Rheology of graphene oxide stabilized Pickering emulsions.

Pickering emulgels stabilized by graphene oxide (GO) with didodecyldimethylammonium bromide (DDAB) as an auxiliary surfactant and liquid paraffin as the oil phase have proved to be an excellent 3D printable ink. This paper elucidates the structure of such emulgels by a combination of microscopy before and after intensive shear as well as broadband dielectric spectroscopy and rheology in the linear and nonlinear regime. An increase of the DDAB surfactant and GO-contents leads to a systematic increase of modulus and viscosity, a reduction of the limits of the nonlinear regime and a more complicated variation of the normal forces, with negative normal forces at high shear rate  for low GO-contents and positive normal forces at high GO-contents. The interfacial jamming behavior studied by morphology, rheology and dielectric spectroscopy is explained based on droplet deformation, jamming and recovery phenomena.

Understanding Gel-Powers: Exploring Rheological Marvels of Acrylamide/Sodium Alginate Double-Network Hydrogels.

This study investigates the rheological properties of dual-network hydrogels based on acrylamide and sodium alginate under large deformations. The concentration of calcium ions affects the nonlinear behavior, and all gel samples exhibit strain hardening, shear thickening, and shear densification. The paper focuses on systematic variation of the alginate concentration-which serves as second network building blocks-and the Ca2+-concentration-which shows how strongly they are connected. The precursor solutions show a typical viscoelastic solution behavior depending on alginate content and pH. The gels are highly elastic solids with only relatively small viscoelastic components, i.e., their creep and creep recovery behavior are indicative of the solid state after only a very short time while the linear viscoelastic phase angles are very small. The onset of the nonlinear regime decreases significantly when closing the second network (alginate) upon adding Ca2+, while at the same time the nonlinearity parameters (Q0, I3/I1, S, T, e3/e1, and v3/v1) increase significantly. Further, the tensile properties are significantly improved by closing the alginate network by Ca2+ at intermediate concentrations.

GaOOH Crystallite Growth on Liquid Metal Microdroplets in Water: Influence of the Local Environment.

Gallium-based liquid metals form alloys with a melting point close to or below room temperature. On the surface of these liquid metals, a thin oxide skin is formed once in contact with oxygen, and this oxide skin can be leveraged to stabilize liquid metal micro- and nanodroplets in a liquid. During sonication and storage of these droplets in aqueous solution, gallium oxide hydroxide (GaOOH) forms on these droplets, and given enough time or treatment with heat, a full shape transition and dealloying are observed. In this article, we show that GaOOH can be grown at room temperature and that the growth is dependent on both the local environment and temperature. GaOOH growth on liquid metal microdroplets located at the air/water interface is considerably faster than in the bulk phase. Interestingly, hydrolysis to GaOOH is hampered and stops at 15 °C in bulk water after 6 h. In contrast, hydrolysis commences even at 15 °C for liquid metal microdroplets located at the air/water interface, and full surface coverage is obtained after around 24 h (compared to 12 h at 25 °C at the air/water interface). The X-ray photoelectron spectroscopy (XPS) measurement suggests that gallium oxide is dissolved and Ga(OH)3 is formed as a precursor that reacts in a downstream reaction toward GaOOH. This improved understanding of the GaOOH formation can be leveraged to control the liquid metal micro- and nanodroplet shape and composition (i.e., for biomedical applications).

Ultrathin Diamond Nanofilms-Development, Challenges, and Applications.

Diamond is a highly attractive material for ample applications in material science, engineering, chemistry, and biology because of its favorable properties. The advent of conductive diamond coatings and the steady demand for miniaturization in a plethora of economic and scientific fields resulted in the impetus for interdisciplinary research to develop intricate deposition techniques for thin (≤1000 nm) and ultra-thin (≤100 nm) diamond films on non-diamond substrates. By virtue of the lowered thickness, diamond coatings feature high optical transparency in UV-IR range. Combined with their semi-conductivity and mechanical robustness, they are promising candidates for solar cells, optical devices, transparent electrodes, and photochemical applications. In this review, the difficulty of (ultra-thin) diamond film development and production, introduction of important stepping stones for thin diamond synthesis, and summarization of the main nucleation procedures for diamond film synthesis are elucidated. Thereafter, applications of thin diamond coatings are highlighted with a focus on applications relying on ultrathin diamond coatings, and the excellent properties of the diamond exploited in said applications are discussed, thus guiding the reader and enabling the reader to quickly get acquainted with the research field of ultrathin diamond coatings.

Surface Tension of the Oxide Skin of Gallium-Based Liquid Metals.

Gallium-based alloys have garnered considerable attention in the scientific community, particularly as they are in an atypical liquid state at and near room temperature. Though physical parameters, such as thermal conductivity, electrical conductivity, viscosity, yield stress, and surface tension, of these alloys are broadly known, the surface tension (surface free energy) of the oxide skin remains intangible due to the high yield stress of the oxide skin. In this article, we propose to employ gradually attenuated vibrations to obtain equilibrium shapes, which are analyzed along the lines of the puddle height method. The surface tension of the oxide skin was determined on quartz glass and liquid metal-phobic diamond coating to be around 350-365 mN/m, thus independent of the substrate surface or employed liquid metal (i.e., eutectic Ga-In (EGaIn) and galinstan). The similarity of the surface tension for different alloys was ascribed to the composition of the oxide skin, which predominantly comprises gallium oxides due to thermodynamic constraints. We envision that this method can also be applied to other liquid metal alloys and liquid metal marble systems facilitating modeling, simulation, and optimization processes.

Liquid Metal-Based Soft Microfluidics.

Motivated by the increasing demand of wearable and soft electronics, liquid metal (LM)-based microfluidics has been subjected to tremendous development in the past decade, especially in electronics, robotics, and related fields, due to the unique advantages of LMs that combines the conductivity and deformability all-in-one. LMs can be integrated as the core component into microfluidic systems in the form of either droplets/marbles or composites embedded by polymer materials with isotropic and anisotropic distribution. The LM microfluidic systems are found to have broad applications in deformable antennas, soft diodes, biomedical sensing chips, transient circuits, mechanically adaptive materials, etc. Herein, the recent progress in the development of LM-based microfluidics and their potential applications are summarized. The current challenges toward industrial applications and future research orientation of this field are also summarized and discussed.

Light-Induced Shape Morphing of Liquid Metal Nanodroplets Enabled by Polydopamine Coating.

Shape morphing nanosystems have recently attracted much attention and a number of applications are developed, spanning from autonomous robotics to drug delivery. However, the fabrication of such nanosystems remains at an early stage owing to limited choices of strategies and materials. This work reports a facile method to fabricate liquid metal (LM) nanodroplets by sonication of bulk LM in an aqueous dopamine hydrochloride solution and their application in light-induced shape morphing at the nanoscale. In this method, dopamine acts as a surfactant, which stabilizes the LM nanodroplets dispersion during the sonication, and results in downsizing of the nanodroplets. Furthermore, by adding 2-amino-2-(hydroxymethyl)-1,3-propanediol to the suspension, self-polymerization of dopamine molecules occurs, resulting in the formation of polydopamine (PDA)-coated LM nanodroplets. Owing to the high photothermal conversion of the PDA, PDA-coated LM nanodroplets are transformed from spherical shapes to ellipsoids by NIR laser irradiation. This study paves a simple and reliable pathway for the preparation of functional LM nanodroplets and their application as shape-morphing nanosystems.

Biomimetic Extreme-Temperature- and Environment-Adaptable Hydrogels.

Recently, temperature-resistant hydrogels, hydrogels which are freezing- and dehydration-resistant, have garnered considerable attention in the scientific community as they extend the rage of application of hydrogels to arid and/or cold environments. Besides, these hydrogels exhibit tunable conductivity and mechanical performance while offering excellent biocompatibility and flexibility, making them interesting candidates for flexible and wearable electronics and (bio)sensors. Several biomimetic strategies were developed to fabricate anti-freezing and anti-dehydration hydrogels with a diversity of merits, such as high strain resistance and conductivity, even at sub-zero temperatures, and employed as (bio)sensors, electrodes, and energy-storage devices. This review summarizes the recent advances in the preparation and application of temperature-resistant hydrogels, indicates issues of the state-of-the-art hydrogels, and offers potential future research directions.

Electric Actuation of Liquid Metal Droplets in Acidified Aqueous Electrolyte.

The electric actuation of room-temperature liquid metals, such as Galinstan (gallium-indium-tin), has largely been conducted in alkaline electrolyte. Addition of surface-active anions and a proper acidic pH are expected to influence the interfacial tension of the liquid metal due to a high surface charge density. Hence, it should be possible to actuate liquid metals in such acidic environments. To ascertain this, at first, the dependence of the interfacial tension of Galinstan in NaOH, acidified KI, and acidified NaCl electrolyte on the concentration of the surface-active anions OH-, I-, and Cl-, respectively, were studied. Subsequently, a systematic study of the actuation of Galinstan in acidified KI electrolyte was executed and compared to actuation in alkaline medium. In the presence of HCl and acidified NaCl electrolyte, the interfacial tension of Galinstan is only marginally altered, while acidified KI solution reduced the interfacial tension of Galinstan significantly from 470.8 ± 1.4 (no KI) to 370.6 ± 4.1 mN/m (5 M KI) due to the high surface charge density of the electric double layer. Therefore, in acidified electrolyte in the presence of surface-active anions, the electrically actuated motion of LM can be realized. In particular, the actuation of Galinstan achieves a higher average and maximum speed at lower applied voltage and power consumption for acidified KI electrolyte. The formation of high surface charge density in acidified environments signifies a paradigm shift and opens up new possibilities to tune interfacial tension and controlled LM droplet motion of room-temperature liquid metals.

Liquid Metal-Mediated Mechanochemical Polymerization.

Mechanically controlled polymerization that employs the mechanical energy to fabricate novel synthetic materials has attracted considerable interest. However, only a few examples have been achieved so far, owing to the limited choices of materials and strategies. Herein, a versatile, liquid metal (LM)-mediated mechanochemical polymerization method (LMMMP) is developed for the air-compatible, robust preparation of polymers in an aqueous solution. This method involves the simultaneous disruption of bulk LMs into micro- and nanodroplets and the combination of monomers into polymers during ultrasonic irradiation. The pristine and reactive LM surface continuously generated by ultrasound endows this polymerization method with excellent oxygen tolerance, high reaction rate, and the ability to produce polymers with high molecular weight from a wide variety of water-soluble monomers. Besides, LM droplets are readily reclaimed and reused for polymerization. The authors envision that the LMMMP promotes the utilization of mechanical energy for the synthesis of functional polymers, constitutes a novel fabrication approach for polymer-LM nanocomposites, and provides new insight into the design of LM-based platforms for polymerization.

Analysis and Transformations of Room-Temperature Liquid Metal Interfaces - A Closer Look through Interfacial Tension.

Room-temperature gallium-based liquid metals (e. g. Galinstan) are emerging materials for a wide range of applications spanning from soft robotics to a medium for running chemical reactions. However, Galinstan suffers from rapid oxide formation at the surface, limiting its application. Similarly, this surface oxidation renders it difficult to determine the surface free energy of Galinstan. Herein, the interfacial properties of Galinstan in contact with argon, acid, base, water, and organic solvents were investigated, yielding kinetic information regarding the removal by acids and bases, knowledge about oxo-hydroxide/hydroxide species at the Galinstan/water interface, and an estimated value of the dispersive contribution of the surface free energy. The dispersive contribution of the surface free energy of Galinstan was calculated to be (239.7±9.1) mN/m, around 40 % of the total surface free energy. By employing the dispersive surface tension of Galinstan, the interfacial tension between a liquid and Galinstan can be easily obtained, facilitating the design and application of liquid metal-based devices.

Controlling Directional Liquid Motion on Micro- and Nanocrystalline Diamond/ß-SiC Composite Gradient Films.

In this Article, we report the synthesis of micro- and nanocrystalline diamond/ß-SiC composite gradient films, using a hot filament chemical vapor deposition (HFCVD) technique and its application as a robust and chemically inert means to actuate water and hazardous liquids. As revealed by scanning electron microscopy, the composition of the surface changed gradually from pure nanocrystalline diamond (hydrophobic) to a nanocrystalline ß-SiC surface (hydrophilic). Transmission electron microscopy and Raman spectroscopy were employed to determine the presence of diamond, graphite, and ß-SiC phases. The as-prepared gradient films were evaluated for their ability to actuate water. Indeed, water was transported via the gradient from the hydrophobic (hydrogen-terminated diamond) to the hydrophilic side (hydroxyl-terminated ß-SiC) of the gradient surface. The driving distance and velocity of water is pivotally influenced by the surface roughness. The nanogradient surface showed significant promise as the lower roughness combined with the longer gradient yields in transport distances of up to 3.7 mm, with a maximum droplet velocity of nearly 250 mm/s measured by a high-speed camera. As diamond and ß-SiC are chemically inert, the gradient surfaces can be used to drive hazardous liquids and reactive mixtures, which was signified by the actuation of hydrochloric acid and sodium hydroxide solution. We envision that the diamond/ß-SiC gradient surface has high potential as an actuator for water transport in microfluidic devices, DNA sensors, and implants, which induce guided cell growth.

Softening and Shape Morphing of Stiff Tough Hydrogels by Localized Unlocking of the Trivalent Ionically Cross-Linked Centers.

The mechanical properties (e.g., stiffness, stretchability) of prefabricated hydrogels are of pivotal importance for diverse applications in tissue engineering, soft robotics, and medicine. This study reports a feasible method to fabricate ultrasoft and highly stretchable structures from stiff and tough hydrogels of low stretchability and the application of these switchable hydrogels in programmable shape-morphing systems. Stiff and tough hydrogel structures are first fabricated by the mechanical strengthening of Ca2+ -alginate/polyacrylamide tough hydrogels by addition of Fe3+ ions, which introduces Fe3+ ionically cross-linked centers into the Ca2+ divalent cross-linked hydrogel, forming an additional and much less flexible trivalent ionically cross-linked network. The resulting stiff and tough hydrogels are exposed to an L-ascorbic acid (vitamin C, VC) solution to rapidly reduce Fe3+ to Fe2+ . As a result, flexible divalent ionically cross-linked networks are formed, leading to swift softening of the stiff and tough hydrogels. Moreover, localized stiffness variation of the tough hydrogels can be realized by precise patterning of the VC solution. To validate this concept, sequential steps of VC patterning are carried out for local tuning of the stiffness of the hydrogels. With this strategy, localized softening, unfolding, and sequential folding of the tough hydrogels into complex 3D structures is demonstrated.

Rational Fabrication of Anti-Freezing, Non-Drying Tough Organohydrogels by One-Pot Solvent Displacement.

Tough hydrogels, polymeric network structures with excellent mechanical properties (such as high stretchability and toughness), are emerging soft materials. Despite their remarkably mechanical features, tough hydrogels exhibit two flaws (freezing around the icing temperatures of water and drying under arid conditions). Inspired by cryoprotectants (CPAs) used in the inhibition of the icing of water in biological samples, a versatile and straightforward method is reported to fabricate extreme anti-freezing, non-drying CPA-based organohydrogels with long-term stability by partially displacing water molecules within the pre-fabricated hydrogels. CPA-based Ca-alginate/polyacrylamide (PAAm) tough hydrogels were successfully fabricated with glycerol, glycol, and sorbitol. The CPA-based organohydrogels remain unfrozen and mechanically flexible even up to -70 °C and are stable under ambient conditions or even vacuum.

Detailed Study of BSA Adsorption on Micro- and Nanocrystalline Diamond/ß-SiC Composite Gradient Films by Time-Resolved Fluorescence Microscopy.

The adsorption of bovine serum albumin (BSA) on micro- and nanocrystalline diamond/ß-SiC composite films synthesized using the hot filament chemical vapor deposition (HFCVD) technique has been investigated by confocal fluorescence lifetime imaging microscopy. BSA labeled with fluorescein isothiocyanate (FITC) was employed as a probe. The BSAFITC conjugate was found to preferentially adsorb on both O-/OH-terminated microcrystalline and nanocrystalline diamond compared to the OH-terminated ß-SiC, resulting in an increasing amount of BSA adsorbed to the gradient surfaces with an increasing diamond/ß-SiC ratio. The different strength of adsorption (>30 times for diamond with a grain size of 570 nm) coincides with different surface energy parameters and differing conformational changes upon adsorption. Fluorescence data of the adsorbed BSAFITC on the gradient film with different diamond coverage show a four-exponential decay with decay times of 3.71, 2.54, 0.66, and 0.13 ns for a grain size of 570 nm. The different decay times are attributed to the fluorescence of thiourea fluorescein residuals of linked FITC distributed in BSA with different dye-dye and dye-surface distances. The longest decay time was found to correlate linearly with the diamond grain size. The fluorescence of BSAFITC undergoes external dynamic fluorescence quenching on the diamond surface by H- and/or sp2-defects and/or by amorphous carbon or graphite phases. An acceleration of the internal fluorescence concentration quenching in BSAFITC because of structural changes of albumin due to adsorption, is concluded to be a secondary contributor. These results suggest that the micro- and nanocrystalline diamond/ß-SiC composite gradient films can be utilized to spatially control protein adsorption and diamond crystallite size, which facilitates systematic studies at these interesting (bio)interfaces.

Enzyme degradable polymersomes from hyaluronic acid-block-poly(ε-caprolactone) copolymers for the detection of enzymes of pathogenic bacteria.

We introduce a new hyaluronidase-responsive amphiphilic block copolymer system, based on hyaluronic acid (HYA) and polycaprolactone (PCL), that can be assembled into polymersomes by an inversed solvent shift method. By exploiting the triggered release of encapsulated dye molecules, these HYA-block-PCL polymersomes lend themselves as an autonomous sensing system for the detection of the presence of hyaluronidase, which is produced among others by the pathogenic bacterium Staphylococcus aureus. The synthesis of the enzyme-responsive HYA-block-PCL block copolymers was carried out by copper-catalyzed Huisgen 1,3-dipolar cycloaddition of ω-azide-terminated PCL and ω-alkyne-functionalized HYA. The structure of the HYA-block-PCL assemblies and their enzyme-triggered degradation and concomitant cargo release were investigated by dynamic light scattering, fluorescence spectroscopy, confocal laser-scanning microscopy, scanning and transmission electron, and atomic force microscopy. As shown, a wide range of reporter dye molecules as well as antimicrobials can be encapsulated into the vesicles during formation and are released upon the addition of hyaluronidase.

Controlled surface chemistry of diamond/ß-SiC composite films for preferential protein adsorption.

Diamond and SiC both process extraordinary biocompatible, electronic, and chemical properties. A combination of diamond and SiC may lead to highly stable materials, e.g., for implants or biosensors with excellent sensing properties. Here we report on the controllable surface chemistry of diamond/ß-SiC composite films and its effect on protein adsorption. For systematic and high-throughput investigations, novel diamond/ß-SiC composite films with gradient composition have been synthesized using the hot filament chemical vapor deposition (HFCVD) technique. As revealed by scanning electron microscopy (SEM), the diamond/ß-SiC ratio of the composite films shows a continuous change from pure diamond to ß-SiC over a length of ∼ 10 mm on the surface. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to unveil the surface termination of chemically oxidized and hydrogen treated surfaces. The surface chemistry of the composite films was found to depend on diamond/ß-SiC ratio and the surface treatment. As observed by confocal fluorescence microscopy, albumin and fibrinogen were preferentially adsorbed from buffer: after surface oxidation, the proteins preferred to adsorb on diamond rather than on ß-SiC, resulting in an increasing amount of proteins adsorbed to the gradient surfaces with increasing diamond/ß-SiC ratio. By contrast, for hydrogen-treated surfaces, the proteins preferentially adsorbed on ß-SiC, leading to a decreasing amount of albumin adsorbed on the gradient surfaces with increasing diamond/ß-SiC ratio. The mechanism of preferential protein adsorption is discussed by considering the hydrogen bonding of the water self-association network to OH-terminated surfaces and the change of the polar surface energy component, which was determined according to the van Oss method. These results suggest that the diamond/ß-SiC gradient film can be a promising material for biomedical applications which require good biocompatibility and selective adsorption of proteins and cells to direct cell migration.

Long-Term Corrosion of Eutectic Gallium, Indium, and Tin (EGaInSn) Interfacing with Diamond.

Thermal transport is of grave importance in many high-value applications. Heat dissipation can be improved by utilizing liquid metals as thermal interface materials. Yet, liquid metals exhibit corrosivity towards many metals used for heat sinks, such as aluminum, and other electrical devices (i.e., copper). The compatibility of the liquid metal with the heat sink or device material as well as its long-term stability are important performance variables for thermal management systems. Herein, the compatibility of the liquid metal Galinstan, a eutectic alloy of gallium, indium, and tin, with diamond coatings and the stability of the liquid metal in this environment are scrutinized. The liquid metal did not penetrate the diamond coating nor corrode it. However, the liquid metal solidified with the progression of time, starting from the second year. After 4 years of aging, the liquid metal on all samples solidified, which cannot be explained by the dissolution of aluminum from the titanium alloy. In contrast, the solidification arose from oxidation by oxygen, followed by hydrolysis to GaOOH due to the humidity in the air. The hydrolysis led to dealloying, where In and Sn remained an alloy while Ga separated as GaOOH. This hydrolysis has implications for many devices based on gallium alloys and should be considered during the design phase of liquid metal-enabled products.

Microrobots for Targeted Delivery and Therapy in Digestive System.

Untethered miniature robots enable targeted delivery and therapy deep inside the gastrointestinal tract in a minimally invasive manner. By combining actuation systems and imaging tools, significant progress has been made toward the development of functional microrobots. These robots can be actuated by external fields and fuels while featuring real-time tracking feedback toward certain regions and can perform the therapeutic process by rational exertion of the local environment of the gastrointestinal tract (e.g., pH, enzyme). Compared with conventional surgical tools, such as endoscopic devices and catheters, miniature robots feature minimally invasive diagnosis and treatment, multifunctionality, high safety and adaptivity, embodied intelligence, and easy access to tortuous and narrow lumens. In addition, the active motion of microrobots enhances local penetration and retention of drugs in tissues compared to common passive oral drug delivery. Based on the dissimilar microenvironments in the various sections of the gastrointestinal tract, this review introduces the advances of miniature robots for minimally invasive targeted delivery and therapy of diseases along the gastrointestinal tract. The imaging modalities for the tracking and their application scenarios are also discussed. We finally evaluate the challenges and barriers that retard their applications and hint on future research directions in this field.

Small-Scale Robotics with Tailored Wettability.

Small-scale robots (SSRs) have emerged as promising and versatile tools in various biomedical, sensing, decontamination, and manipulation applications, as they are uniquely capable of performing tasks at small length scales. With the miniaturization of robots from the macroscale to millimeter-, micrometer-, and nanometer-scales, the viscous and surface forces, namely adhesive forces and surface tension have become dominant. These forces significantly impact motion efficiency. Surface engineering of robots with both hydrophilic and hydrophobic functionalization presents a brand-new pathway to overcome motion resistance and enhance the ability to target and regulate robots for various tasks. This review focuses on the current progress and future perspectives of SSRs with hydrophilic and hydrophobic modifications (including both tethered and untethered robots). The study emphasizes the distinct advantages of SSRs, such as improved maneuverability and reduced drag forces, and outlines their potential applications. With continued innovation, rational surface engineering is expected to endow SSRs with exceptional mobility and functionality, which can broaden their applications, enhance their penetration depth, reduce surface fouling, and inhibit bacterial adhesion.