Textile-Integrated Liquid Metal Electrodes for Electrophysiological Monitoring.

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.

Gallium-indium nanoparticles as phase change material additives for tunable thermal fluids.

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.

Oxidation of Gallium-based Liquid Metal Alloys by Water.

Gallium alloys with other low melting point metals, such as indium or tin, to form room-temperature liquid eutectic systems. The gallium in the alloys rapidly forms a thin surface oxide when exposed to ambient oxygen. This surface oxide has been previously exploited for self-stabilization of liquid metal nanoparticles, retention of metastable shapes, and imparting stimuli-responsive behavior to the alloy surface. In this work, we study the effect of water as an oxidant and its role in defining the alloy surface chemistry. We identify several pathways that can lead to the formation of gallium oxide hydroxide (GaOOH) crystallites, which may be undesirable in many applications. Furthermore, we find that some crystallite formation pathways can be reinforced by typical top-down particle synthesis techniques like sonication. This improved understanding of interfacial interactions provides critical insight for process design and implementation of advanced devices that utilize the unique coupling of flexibility and conductivity offered by these gallium-based liquid metal alloys.

Chemically modifying the mechanical properties of core-shell liquid metal nanoparticles.

Eutectic gallium-indium (EGaIn) is a room temperature liquid metal that can be readily fabricated into nanoparticles which naturally form a thin, passivating gallium oxide shell. These core-shell nanoparticles are of interest for a variety of stimuli-responsive applications, which often utilize physical deformation of the particles to release the molten, conductive payload from within the gallium oxide shell. In the present work, we introduce a variety of chemical strategies to produce EGaIn nanoparticles which exhibit a wide range of gallium oxide shell thicknesses. These chemically modified oxide thicknesses are then correlated to the core-shell liquid nanoparticles' mechanical properties by subjecting the particles to orthogonal characterization techniques; XPS for measurement of the gallium oxide shell thickness and nanoindentation for measurement of particle stiffness and elastic modulus. Additionally, nanoindentation is used to determine the onset of particle rupture and resultant conductivity. Ultimately, quantification of the relationships between chemical treatment and derivative mechanical properties in liquid metal nanoparticles will enable advanced applications of these colloids, such as in tailored self-healing and responsive electronic devices.

Mechanoresponsive Polymerized Liquid Metal Networks.

Room-temperature liquid metals, such as nontoxic gallium alloys, show enormous promise to revolutionize stretchable electronics for next-generation soft robotic, e-skin, and wearable technologies. Core-shell particles of liquid metal with surface-bound acrylate ligands are synthesized and polymerized together to create cross-linked particle networks comprising >99.9% liquid metal by weight. When stretched, particles within these polymerized liquid metal networks (Poly-LMNs) rupture and release their liquid metal payload, resulting in a rapid 108 -fold increase in the network's conductivity. These networks autonomously form hierarchical structures that mitigate the deleterious effects of strain on electronic performance and give rise to emergent properties. Notable characteristics include nearly constant resistances over large strains, electronic strain memory, and increasing volumetric conductivity with strain to over 20 000 S cm-1 at >700% elongation. Furthermore, these Poly-LMNs exhibit exceptional performance as stretchable heaters, retaining 96% of their areal power across relevant physiological strains. Remarkable electromechanical properties, responsive behaviors, and facile processing make Poly-LMNs ideal for stretchable power delivery, sensing, and circuitry.

Computational and Experimental Investigation of Janus-like Monolayers on Ultrasmall Noble Metal Nanoparticles.

Detection of monolayer morphology on nanoparticles smaller than 10 nm has proven difficult with traditional visualization techniques. Here matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) is used in conjunction with atomistic simulations to detect the formation of Janus-like monolayers on noble metal nanoparticles. Silver metal nanoparticles were synthesized with a monolayer consisting of dodecanethiol (DDT) and mercaptoethanol (ME) at varying ratios. The nanoparticles were then analyzed using MALDI-MS, which gives information on the local ordering of ligands on the surface. The MALDI-MS analysis showed large deviations from random ordering, suggesting phase separation of the DDT/ME monolayers. Atomistic Monte Carlo (MC) calculations were then used to simulate the nanoscale morphology of the DDT/ME monolayers. In order to quantitatively compare the computational and experimental results, we developed a method for determining an expected MALDI-MS spectrum from the atomistic simulation. Experiments and simulations show quantitative agreement, and both indicate that the DDT/ME ligands undergo phase separation, resulting in Janus-like nanoparticle monolayers with large, patchy domains.

Control of Gallium Oxide Growth on Liquid Metal Eutectic Gallium/Indium Nanoparticles via Thiolation.

Eutectic gallium-indium alloy (EGaIn, a room-temperature liquid metal) nanoparticles are of interest for their unique potential uses in self-healing and flexible electronic devices. One reason for their interest is due to a passivating oxide skin that develops spontaneously on exposure to ambient atmosphere which resists deformation and rupture of the resultant liquid particles. It is then of interest to develop methods for control of this oxide growth process. It is hypothesized here that functionalization of EGaIn nanoparticles with thiolated molecules could moderate oxide growth based on insights from the Cabrera-Mott oxidation model. To test this, the oxidation dynamics of several thiolated nanoparticle systems were tracked over time with X-ray photoelectron spectroscopy. These results demonstrate the ability to suppress gallium oxide growth by up to 30%. The oxide progressively matures over a 28 day period, terminating in different final thicknesses as a function of thiol selection. These results indicate not only that thiols moderate gallium oxide growth via competition with oxygen for surface sites but also that different thiols alter the thermodynamics of oxide growth through modification of the EGaIn work function.

Theoretical and Experimental Investigation of Microphase Separation in Mixed Thiol Monolayers on Silver Nanoparticles.

Silver nanoparticles with mixed ligand self-assembled monolayers were synthesized from dodecanethiol and another ligand from a homologous series of alkanethiols (butanethiol, pentanethiol, heptanethiol, octanethiol, nonanethiol, decanethiol, undecanethiol, or dodecanethiol[D25]). These were hypothesized to exhibit ligand phase separation that increases with degree of physical mismatch between the ligands based on the difference in the number of carbons in the two ligands. Dodecanethiol/dodecanethiol[D25] was expected to exhibit minimal phase separation as the ligands have only isotopic differences, while dodecanethiol/butanethiol was hypothesized to exhibit the most phase separation due to the difference in chain length. Phase separation of all other ligand mixtures was expected to fall between these two extremes. Matrix-assisted laser desorption ionization (MALDI) mass spectroscopy provided a value for ligand phase separation by comparison with a binomial (random) model and subsequent calculation of the sum-of-squares error (SSR). These nanoparticle systems were also modeled using the Scheutjens and Fleer self-consistent mean-field theory (SCFT), which determined the most thermodynamically favorable arrangement of ligands on the surface. From MALDI, it was found that dodecanethiol/dodecanethiol[D25] formed a well-mixed monolayer with SSR = 0.002, and dodecanethiol/butanethiol formed a microphase separated monolayer with SSR = 0.164; in intermediate dodecanethiol/alkanethiol mixtures, SSR increased with increasing ligand length difference as expected. For comparison with experiment, an effective SSR value was calculated from SCFT simulations. The SSR values obtained by experiment and theory show good agreement and provide strong support for the validity of SCFT predictions of monolayer structure. These approaches represent robust methods of characterization for ligand phase separation on silver nanoparticles.

Development of experiment and theory to detect and predict ligand phase separation on silver nanoparticles.

MALDI mass-spectrometry measurements are combined with self-consistent mean-field (SCF) calculations to detect and predict ligand phase separation on Ag nanoparticles. The experimental and theoretical techniques complement each other by enabling quantification of the nearest-neighbor distribution of a ligand mixture in a monolayer shell. By tracking a characteristic metallic fragment family, analysis of a MALDI spectrum produces a frequency distribution corresponding to specific ligand patterning. Inherent to the SCF calculation is the enumeration of local interactions that dictate ligand assembly. Interweaving MALDI and SCF facilitates a comparison between the experimentally and theoretically derived frequency distributions as well as their deviation from a well-mixed state. Thus, we combine these techniques to detect and predict phase separation in monolayers that mix uniformly or experience varying degrees of de-mixing, including microphase separation and stripe formation. Definition of MALDI removed as this is a commonly recognized technique.

Straightforward, one-step synthesis of alkanethiol-capped silver nanoparticles from an aggregative model of growth.

Classical nucleation theory and Derjaguin, Landau, Verwey, Overbeek (DLVO) theory for colloidal stability were applied to gain insight into the synthesis of dodecanethiol (DDT) functionalized silver nanoparticles (NPs) by reduction of silver nitrate with sodium borohydride in ethanol. This analysis indicated the importance of quickly establishing a dense DDT ligand brush on inherently unstable primary particles to achieve colloidal stability. The DLVO calculations also indicated that the electrostatic potential was a minor contributor to repulsive interactions, signifying that it would be possible to control NP size and uniformity in solutions with high ionic strength, as long as sufficient DDT was available to form a densely packed ligand layer on the NPs. These insights were applied to design a new straightforward, one-step, one-phase synthesis for the production of alkanethiol-functionalized silver NPs. To test the insights from DLVO theory, 16 samples were synthesized in the parameter space R = 3-12, S = 1-12 where R = [NaBH4]/[AgNO3], S = [DDT]/[AgNO3], and [AgNO3] = 10 mM. In general, samples with R = 3 or S = 1 were polydisperse; however, samples in the R = 6-12 and S = 3-12 range had uniform particle sizes with average diameters between 3.5 and 4.7 nm. Additionally, samples with R = 72-108 and S = 12 were synthesized to test particle stability at high ionic strength; again, uniform NPs with average diameters from 3.5 to 3.8 nm were produced. Ultimately, the insights gained from DLVO theory successfully guided the development of a one-step, one-phase technique for the synthesis of uniform, spherical alkanethiol-functionalized silver NPs.