Combining droplets of liquid metal (LM) with nanomaterials often introduces synergistic thermal or electrical properties that are not found in the constituent materials alone. However, in these existing systems, LM droplets maintain a statistically uniform dispersion and are not capable of self-assembly or aggregation. These composites are limited by their need for high volume fractions of LM (>60 vol %) to achieve high thermal properties, introducing LM leaking as a drawback for thermal management and wearable electronic applications. In this work, we show that coating nanoscale droplets of eutectic gallium-indium (EGaIn) LM with small volume fractions of Ti3C2Tx MXenes (0.25 vol %) results in a unique LM morphology in which droplets self-assemble to form semisolid aggregates. This is accomplished by wrapping MXene sheets around individual LM droplets to create "sticky" particles that form self-assembled aggregates when mixed with a silicone oil. By introducing aggregation as a design parameter in soft LM composites, the thermal and electric resistance of the composite is shown to change dramatically. In contrast to silicone-based composites containing LM droplets or MXene nanosheets alone, these MXene-LM-silicone-based composites exhibit an exponential increase in thermal and electrical conductivity with decreasing interfacial thickness with significantly lower LM volume fractions (25 vol %) while avoiding LM rupture and bleed-out. This could enable more effective composites, reducing the amount of filler material required for thermal interface materials (TIM) and printed electronics.

The use of magnetically aligned anisotropic conductive epoxy (MA-ACE) as a versatile interconnection technology for flexible and stretchable electronics is explored. The MA-ACE leverages magnetically induced self-assembly of vertically conductive columns within a polymer matrix. Additionally, it offers low curing temperatures and pressure-less assembly while providing strong adhesion to a wide range of materials. Moreover, it integrates well with traditional surface mount technology (SMT) lines. The MA-ACE was used to interconnect microelectronic components with different mechanical properties: (1) printed polymerized liquid metal networks (Poly-LMNs) to surface-mount devices (SMDs), electronic textiles (E-textiles), and copper-clad laminates; (2) E-textiles to SMDs; (3) E-textiles to other E-textiles; and (4) screen-printed flexible silver to SMDs at variable pitch sizes on either polyethylene terephthalate (PET) or polyimide (PI) substrates. The performance of the MA-ACE interconnects was assessed under mechanical stress conditions, including tensile tests, shear tests, fatigue cycling, and exposure to various temperature and humidity conditions. The MA-ACE was found to be highly compliant, successfully bonding different materials while maintaining electrical continuity and mechanical reliability. The results showed that the MA-ACE exhibited a high yield of 86-100%, strong adhesion to all surfaces, low contact resistance (~50-150 mΩ per connection), and high x-y isolation (>1011 Ω). While the MA-ACE was mechanically robust under stress across all interconnection configurations, other components, including E-textiles, Poly-LMNs, and printed flexible silver traces, demonstrated failures such as leakage and delamination.

Liquid metal (LM) and its exceptional thermal and electrical conductivity properties have gained interest due to its fluidic mechanics at room temperature, enabling shape transformations that are beneficial for flexible electronics. These same properties in particle form allow for the integration of this metal to create complex materials with electrical characteristics attributed to percolation networks linked between LM particles (LMPs). We hypothesize that embedding the percolated network of LMPs within elastomeric nanofibers will yield a fibrous structure that maintains electrical conductivity unaffected by mechanical deformations for flexible electronics. The fibrous mat will maintain its fibrous architecture with added conductivity while exhibiting stretchable mechanics. For applications in three-dimensional cell culturing, we expect the inoculation of the percolation network of LMPs into the elastomeric fibers, the sintering procedure to fuse LMPs, creating a conductive core fiber, and assessing the conductive substrate's application to be applicable for other flexible electronics (e.g., electrophysiology, electrochemistry, wound stimulation).

Room‐temperature liquid metal particles based on gallium have become significantly important for developing next‐generation soft electronics. Eutectic gallium‐indium (EGaIn) alloys can be fabricated into core‐shell particles discretely encapsulated by a passivating oxide. Application of mechanical stimuli results in rupturing the molten core through the oxide shell, merging EGaIn particles to form conductive pathways. Generally, the mechanical properties of EGaIn are largely defined by the native oxide which imparts viscoelastic properties to the fluid. In this work, the practical implications of EGaIn deformation and fracture behavior with the native oxide and a non‐native inorganic silica shell are demonstrated. To augment the mechanical properties of EGaIn, silica nanoshells are introduced as a chemically inert coating to enable brittle fracture of particles. In situ single‐particle nanoindentation characterization reveals the environmental and geometrical considerations for particle deformation and fracture. Silica‐coated EGaIn particles reach stiffness values at least an order of magnitude higher than that of native EGaIn particles. The thickness of the silica shell can be tailored to further modify the mechanical behavior of EGaIn, enabling potential pressure‐sensitive conductivity. These results provide additional pathways to understand the design and implementation of functionalized EGaIn particles for future applications in mechanoresponsive electronics, plasmonics, and therapeutics.

Room-temperature liquid metal alloys encompass a highly versatile family of materials possessing a unique set of chemical, electronic, biological, and mechanical properties. The surface oxide of liquid metals has a direct influence on these properties and is often composed of one of the major alloy components (i.e., gallium or indium). However, this is not a foregone conclusion, as the identity of the surface oxide can be altered by the addition of minority elements into the liquid metal. Through judicious choice of a minority alloying metal, the composition of the oxide and therefore the resulting molten alloy's properties are significantly modified. We demonstrate this by adding a small amount (similar to 5%) of several thermodynamically favorable alloying elements (X = Zn, Mg, Al) to eutectic gallium indium (EGaIn), resulting in a new class of alloys with designed surface oxide compositions that we term XGaIn. Using both STEM-EDS and XPS, XGaIn alloys are shown to form oxide layers enriched in the lowest-redox element as expected based on the thermodynamics of the alloy system. This approach is shown to be generalizable across both Ga and non-Ga-based liquid metal alloy compositions. XGaIn alloys with added Zn and Mg are shown to have strong antimicrobial activity, which has exciting implications for the development of flexible electronic medical devices and sensors.

Electronic skins (e-skins) are widely used in wearables, robotics, and medical field. In this paper we introduce a fabrication and characterization of a bio-inspired robot electronic skin, with multiparameter sensing capabilities that include detection of changes in temperature, and locomotion of a sensi-worm robot. Soft robotics has gained much attention in recent years due to its stretchability and conformability in reaching confined areas and uneven surfaces. Soft robots enabled by advances in flexible and hybrid electronics (FHE) offer the potential to navigate through unusual environments with robust, compliant structures that integrate sensors and actuators suitable for industrial inspection, maintenance, and repair requirements. Liquid metal inks have demonstrated immense promise in highly stretchable electronics, offering good conductivity and stable resistance over large strains. However, these inks present adhesion and wetting challenges during printing. Herein, we developed an e-skin fabrication process for multiparameter sensing using polymerized liquid metal ink printed on styrene-ethylene-butadiene-styrene (SEBS) and Thermoplastic Polyurethane (TPU) substrates, using direct write printing method, followed by an electromechanical evaluation against mechanical and thermal stresses. The thermomechanical behavior of the printed traces was investigated with a series of uniaxial stretching and cycling, thermal shock, and thermal stability. The change in electrical resistance with respect to strain amplitude up to 50% during uniaxial stretching was monitored. The skin strain sensors have gauge factor $\sim \mathbf{1.3}$ for TPU and 1.8 for SEBS. On the other hand, the printed thermistor design showed stable performance over thermal cycling with a thermal coefficient of resistance (TCR) 5.7e-4 ppm/C which is smaller to other regular metal-based inks.

Eutectic gallium indium liquid metal (LM) is a promising conductive liquid for various electronic applications. In particular, the directional transport of LM droplets has potential applications in soft electronics to control electrical conductivity. Existing methods transport LM droplets by applying an electric field to generate an interfacial tension difference within the LM droplet due to nonuniform ionic distribution of the electrical double layer. However, these methods require confined channels and tethered systems to apply the electric field. In this study, channel-free wireless transport of LM droplets is demonstrated via on-demand magnetomechanical actuation of asymmetric microwall arrays comprising vertically aligned ferromagnetic iron particles embedded in polydimethylsiloxane. Asymmetric microwall of two different widths is designed to generate an asymmetric bending stiffness at a given magnetic field. The asymmetric microwall bends gradually in response to a linear external magnetic field perpendicular to the alignment axis of the iron particles. Therefore, nonuniform magnetomechanical bending induces a local height gradient along the microwall, causing gravitational-force-driven roll-off motion of the LM droplet. Transport direction of LM droplet is modulated by varying the geometric parameters. Finally, the opposite-directional transport of two LM droplets is demonstrated under a linear magnetic field by pre-programming the asymmetric bending direction of each microwall array.

Next generation on-skin electrodes will require soft, flexible, and gentle materials to provide both high-fidelity sensing and wearer comfort. However, many commercially available on-skin electrodes lack these key properties due to their use of rigid hardware, harsh adhesives, uncomfortable support structures, and poor breathability. To address these challenges, this work presents a new device paradigm by joining biocompatible electrospun spider silk with printable liquid metal to yield an incredibly soft and scalable on-skin electrode that is strain-tolerant, conformable, and gentle on-skin. These electrodes, termed silky liquid metal (SLiM) electrodes, are found to be over five times more breathable than commercial wet electrodes, while the silk's intrinsic adhesion mechanism allows SLiM electrodes to avoid the use of harsh artificial adhesives, potentially decreasing skin irritation and inflammation over long-term use. Finally, the SLiM electrodes provide comparable impedances to traditional wet and other liquid metal electrodes, offering a high-fidelity sensing alternative with increased wearer comfort. Human subject testing confirmed the SLiM electrodes ability to sense electrophysiological signals with high fidelity and minimal irritation to the skin. The unique properties of the reported SLiM electrodes offer a comfortable electrophysiological sensing solution especially for patients with pre-existing skin conditions or surface wounds.

GalliumGallium and gallium alloys have received significant attention in recent years due to their unique thermal and electrical properties while maintaining liquidus properties. Applications range from reconfigurable antennas to strain-tolerant conductors for flexible hybrid electronics. Several commercial embodiments of gallium alloys for these applications are now becoming available, however their interaction with solid metals such as aluminum, copper, and nickel raise concern as gallium is known to aggressively attack many of these and weaken them. Herein, we detail these effects with one particular system that is formulated specifically for printed stretchable electronics, namelyELMNT ELMNT™ Inks, which are composed of colloidal nanoparticles of gallium/indium alloy and allow for readily manufactured stretchable circuits using techniques such as screen-printing and ink jet printing. The interactions between a range of common metals andELMNT ELMNT™ Inks are reported and compared to bulk galliumGallium/indium interactions.

The characterization of an RF transmitter composed of insulating and conducting regions of liquid‐metal embedded elastomer (LMEE) is presented along with in situ measurements of LMEE microstrip lines as they are subjected to local material strains up to 40%. The LMEE is comprised of microscale droplets of a gallium–indium alloy that is liquid at room temperature and suspended within a cured elastomer matrix of polydimethylsiloxane (PDMS). The liquid metal microparticles were initially electrically isolated, but applying mechanical loading caused the permanent formation of highly localized conductive traces. Bonding films of LMEE onto a PDMS dielectric layer resulted in a stretchable microstrip structure that is capable of radio frequency (RF) transmission. Scattering parameter ( S ‐parameter) measurements for reflection and transmission are presented for these microstrip lines as the electrical length increased up to 19%. A customized clamp was utilized to isolate the mechanical strain on the material from the electrical connectors and allow for transmission line characterization under applied strain and dielectric characterization of the LMEE material was performed. The stretchable microstrip lines show remarkable consistency in transmission response at 0.5–5 GHz when mechanically loaded to 40% strain for 1000 loading cycles.

Liquid metals are ideally suited for flexible and wearable electronics due to their compatibility with additive manufacturing and high electrical conductivity that is maintained following mechanical perturbation. While printing of eutectic gallium–indium (eGaIn) liquid metal nanoparticles has been demonstrated, previous techniques for activating electrical conductivity in the as‐printed insulating eGaIn nanoparticles limit throughput in roll‐to‐roll manufacturing processes. Here, ultrafast photonic sintering of eGaIn nanoparticles is demonstrated, which is further enhanced through the use of nitrocellulose as a carrier polymer that undergoes optically triggered combustion to produce eGaIn thin films with electrical conductivities exceeding 104 S cm–1. This combustion‐assisted photonic sintering (CAPS) is two orders of magnitude faster than previously demonstrated noncontact sintering techniques. By circumventing the established tradeoff between electrical conductivity and activation speed, CAPS will facilitate the use of eGaIn liquid metal nanoparticles in high‐throughput additive manufacturing of flexible and wearable electronics, sensors, and related technologies.

Next generation textile-based wearable sensing systems will require flexibility and strength to maintain capabilities over a wide range of deformations. However, current material sets used for textile-based skin contacting electrodes lack these key properties, which hinder applications such as electrophysiological sensing. In this work, a facile spray coating approach to integrate liquid metal nanoparticle systems into textile form factors for conformal, flexible, and robust electrodes is presented. The liquid metal system employs functionalized liquid metal nanoparticles that provide a simple "peel-off to activate" means of imparting conductivity. The spray coating approach combined with the functionalized liquid metal system enables the creation of long-term reusable textile-integrated liquid metal electrodes (TILEs). Although the TILEs are dry electrodes by nature, they show equal skin-electrode impedances and sensing capabilities with improved wearability compared to commercial wet electrodes. Biocompatibility of TILEs in an in vivo skin environment is demonstrated, while providing improved sensing performance compared to previously reported textile-based dry electrodes. The "spray on dry-behave like wet" characteristics of TILEs opens opportunities for textile-based wearable health monitoring, haptics, and augmented/virtual reality applications that require the use of flexible and conformable dry electrodes.

Stretchable electronic devices are hybrid in nature, comprised of soft and rigid electronic components. Robust and reliable electrical interconnections that accommodate rigid semiconductors, communications, and sensor components on an otherwise flexible circuit are necessary to ensure the proper functionality of the device. Magnetically aligned anisotropic conductive epoxy (ACE) takes advantage of magnetically induced self-assembly of Z-axis wires in a flexible and durable matrix is optimized to suit flexible hybrid electronic applications. ACE interconnect technology was chosen for this study because it allows for pressure-less assembly and provides adhesion to an unusually wide range of materials. Of equal importance, the technology integrates well into traditional Surface Mount Technology (SMT) lines. Presented here is a subset of our ongoing evaluation of this ACE technology using various test vehicles and rigorous electromechanical testing. Herein, the ACE is used to fabricate bonds between e-textile to e-textile, e-textile to SMD resistor, and e-textile to electronic module board. The conductors on the e-textile consist of either silver-plated or laminated screen-printed multilayers of silver and carbon on thermoplastic polyurethane. The electrical resistance of the test vehicle is monitored while exposing the test samples to various mechanical stressing including tensile tests, fatigue cycling, and shear test. Magnetically aligned anisotropic conductive epoxy has shown to offer mechanically robust flex-to-flex and flex-to-rigid electrical connections. As a demonstration, the ACE is used to create connections between e-textile and wearable electronic module. In addition, the durability before and after repeated washing in commercial home clothes washing machine is demonstrated.

Recent developments in autonomous engineered matter have introduced the ability for intelligent materials to process environmental stimuli and functionally adapt 1 – 4 . To formulate a foundation for such an engineered living material paradigm, researchers have introduced sensing 5 – 11 and actuating 12 – 16 functionalities in soft matter. Yet, information processing is the key functional element of autonomous engineered matter that has been recently explored through unconventional techniques with limited computing scalability 17 – 20 . Here we uncover a relation between Boolean mathematics and kinematically reconfigurable electrical circuits to realize all combinational logic operations in soft, conductive mechanical materials. We establish an analytical framework that minimizes the canonical functions of combinational logic by the Quine–McCluskey method, and governs the mechanical design of reconfigurable integrated circuit switching networks in soft matter. The resulting mechanical integrated circuit materials perform higher-level arithmetic, number comparison, and decode binary data to visual representations. We exemplify two methods to automate the design on the basis of canonical Boolean functions and individual gate-switching assemblies. We also increase the computational density of the materials by a monolithic layer-by-layer design approach. As the framework established here leverages mathematics and kinematics for system design, the proposed approach of mechanical integrated circuit materials can be realized on any length scale and in a wide variety of physics.

Stretchable wearable electronics often require integrating mechanically different materials to fabricate functional devices that are stretchable, conformal, and sufficiently reliable. Common methods to integrate highly stretchable conductors with other Flexible Hybrid Electronics (FHE) components include solder and isotropic/anisotropic conductive adhesives or films. Due to the nature of the stretchable conductors/substrate, each of those methods has its limitations in wearable and stretchable devices. A magnetically aligned Anisotropic Conductive Epoxy (ACE) has promising potential for such integration due to its excellent adhesion to most substrates of interest, low curing temperature, and no pressure processing requirements. This study investigates the effect of the mechanical and environmental stresses on a magnetically aligned ACE as a bonding material between polymerized liquid metal networks (Poly-LMNs) and copper, e-textile, and Surface Mount Device (SMD) resistor. The test coupons were subjected to various mechanical and environmental stresses through a tensile test, fatigue cycling, and exposure to temperature and humidity conditions. The results showed that the ACE exhibited robustness and low contact resistance during electromechanical testing. In addition, the Poly-LMNs to e-textile or Cu-Flex assembly through magnetically aligned ACE showed no noticeable increase in the electrical resistance from cycle to cycle during fatigue cycling. The environmental conditions showed no significant impact on direct contact joints; however, there was a permanent failure to via connections due to moisture absorption, which led to cracks/delamination.

Integrated circuits utilize networked logic gates to compute Boolean logic operations that are the foundation of modern computation and electronics. With the emergence of flexible electronic materials and devices, an opportunity exists to formulate digital logic from compliant, conductive materials. Here, we introduce a general method of leveraging cellular, mechanical metamaterials composed of conductive polymers to realize all digital logic gates and gate assemblies. We establish a method for applying conductive polymer networks to metamaterial constituents and correlate mechanical buckling modes with network connectivity. With this foundation, each of the conventional logic gates is realized in an equivalent mechanical metamaterial, leading to soft, conductive matter that thinks about applied mechanical stress. These findings may advance the growing fields of soft robotics and smart mechanical matter, and may be leveraged across length scales and physics.

One of the most critical limitations for high-power electronics today is thermal management and routing thermal energy efficiently away from thermally sensitive components. A potential solution to this problem is the integration of cooling channels in close proximity to thermally sensitive materials for increased heat removal efficiency. These channels typically use single phase fluids (liquid), dual phase fluids (vapor-liquid), or suspended organic/polymer phase change material particles in a fluid (PCM slurry). Expanding upon the latter, this work demonstrates the use of inorganic Ga-In alloy nanoparticles (NPs) suspended in a traditional thermal transport fluid to simultaneously (1) increase the overall thermal diffusivity of the fluid and (2) serve as a cyclable solid-liquid PCM slurry which provides a thermal sink that is definable over a wide range of relevant temperatures for power electronics. Herein, the relationship between particle size, composition, and volume fraction are explored as they relate to the PCM slurry optimum working temperature, total energy absorption, and rheological properties. A mere 0.10 volume fraction of Ga-In NPs is reported to increase the overall thermal conductivity by nearly 50% and can be optimized to melt at temperatures as low as -46 °C. Based on thermal measurements, it was observed that these nanoparticle systems lack the preference to form αGa and have a large thermal hysteresis due to exhibiting extreme undercooling, with crystallization temperatures near -130 °C, enabling opportunities within extreme environments such as space applications or low temperature imaging systems.

Room-temperature liquid metals such as eutectic gallium-indium (EGaIn) alloys have the potential to realize physically reconfigurable microwave components and circuits. Integrating microfluidic precision control of these liquid conductors with standard microwave board circuits enables a new paradigm of tunability for microwave components. One area that would greatly benefit from passive component agility is impedance-matching networks, where fine tuning the reactance is essential for reconfigurability. We demonstrate a microfluidic-integrated microwave inductor device that employs a high-impedance microstrip trace with three switchable taps to provide inductances of 1.2 nH, 1.9 nH and 2.3 nH at 5 GHz. Precise volumes of liquid metal are dispensed and confined to bridge existing gaps between copper microstrip traces to reconfigure the total length of the narrow trace, providing an effective method to tune the component's inductance.

Gallium is a metal that literally melts in your hand. It has low toxicity, near-zero vapor pressure, and a viscosity similar to water. Despite possessing a surface tension larger than any other liquid (near room temperature), gallium can form nonspherical shapes due to the thin, solid native oxide skin that forms rapidly in oxygen. These properties enable new ways to pattern metals (e.g., injection and printing) to create stretchable and soft devices with an unmatched combination of mechanical and electrical properties. The oxide skin can be transferred to other substrates and manipulated electrochemically to lower the interfacial tension to near zero. The reactivity of gallium can drive a wide range of reactions. The liquid state of gallium makes it easy to break into particles for making colloids and soft composites that have unusual properties due to the deformable nature of the filler. This review summarizes the truly unique and exciting properties of gallium liquid metals.

Advances in materials, designs, and controls are propelling the field of soft robotics at an incredible rate; however, current methods for prototyping soft robots remain cumbersome and struggle to incorporate desirable geometric complexity. Herein, a vat photopolymerizable self-healing elastomer system capable of extreme elongations up to 1000% is presented. The material is formed from a combination of thiol/acrylate mixed chain/step-growth polymerizations and uses a combination of physical processes and dynamic-bond exchange via thioethers to achieve full self-healing capacity over multiple damage/healing cycles. These elastomers can be three dimensional (3D) printed with modular designs capable of healing together to form highly complex and large functional soft robots. Additionally, these materials show reprogrammable resting shapes and compatibility with self-healing liquid metal electronics. Using these capabilities, subcomponents with multiple internal channel systems were printed, healed together, and combined with functional liquid metals to form a high-wattage pneumatic switch and a humanoid-scale soft robotic gripper. The combination of 3D printing and self-healing elastomeric materials allows for facile production of support-free parts with extreme complexity, resulting in a paradigm shift for the construction of modular soft robotics.