy and toughen the ionogel; and an elastic solvent-rich phase that enables for large strain. These ionogels have high fracture strength (12.6 MPa), fracture energy (~24 kJ m
) and Young's modulus (46.5 MPa), while being highly stretchable (~600% strain) and having self-healing and shape-memory properties. This concept can be applied to other monomers and ionic liquids, offering a promising way to tune ionogel microstructure and properties in situ during one-step polymerization.
This review discusses methods, challenges, and opportunities for direct‐write and 3D printing of low melting point, gallium‐based liquid metal alloys at room temperature. Alloys of gallium exhibit ...high conductivity and high stretchability making them well suited for use in soft circuitry for stretchable electronics and soft robotics. In addition, the liquid nature of the metal enables entirely new ways to pattern metals at room temperature; herein, the focus is placed on additive printing via nozzle‐based methods. Room temperature printing of liquid metals enables rapid fabrication of complex geometries (with dimensions as small as 10 µm) on a wide range of materials, such as polymers. These processes can be used to make metallic conductors for devices with self‐healing capabilities, soft/stretchable electrodes, and sensors.
Herein, methods for direct‐write printing of gallium‐based alloys at room temperature are summarized. Such liquid metals exhibit high thermal conductivity, electrical conductivity, and stretchability, making them well suited for soft and stretchable devices. These alloys have unique rheology that presents challenges and opportunities for printing.
The ability to pattern, structure, re-shape and actuate hydrogels is important for biomimetics, soft robotics, cell scaffolding and biomaterials. Here we introduce an 'ionoprinting' technique with ...the capability to topographically structure and actuate hydrated gels in two and three dimensions by locally patterning ions via their directed injection and complexation, assisted by electric fields. The ionic binding changes the local mechanical properties of the gel to induce relief patterns and, in some cases, evokes localized stress large enough to cause rapid folding. These ionoprinted patterns are stable for months, yet the ionoprinting process is fully reversible by immersing the gel in a chelator. The mechanically patterned hydrogels exhibit programmable temporal and spatial shape transitions, and serve as a basis for a new class of soft actuators that can gently manipulate objects both in air and in liquid solutions.
Stable suspensions of eutectic gallium indium (EGaIn) liquid metal nanoparticles form by probe-sonicating the metal in an aqueous solution. Positively-charged molecular or macromolecular surfactants ...in the solution, such as cetrimonium bromide or lysozyme, respectively, stabilize the suspension by interacting with the negative charges of the surface oxide that forms on the metal. The liquid metal breaks up into nanospheres
sonication, yet can transform into rods of gallium oxide monohydroxide (GaOOH)
moderate heating in solution either during or after sonication. Whereas heating typically drives phase transitions from solid to liquid (
melting), here heating drives the transformation of particles from liquid to solid
oxidation. Interestingly, indium nanoparticles form during the process of shape transformation due to the selective removal of gallium. This dealloying provides a mechanism to create indium nanoparticles at temperatures well below the melting point of indium. To demonstrate the versatility, we show that it is possible to shape transform and dealloy other alloys of gallium including ternary liquid metal alloys. Scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS) mapping, and X-ray diffraction (XRD) confirm the dealloying and transformation mechanism.
This paper describes a method to direct‐write 3D liquid metal microcomponents at room temperature. The thin oxide layer on the surface of the metal allows the formation of mechanically stable ...structures strong enough to stand against gravity and the large surface tension of the liquid. The method is capable of printing wires, arrays of spheres, arches, and interconnects.
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