The Potential of Liquid Marbles for Biomedical Applications: A Critical Review

Polyhedral Vinyl Polymer Particles Synthesized Via Solvent-Free Radical Polymerization

Liquid marbles (LMs) with a cubic shape are created by using various vinyl monomers as an inner liquid and polymer plates with mm size as a stabilizer. The relationship between the surface tension of the vinyl monomers and formability of the LMs is investigated. LMs can be fabricated using vinyl monomers with surface tensions of 42.7-40.3 mN m-1. The cubic polymer particles are successively synthesized via free-radical polymerizations by irradiation of the cubic LMs with UV light in a solvent-free manner. In addition, controlling the number of polymer plates per one LM, the shape of the plate or the coalescence of the LMs can lead to production of polymer particles with desired forms (e.g., Platonic and rectangular solids) that correspond to the shapes of the original LMs.

Non-Aqueous Polyhedral Liquid Marbles Stabilized with Polymer Plates Having Surface Roughness

Hydrophobic polymer plates with smooth and rough surfaces are used as a stabilizer for cubic liquid marbles (LMs) to study the effect of surface roughness on their formation. The smooth and rough polymer plates can stabilize LMs using liquids with surface tensions of 72.8-26.6 and 72.8-22.9 mN m-1, respectively. It is clarified that the higher the surface roughness, the lower the surface tension of the liquids are stabilized to form the LMs. These results indicated that the introduction of surface roughness improves the hydrophobicity of the polymer plates and the rough polymer plates can stabilize LMs using liquids with a wider surface tension range. Electron microscopy studies and numerical analyses confirmed that the LMs can be formed, when the Cassie-Baxter wetting state, where θY>90° (θY: the contact angle on smooth surfaces) and θR>90° (θR: the contact angle on rough surfaces), and the metastable Cassie-Baxter wetting state, where θY<90° and θR>90°, are realized. Finally, the synthesis of cubic polymer particles are succeeded by free radical polymerization of the cubic LMs containing a hydrophobic vinyl monomer (dodecyl acrylate) in a solvent-free manner.

Biocompatible Hydrogel-Based Liquid Marbles with Magnetosomes

Liquid marbles are widely known for their potential biomedical applications, especially due to their versatility and ease of preparation. In the present work, we prepared liquid marbles with various cores composed of water, agar-based hydrogels, magnetic fluids, or non-aqueous substances. As a coating material, we used biocompatible particles of plant origin, such as turmeric grains and Lycopodium pollen. Additionally, we provided marbles with magnetic properties by incorporating either magnetosomes or iron oxide nanoparticles as a powder or by injecting another magnetic fluid. Structures obtained in this way were stable and susceptible to manipulation by an external magnetic field. The properties of the magnetic components of our marbles were verified using electron paramagnetic resonance (EPR) spectroscopy and vibrating sample magnetometry (VSM). Our approach to encapsulation of active substances such as antibiotics within a protective hydrogel core opens up new perspectives for the delivery of hydrophobic payloads to the inherently hydrophilic biological environment. Additionally, hydrogel marbles enriched with magnetic materials showed promise as biocompatible heating agents under alternating magnetic fields. A significant innovation of our research was also the fabrication of composite structures in which the gel-like core was surrounded without mixing by a magnetic fluid covered on the outside by the particle shell. Our liquid marbles, especially those with a hydrogel core and magnetic content, due to the ease of preparation and favorable properties, have great potential for biomedical use. The fact that we were able to simultaneously produce, functionalize (by filling with predefined cargo), and manipulate (by means of an external magnetic field) several marbles also seems to be important from an application point of view.

Liquid marbles: review of recent progress in physical properties, formation techniques, and lab-in-a-marble applications in microreactors and biosensors

Liquid marbles (LMs) are nonsticking droplets whose surfaces are covered with low-wettability particles. Owing to their high mobility, shape reconfigurability, and widely accessible liquid/particle possibilities, the research on LMs has flourished since 2001. Their physical properties, fabrication mechanisms, and functionalisation capabilities indicate their potential for various applications. This review summarises the fundamental properties of LMs, the recent advances (mainly works published in 2020-2023) in the concept of LMs, physical properties, formation methods, LM-templated material design, and biochemical applications. Finally, the potential development and variations of LMs are discussed.

Emerging open-channel droplet arrays for biosensing

Open-channel droplet arrays have attracted much attention in the fields of biochemical analysis, biofluid monitoring, biomarker recognition and cell interactions, as they have advantages with regard to miniaturization, parallelization, high-throughput, simplicity and accessibility. Such droplet arrays not only improve the sensitivity and accuracy of a biosensor, but also do not require sophisticated equipment or tedious processes, showing great potential in next-generation miniaturized sensing platforms. This review summarizes typical examples of open-channel microdroplet arrays and focuses on diversified biosensing integrated with multiple signal-output approaches (fluorescence, colorimetric, surface-enhanced Raman scattering (SERS), electrochemical, etc.). The limitations and development prospects of open-channel droplet arrays in biosensing are also discussed with regard to the increasing demand for biosensors.

Electrostatic Adsorption Behaviors of Polymer Plates to a Droplet

Electrostatic transfer and adsorption of electrically conductive polymer-coated poly(ethylene terephthalate) plates from a particle bed to a water droplet were studied, with the influence of plate thickness and shape observed. After synthesis and confirmation of the particles' properties using stereo and scanning electron microscopies, elemental microanalysis, and water contact angle measurement, the electric field strength and droplet-bed separation distance required for transfer were measured. An electrometer and high-speed video footage were used to measure the charge transferred by each particle, and its orientation and adsorption behavior during transfer and at the droplet interface. The use of plates of consistent square cross section allowed the impact of contact-area-dependent particle cohesion and gravity on the electrostatic transfer of particles to be decoupled for the first time. The electrostatic force required to extract a plate was directly proportional to the plate mass (thickness), a trend very different from that previously observed for spherical particles of varied diameter (mass). This reflected the different relationship between mass, surface area, and cohesive forces for spherical and plate-shaped particles of different sizes. Thicker plates transferred more charge to the droplet, probably due to their remaining at the bed at higher field strengths. The impact of plate cross-sectional geometry was also assessed. Differences in the ease of transfer of square, hexagonal, and circular plates seemed to depend only on their mass, while other aspects of their comparative behavior are attributed to the more concentrated charge distribution present on particles with sharper vertices.

Nonspherical Epoxy Resin Polymer Particles Synthesized via Solvent-Free Polyaddition Reactions

Cubic liquid marbles (LMs) were fabricated by using various epoxy monomers as internal liquids and millimeter-sized polymer plates as stabilizers. Successively, cubic polymer particles were synthesized via solvent-free polyaddition reactions by exposing the cubic LMs to NH₃ vapor used as a curing agent. The effect of the solubility parameters (SPs) for the epoxy monomers on the formation of the cubic polymer particles was investigated. As a result, we succeeded in fabricating cubic polymer particles reflecting the shapes of the original LMs by using epoxy monomers with SP values of 23.70-21.66 (MPa)^1/2. Furthermore, the shapes of the LMs could be controlled on demand (e.g., pentahedral and rectangular) by control of the number of polymer plates per LM and/or coalescence of the LMs, resulting in fabrication of polymer particles with shapes reflecting those of the LMs.

Bovine in vitro oocyte maturation and embryo culture in liquid marbles 3D culture system

Despite the advances in in vitro embryo production (IVP) over the years, the technique still has limitations that need to be overcome. In cell cultures, it is already well established that three-dimensional culture techniques are more physiological and similar to the in vivo development. Liquid marble (LM) is a three-dimensional system based on the use of a hydrophobic substance to create in vitro microbioreactors. Thus, we hypothesized that the LM system improves bovine in vitro oocyte maturation and embryo culture. In experiment I, bovine cumulus-oocyte complexes (COCs) were placed for in vitro maturation for 22h in two different groups: control (conventional 2D culture) and LM (three-dimensional culture). We found that oocyte nuclear maturation was not altered by the LM system, however it was observed a decrease in expression of genes important in the oocyte maturation process in cumulus cells of LM group (BCL2, EIF4E, and GAPDH). In experiment II, the COCs were conventionally matured and fertilized, and for culture, they were divided into LM or control groups. There was a decrease in blastocyst rate and cell counting, a down-regulation of miR-615 expression, and an increase in the DNA global methylation and hydroxymethylation in embryos of LM group. Therefore, for the bovine in vitro embryo production, this specific three-dimensional system did not present the advantages that we expected, but demonstrated that the embryos changed their development and epigenetics according to the culture system.

Effect of surface roughness on the solar evaporation of liquid marbles

HYPOTHESIS

Nanostructured materials are widely used for solar energy harvesting and conversion due to their excellent photothermal properties. It is generally accepted that the better the light absorption ability, the better the photothermal conversion efficiency.

EXPERIMENT

A series of experiments in solar evaporation of liquid marbles (LMs) by coating the droplets with Fe3O4, Ni nanoparticles (NPs) and carbon nanotubes (CNTs) are conducted.

FINDINGS

Conversely, we found that the surface roughness of solar absorber plays a significant role in solar evaporation rather than the light absorption. The results disclose that the Fe3O4 NPs with the lowest absorptivity has the largest roughness on drop surface, while that of CNTs show the opposite properties. The evaporation dynamics of LMs are featured with dome or constant spherical collapse with different roughness. Such dynamic difference arises from the mechanical competition between the capillary force and interparticle interaction. Besides, the strong light-harvesting and near-field radiation enabled by the rough surfaces enhance the solar evaporation. The Fe3O4-LM shows the highest evaporation rate of 6.55 μg/s, which is 1.09 and 1.30 times larger than that of Ni-LM and CNT-LM, respectively. Numerical analysis reveals that the rough surface with stacking arrangement of NPs greatly enhances the light-induced electromagnetic field and heat concentration over the interface, leading to a plasmon-coupling boundary with high temperature for the fast evaporation. Uncovering these properties could be of much help for developments of heatable miniature evaporators or reactors and their counterparts, permitting a broad range of processes with precise temperature and kinetic control.

Reciprocating Oscillation of a Floating Ferrofluid Marble Triggered by Magnetic Fields

Liquid marbles have the potential for microfluidic transport, medical diagnostics, and chemical analysis due to their negligible stickiness, environmental independence, and excellent mobility. Here, we report a non-contact manipulation strategy to arouse a reciprocating oscillation of ferrofluid marbles floating on the water surface, which can be used as microreactors. We experimentally investigated the quantitative relationship between the oscillation behavior, the applied magnetic field parameters, and the field regulation mechanism. The variables, including the magnetic field strength, marble volume, and switching period, are vital in determining the final state. The oscillation can be separated into three stages: transitional movement, compressive deformation, and rebound, before entering the next cycle. Accordingly, we created a manipulation technique for improving the mixing of inner reactants inside this marble container by remote-controlled shaking after optimizing with an oscillation model.

Magnetically Driven Manipulation of Nonmagnetic Liquid Marbles: Billiards with Liquid Marbles

Liquid marbles are droplets encapsulated by a layer of hydrophobic nanoparticles and have been extensively employed in digital microfluidics and lab-on-a-chip systems in recent years. In this study, magnetic liquid marbles were used to manipulate nonmagnetic liquid marbles. To achieve this purpose, a ferrofluid liquid marble (FLM) was employed and attracted toward an electromagnet, resulting in an impulse to a water liquid marble (WLM) on its way to the electromagnet. It was observed that the manipulation of the WLM by the FLM was similar to the collision of billiard balls except that the liquid marbles exhibited an inelastic collision. Taking the FLM as the projectile ball and the WLM as the other target balls, one can adjust the displacement and direction of the WLM precisely, similar to an expert billiard player. Firstly, the WLM displacement can be adjusted by altering the liquid marble volumes, the initial distances from the electromagnet, and the coil current. Secondly, the WLM direction can be adjusted by changing the position of the WLM relative to the connecting line between the FLM center and the electromagnet. Results show that when the FLM or WLM volume increases by five times, the WLM shooting distance approximately increases by 200% and decreases by 75%, respectively.

Choosing the Right Path for the Successful Storage of Seeds

Seeds are the most commonly used source of storage material to preserve the genetic diversity of plants. However, prior to the deposition of seeds in gene banks, several questions need to be addressed. Here, we illustrate the scheme that can be used to ensure that the most optimal conditions are identified to enable the long-term storage of seeds. The main questions that need to be answered pertain to the production of viable seeds by plants, the availability of proper protocols for dormancy alleviation and germination, seed tolerance to desiccation and cold storage at -20 °C. Finally, it is very important to fully understand the capability or lack thereof for seeds or their explants to tolerate cryogenic conditions. The proper storage regimes for orthodox, intermediate and recalcitrant seeds are discussed.

Effect of particle size on the stripping dynamics during impact of liquid marbles onto a liquid film

The robust attachment of particles at fluid interfaces is favorable for engineering new materials due to the large capillary energy, but it meets significant challenges when particle removal is a requirement. A previous study has shown that soap films can be utilized to achieve particle separation from liquid marbles. Here, we investigate the effects of particle size on the particle separation from liquid marbles using fast dynamics of drop impact on a soap film. Experimental observations disclose that the fast dynamics of the liquid marble involves coalescence, bouncing, stripping, or tunneling through the film by controlling the falling height and drop volume. More importantly, the active regime of the stripping mode can be selective-controlled by tuning the particle size, and the smaller stabilizing particles make a wider stripping regime. This is attributed to the smaller change of the surface energy resulting from the larger surface tension of LMs wrapped by smaller particles. Theoretical analysis reveals that the stripping thresholds are determined by the energy competition between kinetic energy, the increased surface energy and viscous dissipation, which offers important insights into particle separation by tuning the particle size. The present study provides guidelines for applications that involve phase separation.

"Foam Marble" Stabilized with One Type of Polymer Particle

There has been increasing interest in colloidal particles adsorbed at the air-water interface, which lead to stabilization of aqueous foams and liquid marbles. The wettability of the particles at the interface is known to play an important role in determining the type of air/water dispersed system. Foams are preferably formed using relatively hydrophilic particles, and liquid marbles tend to be formed using relatively hydrophobic particles. In this study, submicrometer-sized polystyrene particles carrying poly(N,N-diethylaminoethyl methacrylate) hairs (PDEA-PS particles), which are synthesized by dispersion polymerization, are demonstrated to work as a particulate stabilizer for both aqueous foams and liquid marbles. A key point for the hydrophilic PDEA-PS particles to stabilize both aqueous foams and liquid marbles, which have been generally stabilized with hydrophilic and hydrophobic particles, respectively, is the wetting mode of the particles with respect to water. The flocculates of PDEA-PS particles adsorb to the air-water interface from the aqueous phase to stabilize foam in a Wenzel mode, and the dried PDEA-PS particles adsorb to the interface as aggregates from the air phase to stabilize liquid marbles in a metastable Cassie-Baxter mode. On the basis of the difference in the wetting mode, stabilization of an air-in-water-in-air multiple gas-liquid dispersed system, named "foam marble", is realized. After the evaporation of water from the foam marble, a porous sphere is successfully obtained with pore sizes of a few tens of micrometers (reflecting the bubble sizes) and a few tens of nanometers (reflecting the gap sizes among the PDEA-PS particles).

Interparticle Repulsion of Microparticles Delivered to a Pendent Drop by an Electric Field

We report an unusually large spacing observed between microparticles after delivery to the surface of a pendent water droplet using a DC nonuniform electrostatic field, primarily via dielectrophoresis. The influence of particle properties was investigated using core particles, which were either coated or surface-modified to alter their wettability and conductivity. Particles that exhibited this spacing were both hydrophobic and possessed some dielectric material exposed to the external field, such as a coating or exposed dielectric core. The origin of this behavior is proposed to be the induced dipole-dipole repulsion between particles, which increases with particle size and decreases when the magnitude of the electric field is reduced. When the particles were no longer subjected to an external field, this large interparticle repulsion ceased and the particles settled to the bottom of the droplet under the force of gravity. We derive a simple model to predict this spacing, with the dipole-dipole repulsion balanced against particle weight. The external electric field was calculated using the existing electric field models. The spacing was found to be dependent on particle density and the induced dipole moment as well as the number of particles present on the droplet interface. As the number of particles increased, a decrease in interparticle spacing was observed.

Controllable Positive/Negative Phototaxis of Millimeter-Sized Objects with Sensing Function

Phototaxis, which is the directional motion toward or away from light, is common in nature and inspires development of artificial light-steered active objects. Most of the light-steered objects developed so far exhibit either positive or negative phototaxis, and there are few examples of research on objects that exhibit both positive and negative phototaxis. Herein, small objects showing both positive and negative phototaxis on the water surface upon near-infrared (NIR) light irradiation, with the direction controlled by the position of light irradiation, are reported. The millimeter-sized tetrahedral liquid marble containing gelled water coated by one polymer plate with light-to-heat photothermal characteristic, which adsorbs onto the bottom of the liquid marble, and three polymer plates with highly transparent characteristic, which adsorb onto the upper part of the liquid marble, is utilized as a model small object. Light irradiation on the front side of the object induces negative phototaxis and that on the other side induces positive phototaxis, and the motion can be controlled to 360° arbitrary direction by precise control of the light irradiation position. Thermographic studies confirm that the motions are realized through Marangoni flow generated around the liquid marble, which is induced by position-selective NIR light irradiation. The object can move centimeter distances, and numerical analysis indicates that average velocity and acceleration are approximately 12 mm/s and 71 mm/s², respectively, which are independent of the direction of motions. The generated force is estimated to be approximately 0.4 μN based on Newton's equation. Furthermore, functional cargo can be loaded into the inner phase of the small objects, which can be delivered and released on demand and endows them with environmental sensing ability.

How particle-particle and liquid-particle interactions govern the fate of evaporating liquid marbles

Liquid marbles refer to droplets that are covered with a layer of non-wetting particles. They are observed in nature and have practical significance. These squishy objects bounce, coalesce, break, inflate, and deflate while the liquid does not touch the substrate underneath. Despite the considerable cross-disciplinary interest and value of the research on liquid marbles, a unified framework for describing the mechanics of deflating liquid marbles-as the liquid evaporates-is unavailable. For instance, analytical approaches for modeling the evaporation of liquid marbles exploit empirical parameters that are not based on liquid-particle and particle-particle interactions. Here, we have combined complementary experiments and theory to fill this gap. To unentangle the contributions of particle size, roughness, friction, and chemical make-up, we investigated the evaporation of liquid marbles formed with particles of sizes varying over 7 nm-300 μm and chemical compositions ranging from hydrophilic to superhydrophobic. We demonstrate that the potential final states of evaporating liquid marbles are characterized by one of the following: (I) constant surface area, (II) particle ejection, or (III) multilayering. Based on these insights, we developed an evaporation model for liquid marbles that takes into account their time-dependent shape evolution. The model fits are in excellent agreement with our experimental results. Furthermore, this model and the general framework can provide mechanistic insights into extant literature on the evaporation of liquid marbles. Altogether, these findings advance our fundamental understanding of liquid marbles and should contribute to the rational development of technologies.

Self-Propelled Aero-GaN Based Liquid Marbles Exhibiting Pulsed Rotation on the Water Surface

We report on self-propelled rotating liquid marbles fabricated using droplets of alcoholic solution encapsulated in hollow microtetrapods of GaN with hydrophilic free ends of their arms and hydrophobic lateral walls. Apart from stationary rotation, elongated-spheroid-like liquid marbles were found, for the first time, to exhibit pulsed rotation on water surfaces characterized by a threshold speed of rotation, which increased with the weight of the liquid marble while the frequency of pulses proved to decrease. To throw light upon the unusual behavior of the developed self-propelled liquid marbles, we propose a model which takes into account skimming of the liquid marbles over the water surface similar to that inherent to flying water lily beetle and the so-called helicopter effect, causing a liquid marble to rise above the level of the water surface when rotating.

How Liquid Marbles Break Down: Direct Evidence for Two Breakage Scenarios

Liquid marbles are nonsticking droplets wrapped with hydrophobic nano- to micrometer particles and are expected to be useful for various applications, especially in industrial and biomedical fields. However, the practical use of liquid marbles is limited by their fragility. In this study, the dynamics of particle monolayer-stabilized liquid marble breakage upon impacting a solid surface are monitored in situ by high-speed interfacial microscopy. The experiments show that the breakage of liquid marbles can be induced by either i) cracking or ii) water penetration depending on the impact energy. The applicable scenario is determined by whether a jamming transition of the wrapping particles occurs during impact. The breakage mechanisms provide insights on how to improve the robustness of liquid marbles in accordance with these scenarios.

Boxes fabricated from plate-stabilized liquid marbles

Polyhedral liquid marbles were fabricated using hydrophobic polymer plates in the shape of a circle, a heart and a star as a stabilizer and water as an inner liquid phase. Boxes could be fabricated by the evaporation of the inner water from the liquid marbles. The fabrication efficiency and stability of these boxes as a function of the plate shape were investigated. Functional materials such as polymers and colloidal particles were successfully introduced into the boxes.

Fighting Like Cats and Dogs: Challenges in Domestic Carnivore Oocyte Development and Promises of Innovative Culture Systems

In vitro embryo production in cats and dogs still presents some challenges, and it needs to be optimized to transfer efficient protocols to related wild, endangered species. While the chemical composition of culture media has been the focus of several studies, the importance of culture substrates for oocyte and embryo culture has often been neglected. Traditional in vitro systems, i.e., two-dimensional cultures, do not resemble the physiological environments where cells develop, and they may cause morphological and functional alterations to oocytes and embryos. More modern three-dimensional and microfluidic culture system better mimic the structure and the stimuli found in in vivo conditions, and they could better support the development of oocytes and embryos in vitro, as well as the maintenance of more physiological behaviors. This review describes the different culture systems tested for domestic carnivore reproductive cells along the years, and it summarizes their effects on cultured cells with the purpose of analyzing innovative options to improve in vitro embryo production outcomes.

Multifunctional liquid marbles to stabilize and transport reactive fluids

The self-organized transport and delivery of reactive liquids without spillage or loss of activity have been among the most daunting challenges for a long time. In this direction, we employ the concept of forming "liquid marbles" (LMs) to encapsulate and transport reactive hydrogen peroxide (H2O2) coated with functional microparticles. For example, peroxide marbles coated with a toner ink display remote-controlled magnetotactic movement inside a fluidic medium, thus overcoming the weaknesses associated with use of the bare droplets. Interestingly, in such a scenario, the coating of the marbles could also be removed or reformed by bringing the magnet towards or away from the marble. In this way, this process could ensure an on-demand remotely guided coating on the peroxide droplet or its removal. The liquid marbles carrying peroxide solutions are found to preserve the activity of the peroxide and exhibit a low evaporation rate compared with the uncoated peroxide fuel. Interestingly, oil droplets floating on the water could be recovered by introducing the armoured LMs into water under magnetic guidance. Further, the functionalized marbles could be employed as suicide bags for the on-demand delivery of reactive materials in targeted locations. Preliminary research on the antibacterial activity of such liquid marbles has proven to be effective in bacterial killing, which may create new avenues for emerging antibacterial and antibiofilm applications. Finally, such functionalized LMs have been employed to investigate the effects of surface charge on attachment of recombinant Escherichia coli bacteria expressing green fluorescent protein and monitoring the real-time imaging of bacterial death attached to the marble surface.

Locomotion of a Nonaqueous Liquid Marble Induced by Near-Infrared-Light Irradiation

Micrometer-sized hydrophobic polyaniline (PANI) grains were synthesized via an aqueous chemical oxidative polymerization protocol in the presence of dopant carrying perfluoroalkyl or alkyl groups. The critical surface tensions of the PANIs synthesized in the presence of heptadecafluorooctanesulfonic acid and sodium dodecyl sulfate dopants were lower than that of PANI synthesized in the absence of dopant, indicating the presence of hydrophobic dopant on the grain surfaces. The PANI grains could adsorb to air-liquid interfaces, and aqueous and nonaqueous liquid marbles (LMs) were successfully fabricated using liquids with surface tensions ranging between 72.8 and 42.9 mN/m. Thermography studies confirmed that the surface temperature of the LMs increased by near-infrared light irradiation thanks to the photothermal property of the PANI, and the maximum temperatures measured for nonaqueous LMs were higher than that measured for aqueous LM. We demonstrated that transport of the LMs on a planar water surface can be achieved via Marangoni flow generated by the near-infrared light-induced temperature gradient. Numerical analyses indicated that the LMs containing liquids with lower specific heat and thermal conductivity and higher density showed longer path length per one light irradiation shot and longer decay time. This is because generated heat could efficiently transfer from the LMs to the water surface and larger inertial force could work on the LMs. The LMs could also move over the solid substrate thanks to their near-spherical shapes. Furthermore, it was also demonstrated that the inner liquids of the LMs could be released on site by an external stimulus.

Magnetothermal Miniature Reactors Based on Fe3 O4 Nanocube-Coated Liquid Marbles

Liquid marbles have recently attracted much interest in various scientific fields because of their isolated environment and robustness. However, conventional liquid marbles lack a reliable heating mechanism, which is critical in many potential applications. Here, the development of iron oxide (Fe₃ O₄ ) nanocube-coated liquid marbles (iNLMs), which can be homogeneously heated with an alternating magnetic field (AMF) to as high as 86 °C, is reported. Through tuning the power of the AMF, the iNLMs canbe heated to desired temperatures in controllable patterns. Furthermore, multicenter and selective heating is realized based on the unique magnetothermal properties of iNLMs. As heatable miniature reactors, the iNLMs are further demonstrated to facilitate the kinetic study of temperature-dependent chemical reactions. DNA amplification is successfully performed in liquid marbles, achieving a 25% superior amplification rate compared with that in a common thermal cycler. These results confirm the feasibility of coating liquid marbles with Fe₃ O₄ nanocubes to form delicate magnetothermal miniature reactors, which provides a reliable method of applying liquid marbles in areas such as biosensor technology, point-of-care testing, and theranostics.

Liquid marble technology to create cost-effective 3D cardiospheres as a platform for in vitro drug testing and disease modelling

Three-dimensional (3D) cell culturing has several advantages over 2D cultures. 3D cell cultures more accurately mimic the in vivo environment, which is vital to obtain reliable results in disease modelling and toxicity testing. With the introduction of the Yamanaka factors, reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) became available. This iPSC technology provides a scalable source of differentiated cells. iPSCs can be programmed to differentiate into any cell type of the body, including cardiomyocytes. These heart-specific muscle cells, can then serve as a model for therapeutic drug screening or assay development. Current methods to achieve multicellular spheroids by 3D cell cultures, such as hanging drop and spinner flasks are expensive, time-consuming and require specialized materials and training. Hydrophobic powders can be used to create a micro environment for cell cultures, which are termed liquid marbles (LM). In this procedure we describe the first use of the LM technology for 3D culturing in vitro derived human cardiomyocytes which results in the formation of cardiospheres within 24h. The cardiospheres could be used for several in depth and high-throughput analyses.

Redox-responsive functionalized hydrogel marble for the generation of cellular spheroids

Liquid marbles (LMs) have recently shown a great promise as microbioreactors to construct self-supported aqueous compartments for chemical and biological reactions. However, the evaporation of the inner aqueous liquid core has limited their application, especially in studying cellular functions. Hydrogels are promising scaffolds that provide a spatial environment suitable for three-dimensional cell culture. Here, we describe the fabrication of redox-responsive hydrogel marbles (HMs) as a three-dimensional cell culture platform. The HMs are prepared by introducing an aqueous mixture of a tetra-thiolated polyethylene glycol (PEG) derivative, thiolated gelatin (Gela-SH), horseradish peroxidase, a small phenolic compound, and human hepatocellular carcinoma cells (HepG2) to the inner aqueous phase of LMs. Eventually, HepG2 cells are encapsulated in the HMs then immersed in culture media, where they proliferate and form cellular spheroids. Experimental results show that the Gela-SH concentration strongly influences the physicochemical and microstructure properties of the HMs. After 6 days in culture, the spheroids were recovered from the HMs by degrading the scaffold, and examination showed that they had reached up to about 180 μm in diameter depending on the Gela-SH concentration, compared with 60 μm in conventional HMs without Gela-SH. After long-term culture (over 12 days), the liver-specific functions (secretion of albumin and urea) and DNA contents of the spheroids cultured in the HMs were elevated compared with those cultured in LMs. These results suggest that the developed HMs can be useful in designing a variety of microbioreactors for tissue engineering applications.

Liquid Marbles as Miniature Reactors for Chemical and Biological Applications

The need for miniaturised reaction systems has led to the development of various microreactor platforms, such as droplet-based microreactors. However, these microreactors possess inherent drawbacks, such as rapid evaporation and difficult handling, that limit their use in practical applications. Liquid marbles are droplets covered with hydrophobic particles and are a potential platform that can overcome the weaknesses of bare droplets. The coating particles completely isolate the interior liquids from the surrounding environment, thus conveniently encapsulating the reactions. Great efforts have been made over the past decade to demonstrate the feasibility of liquid marble-based microreactors for chemical and biological applications. This review systemically summarises state-of-the-art implementations of liquid marbles as microreactors. This paper also discusses the various aspects of liquid marble-based microreactors, such as the formation, manipulation, and future perspectives.

CO2-Gas-Responsive Liquid Marble

Liquid marbles were prepared using a water droplet and nonprotonated hydrophobic poly(2-N,N-diisopropylaminoethyl methacrylate) (PDiPAEMA) powder. Although the nonprotonated PDiPAEMA was hydrophobic, PDiPAEMA became hydrophilic because of the protonation of the pendant tertiary amino groups under acidic conditions. Therefore, liquid marbles stabilized with PDiPAEMA powder could float on a neutral to basic water surface, but they immediately disintegrated on an acidic water surface. Furthermore, the liquid marbles floating on the water surface disintegrated in response to CO₂ gas because the water became acidic as a result of carbonic acid formation.

Shear-induced deformation and interfacial jamming of solid-stabilized droplets

We use rheo-microscopy to directly investigate the dynamics of solid-stabilized droplets subjected to shear flow of a surrounding bulk fluid. In our system, the stabilizing particles are weakly attractive through the continuous fluid phase and along the droplet interface. Under shear, droplets stabilized by these particles at near-complete surface coverage exhibit a number of previously unforeseen phenomena, including negative-then-positive deviations from the predictions of Taylor and the behavior of bare droplets, evolution toward spherocylindrical shapes, and an earlier onset of rupture than their bare counterparts, which we explain in light of the weak attractive interparticle interactions along the droplet interface. We also demonstrate the formation of long-lived anisotropic particle-coated droplets by flow cessation, and provide evidence that this is due to the formation of a jammed, disordered particle network along the interface at surface coverage lower than the starting conditions. Importantly, these newly observed phenomena are shown to be sensitive to the droplets' initial surface coverage. Our findings provide new technologically-relevant insights into the physics of particle-coated droplets under fluidic or other external stresses, and introduce avenues for future research to better understand the roles of interparticle interactions and surface coverage in mediating the interfacial rheology of particle-laden interfaces and solid-stabilized emulsions.

Contactless sensing of liquid marbles for detection, characterisation & computing

Liquid marbles (LMs) are of growing interest in many fields, including microfluidics, microreactors, sensors, and signal carriers. The generation of LMs is generally performed manually, although there has recently been a burst of publications involving 'automatic marble makers'. The characteristics of a LM is dependent on many things, including how it is generated, it is therefore important to be able to characterise LMs once made. Here is presented a novel contactless LM sensor, constructed on a PCB board with a comb-like structure of 36 interlacing electrical traces, 100 μm wide and 100 μm apart. This cheap, scalable, and easy to use sensor exploits the inherent impedance (comprised of the electrical resistance, capacitive reactance and inductive reactance) of different LMs. With it, parameters of a LM can be easily determined, without interfering with the LM. These parameters are (1) particle size of the LM coating, (2) the concentration of a NaCl solution used as the LM core, and (3) the volume of the LM. Additionally, due to the comb-like nature of the sensor, the accurate positioning (down to the inter-trace spacing) of the LM can be ascertained. The new sensor has been shown to work under both static and dynamic (mobile) conditions. The capacitance of a LM was recorded to be 0.10 pF, which compares well with the calculated value of 0.12 pF.

Neuromorphic Liquid Marbles with Aqueous Carbon Nanotube Cores

Neuromorphic computing devices attempt to emulate features of biological nervous systems through mimicking the properties of synapses toward implementing the emergent properties of their counterparts, such as learning. Inspired by recent advances in the utilization of liquid marbles (LMs, microliter quantities of fluid coated in hydrophobic powder) for the creation of unconventional computing devices, we describe the development of LMs with neuromorphic properties through the use of copper coatings and 1.0 mg mL-1 carbon nanotube (CNT)-containing fluid cores. Experimentation was performed through sandwiching the LMs between two cup-style electrodes and stimulating them with repeated dc pulses at 3.0 V. Our results demonstrate that "entrainment" of CNT-filled copper LMs via periodic pulses can cause their electrical resistance to rapidly switch between high to low resistance profiles upon inverting the polarity of stimulation: the reduction in resistance between high and low profiles was approximately 88% after two rounds of entrainment. This effect was found to be reversible through reversion to the original stimulus polarity and was strengthened by repeated experimentation, as evidenced by a mean reduction in time to switching onset of 43%. These effects were not replicated in nanotube solutions not bound inside LMs. Our electrical characterization also reveals that nanotube-filled LMs exhibit pinched loop hysteresis IV profiles consistent with the description of memristors. We conclude by discussing the applications of this technology to the development of unconventional computing devices and the study of emergent characteristics in biological neural tissue.

On-Demand Coalescence and Splitting of Liquid Marbles and Their Bioapplications

Coalescence and splitting of liquid marbles (LMs) are critical for the mixture of precise amount precursors and removal of the wastes in the microliter range. Here, the coalescence and splitting of LMs are realized by a simple gravity-driven impact method and the two processes are systematically investigated to obtain the optimal parameters. The formation, coalescence, and splitting of LMs can be realized on-demand with a designed channel box. By selecting the functional channels on the device, gravity-based fusion and splitting of LMs are performed to mix medium/drugs and remove spent culture medium in a precise manner, thus ensuring that the microenvironment of the cells is maintained under optimal conditions. The LM-based 3D stem cell spheroids are demonstrated to possess an approximately threefold of cell viability compared with the conventional spheroid obtained from nonadhesive plates. Delivery of the cell spheroid to a hydrophilic surface results in the in situ respreading of cells and gradual formation of typical 2D cell morphology, which offers the possibility for such spheroid-based stem cell delivery in regenerative medicine.

Mapping outcomes of liquid marble collisions

Liquid marbles (LMs) have many promising roles in the ongoing development of microfluidics, microreactors, bioreactors, and unconventional computing. In many of these applications, the coalescence of two LMs is either required or actively discouraged, therefore it is important to study liquid marble collisions and establish parameters which enable the desired collision outcome. Recent reports on LM coalescence have focused on either two mobile LMs colliding, or an accelerating LM hitting a sessile LM with a backstop. A further possible scenario is the impact of a mobile LM against a non-supported static LM. This paper investigates such a collision, using high-speed videography for single-frame analysis. Multiple collisions were undertaken whilst varying the modified Weber number (We*) and offset ratios (X*). Parameter ranges of 1.0 < We* < 1.4 and 0.0 < X* < 0.1, resulted in a coalescence rate of approximately 50%. Whereas, parameter ranges X* > 0.25, and We* < 0.95 or We* > 1.55 resulted in 100% non-coalescence. Additionally, observations of LMs moving above a threshold velocity of 0.6 m s-1 have revealed a new and unusual deformation. Comparisons of the outcome of collisions whilst varying both the LM volume and the powder grain size have also been made, revealing a strong link. The results of this work provide a deeper understanding of LM coalescence, allowing improved control when designing future collision experiments.

Surfactant-Mediated Collapse of Liquid Marbles and Directed Assembly of Particles at the Liquid Surface

Extensive research is being devoted to both the fundamental and applied aspects of liquid marbles (LMs). However, influence of the surface tension of the liquid substrate on the stability of the LMs and LM-mediated capillary interaction remains unexplored. In this work, we unveil the role of the surface tension of the liquid substrate on the collapse of multilayered LMs and apply this knowledge for realizing a dense planar assembly of microparticles triggered by LM-mediated capillary interactions. Experiments and analysis show that the required surface tension for the collapse is dependent on the volume of the LMs. The larger LMs are less stable, and thus collapse at a higher surface tension than that required for smaller LMs. The results are explained on the basis of the balance between surface tension forces acting on the LM ( F_s) and its weight ( F_w). Force analysis reveals that the collapse of the LM on the liquid substrate occurs when the surface tension force approaches to its weight, that is, when F_s ≈ F_w. This has been verified for LMs having volume in the range 6-10 μL. The experiments with different surfactants (an anionic and a cationic) lead to similar results which indicate that the collapse condition of the LMs is mainly dependent on their weight and the surface tension of the liquid substrate. Further, we demonstrate the LM-mediated assembly of particles at the liquid surface, and interestingly, the LM can be collapsed once the assembly is completed, leading to a denser well-packed assembled structure. We believe that the presented results could provide new insights in the fields of microfluidics, particle patterning, and assembly.

Influence of water evaporation/absorption on the stability of glycerol–water marbles

The porous shell structure of liquid marbles allows liquid vapor to enter in/out of the liquid marbles, leading to the deformation/collapse of liquid marbles, which limits their application as miniature reactors for long-term chemical reactions. In this study, to prevent volatilization and maintain long-term stability, stable liquid marbles were fabricated by encapsulating glycerol/water droplets using superhydrophobic cellulose nanocrystals. The influence of water evaporation and absorption on the stability of aqueous glycerol marbles at different relative humidities (RHs) was investigated. At the same RH, the evaporation/absorption rates of the liquid marbles decreased on increasing the glycerol concentration. For the liquid marbles with the same glycerol volume concentration, the evaporation rates decreased with the increase in RH. The liquid marbles exhibited higher evaporation/absorption resistance compared with pure naked liquid droplets.

The influence of water evaporation and absorption on the stability of aqueous glycerol marbles was investigated.

Cryoprotectant-Free Freezing of Cells Using Liquid Marbles Filled with Hydrogel

Cryopreservation without cryoprotectant remains a significant challenge for the re-establishment of cell culture after freeze-thaw. Thus, finding an alternative and a simple cryopreservation method is necessary. Liquid marble (LM)-based digital microfluidics is a promising approach for cryoprotectant-free cryopreservation. However, the use of this platform to efficiently preserve samples with low cell density and well-controlled serum concentrations has not been investigated. We addressed this issue by embedding an agarose-containing fetal bovine serum (FBS) inside the LM. A low density of 500 cells/μL of murine 3T3 cells was selected for evaluating the postcryogenic survivability. The effects on the post-thaw cell viability of the concentration of agarose, the amount of FBS inside the agarose, and the volume of the LM were investigated systematically. This paper also presents an analysis on the changes in shape and crack size of post-thawed agarose. The results revealed that the embedded agarose gel serves as a controlled release mechanism of FBS and significantly improves cell viability. Post-thaw recovery sustains major cellular features, such as viability, cell adhesion, and morphology. The platform technology reported here opens up new possibilities to cryopreserve rare biological samples without the toxicity risk of cryoprotectants.

Manipulation of a floating liquid marble using dielectrophoresis

A liquid marble is a microliter-sized droplet coated with hydrophobic powder. The porous coating prevents the liquid content from being in direct physical contact with its surroundings, making the liquid marble perfectly non-wetting. On the one hand, the non-wetting ability allows the liquid marble to float and move across a liquid surface with little resistance. On the other hand, the porosity enables gas exchange between the liquid marble and its surroundings. These properties allow the liquid marble to serve as a bioreactor platform for important applications such as cell culture. Liquid marbles floating on a free liquid surface prevent evaporation due to the high humidity near the liquid surface. Moving a floating liquid marble allows for stirring and mixing inside the liquid marble. This paper reports a novel technique for manipulating a floating liquid marble using dielectrophoresis. A relatively simple setup can move liquid marbles of various sizes across the water surface at high speeds. We also present an analytical model to model and accurately predict the motion of the floating liquid marble. The technique reported here potentially allows for high-throughput and efficient handling of floating liquid marbles as a digital microfluidics platform.

Self-healing cellulose nanocrystal-stabilized droplets for water collection under oil

The removal of small water droplets from oils is imperative because oils can easily be contaminated or deteriorated by water during transportation, storage and usage. Here, we report superhydrophobic magnetic cellulose nanocrystal (CNC) nanoparticles, which are capable of removing tiny water droplets from oil by forming under oil particle-stabilized droplets. These particle-stabilized droplets can be collected and moved by a magnetic force, and self-healed upon damage by an external force.

Bioactive Hydrogel Marbles

Liquid marbles represented a significant advance in the manipulation of fluids as they used particle films to confine liquid drops, creating a robust and durable soft solid. We exploit this technology to engineering a bioactive hydrogel marble (BHM). Specifically, pristine bioactive glass nanoparticles were chemically tuned to produce biocompatible hydrophobic bioactive glass nanoparticles (H-BGNPs) that shielded a gelatin-based bead. The designed BHM shell promoted the growth of a bone-like apatite layer upon immersion in a physiological environment. The fabrication process allowed the efficient incorporation of drugs and cells into the engineered structure. The BHM provided a simultaneously controlled release of distinct encapsulated therapeutic model molecules. Moreover, the BHM sustained cell encapsulation in a 3D environment as demonstrated by an excellent in vitro stability and cytocompatibility. The engineered structures also showed potential to regulate a pre-osteoblastic cell line into osteogenic commitment. Overall, these hierarchical nanostructured and functional marbles revealed a high potential for future applications in bone tissue engineering.

Electrostatically Interactive Injectable Hydrogels for Drug Delivery

BACKGROUND

Several injectable hydrogels have been developed extensively for a broad range of biomedical applications. Injectable hydrogels forming in situ through the change in external stimuli have the distinct properties of easy management and minimal invasiveness, and thus provide the advantage of bypassing surgical procedures for administration resulting in better patient compliance.

METHODS

The injectable in situ-forming hydrogels can be formed irreversibly or reversibly under physiological stimuli. Among several external stimuli that induce formation of hydrogels in situ, in this review, we focused on the electrostatic interactions as the most simple and interesting stimulus.

RESULTS

Currently, numerous polyelectrolytes have been reported as potential electrostatically interactive in situ-forming hydrogels. In this review, a comprehensive overview of the rapidly developing electrostatically interactive in situ-forming hydrogels, which are produced by various anionic and cationic polyelectrolytes such as chitosan, celluloses, and alginates, has been outlined and summarized. Further, their biomedical applications have also been discussed.

CONCLUSION

The review concludes with perspectives on the future of electrostatically interactive in situ-forming hydrogels.

Liquid Marble Actuator for Microfluidic Logic Systems

A mechanical flip-flop actuator has been developed that allows for the facile re-routing and distribution of liquid marbles (LMs) in digital microfluidic devices. Shaped loosely like a triangle, the actuating switch pivots from one bistable position to another, being actuated by the very low mass and momentum of a LM rolling under gravity (~4 × 10⁻⁶ kg ms⁻¹). The actuator was laser-cut from cast acrylic, held on a PTFE coated pivot, and used a PTFE washer. Due to the rocking motion of the switch, sequential LMs are distributed along different channels, allowing for sequential LMs to traverse parallel paths. This distributing effect can be easily cascaded, for example to evenly divide sequential LMs down four different paths. This lightweight, cheap and versatile actuator has been demonstrated in the design and construction of a LM-operated mechanical multiplication device — establishing its effectiveness. The actuator can be operated solely by gravity, giving it potential use in point-of-care devices in low resource areas.

Liquid Marbles as an Easy-to-Handle Compartment for Cell-Free Synthesis and In Situ Immobilization of Recombinant Proteins

Liquid marble (LM), a self-standing micro-scale aqueous droplet, emerges as a micro-bioreactor in biological applications. Herein, the potential of LM as media for cell-free synthesis and simultaneous immobilization of recombinant proteins is explored. Initially, formation of hydrogel marble (HM) by using an enzymatic disulfide-based hydrogelation technique is confirmed by incorporating three components, horseradish peroxidase (HRP), a tetra-thiolated poly(ethylene glycol) derivative, and glycyl-L-tyrosine, in LM. The compatibility of the enzymatic hydrogelation with cell-free protein synthesis in LM is then validated. Although the hydrogelation reduces the level of protein synthesis in LM when compared with that in a test tube, the biosynthesis of enhanced green fluorescent protein (EGFP) is achieved. Interestingly, EGFP synthesized in LM is entrapped in the HM, and the introduction of a cysteine residue to EGFP by genetic engineering further increases the amount of protein immobilization in the hydrogel matrices. These results suggest that the cell-free synthesis and HRP-catalyzed hydrogelation can be conducted in parallel in LM, and the eventual entrapment of the key components in HM is possible. Facile recovery of macromolecular products immobilized in HM by degrading the hydrogel network under reducing conditions should lead to the design of an easy-to-handle system to screen protein functions.