Liquid marbles as bioreactors for the study of three-dimensional cell interactions

Liquid Marbles and Drops on Superhydrophobic Surfaces: Interfacial Aspects and Dynamics of Formation: A Review

Liquid marbles (LMs) are droplets encapsulated with powders presenting varied roughness and wettability. These LMs have garnered a lot of attention due to their dual properties of leakage-free and quick transport on both solid and liquid surfaces. These droplets are in a Cassie-Baxter wetting state sitting on both roughness and air pockets existing between particles. They are also reminiscent of the state of a drop on a superhydrophobic (SH) surface. In this review, LMs and bare droplets on SH surfaces are comparatively investigated in terms of two aspects: interfacial and dynamical. LMs present a fascinating class of soft matter due to their superior interfacial activity and their remarkable stability. Inherently hydrophobic powders form stable LMs by simple rolling; however, particles with defined morphologies and chemistries contribute to the varied stability of LMs. The factors contributing to this interesting robustness with respect to bare droplets are then identified by tests of stability such as evaporation and compression. Next, the dynamics of the impact of a drop on a hydrophobic powder bed to form LMs is studied vis-à̀-vis that of drop impact on flat surfaces. The knowledge from drop impact phenomena on flat surfaces is used to build and complement insights to that of drop impact on powder surfaces. The maximum spread of the drop is empirically understood in terms of dimensionless numbers, and their drawbacks are highlighted. Various stages of drop impact-spreading, retraction and rebound, splashing, and final outcome-are systematically explored on both solid and hard surfaces. The implications of crater formation and energy dissipations are discussed in the case of granular beds. While the drop impact on solid surfaces is extensively reviewed, deep interpretation of the drop impact on granular surfaces needs to be improved. Additionally, the applications of each step in the sequence of drop impact phenomena on both substrates are also identified. Next, the criterion for the formation of peculiar jammed LMs was examined. Finally, the challenges and possible future perspectives are envisaged.

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.

Oscillation-Induced Mixing Advances the Functionality of Liquid Marble Microreactors

Droplet-based microreactors often uncover fascinating phenomena and exhibit diverse functionality, which make them applicable in various fields. Liquid marbles (LMs) are non-wetting droplets coated with particles, and these features highlight their potential as microreactors. However, sophisticated experimental designs are typically hindered because it is difficult to obtain sufficient substance mixing in these miniature, damage-prone, self-supporting liquid containers. Here, we demonstrate that subjecting LMs to vertical oscillations by audio signals represents a controllable approach that allows sufficient mixing with variable dynamic modes. The characteristics and key issues in LM oscillation are systematically explored. The effects of oscillation on application potential are examined. Under oscillation conditions, homogeneous mixing can be achieved within a few seconds in LMs consisting of either water or viscous liquids. Importantly, the structures of materials synthesized in LMs can be regulated by modulating the oscillation modes. The variable modes, flexible adjustability, high efficiency, and wide applicability of this oscillation method make it a verified manipulation strategy for advancing the functionality of LM microreactors.

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.

Liquid marble-based digital microfluidics - fundamentals and applications

Liquid marbles are droplets with volume typically on the order of microliters coated with hydrophobic powder. Their versatility, ease of use and low cost make liquid marbles an attractive platform for digital microfluidics. This paper provides the state of the art of discoveries in the physics of liquid marbles and their practical applications. The paper first discusses the fundamental properties of liquid marbles, followed by the summary of different techniques for the synthesis of liquid marbles. Next, manipulation techniques for handling liquid marbles are discussed. Applications of liquid marbles are categorised according to their use as chemical and biological reactors. The paper concludes with perspectives on the future development of liquid marble-based digital microfluidics.

Current Advances in 3D Tissue and Organ Reconstruction

Bi-dimensional culture systems have represented the most used method to study cell biology outside the body for over a century. Although they convey useful information, such systems may lose tissue-specific architecture, biomechanical effectors, and biochemical cues deriving from the native extracellular matrix, with significant alterations in several cellular functions and processes. Notably, the introduction of three-dimensional (3D) platforms that are able to re-create in vitro the structures of the native tissue, have overcome some of these issues, since they better mimic the in vivo milieu and reduce the gap between the cell culture ambient and the tissue environment. 3D culture systems are currently used in a broad range of studies, from cancer and stem cell biology, to drug testing and discovery. Here, we describe the mechanisms used by cells to perceive and respond to biomechanical cues and the main signaling pathways involved. We provide an overall perspective of the most recent 3D technologies. Given the breadth of the subject, we concentrate on the use of hydrogels, bioreactors, 3D printing and bioprinting, nanofiber-based scaffolds, and preparation of a decellularized bio-matrix. In addition, we report the possibility to combine the use of 3D cultures with functionalized nanoparticles to obtain highly predictive in vitro models for use in the nanomedicine field.

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.

Mechanical robustness of monolayer nanoparticle-covered liquid marbles

Powder-derived liquid marbles (LMs) are versatile nonwetting systems but are confronted with many limitations in application, as their surface particles are usually large and agglomerated. Recently, sol-gel film-derived LMs have come on the scene that are termed monolayer nanoparticle-covered (mNPc) LMs based on their unique characteristics, revealing great application potential but also generating many questions. Here, mechanical robustness, a very important yet to be addressed property, of mNPc LMs was systematically studied. Rolling, pendant contact, and compression experiments were designed using bare and coated glasses with water contact angles (WCAs) ranging from 23° to 157°. With rupture as a quality criteria, the mechanical robustness of mNPc LMs enhanced with the hydrophobicity of solid surfaces that exerted pressure on them, but maintained much weaker than typical powder LMs until the solid surface was superhydrophobic. In particular, when contacting hydrophilic surfaces of WCAs ≤53°, an mNPc LM did not have the capacity for nonwetting and ruptured immediately, even if the pressure approached zero. This was distinct from powder LMs and indicated that a particle shell as thin as ∼20 nm could not prevent intermolecular attractions between the internal liquid and external solid surface. An interface scenario consisting of solid surface microroughness was proposed to address this issue. On the other hand, mNPc LMs remained unruptured on superhydrophobic surfaces but presented degraded elasticity under extreme compression. Uncovering these properties could be of much help for developments of mNPc LMs and their counterparts, the mNPc liquid plasticines.

Liquid marbles and liquid plasticines with nanoparticle monolayers

Liquid marbles, as particle-covered macroscopic liquid drops in an air environment, have exhibited great value as self-standing liquid containers in various areas, such as material synthesis, chemical analysis, and cell culture. However, conventional liquid marbles obtained by the rolling-on-powder-bed method usually feature micron-sized or larger particle agglomerates, which harm marble transparency and fine control of marble shape and thus results in considerable limitations for marble applications. Recently, monolayer nanoparticle (NP) coverage has been achieved using a sol-gel film instead of a powder as the particle source. The NP monolayer structure can not only result in highly transparent liquid marbles with very smooth and symmetrical profiles, but can also lead to liquid entities with arbitrarily designable shapes, as called liquid plasticines. Monolayer NP-covered (mNPc) liquid marbles and plasticines have generated important results in both fundamental and practical applications, as ideal physical models or advanced self-standing containers, showing great advantages in some conditions over conventional powder-derived liquid marbles. In this review, the preparations and current applications of the two mNPc systems are summarized and perspectives on their advantages, unclear issues, and application extension are provided.

Rod-shaped liquid plasticine for gas diffusion detection

A rod-shaped liquid plasticine was produced here, which was then shown to serve as a versatile gas detector based on a coloration mechanism. It not only indicated gas existence but also visually revealed the gas frontier positions, which allowed the calculation of diffusion speeds and gas concentrations. This study demonstrated the feasibility of multifunctional applications in a liquid plasticine using its shape and optical advantages.

Grow with the Flow: When Morphogenesis Meets Microfluidics

Developmental biology has advanced the understanding of the intricate and dynamic processes involved in the formation of an organism from a single cell. However, many gaps remain in the knowledge of embryonic development, especially regarding tissue morphogenesis. A possible approach to mimic such phenomena uses pluripotent stem cells in in vitro morphogenetic models. Herein, these systems are summarized with emphasis on the ability to better manipulate and control cellular interfaces with either liquid or solid materials using microengineered tools, which is critical for attaining deeper insights into pattern formation and stem cell differentiation during organogenesis. The role of conventional and customized cell-culture systems in supporting important advances in the field of morphogenesis is discussed, and the fascinating role that material sciences and microengineering currently play and are expected to play in the future is highlighted. In conclusion, it is proffered that continued microfluidics innovations when applied to morphogenesis promise to provide important insights to advance many multidisciplinary fields, including regenerative medicine.

Transparent Bioreactors Based on Nanoparticle-Coated Liquid Marbles for in Situ Observation of Suspending Embryonic Body Formation and Differentiation

Transparent liquid marbles coated with hydrophobic silica nanoparticles were used as micro-bioreactors for embryonic stem cell (ESC) culturing. The high transparency of silica liquid marbles enables real-time and in situ monitoring of embryonic body (EB) formation and differentiation. The experimental result shows that ESCs can aggregate with each other close to the bottom of the liquid marble and form EBs, while remaining suspended in the culture media. The differentiation of the suspending EBs into contractile cardiomyocytes has been demonstrated inside the transparent liquid marbles, which enable the in situ microscopic observation. It was also found, through comparison, that ESCs in a bare sessile drop placed on a superhydrophobic substrate tend to anchor onto the substrate and then differentiate following the normal way of cell spreading, i.e., withdrawal from the cell cycle, fusion with nascent myotubes, and final differentiation into cardiomyocytes. In contrast, liquid marble particle shells weaken the adhesion of spherical EBs to the substrate, encouraging them to differentiate in suspension into cardiomyocytes, without anchoring. The results of this study highlight the promising performance of liquid marbles as "one-pot" micro-bioreactors for EB formation and differentiation.

A capillary rise method for studying the effective surface tension of monolayer nanoparticle-covered liquid marbles

The liquid marble covered with a xerogel-derived nanoparticle monolayer is emerging as a simple and ideal physical model for fundamental study. In this work, the effective surface tension, γeff, of this kind of water marble was studied by employing an optimized capillary rise method. This method is distinctive as it can reveal γeff at a specific point rather than the average γeff over the entire marble. γeff at the maximum lateral diameter position was investigated with particle coverage varying from 100% to 10%, showing a quite slow increasing trend from ∼64 to 72 mN m-1. The delay in the evolution of the γeff was attributed to the slow decline in the areal particle density on the side area of the marble. For comparative purposes, the maximum height method was also employed to determine γeff. The results of the two methods supported the position that a liquid marble's γeff depended on very specific conditions including the measurement method and all marble parameters.

Digital polymerase chain reaction technology - recent advances and future perspectives

Digital polymerase chain reaction (dPCR) technology has remained a "hot topic" in the last two decades due to its potential applications in cell biology, genetic engineering, and medical diagnostics. Various advanced techniques have been reported on sample dispersion, thermal cycling and output monitoring of digital PCR. However, a fully automated, low-cost and handheld digital PCR platform has not been reported in the literature. This paper attempts to critically evaluate the recent developments in techniques for sample dispersion, thermal cycling and output evaluation for dPCR. The techniques are discussed in terms of hardware simplicity, portability, cost-effectiveness and suitability for automation. The present paper also discusses the research gaps observed in each step of dPCR and concludes with possible improvements toward portable, low-cost and automatic digital PCR systems.

Liquid marble coalescence via vertical collision

The coalescence process of liquid marbles is vital to their promising roles as reactors or mixers in digital microfluidics. However, the underlying mechanisms and critical conditions of liquid marble coalescence are not well understood. This paper studies the coalescence process of two equally-sized liquid marbles via vertical collision aided by dielectrophoretic handling. A liquid marble was picked up using the dielectrophoretic force and then dropped vertically onto another liquid marble resting on a hydrophobic powder bed. The whole collision process was recorded by a high-speed camera and the recorded images were then analysed to derive the generalised conditions of liquid marble coalescence. By varying the marble volume, impact velocity and offset ratio in the experiments, we concluded that liquid marble coalescence may occur through the coating pore opening mechanism. We quantitatively measured the radius change versus time of the liquid neck formed between two coalescing marbles and estimated the maximum deformation of impacting marbles before rupture in rebound cases. We also qualitatively described the redistribution of coating particles at the impact area during coalescence as well as the consequent ejection of particles. Finally, we summarised the critical conditions for liquid marble coalescence, providing a frame for future applications involving liquid marbles as micromixers and microreactors.

Evaporation dynamics of liquid marbles at elevated temperatures

Study of evaporation dynamics of liquid marbles at elevated temperature is essential to determine the feasibility of liquid marbles to be used as micro compartments for digital polymerase chain reaction (PCR). We have modified an existing theoretical model of evaporation of a liquid droplet and verified its applicability on the evaporation of liquid marbles. The evaporation dynamics of an individual and a group of liquid marbles are analysed. This paper demonstrates that the evaporation dynamics of liquid marbles obeys the theoretical framework for elevated temperatures. The evaporation of a group of liquid marbles are observed as a coupled function of their diameter, their number in a group, the vapour density of the surrounding atmosphere and their spatial distribution.

We investigate the evaporation behaviour of a group of liquid marbles at elevated temperature under various conditions.

Coalescence Processes of Droplets and Liquid Marbles

The coalescence process of droplets and, more recently, of liquid marbles, has become one of the most essential manipulation schemes in digital microfluidics. This process is indispensable for realising microfluidic functions such as mixing and reactions at microscale. This paper reviews previous studies on droplet coalescence, paying particular attention to the coalescence of liquid marbles. Four coalescence systems have been reviewed, namely, the coalescence of two droplets freely suspended in a fluid; the coalescence of two sessile droplets on a solid substrate; the coalescence of a falling droplet and a sessile droplet on a solid substrate; and liquid marble coalescence. The review is presented according to the dynamic behaviors, physical mechanisms and experimental parameters of the coalescence process. It also provides a systematic overview of how the coalescence process of droplets and liquid marbles could be induced and manipulated using external energy. In addition, the practical applications of liquid marble coalescence as a novel microreactor are highlighted. Finally, future perspectives on the investigation of the coalescence process of liquid marbles are proposed. This review aims to facilitate better understanding of the coalescence of droplets and of liquid marbles as well as to shed new insight on future studies.