Polymeric aqueous biphasic systems for non-contact cell printing on cells: engineering heterocellular embryonic stem cell niches

Outermost Cationic Surface Charge of Layer-by-Layer Films Prevents Endothelial Cells Migration for Cell Compartmentalization in Three-Dimensional Tissues

Tissues and organs possess an organized cellular arrangement that enables their unique functions. However, conventional three-dimensional (3D) encapsulation techniques fail to recapitulate this complexity due to the cell migration during cell culture. In biological tissues, basement membranes (BMs) are essential to mechanically support cellular organization. This study finds that a positively charged outermost surface of multilayered nanofilms, fabricated through layer-by-layer assembly of poly-l-lysine (PLL) and dextran (Dex) via hydrogen bonds, stimulates the barrier functions of BMs. This type of artificial BM (A-BM) demonstrates enhanced barrier properties in comparison to other types of A-BMs composed of BM components such as collagen type IV and laminin. Such an enhancement is potentially associated with the outermost cationic layer, which inhibits the sprouting of endothelial cells (ECs) and effectively prevents EC migration over a 14-d period, aligning with the formation timeline of natural BMs in 3D tissues. Finally, 3D organized vascular channels are successfully engineered with the support of shape-adaptable PLL/Dex nanofilms. This approach offers a guideline for engineering organized 3D tissue models by regulating cell migration, which can provide reliable platforms for in vitro permeability assay of new drugs or drug delivery carriers.

Next Generation Aqueous Two-Phase System for Gentle, Effective, and Timely Extracellular Vesicle Isolation and Transcriptomic Analysis

The isolation of extracellular vesicles (EVs) using currently available methods frequently compromises purity and yield to prioritize speed. Here, we present a next-generation aqueous two-phase system (next-gen ATPS) for the isolation of EVs regardless of scale and volume that is superior to conventional methods such as ultracentrifugation (UC) and commercial kits. This is made possible by the two aqueous phases, one rich in polyethylene glycol (PEG) and the other rich in dextran (DEX), whereby fully encapsulated lipid vesicles preferentially migrate to the DEX-rich phase to achieve a local energy minimum for the EVs. Isolated EVs as found in the DEX-rich phase are more amenable to biomarker analysis such as nanoscale flow cytometry (nFC) when using various pre-conjugated antibodies specific for CD9, CD63 and CD81. TRIzol RNA isolation is further enabled by the addition of dextranase, a critical component of this next-gen ATPS method. RNA yield of next-gen ATPS-isolated EVs is superior to UC and other commercial kits. This negates the use of specialized EV RNA extraction kits. The use of dextranase also enables more accurate immunoreactivity of pre-conjugated antibodies for the detection of EVs by nFC. Transcriptomic analysis of EVs isolated using the next-gen ATPS revealed a strong overlap in microRNA (miRNA), circular RNA (circRNA) and small nucleolar RNA (snoRNA) profiles with EV donor cells, as well as EVs isolated by UC and the exoRNeasy kit, while detecting a superior number of circRNAs compared to the kit in human samples. Overall, this next-gen ATPS method stands out as a rapid and highly effective approach to isolate high-quality EVs in high yield, ensuring optimal extraction and analysis of EV-encapsulated nucleic acids.

Affinity-Controlled Partitioning of Biomolecules at Aqueous Interfaces and Their Bioanalytic Applications

All-aqueous phase separation systems play essential roles in bioanalytical and biochemical applications. Compared to conventional oil and organic solvent-based systems, these systems are characterized by their rich bulk and interfacial properties, offering superior biocompatibility. In particular, phase separation in all-aqueous systems facilitates the creation of compartments with specific physicochemical properties, and therefore largely enhances the accessibility of the systems. In addition, the all-aqueous compartments have diverse affinities, with an important property known as partitioning, which can concentrate (bio)molecules toward distinct immiscible phases. This partitioning affinity imparts all-aqueous interfaces with selective permeability, enabling the controlled enrichment of target (bio)molecules. This review introduces the basic principles and applications of partitioning-induced interfacial phenomena in a typical all-aqueous system, namely aqueous two-phase systems (ATPSs); these applications include interfacial chemical reactions, bioprinting, and assembly, as well as bio-sensing and detection. The primary challenges associated with designing all-aqueous phase separation systems and several future directions are also discussed, such as the stabilization of aqueous interfaces, the handling of low-volume samples, and exploration of suitable ATPSs compositions with the efficient protocol.

Exploring the potential of all-aqueous immiscible systems for preparing complex biomaterials and cellular constructs

All-aqueous immiscible systems derived from liquid-liquid phase separation of incompatible hydrophilic agents such as polymers and salts have found increasing interest in the biomedical and tissue engineering fields in the last few years. The unique characteristics of aqueous interfaces, namely their low interfacial tension and elevated permeability, as well as the non-toxic environment and high water content of the immiscible phases, confer to these systems optimal qualities for the development of biomaterials such as hydrogels and soft membranes, as well as for the preparation of in vitro tissues derived from cellular assembly. Here, we overview the main properties of these systems and present a critical review of recent strategies that have been used for the development of biomaterials with increased levels of complexity using all-aqueous immiscible phases and interfaces, and their potential as cell-confining environments for micropatterning approaches and the bioengineering of cell-rich structures. Importantly, due to the relatively recent emergence of these areas, several key design considerations are presented, in order to guide researchers in the field. Finally, the main present challenges, future directions, and adaptability to develop advanced materials with increased biomimicry and new potential applications are briefly evaluated.

Free-Boundary Microfluidic Platform for Advanced Materials Manufacturing and Applications

Microfluidics, with its remarkable capacity to manipulate fluids and droplets at the microscale, has emerged as a powerful platform in numerous fields. In contrast to conventional closed microchannel microfluidic systems, free-boundary microfluidic manufacturing (FBMM) processes continuous precursor fluids into jets or droplets in a relatively spacious environment. FBMM is highly regarded for its superior flexibility, stability, economy, usability, and versatility in the manufacturing of advanced materials and architectures. In this review, a comprehensive overview of recent advancements in FBMM is provided, encompassing technical principles, advanced material manufacturing, and their applications. FBMM is categorized based on the foundational mechanisms, primarily comprising hydrodynamics, interface effects, acoustics, and electrohydrodynamic. The processes and mechanisms of fluid manipulation are thoroughly discussed. Additionally, the manufacturing of advanced materials in various dimensions ranging from zero-dimensional to three-dimensional, as well as their diverse applications in material science, biomedical engineering, and engineering are presented. Finally, current progress is summarized and future challenges are prospected. Overall, this review highlights the significant potential of FBMM as a powerful tool for advanced materials manufacturing and its wide-ranging applications.

Phase-separation facilitated one-step fabrication of multiscale heterogeneous two-aqueous-phase gel

Engineering heterogeneous hydrogels with distinct phases at various lengths, which resemble biological tissues with high complexity, remains challenging by existing fabricating techniques that require complicated procedures and are often only applicable at bulk scales. Here, inspired by ubiquitous phase separation phenomena in biology, we present a one-step fabrication method based on aqueous phase separation to construct two-aqueous-phase gels that comprise multiple phases with distinct physicochemical properties. The gels fabricated by this approach exhibit enhanced interfacial mechanics compared with their counterparts obtained from conventional layer-by-layer methods. Moreover, two-aqueous-phase gels with programmable structures and tunable physicochemical properties can be conveniently constructed by adjusting the polymer constituents, gelation conditions, and combining different fabrication techniques, such as 3D-printing. The versatility of our approach is demonstrated by mimicking the key features of several biological architectures at different lengths: macroscale muscle-tendon connections; mesoscale cell patterning; microscale molecular compartmentalization. The present work advances the fabrication approach for designing heterogeneous multifunctional materials for various technological and biomedical applications.

One-Step Fabrication of Coconut-Like Capsules via Competitive Reactions at an All-Aqueous Interface for Enzyme Immobilization

A concept of interfacial competitive reaction between biomineralization and alginate gelation at an all-aqueous single-emulsion droplet interface to prepare robust coconut-like capsules (inner hard wall and outer soft wall) is developed. The concept is further applied for enzyme immobilization with high encapsulation efficiency, enzyme loading, mass transfer coefficient, and recyclability. The thickness and swelling properties of the shell are simply tunable by a competitive reaction. Our platform may open a green, facile, and efficient way to prepare organic-inorganic hybrid sustainable materials with tailored compositions and structures.

Water-in-water droplet microfluidics: A design manual

Droplet microfluidics is utilized in a wide range of applications in biomedicine and biology. Applications include rapid biochemical analysis, materials generation, biochemical assays, and point-of-care medicine. The integration of aqueous two-phase systems (ATPSs) into droplet microfluidic platforms has potential utility in oil-free biological and biomedical applications, namely, reducing cytotoxicity and preserving the native form and function of costly biomolecular reagents. In this review, we present a design manual for the chemist, biologist, and engineer to design experiments in the context of their biological applications using all-in-water droplet microfluidic systems. We describe the studies achievable using these systems and the corresponding fabrication and stabilization methods. With this information, readers may apply the fundamental principles and recent advancements in ATPS droplet microfluidics to their research. Finally, we propose a development roadmap of opportunities to utilize ATPS droplet microfluidics in applications that remain underexplored.

Leveraging ultra-low interfacial tension and liquid-liquid phase separation in embedded 3D bioprinting

Many recently developed 3D bioprinting strategies operate by extruding aqueous biopolymer solutions directly into a variety of different support materials constituted from swollen, solvated, aqueous, polymer assemblies. In developing these 3D printing methods and materials, great care is often taken to tune the rheological behaviors of both inks and 3D support media. By contrast, much less attention has been given to the physics of the interfaces created when structuring one polymer phase into another in embedded 3D printing applications. For example, it is currently unclear whether a dynamic interfacial tension between miscible phases stabilizes embedded 3D bioprinted structures as they are shaped while in a liquid state. Interest in the physics of interfaces between complex fluids has grown dramatically since the discovery of liquid-liquid phase separation (LLPS) in living cells. We believe that many new insights coming from this burst of investigation into LLPS within biological contexts can be leveraged to develop new materials and methods for improved 3D bioprinting that leverage LLPS in mixtures of biopolymers, biocompatible synthetic polymers, and proteins. Thus, in this review article, we highlight work at the interface between recent LLPS research and embedded 3D bioprinting methods and materials, and we introduce a 3D bioprinting method that leverages LLPS to stabilize printed biopolymer inks embedded in a bioprinting support material.

Multilayer-Stabilized Water-in-Water Emulsions

Water-in-water (W/W) emulsions are of interest for various applications due to their inherent biocompatibility, ultralow interfacial tensions, and large interface thickness. However, it is still challenging to prepare stable W/W emulsions with tailored phase architectures compared to oil-in-water (O/W) and water-in-oil (W/O) emulsions. Here, we report a multilayer-stabilized W/W emulsion composed of poly(ethylene glycol)/dextran in the presence of DNA strands. The W/W emulsions present onion-ring-like structures, which are interpreted by a nanofluid film model. Emulsion behavior, e.g., stability, interface tension, etc., can be controlled by the type of DNA (single or double strands), DNA concentration, and volume fraction of dispersed phase. Our findings could broaden the preparation of novel emulsions for potential applications in emulsion polymerization, new media of homogeneous catalysis, and DNA transportation of water-in-water media.

Reconstructing the tumor architecture into organoids

Cancer remains a leading health burden worldwide. One of the challenges hindering cancer therapy development is the substantial discrepancies between the existing cancer models and the tumor microenvironment (TME) of human patients. Constructing tumor organoids represents an emerging approach to recapitulate the pathophysiological features of the TME in vitro. Over the past decade, various approaches have been demonstrated to engineer tumor organoids as in vitro cancer models, such as incorporating multiple cellular populations, reconstructing biophysical and chemical traits, and even recapitulating structural features. In this review, we focus on engineering approaches for building tumor organoids, including biomaterial-based, microfabrication-assisted, and synthetic biology-facilitated strategies. Furthermore, we summarize the applications of engineered tumor organoids in basic cancer research, cancer drug discovery, and personalized medicine. We also discuss the challenges and future opportunities in using tumor organoids for broader applications.

Aqueous Two-Phase Systems and Microfluidics for Microscale Assays and Analytical Measurements

Phase separation is a common occurrence in nature. Synthetic and natural polymers, salts, ionic liquids, surfactants, and biomacromolecules phase separate in water, resulting in an aqueous two-phase system (ATPS). This review discusses the properties, handling, and uses of ATPSs. These systems have been used for protein, nucleic acid, virus, and cell purification and have in recent years found new uses for small organics, polysaccharides, extracellular vesicles, and biopharmaceuticals. Analytical biochemistry applications such as quantifying protein-protein binding, probing for conformational changes, or monitoring enzyme activity have been performed with ATPSs. Not only are ATPSs biocompatible, they also retain their properties at the microscale, enabling miniaturization experiments such as droplet microfluidics, bacterial quorum sensing, multiplexed and point-of-care immunoassays, and cell patterning. ATPSs include coacervates and may find wider interest in the context of intracellular phase separation and origin of life. Recent advances in fundamental understanding and in commercial application are also considered.

Modeling adaptive drug resistance of colorectal cancer and therapeutic interventions with tumor spheroids

Drug resistance is a major barrier against successful treatments of cancer patients. Various intrinsic mechanisms and adaptive responses of tumor cells to cancer drugs often lead to failure of treatments and tumor relapse. Understanding mechanisms of cancer drug resistance is critical to develop effective treatments with sustained anti-tumor effects. Three-dimensional cultures of cancer cells known as spheroids present a biologically relevant model of avascular tumors and have been increasingly incorporated in tumor biology and cancer drug discovery studies. In this review, we discuss several recent studies from our group that utilized colorectal tumor spheroids to investigate responses of cancer cells to cytotoxic and molecularly targeted drugs and uncover mechanisms of drug resistance. We highlight our findings from both short-term, one-time treatments and long-term, cyclic treatments of tumor spheroids and discuss mechanisms of adaptation of cancer cells to the treatments. Guided by mechanisms of resistance, we demonstrate the feasibility of designing specific drug combinations to effectively block growth and resistance of cancer cells in spheroid cultures. Finally, we conclude with our perspectives on the utility of three-dimensional tumor models and their shortcomings and advantages for phenotypic and mechanistic studies of cancer drug resistance.

Immunostaining Extracellular Vesicles Based on an Aqueous Two-Phase System: For Analysis of Tetraspanins

Immunostaining of extracellular vesicles (EVs) has become necessary for the characterization of EV subtypes, clarification of the EV biogenesis/cellular uptake pathway, drug delivery, etc. Immunostained EVs must be in suspension for further downstream analyses or uses. However, conventional EV immunostaining methods yielding EVs in suspension lack either sufficient recovery or staining specificity because of the washing steps. In this study, we have devised and tested a method to wash immunostained EVs with successive aqueous two-phase system (ATPS) separations. The ATPS is a liquid-liquid extraction procedure that ensures a gentle separation of target molecules. The ATPS has been successfully employed to separate EVs from other impurities with high yield and high purity. Immunostained EVs were washed with the ATPS and compared with other immunostaining methods to confirm the proposed method's high EV recovery and staining accuracy. According to the result, the ATPS-based EV immunostaining method required as low as ∼1 μg without compromise of accuracy and recovery.

Predicting the Mixing Behavior of Aqueous Solutions Using a Machine Learning Framework

The most direct approach to determining if two aqueous solutions will phase-separate upon mixing is to exhaustively screen them in a pair-wise fashion. This is a time-consuming process that involves preparation of numerous stock solutions, precise transfer of highly concentrated and often viscous solutions, exhaustive agitation to ensure thorough mixing, and time-sensitive monitoring to observe the presence of emulsion characteristics indicative of phase separation. Here, we examined the pair-wise mixing behavior of 68 water-soluble compounds by observing the formation of microscopic phase boundaries and droplets of 2278 unique 2-component solutions. A series of machine learning classifiers (artificial neural network, random forest, k-nearest neighbors, and support vector classifier) were then trained on physicochemical property data associated with the 68 compounds and used to predict their miscibility upon mixing. Miscibility predictions were then compared to the experimental observations. The random forest classifier was the most successful classifier of those tested, displaying an average receiver operator characteristic area under the curve of 0.74. The random forest classifier was validated by removing either one or two compounds from the input data, training the classifier on the remaining data and then predicting the miscibility of solutions involving the removed compound(s) using the classifier. The accuracy, specificity, and sensitivity of the random forest classifier were 0.74, 0.80, and 0.51, respectively, when one of the two compounds to be examined was not represented in the training data. When asked to predict the miscibility of two compounds, neither of which were represented in the training data, the accuracy, specificity, and sensitivity values for the random forest classifier were 0.70, 0.82 and 0.29, respectively. Thus, there is potential for this machine learning approach to improve the design of screening experiments to accelerate the discovery of aqueous two-phase systems for numerous scientific and industrial applications.

Evaporation-Driven Water-in-Water Droplet Formation

We present new observations of aqueous two-phase system (ATPS) thermodynamic and interfacial phenomena that occur inside sessile droplets due to water evaporation. Sessile droplets that contain polymeric solutions, which are initially in equilibrium in a single phase, are observed at their three-phase liquid-solid-air contact line. As evaporation of a sessile droplet proceeds, we find that submicron secondary water-in-water (W/W) droplets emerge spontaneously at the edges of the mother sessile droplet due to the resulting phase separation from water evaporation. To understand this phenomenon, we first study the secondary W/W droplet formation process on different substrate materials, namely, glass, polycarbonate (PC), thermoplastic elastomer (TPE), poly(dimethylsiloxane)-coated glass slide (PDMS substrate), and Teflon-coated glass slide (Teflon substrate), and show that secondary W/W droplet formation arises only in lower-contact-angle substrates near the three-phase contact line. Next, we characterize the size of the emergent secondary W/W droplets as a function of time. We observe that W/W drops are formed, coalesced, aligned, and trapped along the contact line of the mother droplet. We demonstrate that this W/W multiple emulsion system can be used to encapsulate magnetic particles and blood cells, and achieve size-based separation. Finally, we show the applicability of this system for protein sensing. This is the first experimental observation of evaporation-induced secondary W/W droplet generation in a sessile droplet. We anticipate that the phenomena described here may be applicable to some biological assay applications, for example, biomarker detection, protein sensing, and point-of-care diagnostic testing.

Photosynthetic hydrogen production by droplet-based microbial micro-reactors under aerobic conditions

The spontaneous self-assembly of multicellular ensembles into living materials with synergistic structure and function remains a considerable challenge in biotechnology and synthetic biology. Here, we exploit the aqueous two-phase separation of dextran-in-PEG emulsion micro-droplets for the capture, spatial organization and immobilization of algal cells or algal/bacterial cell communities to produce discrete multicellular spheroids capable of both aerobic (oxygen producing) and hypoxic (hydrogen producing) photosynthesis in daylight under air. We show that localized oxygen depletion results in hydrogen production from the core of the algal microscale reactor, and demonstrate that enhanced levels of hydrogen evolution can be achieved synergistically by spontaneously enclosing the photosynthetic cells within a shell of bacterial cells undergoing aerobic respiration. Our results highlight a promising droplet-based environmentally benign approach to dispersible photosynthetic microbial micro-reactors comprising segregated cellular micro-niches with dual functionality, and provide a step towards photobiological hydrogen production under aerobic conditions.

The development of techniques capable of orchestrating the assembly of living cells into multicellular ensembles with synergistic and function is challenge. Here, the authors construct algal or algal/bacterial cells-based core shell-like structure based on aqueous two-phase system for synergic photosynthetic H₂ production.

Effects of Processing Parameters of 3D Bioprinting on the Cellular Activity of Bioinks

In this review, few established cell printing techniques along with their parameters that affect the cell viability during bioprinting are considered. 3D bioprinting is developed on the principle of additive manufacturing using biomaterial inks and bioinks. Different bioprinting methods impose few challenges on cell printing such as shear stress, mechanical impact, heat, laser radiation, etc., which eventually lead to cell death. These factors also cause alteration of cells phenotype, recoverable or irrecoverable damages to the cells. Such challenges are not addressed in detail in the literature and scientific reports. Hence, this review presents a detailed discussion of several cellular bioprinting methods and their process-related impacts on cell viability, followed by probable mitigation techniques. Most of the printable bioinks encompass cells within hydrogel as scaffold material to avoid the direct exposure of the harsh printing environment on cells. However, the advantages of printing with scaffold-free cellular aggregates over cell-laden hydrogels have emerged very recently. Henceforth, optimal and favorable crosslinking mechanisms providing structural rigidity to the cell-laden printed constructs with ideal cell differentiation and proliferation, are discussed for improved understanding of cell printing methods for the future of organ printing and transplantation.

Silk fibroin reactive inks for 3D printing crypt-like structures

A reactive silk fibroin ink formulation designed for extrusion three-dimensional (3D) printing of protein-based hydrogels at room temperature is reported. This work is motivated by the need to produce protein hydrogels that can be printed into complex shapes with long-term stability using extrusion 3D printing at ambient temperature without the need for the addition of nanocomposites, synthetic polymers, or sacrifical templates. Silk fibroin from the Bombyx mori silkworm was purified and synthesized into reactive inks by enzyme-catalyzed dityrosine bond formation. Rheological and printing studies showed that tailoring the peroxide concentration in the reactive ink enables the silk to be extruded as a filament and printed into hydrogel constructs, supporting successive printed layers without flow of the construct or loss of desired geometry. To enable success of longer-term in vitro studies, 3D printed silk hydrogels were found to display excellent shape retention over time, as evidenced by no change in construct dimensions or topography when maintained for nine weeks in culture medium. Caco-2 (an intestinal epithelial cell line) attachment, proliferation, and tight junction formation on the printed constructs was not found to be affected by the geometry of the constructs tested. Intestinal myofibroblasts encapsulated within reactive silk inks were found to survive shearing during printing and proliferate within the hydrogel constructs. The work here thus provides a suitable route for extrusion 3D printing of protein hydrogel constructs that maintain their shape during printing and culture, and is expected to enable longer-term cellular studies of hydrogel constructs that require complex geometries and/or varying spatial distributions of cells on demand via digital printing.

Fabrication of Artificial Nanobasement Membranes for Cell Compartmentalization in 3D Tissues

In recent decades, tissue engineering techniques have attracted much attention in the construction of 3D tissues or organs. However, even though precise control of cell locations in 3D has been achieved, the organized cell locations are easily destroyed because of the cell migration during the cell culture period. In human body, basement membranes (BMs) maintain the precise cell locations in 3D (compartmentalization). Constructing artificial BMs that mimic the structure and biofunctions of natural BMs remains a major challenge. Here, a nanometer-sized artificial BM through layer-by-layer assembly of collagen type IV (Col-IV) and laminin (LM), chosen because they are the main components of natural BMs, is reported. This multilayered Col-IV/LM nanofilm imitates natural BM structure closely, showing controllable and similar components, thickness, and fibrous network. The Col-IV/LM nanofilms have high cell adhesion properties and maintain the spreading morphology effectively. Furthermore, the barrier effect of preventing cell migration but permitting effective cell-cell crosstalk between fibroblasts and endothelial cells demonstrates the ability of Col-IV/LM nanofilms for cell compartmentalization in 3D tissues, providing more reliable tissue models for evaluating drug efficacy, nanotoxicology, and implantation.

Gravitational sedimentation-based approach for ultra-simple and flexible cell patterning coculture on microfluidic device

Combining patterning coculture technique with microfluidics enables the reconstruction of complex in-vivo system to facilitate in-vitro studies on cell-cell and cell-environment interactions. However, simple and versatile approaches for patterning coculture of cells on microfluidic platforms remain lacking. In this study, a novel gravitational sedimentation-based approach is presented to achieve ultra-simple and flexible cell patterning coculture on a microfluidic platform, where multiple cell types can be patterned simultaneously to form a well-organized cell coculture. In contrast to other approaches, the proposed approach allows the rapid patterning of multiple cell types in microfluidic channels without the use of sheath flow and a prepatterned functional surface. This feature greatly simplifies the experimental setup, operation, and chip fabrication. Moreover, cell patterning can be adjusted by simply modifying the cell-loading tubing direction, thereby enabling great flexibility for the construction of different cell patterns without complicating the chip design and flow control. A series of physical and biological experiments are conducted to validate the proposed approach. This research paves a new way for building physiologically realistic in-vitro coculture models on microfluidic platforms for various applications, such as cell-cell interaction and drug screening.

Microfluidic Generation of All-Aqueous Double and Triple Emulsions

Higher order emulsions are used in a variety of different applications in biomedicine, biological studies, cosmetics, and the food industry. Conventional droplet generation platforms for making higher order emulsions use organic solvents as the continuous phase, which is not biocompatible and as a result, further washing steps are required to remove the toxic continuous phase. Recently, droplet generation based on aqueous two-phase systems (ATPS) has emerged in the field of droplet microfluidics due to their intrinsic biocompatibility. Here, a platform to generate all-aqueous double and triple emulsions by introducing pressure-driven flows inside a microfluidic hybrid device is presented. This system uses a conventional microfluidic flow-focusing geometry coupled with a coaxial microneedle and a glass capillary embedded in flow-focusing junctions. The configuration of the hybrid device enables the focusing of two coaxial two-phase streams, which helps to avoid commonly observed channel-wetting problems. It is shown that this approach achieves the fabrication of higher-order emulsions in a poly(dimethylsiloxane)-based microfluidic device, and controls the structure of the all-aqueous emulsions. This hybrid microfluidic approach allows for facile higher-order biocompatible emulsion formation, and it is anticipated that this platform will find utility for generating biocompatible materials for various biotechnological applications.

Emerging aqueous two-phase systems: from fundamentals of interfaces to biomedical applications

This review summarizes recent advances of aqueous two-phase systems (ATPSs), particularly their interfaces, with a focus on biomedical applications.

An open-space microfluidic chip with fluid walls for online detection of VEGF via rolling circle amplification

We report an open-space microfluidic chip with fluid walls, integrating functions of cell culture and online detection of secreted proteins controlled by the interfacial tension value.

Despite traditional poly-dimethyl siloxane (PDMS) microfluidic devices having great potential in various biological studies, they are limited by sophisticated fabrication processes and low utilization. An easily controlled microfluidic platform with high efficiency and low cost is desperately required. In this work, we present an open-space microfluidic chip with fluid walls, integrating cell culture and online semi-quantitative detection of vascular endothelial growth factor (VEGF) via rolling circle amplification (RCA) reaction. In comparison with conventional co-culture detecting platforms, this method features the prominent advantages of saving reagents and time, a simplified chip fabrication process, and avoiding additional assistance for online detection with the help of an interfacial tension valve. On such a multi-functional microfluidic chip, cells (human umbilical vein endothelial cells and malignant glioma cells) could maintain regular growth and cell viability. VEGF could be detected with excellent specificity and good linearity in the range of 10–250 pg mL^–1. Meanwhile, VEGF secreted by malignant glioma cells was also detected online and obviously increased when cells were stimulated by deferoxamine (DFO) to mimic a hypoxic microenvironment. The designed biochip with fluid walls provides a new perspective for micro-total analysis and could be promisingly applied in future clinical diagnosis and drug analysis.

All-Aqueous Thin-Film-Flow-Induced Cell-Based Monolayers

Cells in vitro usually require a solid scaffold to attach and form two-dimensional monolayer structures. To obtain a substrate-free cell monolayer, long culture time and specific detaching procedures are required. In this study, a thin-film-flow-induced strategy is reported to overcome the challenges of assembling in vitro scaffold-free monolayered cell aggregates. The assembly is driven by a dewetting-like thin-film withdrawal along all-aqueous interfaces characterized by a low interfacial tension. The withdrawal process drives the cells adsorbed on the liquid film to aggregate and assemble into an organized and compact monolayer. This strategy is not limited to biological cells but also colloidal particles, as demonstrated by the assembly of hybrid cell-particle monolayers. The versatility offered by this approach suggests new opportunities in understanding early tissue formation and functionalizing cell monolayer aggregates by colloidal particles with customized functions.

Endowing a Light-Inert Aqueous Surfactant Two-Phase System with Photoresponsiveness by Introducing a Trojan Horse

The ability to modulate the phase behavior of an aqueous surfactant two-phase (ASTP) system reversibly with light is of great importance in both fundamental and applied science. Thus far, most of the light-responsive ASTP systems are based on covalent modification of the component molecules. In this article, we, for the first time, achieve photoresponsiveness in a light-inert ASTP system by physically introducing a phototrigger with the aid of a Trojan horse. The ASTP system formed from sodium laurate (SL) and dodecyltributylammonium bromide (DBAB) does not show light responsiveness by physically mixing a light-responsive azobenzene compound, 2-(4-(phenyldiazenyl)phenoxy)acetate sodium (Azo). However, in the presence of the host-guest complex SL@β-CD formed from β-CD and sodium laurate (SL), the ASTP turns quickly into a homogeneous suspension under visible light, which recovers to the original ASTP state again under 365 nm UV irradiation. Because the SL@β-CD complex exists harmonically with the ASTP system, it can be viewed as a "Trojan horse" that becomes fatal only when the encapsulated SL is triggered to release. In the presence of the Trojan horse, the photoresponsiveness of the ASTP system can be manipulated reversibly by alternatively exerting UV and visible light. Using this strategy, we are able to collect trace amounts of oily components from water. The current strategy points out that it is possible to achieve light responsiveness in light-inert systems with a physical method, which may have profound impact on both the fundamental and applied science.

Precision cell delivery in biphasic polymer systems enhances growth of keratinocytes in culture and promotes their attachment on acellular dermal matrices

Current approaches for precision deposition of cells are not optimized for moist environments or for substrates with complex surface topographic features, for example, the surface of dermal matrices and other biomaterials. To overcome these challenges, an approach is presented that utilizes cell confinement in phase-separating polymer solutions of polyethylene glycol and dextran to precisely deliver keratinocytes in well-defined colonies. Using this approach, keratinocyte colonies are produced with superior viability, proliferative capacity, and barrier formation compared with the same number of cells dispersedly seeded across substrate surfaces. It is further demonstrated that keratinocytes delivered in colonies to the surface of acellular dermal matrices form an intact epidermal basal layer more rapidly and more completely than cells delivered by conventional dispersed seeding. These findings demonstrate that delivery of keratinocytes in phase-separating polymer solutions holds potential for enhancing growth of keratinocytes in culture and production of functional skin equivalents.

All-Aqueous-Phase Microfluidics for Cell Encapsulation

Cell-laden hydrogel microcarriers are widely used in diverse biomedical applications like three-dimensional (3D) cell culture, cellular therapy, and tissue engineering, where microcarriers were generally produced by oil, which is the common but not optimal choice, as oil may cause cytotoxicity or protein denaturation. Here, an all-aqueous-phase microfluidics is presented to achieve oil-free emulsification of cell-laden microcapsules and 3D cell culture. Aqueous solutions with different concentration gradients are used as an immiscible continuous phase and a dispersed phase, and oscillation from a solenoid valve facilitates the formation of microcapsules at the water-water interface. By adjusting aqueous-phase flow rates and oscillating frequencies, core-shell microcapsules with controllable structures can be stably and continuously generated. In further 3D cell culture, encapsulated cells maintained good viabilities and aggregated together. These features show that the oil-free microfluidic method may have broad prospects in many biomedical applications.

Development of dual-component protein microparticles in all-aqueous systems for biomedical applications

Protein microparticles assisted by an emulsion droplet template have shown great promise in drug/cell delivery and tissue engineering, as well as diagnosis and treatment of diseases.

The regenerated silk fibroin hydrogel with designed architecture bioprinted by its microhydrogel

The regenerated silk fibroin microhydrogel with thixotropic property could be bioprinted and then ripened to a tough hydrogel because of the change in “the second network” of the microhydrogel.

Stem cell colony interspacing effect on differentiation to neural cells

Efforts to enhance the efficiency of neural differentiation of stem cells are primarily focused on exogenous modulation of physical niche parameters such as surface topography and extracellular matrix proteins, or addition of certain growth factors or small molecules to culture media. We report a novel neurogenic niche to enhance the neural differentiation of embryonic stem cells (ESCs) without any external intervention by micropatterning ESCs into spatially organized colonies of controlled size and interspacing. Using an aqueous two-phase system cell microprinting technology, we generated pairs of uniformly sized isolated ESC colonies at defined interspacing distances over a layer of differentiation-inducing stromal cells. Our comprehensive analysis of temporal expression of neural genes and proteins of cells in colony pairs showed that interspacing two colonies at approximately 0.66 times the colony diameter (0.66D) significantly enhanced neural differentiation of ESCs. Cells in these colonies displayed higher expression of neural genes and proteins and formed thick neurite bundles between the two colonies. A computational model of spatial distribution of soluble factors of cells in interspaced colony pairs showed that the enhanced neural differentiation is due to the presence of stable concentration gradients of soluble signalling factors between the two colonies. Our results indicate that culturing ESCs in colony pairs with defined interspacing is a promising approach to efficiently derive neural cells. Additionally, this approach provides a platform for quantitative studies of molecular mechanisms that regulate neurogenesis of stem cells.

Microscopic evaluation of aqueous two-phase system emulsion characteristics enables rapid determination of critical polymer concentrations for solution micropatterning

Aqueous two-phase systems have emerged as valuable tools for microscale analysis of cell growth and many other biotechnology applications. The most critical step in developing an aqueous two-phase system for a specific application is identifying the critical concentrations at which the polymer solutions phase-separate. Current techniques for determining these critical concentrations rely on laborious methods, highly specialized assays or computational methods that make this step difficult for non-specialists. To overcome these limitations, we present a simplified assay that uses only readily accessible laboratory instruments and consumables (e.g., multichannel micropipettes, 96-well plates and a simple compound microscope) to determine the critical concentrations of aqueous two-phase system-forming polymers. We demonstrate that formulations selected from phase diagrams that describe these critical concentrations can be applied for solution micropatterning of cells.

Statistical analysis of multi-dimensional, temporal gene expression of stem cells to elucidate colony size-dependent neural differentiation

High throughput gene expression analysis using qPCR is commonly used to identify molecular markers of complex cellular processes. However, statistical analysis of multi-dimensional, temporal gene expression data is complicated by limited biological replicates and large number of measurements. Moreover, many available statistical tools for analysis of time series data assume that the data sequence is static and does not evolve over time. With this assumption, the parameters used to model the time series are fixed and thus, can be estimated by pooling data together. However, in many cases, dynamic processes of biological systems involve abrupt changes at unknown time points, making the assumption of stationary time series break down. We addressed this problem using a combination of statistical methods including hierarchical clustering, change point detection, and multiple testing. We applied this multi-step method to multi-dimensional, temporal gene expression data that resulted from our study of colony size-dependent neural cell differentiation of stem cells. The gene expression data were time series as the observations were recorded sequentially over time. Hierarchical clustering segregated the genes into three distinct clusters based on their temporal expression profiles; change point detection identified specific time points at which the entire dataset was divided into several homogenous subsets to allow a separate analysis of each subset; and multiple testing procedure identified the differentially expressed genes in each cluster within each subset of data. We established that our multi-step approach pinpoints specific sets of genes that underlie colony size-mediated neural differentiation of stem cells and demonstrated its advantages over conventional parametric and non-parametric tests that do not take into account temporal dynamics of the data. Importantly, our proposed approach is broadly applicable to any multivariate data sets of limited sample size from high throughput and high content screening such as in drug and biomarker discovery studies.

Emerging Biotechnology Applications of Aqueous Two-Phase Systems

Liquid-liquid phase separation between aqueous solutions containing two incompatible polymers, a polymer and a salt, or a polymer and a surfactant, has been exploited for a wide variety of biotechnology applications throughout the years. While many applications for aqueous two-phase systems fall within the realm of separation science, the ability to partition many different materials within these systems, coupled with recent advances in materials science and liquid handling, has allowed bioengineers to imagine new applications. This progress report provides an overview of the history and key properties of aqueous two-phase systems to lend context to how these materials have progressed to modern applications such as cellular micropatterning and bioprinting, high-throughput 3D tissue assembly, microscale biomolecular assay development, facilitation of cell separation and microcapsule production using microfluidic devices, and synthetic biology. Future directions and present limitations and design considerations of this adaptable and promising toolkit for biomolecule and cellular manipulation are further evaluated.

Microprinted Stem Cell Niches Reveal Compounding Effect of Colony Size on Stromal Cells-Mediated Neural Differentiation

Microenvironmental factors have a major impact on differentiation of embryonic stem cells (ESCs). Here, a novel phenomenon that size of ESC colonies has a significant regulatory role on stromal cells induced differentiation of ESCs to neural cells is reported. Using a robotic cell microprinting technology, defined densities of ESCs are confined within aqueous nanodrops over a layer of supporting stromal cells immersed in a second, immiscible aqueous phase to generate ESC colonies of defined sizes. Temporal protein and gene expression studies demonstrate that larger ESC colonies generate disproportionally more neural cells and longer neurite processes. Unlike previous studies that attribute neural differentiation of ESCs solely to interactions with stromal cells, it is found that increased intercellular signaling of ESCs significantly enhances neural differentiation. This study offers an approach to generate neural cells with improved efficiency for potential use in translational research.

Biomaterials-Based Approaches to Tumor Spheroid and Organoid Modeling

Evolving understanding of structural and biological complexity of tumors has stimulated development of physiologically relevant tumor models for cancer research and drug discovery. A major motivation for developing new tumor models is to recreate the 3D environment of tumors and context-mediated functional regulation of cancer cells. Such models overcome many limitations of standard monolayer cancer cell cultures. Under defined culture conditions, cancer cells self-assemble into 3D constructs known as spheroids. Additionally, cancer cells may recapitulate steps in embryonic development to self-organize into 3D cultures known as organoids. Importantly, spheroids and organoids reproduce morphology and biologic properties of tumors, providing valuable new tools for research, drug discovery, and precision medicine in cancer. This Progress Report discusses uses of both natural and synthetic biomaterials to culture cancer cells as spheroids or organoids, specifically highlighting studies that demonstrate how these models recapitulate key properties of native tumors. The report concludes with the perspectives on the utility of these models and areas of need for future developments to more closely mimic pathologic events in tumors.

Deep-Reaching Hydrodynamic Flow Confinement: Micrometer-Scale Liquid Localization for Open Substrates With Topographical Variations

We present a method for nonintrusive localization and reagent delivery on immersed biological samples with topographical variation on the order of hundreds of micrometers. Our technique, which we refer to as the deep-reaching hydrodynamic flow confinement (DR-HFC), is simple and passive: it relies on a deep-reaching hydrodynamic confinement delivered through a simple microfluidic probe design to perform localized microscale alterations on substrates as deep as 600 μm. Designed to scan centimeter-scale areas of biological substrates, our method passively prevents sample intrusion by maintaining a large gap between the probe and the substrate. The gap prevents collision of the probe and the substrate and reduces the shear stress experienced by the sample. We present two probe designs: linear and annular DR-HFC. Both designs comprise a reagent-injection aperture and aspiration apertures that serve to confine the reagent. We identify the design parameters affecting reagent localization and depth by DR-HFC and study their individual influence on the operation of DR-HFC numerically. Using DR-HFC, we demonstrate localized binding of antihuman immunoglobulin G (IgG) onto an activated substrate at various depths from 50 to 600 μm. DR-HFC provides a readily implementable approach for noninvasive processing of biological samples applicable to the next generation of diagnostic and bioanalytical devices.

All-Aqueous Assemblies via Interfacial Complexation: Toward Artificial Cell and Microniche Development

In nature, the environment surrounding biomolecules and living cells can dictate their structure, function, and properties. Confinement is a key means to define and regulate such environments. For example, the confinement of appropriate constituents in compartments facilitates the assembly, dynamics, and function of biochemical machineries as well as subcellular organelles. Membraneless organelles, in particular, are thought to form via thermodynamic cues defined within the interior space of cells. On larger length scales, the confinement of living cells dictates cellular function for both mammalian and bacterial cells. One promising class of artificial structures that can recapitulate these multiscale confinement effects is based on aqueous two-phase systems (ATPSs). This feature article highlights recent developments in the production and stabilization of ATPS-droplet-based systems, with a focus on interfacial complexation. These systems enable structure formation, modulation, and triggered (dis)assembly, thereby allowing structures to be tailored to fit the desired function and designed for particular confinement studies. Open issues for both synthetic cells and niche studies are identified.

Precise and Arbitrary Deposition of Biomolecules onto Biomimetic Fibrous Matrices for Spatially Controlled Cell Distribution and Functions

Advances in nano-/microfabrication allow the fabrication of biomimetic substrates for various biomedical applications. In particular, it would be beneficial to control the distribution of cells and relevant biomolecules on an extracellular matrix (ECM)-like substrate with arbitrary micropatterns. In this regard, the possibilities of patterning biomolecules and cells on nanofibrous matrices are explored here by combining inkjet printing and electrospinning. Upon investigation of key parameters for patterning accuracy and reproducibility, three independent studies are performed to demonstrate the potential of this platform for: i) transforming growth factor (TGF)-β1-induced spatial differentiation of fibroblasts, ii) spatiotemporal interactions between breast cancer cells and stromal cells, and iii) cancer-regulated angiogenesis. The results show that TGF-β1 induces local fibroblast-to-myofibroblast differentiation in a dose-dependent fashion, and breast cancer clusters recruit activated stromal cells and guide the sprouting of endothelial cells in a spatially resolved manner. The established platform not only provides strategies to fabricate ECM-like interfaces for medical devices, but also offers the capability of spatially controlling cell organization for fundamental studies, and for high-throughput screening of various biomolecules for stem cell differentiation and cancer therapeutics.

Manipulating Living Cells to Construct a 3D Single-Cell Assembly without an Artificial Scaffold

Artificial scaffolds such as synthetic gels or chemically-modified glass surfaces that have often been used to achieve cell adhesion are xenobiotic and may harm cells. To enhance the value of cell studies in the fields of regenerative medicine and tissue engineering, it is becoming increasingly important to create a cell-friendly technique to promote cell–cell contact. In the present study, we developed a novel method for constructing stable cellular assemblies by using optical tweezers in a solution of a natural hydrophilic polymer, dextran. In this method, a target cell is transferred to another target cell to make cell–cell contact by optical tweezers in a culture medium containing dextran. When originally non-cohesive cells are held in contact with each other for a few minutes under laser trapping, stable cell–cell adhesion is accomplished. This method for creating cellular assemblies in the presence of a natural hydrophilic polymer may serve as a novel next-generation 3D single-cell assembly system with future applications in the growing field of regenerative medicine.

Self-regulatory factors of embryonic stem cells in co-culture with stromal cells enhance neural differentiation

Embryonic stem cells (ESCs), due to their intrinsic capability to generate somatic cells of all three germ layers, are potential sources of neural cells for cell replacement therapies. However, the empirical differentiation protocols and the lack of mechanistic understanding of the neural differentiation of ESCs have limited the utility of ESCs as a developmental model or as a cell source for neural cell populations for replacement therapies. Co-culturing ESCs with stromal cells is one of the extensively used methods to induce neural differentiation. Despite several studies to identify neural inducing factors in stromal cell induced neural differentiation, the self-regulatory effects of ESCs in the neural differentiation process remain unexplored. For the first time, we elucidate the self-regulatory role of mESCs in their neural cell differentiation by supplementing conditioned media from differentiating mESCs to mESC-PA6 co-cultures and quantitatively evaluating the change in neural differentiation. Moreover, we use statistical tools to analyze the expression of various growth and trophic factors and distinguish the factors produced primarily by PA6 cells versus mESCs in co-culture. We observe that addition of the medium containing mESC-secreted factors to a single mESC colony co-cultured with PA6 cells significantly enhances the neural differentiation of mESCs compares to the medium extracted from the stromal cells only. Hierarchical clustering of gene expression data from PA6 and co-cultured mESCs segregates two groups of factors that are produced by the stromal cells and differentiating mESCs. Identifying the major soluble factors that drive and regulate the neural differentiation process in the mESC-PA6 co-culture niche will help understand molecular mechanisms of neural development. Moreover, it can be a major step toward developing novel protocols to differentiate stem cells with mESC derived factor supplementation without using feeder cells and with greater efficiency compared to existing approaches.

Colony size effect on neural differentiation of embryonic stem cells microprinted on stromal cells

Controlling cellular microenvironment to induce neural differentiation of embryonic stem cells (ESCs) remains a major challenge. We address this need by introducing a micro-engineered co-culture system that resembles embryonic development in terms of direct intercellular interactions and induces neural differentiation of ESCs. A polymeric aqueous two-phase system (ATPS)-mediated robotic microprinting technology allows precise localization of mouse ESCs (mESCs) over a layer of supporting stromal cells. mESCs proliferate over a 2-week culture period into a single colony of defined size. Physical and chemical cues from the stromal cells guide mESCs to differentiate toward specific neural lineages. We generated mESC colonies of three different sizes from 100, 250 and 500 single cells and showed that size of mESC colonies is an important factor determining the yield of neural cells. Expression of early neural cell markers nestin denoting neural stem cells, NCAM specifying neural progenitors, and β-III tubulin (TuJ) indicating post mitotic neurons escalated from day 4. Differentiation into specific neural cells astrocytes marked by GFAP, oligodendrocytes indicated by CNPase, and TH-positive dopaminergic neurons was observed during the second week of culture. Unexpectedly, analysis of protein expression revealed a disproportionate increase in neural differentiation of mESCs by increase in the colony size. For the first time, our study establishes colony size as an important regulator of fate of ESCs in this heterocellular niche. This approach of deriving neural cells may make a major impact on stem cell research for treating neurodegenerative diseases.

Molecular Analysis of Stromal Cells-Induced Neural Differentiation of Mouse Embryonic Stem Cells

Deriving specific neural cells from embryonic stem cells (ESCs) is a promising approach for cell replacement therapies of neurodegenerative diseases. When co-cultured with certain stromal cells, mouse ESCs (mESCs) differentiate efficiently to neural cells. In this study, a comprehensive gene and protein expression analysis of differentiating mESCs is performed over a two-week culture period to track temporal progression of cells from a pluripotent state to specific terminally-differentiated neural cells such as neurons, astrocytes, and oligodendrocytes. Expression levels of 26 genes consisting of marker genes for pluripotency, neural progenitors, and specific neuronal, astroglial, and oligodendrocytic cells are tracked using real time q-PCR. The time-course gene expression analysis of differentiating mESCs is combined with the hierarchal clustering and functional principal component analysis (FPCA) to elucidate the evolution of specific neural cells from mESCs at a molecular level. These statistical analyses identify three major gene clusters representing distinct phases of transition of stem cells from a pluripotent state to a terminally-differentiated neuronal or glial state. Temporal protein expression studies using immunohistochemistry demonstrate the generation of neural stem/progenitor cells and specific neural lineages and show a close agreement with the gene expression profiles of selected markers. Importantly, parallel gene and protein expression analysis elucidates long-term stability of certain proteins compared to those with a quick turnover. Describing the molecular regulation of neural cells commitment of mESCs due to stromal signaling will help identify major promoters of differentiation into specific cell types for use in cell replacement therapy applications.

Engineered Breast Cancer Cell Spheroids Reproduce Biologic Properties of Solid Tumors

Solid tumors develop as 3D tissue constructs. As tumors grow larger, spatial gradients of nutrients and oxygen and inadequate diffusive supply to cells distant from vasculature develops. Hypoxia initiates signaling and transcriptional alterations to promote survival of cancer cells and generation of cancer stem cells (CSCs) that have self-renewal and tumor-initiation capabilities. Both hypoxia and CSCs are associated with resistance to therapies and tumor relapse. This study demonstrates that 3D cancer cell models, known as tumor spheroids, generated with a polymeric aqueous two-phase system (ATPS) technology capture these important biological processes. Similar to solid tumors, spheroids of triple negative breast cancer cells deposit major extracellular matrix proteins. The molecular analysis establishes presence of hypoxic cells in the core region and expression of CSC gene and protein markers including CD24, CD133, and Nanog. Importantly, these spheroids resist treatment with chemotherapy drugs. A combination treatment approach using a hypoxia-activated prodrug, TH-302, and a chemotherapy drug, doxorubicin, successfully targets drug resistant spheroids. This study demonstrates that ATPS spheroids recapitulate important biological and functional properties of solid tumors and provide a unique model for studies in cancer research.