Interfacial tension based on-chip extraction of microparticles confined in microfluidic Stokes flows

Gravity-Oriented Microfluidic Device for Biocompatible End-to-End Fabrication of Cell-Laden Microgels

Droplet microfluidics are extensively utilized to generate monodisperse cell-laden microgels in biomedical applications. However, maintaining cell viability is still challenging due to overexposure to harsh conditions in subsequent procedures that recover the microgels from the oil phase. Here, a gravity-oriented microfluidic device for end-to-end fabrication of cell-laden microgels is reported, which integrates dispersion, gelation, and extraction into a continuous workflow. This innovative on-chip extraction, driven by native buoyancy and kinetically facilitated by pseudosurfactant, exhibits 100% retrieval efficiency for microgels with a wide range of sizes and stiffnesses. The viability of encapsulated cells is perfectly maintained at ≈98% with minimal variations within and between batches. The end-to-end fabrication remarkably enhances the biocompatibility and practicality of microfluidics-based cell encapsulation and is promising to be compatible with various applications ranging from single-cell analysis to clinical therapy.

Hydrogels for Single-Cell Microgel Production: Recent Advances and Applications

Single-cell techniques have become more and more incorporated in cell biological research over the past decades. Various approaches have been proposed to isolate, culture, sort, and analyze individual cells to understand cellular heterogeneity, which is at the foundation of every systematic cellular response in the human body. Microfluidics is undoubtedly the most suitable method of manipulating cells, due to its small scale, high degree of control, and gentle nature toward vulnerable cells. More specifically, the technique of microfluidic droplet production has proven to provide reproducible single-cell encapsulation with high throughput. Various in-droplet applications have been explored, ranging from immunoassays, cytotoxicity assays, and single-cell sequencing. All rely on the theoretically unlimited throughput that can be achieved and the monodispersity of each individual droplet. To make these platforms more suitable for adherent cells or to maintain spatial control after de-emulsification, hydrogels can be included during droplet production to obtain “microgels.” Over the past years, a multitude of research has focused on the possibilities these can provide. Also, as the technique matures, it is becoming clear that it will result in advantages over conventional droplet approaches. In this review, we provide a comprehensive overview on how various types of hydrogels can be incorporated into different droplet-based approaches and provide novel and more robust analytic and screening applications. We will further focus on a wide range of recently published applications for microgels and how these can be applied in cell biological research at the single- to multicell scale.

Noncovalent reversible binding-enabled facile fabrication of leak-free PDMS microfluidic devices without plasma treatment for convenient cell loading and retrieval

The conventional approach for fabricating polydimethylsiloxane (PDMS) microfluidic devices is a lengthy and inconvenient procedure and may require a clean-room microfabrication facility often not readily available. Furthermore, living cells can't survive the oxygen-plasma and high-temperature-baking treatments required for covalent bonding to assemble multiple PDMS parts into a leak-free device, and it is difficult to disassemble the devices because of the irreversible covalent bonding. As a result, seeding/loading cells into and retrieving cells from the devices are challenging. Here, we discovered that decreasing the curing agent for crosslinking the PDMS prepolymer increases the noncovalent binding energy of the resultant PDMS surfaces without plasma or any other treatment. This enables convenient fabrication of leak-free microfluidic devices by noncovalent binding for various biomedical applications that require high pressure/flow rates and/or long-term cell culture, by simply hand-pressing the PDMS parts without plasma or any other treatment to bind/assemble. With this method, multiple types of cells can be conveniently loaded into specific areas of the PDMS parts before assembly and due to the reversible nature of the noncovalent bonding, the assembled device can be easily disassembled by hand peeling for retrieving cells. Combining with 3D printers that are widely available for making masters to eliminate the need of photolithography, this facile yet rigorous fabrication approach is much faster and more convenient for making PDMS microfluidic devices than the conventional oxygen plasma-baking-based irreversible covalent bonding method.

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The stability of noncovalent PDMS-PDMS binding is dependent on the binding energy instead of the binding strength.

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The noncovalent binding of a special formulation of PDMS is sufficient for reversible assembly of leak-free microfluidic devices.

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The noncovalent binding method enables loading multiple types of cells into PDMS parts before assembling into the final device.

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The PDMS device can be easily dissembled due to the reversible nature of the noncovalent binding for retrieving cells.

Droplet microfluidic devices for organized stem cell differentiation into germ cells: capabilities and challenges

Demystifying the mechanisms that underlie germline development and gamete production is critical for expanding advanced therapies for infertile couples who cannot benefit from current infertility treatments. However, the low number of germ cells, particularly in the early stages of development, represents a serious challenge in obtaining sufficient materials required for research purposes. In this regard, pluripotent stem cells (PSCs) have provided an opportunity for producing an unlimited source of germ cells in vitro. Achieving this ambition is highly dependent on accurate stem cell niche reconstitution which is achievable through applying advanced cell engineering approaches. Recently, hydrogel microparticles (HMPs), as either microcarriers or microcapsules, have shown promising potential in providing an excellent 3-dimensional (3D) biomimetic microenvironment alongside the systematic bioactive agent delivery. In this review, recent studies of utilizing various HMP-based cell engineering strategies for appropriate niche reconstitution and efficient in vitro differentiation are highlighted with a special focus on the capabilities of droplet-based microfluidic (DBM) technology. We believe that a deep understanding of the current limitations and potentials of the DBM systems in integration with stem cell biology provides a bright future for germ cell research.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12551-021-00907-5.

Deep Learning-Enabled Label-Free On-Chip Detection and Selective Extraction of Cell Aggregate-Laden Hydrogel Microcapsules

Microfluidic encapsulation of cells/tissues in hydrogel microcapsules has attracted tremendous attention in the burgeoning field of cell-based medicine. However, when encapsulating rare cells and tissues (e.g., pancreatic islets and ovarian follicles), the majority of the resultant hydrogel microcapsules are empty and should be excluded from the sample. Furthermore, the cell-laden hydrogel microcapsules are usually suspended in an oil phase after microfluidic generation, while the microencapsulated cells require an aqueous phase for further culture/transplantation and long-term suspension in oil may compromise the cells/tissues. Thus, real-time on-chip selective extraction of cell-laden hydrogel microcapsules from oil into aqueous phase is crucial to the further use of the microencapsulated cells/tissues. Contemporary extraction methods either require labeling of cells for their identification along with an expensive detection system or have a low extraction purity (<≈30%). Here, a deep learning-enabled approach for label-free detection and selective extraction of cell-laden microcapsules with high efficiency of detection (≈100%) and extraction (≈97%), high purity of extraction (≈90%), and high cell viability (>95%) is reported. The utilization of deep learning to dynamically analyze images in real time for label-free detection and on-chip selective extraction of cell-laden hydrogel microcapsules is unique and may be valuable to advance the emerging cell-based medicine.

Methods of Generating Dielectrophoretic Force for Microfluidic Manipulation of Bioparticles

Manipulation of microscale bioparticles including living cells is of great significance to the broad bioengineering and biotechnology fields. Dielectrophoresis (DEP), which is defined as the interactions between dielectric particles and the electric field, is one of the most widely used techniques for the manipulation of bioparticles including cell separation, sorting, and trapping. Bioparticles experience a DEP force if they have a different polarization from the surrounding media in an electric field that is nonuniform in terms of the intensity and/or phase of the electric field. A comprehensive literature survey shows that the DEP-based microfluidic devices for manipulating bioparticles can be categorized according to the methods of creating the nonuniformity via patterned microchannels, electrodes, and media to generate the DEP force. These methods together with the theory of DEP force generation are described in this review, to provide a summary of the methods and materials that have been used to manipulate various bioparticles for various specific biological outcomes. Further developments of DEP-based technologies include identifying materials that better integrate with electrodes than current popular materials (silicone/glass) and improving the performance of DEP manipulation of bioparticles by combining it with other methods of handling bioparticles. Collectively, DEP-based microfluidic manipulation of bioparticles holds great potential for various biomedical applications.

Microfluidic and Microscale Assays to Examine Regenerative Strategies in the Neuro Retina

Bioengineering systems have transformed scientific knowledge of cellular behaviors in the nervous system (NS) and pioneered innovative, regenerative therapies to treat adult neural disorders. Microscale systems with characteristic lengths of single to hundreds of microns have examined the development and specialized behaviors of numerous neuromuscular and neurosensory components of the NS. The visual system is comprised of the eye sensory organ and its connecting pathways to the visual cortex. Significant vision loss arises from dysfunction in the retina, the photosensitive tissue at the eye posterior that achieves phototransduction of light to form images in the brain. Retinal regenerative medicine has embraced microfluidic technologies to manipulate stem-like cells for transplantation therapies, where de/differentiated cells are introduced within adult tissue to replace dysfunctional or damaged neurons. Microfluidic systems coupled with stem cell biology and biomaterials have produced exciting advances to restore vision. The current article reviews contemporary microfluidic technologies and microfluidics-enhanced bioassays, developed to interrogate cellular responses to adult retinal cues. The focus is on applications of microfluidics and microscale assays within mammalian sensory retina, or neuro retina, comprised of five types of retinal neurons (photoreceptors, horizontal, bipolar, amacrine, retinal ganglion) and one neuroglia (Müller), but excludes the non-sensory, retinal pigmented epithelium.

Enhanced single-cell encapsulation in microfluidic devices: From droplet generation to single-cell analysis

Single-cell analysis to investigate cellular heterogeneity and cell-to-cell interactions is a crucial compartment to answer key questions in important biological mechanisms. Droplet-based microfluidics appears to be the ideal platform for such a purpose because the compartmentalization of single cells into microdroplets offers unique advantages of enhancing assay sensitivity, protecting cells against external stresses, allowing versatile and precise manipulations over tested samples, and providing a stable microenvironment for long-term cell proliferation and observation. The present Review aims to give a preliminary guidance for researchers from different backgrounds to explore the field of single-cell encapsulation and analysis. A comprehensive and introductory overview of the droplet formation mechanism, fabrication methods of microchips, and a myriad of passive and active encapsulation techniques to enhance single-cell encapsulation efficiency were presented. Meanwhile, common methods for single-cell analysis, especially for long-term cell proliferation, differentiation, and observation inside microcapsules, are briefly introduced. Finally, the major challenges faced in the field are illustrated, and potential prospects for future work are discussed.

Microfluidic technologies to engineer mesenchymal stem cell aggregates-applications and benefits

Three-dimensional cell culture and the forming multicellular aggregates are superior over traditional monolayer approaches due to better mimicking of in vivo conditions and hence functions of a tissue. A considerable amount of attention has been devoted to devising efficient methods for the rapid formation of uniform-sized multicellular aggregates. Microfluidic technology describes a platform of techniques comprising microchannels to manipulate the small number of reagents with unique properties and capabilities suitable for biological studies. The focus of this review is to highlight recent studies of using microfluidics, especially droplet-based types for the formation, culture, and harvesting of mesenchymal stem cell aggregates and their subsequent application in stem cell biology, tissue engineering, and drug screening. Droplet-based microfluidics can be used to form microgels as carriers for delivering cells and to provide biological cues to the target tissue so as to be minimally invasive. Stem cell-laden microgels with a shape-forming property can be used as smart building blocks by injecting them into the injured tissue thereby constituting the cornerstone of tissue regeneration.

Droplet encapsulation of particles in different regimes and sorting of particle-encapsulating-droplets from empty droplets

Encapsulation of microparticles in droplets has profound applications in biochemical assays. We investigate encapsulation of rigid particles (polystyrene beads) and deformable particles (biological cells) inside aqueous droplets in various droplet generation regimes, namely, squeezing, dripping, and jetting. Our study reveals that the size of the positive (particle-encapsulating) droplets is larger or smaller compared to that of the negative (empty) droplets in the dripping and jetting regimes but no size contrast is observed in the squeezing regime. The size contrast of the positive and negative droplets in the different regimes is characterized in terms of capillary number C a and stream width ratio ω (i.e., ratio of stream width at the throat to particle diameter ω = w / d p ). While for deformable particles, the positive droplets are always larger compared to the negative droplets, for rigid particles, the positive droplets are larger in the dripping and jetting regimes for 0.50 ≤ ω ≤ 0.80 but smaller in the jetting regime for ω < 0.50 . We exploit the size contrast of positive and negative droplets for sorting across the fluid-fluid interface based on noninertial lift force (at R e ≪ 1 ), which is a strong function of droplet size. We demonstrate sorting of the positive droplets encapsulating polystyrene beads and biological cells from the negative droplets with an efficiency of ∼95% and purity of ∼65%. The proposed study will find relevance in single-cell studies, where positive droplets need to be isolated from the empty droplets prior to downstream processing.

An integrated microfluidic flow-focusing platform for on-chip fabrication and filtration of cell-laden microgels

We present the development of a stable continuous, and integrated microfluidic platform for the high-throughput fabrication of monodisperse cell-laden microgel droplets with high and maintained cellular viability. This is through combining onto one chip all the required processes from the droplet generation in a flow focusing microfluidic junction passing through on-chip photocrosslinking to the separation of the droplets from the continuous oil phase. To avoid cellular aggregation during the droplet generation process, cells were treated with bovine serum albumin (BSA) before mixing with gelatin methacrylate (GelMA). And, a magnetic mixer was applied to the GelMA prepolymer-cell suspension syringe to eliminate cell sedimentation. These approaches resulted in having a reasonable distribution of cells among monodisperse microdroplets. The microdroplets were irradiated with a 405 nm wavelength laser beam while passing through the crosslinking chamber of the microfluidic device. The produced microgels enter the filtration unit of the same device where they were gently separated from the oil phase into the washing buffer aqueous solution of Tween 80 using the filter microposts array. The viability of the encapsulated cells was around 85% at day 1 and was maintained throughout 5 days. Using this method of controlling cell encapsulation with on-chip crosslinking and oil filtration, highly efficient cell-laden microgel production is achieved. The presented integrated microfluidic platform can be a candidate for standard cell-encapsulation experiments and other tissue engineering applications.

Surface-Acoustic-Wave-Based Lab-on-Chip for Rapid Transport of Cryoprotectants across Cell Membrane for Cryopreservation with Significantly Improved Cell Viability

Cryopreservation is essential to effectively extend the shelf life of delicate biomaterials while maintaining proper levels of cell functions. Cryopreservation requires a cryoprotective agent (CPA) to suppress intracellular ice formation during freezing, but it must be removed prior to clinical use due to its toxicity. Conventional multistep CPA loading and unloading approaches are time consuming, often creating osmotic shocks and causing mechanical injuries for biological samples. An efficient surface-acoustic-wave- (SAW-) based lab-on-a-chip (LoC) for fast loading and removal of CPAs is presented here. With the SAW-based multistep CPA loading/removal approach, high concentration (3 m) CPA can be successfully loaded and removed in less than 1 min. Results show that the technique causes the least harm to umbilical cord matrix mesenchymal stem cells as compared to conventional method, and an average of 24% higher cell recovery rate is achieved, while preserving the integrity and morphology of the cells. This device is the first of its kind to combine high loading/unloading efficiency, high cell viability, and high throughput into one LoC device, offering not only a more efficient and safer route for CPA loading and removal from cells, but also paving the way for other cryopreservation-dependent applications.

Microfluidic Templated Multicompartment Microgels for 3D Encapsulation and Pairing of Single Cells

Controlled encapsulation and pairing of single cells within a confined 3D matrix can enable the replication of the highly ordered cellular structure of human tissues. Microgels with independently controlled compartments that can encapsulate cells within separately confined hydrogel matrices would provide precise control over the route of pairing single cells. Here, a one-step microfluidic method is presented to generate monodisperse multicompartment microgels that can be used as a 3D matrix to pair single cells in a highly biocompatible manner. A method is presented to induce microgels formation on chip, followed by direct extraction of the microgels from oil phase, thereby avoiding prolonged exposure of the microgels to the oil. It is further demonstrated that by entrapping stem cells with niche cells within separate but adjacent compartments of the microgels, it can create complex stem cell niche microenvironments in a controlled manner, which can serve as a useful tool for the study of cell-cell interactions. This microfluidic technique represents a significant step toward high-throughput single cells encapsulation and pairing for the study of intercellular communications at single cell level, which is of significant importance for cell biology, stem cell therapy, and tissue engineering.

Label-Free On-Chip Selective Extraction of Cell-Aggregate-Laden Microcapsules from Oil into Aqueous Solution with Optical Sensor and Dielectrophoresis

Microfluidic encapsulation of cells or tissues in biocompatible solidlike hydrogels has wide biomedical applications. However, the microfluidically encapsulated cells/tissues are usually suspended in oil and need to be extracted into aqueous solution for further culture or use. Current extracting techniques are either nonselective for the cell/tissue-laden hydrogel microcapsules or rely on fluorescence labeling of the cells/tissues, which may be undesired for their further culture or use. Here we developed a microelectromechanical system (MEMS) to achieve label-free on-chip selective extraction of cell-aggregate-laden hydrogel microcapsules from oil into aqueous solution. The system includes a microfluidic device, an optical sensor, a dielectrophoretic (DEP) actuator, and microcontrollers. The microfluidic device is for encapsulating cell aggregates in hydrogel microcapsules using the flow-focusing function with microchannels for extracting microcapsules. The optical sensor is to detect the cell aggregates, based on the difference of the optical properties between the cell aggregates and surrounding solution before their encapsulation in hydrogel microcapsules. This strategy is used because the difference in optical property between the cell-aggregate-laden hydrogel microcapsules and empty microcapsules is too small to tell them apart with a commonly used optical sensor. The DEP actuator, which is controlled by the sensor and microcontrollers, is for selectively extracting the targeted hydrogel microcapsules by DEP force. The results indicate this system can achieve selective extraction of cell-aggregate-laden hydrogel microcapsules with ∼100% efficiency without compromising the cell viability, and can improve the purity of the cell-aggregate-laden microcapsules by more than 75 times compared with nonselective extraction.

Generation and manipulation of hydrogel microcapsules by droplet-based microfluidics for mammalian cell culture

Hydrogel microcapsules provide miniaturized and biocompatible niches for three-dimensional (3D) in vitro cell culture. They can be easily generated by droplet-based microfluidics with tunable size, morphology, and biochemical properties. Therefore, microfluidic generation and manipulation of cell-laden microcapsules can be used for 3D cell culture to mimic the in vivo environment towards applications in tissue engineering and high throughput drug screening. In this review of recent advances mainly since 2010, we will first introduce general characteristics of droplet-based microfluidic devices for cell encapsulation with an emphasis on the fluid dynamics of droplet breakup and internal mixing as they directly influence microcapsule's size and structure. We will then discuss two on-chip manipulation strategies: sorting and extraction from oil into aqueous phase, which can be integrated into droplet-based microfluidics and significantly improve the qualities of cell-laden hydrogel microcapsules. Finally, we will review various applications of hydrogel microencapsulation for 3D in vitro culture on cell growth and proliferation, stem cell differentiation, tissue development, and co-culture of different types of cells.

Microfluidic Encapsulation of Ovarian Follicles for 3D Culture

The ovarian follicle that contains one single oocyte is the fundamental functional tissue unit of mammalian ovary. Therefore, isolation and in vitro culture of ovarian follicles to obtain fertilizable oocytes are regarded as a promising strategy for women to combat infertility. In this communication, we performed a brief survey of studies on microfluidic encapsulation of ovarian follicles in core-shell hydrogel microcapsules for biomimetic 3D culture. These studies highlighted that recapitulation of the mechanical heterogeneity of the extracellular matrix in ovary is crucial for in vitro culture to develop early pre-antral follicles to the antral stage, and for the release of cumulus-oocyte complex (COC) from antral follicles in vitro. The hydrogel encapsulation-based biomimetic culture system and the microfluidic technology may be invaluable to facilitate follicle culture as a viable option for restoring women's fertility in the clinic.

One step fabrication of hydrogel microcapsules with hollow core for assembly and cultivation of hepatocyte spheroids

3D hepatic microtissues can serve as valuable liver analogues for cell-based therapies and for hepatotoxicity screening during preclinical drug development. However, hepatocytes rapidly dedifferentiate in vitro, and typically require 3D culture systems or co-cultures for phenotype rescue. In this work we present a novel microencapsulation strategy, utilizing coaxial flow-focusing droplet microfluidics to fabricate microcapsules with liquid core and poly(ethylene glycol) (PEG) gel shell. When entrapped inside these capsules, primary hepatocytes rapidly formed cell-cell contacts and assembled into compact spheroids. High levels of hepatic function were maintained inside the capsules for over ten days. The microencapsulation approach described here is compatible with difficult-to-culture primary epithelial cells, allows for tuning gel mechanical properties and diffusivity, and may be used in the future for high density suspension cell cultures.

STATEMENT OF SIGNIFICANCE

Our paper combines an interesting new way for making capsules with cultivation of difficult-to-maintain primary epithelial cells (hepatocytes). The microcapsules described here will enable high density suspension culture of hepatocytes or other cells and may be used as building blocks for engineering tissues.

Microscale Biomaterials with Bioinspired Complexity of Early Embryo Development and in the Ovary for Tissue Engineering and Regenerative Medicine

Tissue engineering and regenerative medicine (TERM) are attracting more and more attention for treating various diseases in modern medicine. Various biomaterials including hydrogels and scaffolds have been developed to prepare cells (particularly stem cells) and tissues under 3D conditions for TERM applications. Although these biomaterials are usually homogeneous in early studies, effort has been made recently to generate biomaterials with the spatiotemporal complexities present in the native milieu of the specific cells and tissues under investigation. In this communication, the microfluidic and coaxial electrospray approaches that we used for generating microscale biomaterials with the spatial complexity of both pre-hatching embryos and ovary in the female reproductive system were introduced. This is followed by an overview of our recent work on applying the resultant bioinspired biomaterials for cultivation of normal and cancer stem cells, regeneration of cardiac tissue, and culture of ovarian follicles. The cardiac regeneration studies show the importance of using different biomaterials to engineer stem cells at different stages (i.e., in vitro culture versus in vivo implantation) for tissue regeneration. All the studies demonstrate the merit of accounting for bioinspired complexities in engineering cells and tissues for TERM applications.

In vitro culture of ovarian follicles from Peromyscus

The ovarian follicle is the fundamental functional tissue unit of mammalian ovary. Each ovarian follicle contains one single oocyte. Isolation and in vitro culture of ovarian follicles to obtain fertilizable oocytes have been regarded as a promising strategy for women to combat infertility. The follicles from Peromyscus are considered as a better model than that from inbred mice for studying follicle culture. This is because Peromyscus mice are outbred (as with humans) with an increased life span. In this article, we reviewed studies on this subject conducted using Peromyscus follicles. These studies show that the conventional 2D micro-drop and 3D hanging-drop approaches established for in vitro culture of early preantral follicles from inbred mice are not directly applicable for cultivating the follicles from Peromyscus. However, the efficiency could be significantly improved by culturing multiple early preantral follicles in one hanging drop of Peromyscus ovarian cell-conditioned medium. It is further revealed that the mechanical heterogeneity in the extracellular matrix of ovary is crucial for developing early preantral follicles to the antral stage and for the subsequent ovulation to release cumulus-oocyte complex. These findings may provide valuable guidance for furthering the technology of in vitro follicle culture to restore fertility in the clinic.

The effect of RGD peptide on 2D and miniaturized 3D culture of HEPM cells, MSCs, and ADSCs with alginate hydrogel

Advancements in tissue engineering require the development of new technologies to study cell behavior in vitro. This study focuses on stem cell behavior within various miniaturized three-dimensional (3D) culture conditions of alginate biomaterials modified with the Arg-Gly-Asp (RGD) peptide known for its role in cell adhesion/attachment. Human embryonic palatal mesenchyme (HEPM) cells, bone marrow derived mesenchymal stem cells (MSCs), and human adipose derived stem cells (ADSCs) were cultured on a flat hydrogel of different concentrations of alginate-RGD, and in the miniaturized 3D core of microcapsules with either a 2% alginate or 2% alginate-RGD shell. The core was made of 0%, 0.5%, or 2% alginate-RGD. Cell spreading was observed in all systems containing the RGD peptide, and the cell morphology was quantified by measuring the cell surface area and circularity. In all types of stem cells, there was a significant increase in the cell surface area (p < 0.05) and a significant decrease in cell circularity (p < 0.01) in alginate-RGD conditions, indicating that cells spread much more readily in environments containing the peptide. This control over the cell spreading within a 3D microenvironment can help to create the ideal biomimetic condition in which to conduct further studies on cell behavior.

Alginate Hydrogel Microencapsulation Inhibits Devitrification and Enables Large-Volume Low-CPA Cell Vitrification

Cryopreservation of stem cells is important to meet their ever-increasing demand by the burgeoning cell-based medicine. The conventional slow freezing for stem cell cryopreservation suffers from inevitable cell injury associated with ice formation and the vitrification (i.e., no visible ice formation) approach is emerging as a new strategy for cell cryopreservation. A major challenge to cell vitrification is intracellular ice formation (IIF, a lethal event to cells) induced by devitrification (i.e., formation of visible ice in previously vitrified solution) during warming the vitrified cells at cryogenic temperature back to super-zero temperatures. Consequently, high and toxic concentrations of penetrating cryoprotectants (i.e., high CPAs, up to ~8 M) and/or limited sample volumes (up to ~2.5 μl) have been used to minimize IIF during vitrification. We reveal that alginate hydrogel microencapsulation can effectively inhibit devitrification during warming. Our data show that if ice formation were minimized during cooling, IIF is negligible in alginate hydrogel-microencapsulated cells during the entire cooling and warming procedure of vitrification. This enables vitrification of pluripotent and multipotent stem cells with up to ~4 times lower concentration of penetrating CPAs (up to 2 M, low CPA) in up to ~100 times larger sample volume (up to ~250 μl, large volume).

Stiffness-Independent Highly Efficient On-Chip Extraction of Cell-Laden Hydrogel Microcapsules from Oil Emulsion into Aqueous Solution by Dielectrophoresis

A dielectrophoresis (DEP)-based method achieves highly efficient on-chip extraction of cell-laden microcapsules of any stiffness from oil into aqueous solution. The hydrogel microcapsules can be extracted into the aqueous solution by DEP and interfacial tension forces with no trapped oil, while the encapsulated cells are free from electrical damage due to the Faraday cage effect.

A Biomimetic Core-Shell Platform for Miniaturized 3D Cell and Tissue Engineering

This article describes a biomimetic core-shell platform with a collagen-based core and an alginate hydrogel shell for cell and tissue culture. With this system, chemical and physical properties of extracellular matrix (ECM) in the core microenvironment can be controlled to regulate proliferation and development of cells/tissues under miniaturized three-dimensional (3D) culture.

Interfacial tension based on-chip extraction of microparticles confined in microfluidic Stokes flows

Microfluidics involving two immiscible fluids (oil and water) has been increasingly used to produce hydrogel microparticles with wide applications. However, it is difficult to extract the microparticles out of the microfluidic Stokes flows of oil that have a Reynolds number (the ratio of inertia to viscous force) much less than one, where the dominant viscous force tends to drive the microparticles to move together with the surrounding oil. Here, we present a passive method for extracting hydrogel microparticles in microfluidic Stokes flow from oil into aqueous extracting solution on-chip by utilizing the intrinsic interfacial tension between oil and the microparticles. We further reveal that the thickness of an "extended confining layer" of oil next to the interface between oil and aqueous extracting solution must be smaller than the radius of microparticles for effective extraction. This method uses a simple planar merging microchannel design that can be readily fabricated and further integrated into a fluidic system to extract microparticles for wide applications.