Stem cells in microfluidics

A neurovascular unit-on-a-chip: culture and differentiation of human neural stem cells in a three-dimensional microfluidic environment

Biological studies typically rely on a simple monolayer cell culture, which does not reflect the complex functional characteristics of human tissues and organs, or their real response to external stimuli. Microfluidic technology has advantages of high-throughput screening, accurate control of the fluid velocity, low cell consumption, long-term culture, and high integration. By combining the multipotential differentiation of neural stem cells with high throughput and the integrated characteristics of microfluidic technology, an in vitro model of a functionalized neurovascular unit was established using human neural stem cell-derived neurons, astrocytes, oligodendrocytes, and a functional microvascular barrier. The model comprises a multi-layer vertical neural module and vascular module, both of which were connected with a syringe pump. This provides controllable conditions for cell inoculation and nutrient supply, and simultaneously simulates the process of ischemic/hypoxic injury and the process of inflammatory factors in the circulatory system passing through the blood-brain barrier and then acting on the nerve tissue in the brain. The in vitro functionalized neurovascular unit model will be conducive to central nervous system disease research, drug screening, and new drug development.

Microfluidics for Neuronal Cell and Circuit Engineering

The widespread adoption of microfluidic devices among the neuroscience and neurobiology communities has enabled addressing a broad range of questions at the molecular, cellular, circuit, and system levels. Here, we review biomedical engineering approaches that harness the power of microfluidics for bottom-up generation of neuronal cell types and for the assembly and analysis of neural circuits. Microfluidics-based approaches are instrumental to generate the knowledge necessary for the derivation of diverse neuronal cell types from human pluripotent stem cells, as they enable the isolation and subsequent examination of individual neurons of interest. Moreover, microfluidic devices allow to engineer neural circuits with specific orientations and directionality by providing control over neuronal cell polarity and permitting the isolation of axons in individual microchannels. Similarly, the use of microfluidic chips enables the construction not only of 2D but also of 3D brain, retinal, and peripheral nervous system model circuits. Such brain-on-a-chip and organoid-on-a-chip technologies are promising platforms for studying these organs as they closely recapitulate some aspects of in vivo biological processes. Microfluidic 3D neuronal models, together with 2D in vitro systems, are widely used in many applications ranging from drug development and toxicology studies to neurological disease modeling and personalized medicine. Altogether, microfluidics provide researchers with powerful systems that complement and partially replace animal models.

Gradient biomimetic platforms for neurogenesis studies

There is a need for the development of new cellular therapies for the treatment of many diseases, with the central nervous system (CNS) currently an area of specific focus. Due to the complexity and delicacy of its biology, there is currently a limited understanding of neurogenesis and consequently a lack of reliable test platforms, resulting in several CNS based diseases having no cure. The ability to differentiate pluripotent stem cells into specific neuronal sub-types may enable scalable manufacture for clinical therapies, with a focus also on the purity and quality of the cell population. This focus is targeted towards an urgent need for the diseases that currently have no cure, e.g. Parkinson's disease. Differentiation studies carried out using traditional 2D cell culture techniques are designed using biological signals and morphogens known to be important for neurogenesis in vivo. However, such studies are limited by their simplistic nature, including a general poor efficiency and reproducibility, high reagent costs and an inability to scale-up the process to a manufacture-wide design for clinical use. Biomimetic approaches to recapitulate a more in vivo-like environment are progressing rapidly within this field, with application of bio(chemical) gradients presented both as 2D surfaces and within a 3D volume. This review focusses on the development and application of these advanced extracellular environments particularly for the neural niche. We emphasise the progress that has been made specifically in the area of stem cell derived neuronal differentiation. Increasing developments in biomaterial approaches to manufacture stem cells will enable the improvement of differentiation protocols, enhancing the efficiency and repeatability of the process with a move towards up-scaling. Progress in this area brings these techniques closer to enabling the development of therapies for the clinic.

Human Blood Serum Induces p38-MAPK- and Hsp27-Dependent Migration Dynamics of Adult Human Cardiac Stem Cells: Single-Cell Analysis via a Microfluidic-Based Cultivation Platform

Simple Summary

Adult human stem cells possess the ability to contribute to endogenous regeneration processes of injured tissue by migrating to specific locations. For stem cell-based clinical applications it is highly important to gain knowledge about the migration behavior of adult human stem cells and the underlying molecular mechanisms of this ability. Human blood serum has been shown to have beneficial effects on other regenerative capacities of adult human stem cells. Within this study we tested the effect of human blood serum on the migration behavior of stem cells from the human heart. We used a microfluidic cultivation device, which allowed us to monitor the living cells and their movement behavior in real time. After addition of human blood serum, the heart stem cells increased their speed of movement and covered distance. Further, we observed that this effect could be diminished by inhibition of a specific kinase, p38-MAPK. Thus, our data suggest beneficial effects of human blood serum on adult human heart stem cells dependent on p38-MAPK. Our study contributes to a deeper understanding of the dynamics of stem cell migration and introduces a new platform to monitor stem cell movement in real time.

Abstract

Migratory capabilities of adult human stem cells are vital for assuring endogenous tissue regeneration and stem cell-based clinical applications. Although human blood serum has been shown to be beneficial for cell migration and proliferation, little is known about its impact on the migratory behavior of cardiac stem cells and underlying signaling pathways. Within this study, we investigated the effects of human blood serum on primary human cardiac stem cells (hCSCs) from the adult heart auricle. On a technical level, we took advantage of a microfluidic cultivation platform, which allowed us to characterize cell morphologies and track migration of single hCSCs via live cell imaging over a period of up to 48 h. Our findings showed a significantly increased migration distance and speed of hCSCs after treatment with human serum compared to control. Exposure of blood serum-stimulated hCSCs to the p38 mitogen-activated protein kinase (p38-MAPK) inhibitor SB239063 resulted in significantly decreased migration. Moreover, we revealed increased phosphorylation of heat shock protein 27 (Hsp27) upon serum treatment, which was diminished by p38-MAPK-inhibition. In summary, we demonstrate human blood serum as a strong inducer of adult human cardiac stem cell migration dependent on p38-MAPK/Hsp27-signalling. Our findings further emphasize the great potential of microfluidic cultivation devices for assessing spatio-temporal migration dynamics of adult human stem cells on a single-cell level.

Microfluidics as efficient technology for the isolation and characterization of stem cells

The recent years have been passed with significant progressions in the utilization of microfluidic technologies for cellular investigations. The aim of microfluidics is to mimic small-scale body environment with features like optical transparency. Microfluidics can screen and monitor different cell types during culture and study cell function in response to stimuli in a fully controlled environment. No matter how the microfluidic environment is similar to in vivo environment, it is not possible to fully investigate stem cells behavior in response to stimuli during cell proliferation and differentiation. Researchers have used stem cells in different fields from fundamental researches to clinical applications. Many cells in the body possess particular functions, but stem cells do not have a specific task and can turn into almost any type of cells. Stem cells are undifferentiated cells with the ability of changing into specific cells that can be essential for the body. Researchers and physicians are interested in stem cells to use them in testing the function of the body's systems and solving their complications. This review discusses the recent advances in utilizing microfluidic techniques for the analysis of stem cells, and mentions the advantages and disadvantages of using microfluidic technology for stem cell research.

An integrated microfluidic device for stem cell differentiation based on cell-imprinted substrate designed for cartilage regeneration in a rabbit model

Separating cells from the body and cultivating them in vitro will alter the function of cells. Therefore, for optimal cell culture in the laboratory, conditions similar to those of their natural growth should be provided. In previous studies, it has been shown that the use of cellular shape at the culture surface can regulate cellular function. In this work, the efficiency of the imprinting method increased by using microfluidic chip design and fabrication. In this method, first, a cell-imprinted substrate of chondrocytes was made using a microfluidic chip. Afterwards, stem cells were cultured on a cell-imprinted substrate using a second microfluidic chip aligned with the substrate. Therefore, stem cells were precisely placed on the chondrocyte patterns on the substrate and their fibroblast-like morphology was changed to chondrocyte's spherical morphology after 14-days culture in the chip without using any chemical growth factor. After chondrogenic differentiation and in vitro assessments (real-time PCR and immunocytotoxicity), differentiated stem cells were transferred on a collagen-hyaluronic acid scaffold and transplanted in articular cartilage defect of the rabbit. After 6 months, the post-transplantation analysis showed that the articular cartilage defect had been successfully regenerated in differentiated stem cell groups in comparison with the controls. In conclusion, this study showed the potency of the imprinting method for inducing chondrogenicity in stem cells, which can be used in clinical trials due to the safety of the procedure.

Spatially controlled stem cell differentiation via morphogen gradients: A comparison of static and dynamic microfluidic platforms

The ability to harness the processes by which complex tissues arise during embryonic development would improve the ability to engineer complex tissuelike constructs in vitro-a longstanding goal of tissue engineering and regenerative medicine. In embryos, uniform populations of stem cells are exposed to spatial gradients of diffusible extracellular signaling proteins, known as morphogens. Varying levels of these signaling proteins induce stem cells to differentiate into distinct cell types at different positions along the gradient, thus creating spatially patterned tissues. Here, the authors describe two straightforward and easy-to-adopt microfluidic strategies to expose human pluripotent stem cells in vitro to spatial gradients of desired differentiation-inducing extracellular signals. Both approaches afford a high degree of control over the distribution of extracellular signals, while preserving the viability of the cultured stem cells. The first microfluidic platform is commercially available and entails static culture, whereas the second microfluidic platform requires fabrication and dynamic fluid exchange. In each platform, the authors first computationally modeled the spatial distribution of differentiation-inducing extracellular signals. Then, the authors used each platform to expose human pluripotent stem cells to a gradient of these signals (in this case, inducing a cell type known as the primitive streak), resulting in a regionalized culture with differentiated primitive streak cells predominately localized on one side and undifferentiated stem cells at the other side of the device. By combining this approach with a fluorescent reporter for differentiated cells and live-cell fluorescence imaging, the authors characterized the spatial and temporal dynamics of primitive streak differentiation within the induced signaling gradients. Microfluidic approaches to create precisely controlled morphogen gradients will add to the stem cell and developmental biology toolkit, and may eventually pave the way to create increasingly spatially patterned tissuelike constructs in vitro.

Spatially controlled stem cell differentiation via morphogen gradients: A comparison of static and dynamic microfluidic platforms

The ability to harness the processes by which complex tissues arise during embryonic development would improve the ability to engineer complex tissuelike constructs in vitro-a longstanding goal of tissue engineering and regenerative medicine. In embryos, uniform populations of stem cells are exposed to spatial gradients of diffusible extracellular signaling proteins, known as morphogens. Varying levels of these signaling proteins induce stem cells to differentiate into distinct cell types at different positions along the gradient, thus creating spatially patterned tissues. Here, the authors describe two straightforward and easy-to-adopt microfluidic strategies to expose human pluripotent stem cells in vitro to spatial gradients of desired differentiation-inducing extracellular signals. Both approaches afford a high degree of control over the distribution of extracellular signals, while preserving the viability of the cultured stem cells. The first microfluidic platform is commercially available and entails static culture, whereas the second microfluidic platform requires fabrication and dynamic fluid exchange. In each platform, the authors first computationally modeled the spatial distribution of differentiation-inducing extracellular signals. Then, the authors used each platform to expose human pluripotent stem cells to a gradient of these signals (in this case, inducing a cell type known as the primitive streak), resulting in a regionalized culture with differentiated primitive streak cells predominately localized on one side and undifferentiated stem cells at the other side of the device. By combining this approach with a fluorescent reporter for differentiated cells and live-cell fluorescence imaging, the authors characterized the spatial and temporal dynamics of primitive streak differentiation within the induced signaling gradients. Microfluidic approaches to create precisely controlled morphogen gradients will add to the stem cell and developmental biology toolkit, and may eventually pave the way to create increasingly spatially patterned tissuelike constructs in vitro.

A Potential Application of Triangular Microwells to Entrap Single Cancer Cells: A Canine Cutaneous Mast Cell Tumor Model

Cellular heterogeneity is a major hindrance, leading to the misunderstanding of dynamic cell biology. However, single cell analysis (SCA) has been used as a practical means to overcome this drawback. Many contemporary methodologies are available for single cell analysis; among these, microfluidics is the most attractive and effective technology, due to its advantages of low-volume specimen consumption, label-free evaluation, and real-time monitoring, among others. In this paper, a conceptual application for microfluidic single cell analysis for veterinary research is presented. A microfluidic device is fabricated with an elastomer substrate, polydimethylsiloxane (PDMS), under standard soft lithography. The performance of the microdevice is high-throughput, sensitive, and user-friendly. A total of 53.1% of the triangular microwells were able to trap single canine cutaneous mast cell tumor (MCT) cells. Of these, 38.82% were single cell entrapments, while 14.34% were multiple cell entrapments. The ratio of single-to-multiple cell trapping was high, at 2.7:1. In addition, 80.5% of the trapped cells were viable, indicating that the system was non-lethal. OCT4A-immunofluorescence combined with the proposed system can assess OCT4A expression in trapped single cells more precisely than OCT4A-immunohistochemistry. Therefore, the results suggest that microfluidic single cell analysis could potentially reduce the impact of cellular heterogeneity.

Mesenchymal stem cell 3D encapsulation technologies for biomimetic microenvironment in tissue regeneration

Mesenchymal stem cell (MSC) encapsulation technique has long been emerged in tissue engineering as it plays an important role in implantation of stem cells to regenerate a damaged tissue. MSC encapsulation provides a mimic of a three-dimensional (3D) in vivo environment to maintain cell viability and to induce the stem cell differentiation which regulates MSC fate into multi-lineages. Moreover, the 3D matrix surrounding MSCs protects them from the human innate immune system and allows the diffusion of biomolecules such as oxygen, cytokines, and growth factors. Therefore, many technologies are being developed to create MSC encapsulation platforms with diverse materials, shapes, and sizes. The conditions of the platform are determined by the targeted tissue and translation method. This review introduces several details of MSC encapsulation technologies such as micromolding, electrostatic droplet extrusion, microfluidics, and bioprinting and their application for tissue regeneration. Lastly, some of the challenges and future direction of MSC encapsulation technologies as a cell therapy-based tissue regeneration method will be discussed.

Viable cell culture in PDMS-based microfluidic devices

Microfluidics has played a vital role in developing novel methods to investigate biological phenomena at the molecular and cellular level during the last two decades. Microscale engineering of cellular systems is nevertheless a nascent field marked inherently by frequent disruptive advancements in technology such as PDMS-based soft lithography. Viable culture and manipulation of cells in microfluidic devices requires knowledge across multiple disciplines including molecular and cellular biology, chemistry, physics, and engineering. There has been numerous excellent reviews in the past 15 years on applications of microfluidics for molecular and cellular biology including microfluidic cell culture (Berthier et al., 2012; El-Ali, Sorger, & Jensen, 2006; Halldorsson et al., 2015; Kim et al., 2007; Mehling & Tay, 2014; Sackmann et al., 2014; Whitesides, 2006; Young & Beebe, 2010), cell culture models (Gupta et al., 2016; Inamdar & Borenstein, 2011; Meyvantsson & Beebe, 2008), cell secretion (Schrell et al., 2016), chemotaxis (Kim & Wu, 2012; Wu et al., 2013), neuron culture (Millet & Gillette, 2012a, 2012b), drug screening (Dittrich & Manz, 2006; Eribol, Uguz, & Ulgen, 2016; Wu, Huang, & Lee, 2010), cell sorting (Autebert et al., 2012; Bhagat et al., 2010; Gossett et al., 2010; Wyatt Shields Iv, Reyes, & López, 2015), single cell studies (Lecault et al., 2012; Reece et al., 2016; Yin & Marshall, 2012), stem cell biology (Burdick & Vunjak-Novakovic, 2009; Wu et al., 2011; Zhang & Austin, 2012), cell differentiation (Zhang et al., 2017a), systems biology (Breslauer, Lee, & Lee, 2006), 3D cell culture (Huh et al., 2011; Li et al., 2012; van Duinen et al., 2015), spheroids and organoids (Lee et al., 2016; Montanez-Sauri, Beebe, & Sung, 2015; Morimoto & Takeuchi, 2013; Skardal et al., 2016; Young, 2013), organ-on-chip (Bhatia & Ingber, 2014; Esch, Bahinski, & Huh, 2015; Huh et al., 2011; van der Meer & van den Berg, 2012), and tissue engineering (Andersson & Van Den Berg, 2004; Choi et al., 2007; Hasan et al., 2014). In this chapter, we provide an overview of PDMS-based microdevices for microfluidic cell culture. We discuss the advantages and challenges of using PDMS-based soft lithography for microfluidic cell culture and highlight recent progress and future directions in this area.

Caring for cells in microsystems: principles and practices of cell-safe device design and operation

Microfluidic device designers and users continually question whether cells are 'happy' in a given microsystem or whether they are perturbed by micro-scale technologies. This issue is normally brought up by engineers building platforms, or by external reviewers (academic or commercial) comparing multiple technological approaches to a problem. Microsystems can apply combinations of biophysical and biochemical stimuli that, although essential to device operation, may damage cells in complex ways. However, assays to assess the impact of microsystems upon cells have been challenging to conduct and have led to subjective interpretation and evaluation of cell stressors, hampering development and adoption of microsystems. To this end, we introduce a framework that defines cell health, describes how device stimuli may stress cells, and contrasts approaches to measure cell stress. Importantly, we provide practical guidelines regarding device design and operation to minimize cell stress, and recommend a minimal set of quantitative assays that will enable standardization in the assessment of cell health in diverse devices. We anticipate that as microsystem designers, reviewers, and end-users enforce such guidelines, we as a community can create a set of essential principles that will further the adoption of such technologies in clinical, translational and commercial applications.

Translational Application of Microfluidics and Bioprinting for Stem Cell-Based Cartilage Repair

Cartilage defects can impair the most elementary daily activities and, if not properly treated, can lead to the complete loss of articular function. The limitations of standard treatments for cartilage repair have triggered the development of stem cell-based therapies. In this scenario, the development of efficient cell differentiation protocols and the design of proper biomaterial-based supports to deliver cells to the injury site need to be addressed through basic and applied research to fully exploit the potential of stem cells. Here, we discuss the use of microfluidics and bioprinting approaches for the translation of stem cell-based therapy for cartilage repair in clinics. In particular, we will focus on the optimization of hydrogel-based materials to mimic the articular cartilage triggered by their use as bioinks in 3D bioprinting applications, on the screening of biochemical and biophysical factors through microfluidic devices to enhance stem cell chondrogenesis, and on the use of microfluidic technology to generate implantable constructs with a complex geometry. Finally, we will describe some new bioprinting applications that pave the way to the clinical use of stem cell-based therapies, such as scaffold-free bioprinting and the development of a 3D handheld device for the in situ repair of cartilage defects.

Deformability-based microfluidic separation of pancreatic islets from exocrine acinar tissue for transplant applications

The long-term management of type-1 diabetes (T1D) is currently achieved through lifelong exogenous insulin injections. Although there is no cure for T1D, transplantation of pancreatic islets of Langerhans has the potential to restore normal endocrine function versus the morbidity of hypoglycemic unawareness that is commonly associated with sudden death among fragile diabetics. However, since endocrine islet tissues form a small proportion of the pancreas, sufficient islet numbers can be reached only by combining islets from multiple organ donors and the transplant plug contains significantly high levels of exocrine acinar tissue, thereby exacerbating immune responses. Hence, lifelong administration of immunosuppressants is required after transplantation, which can stress islet cells. The density gradient method that is currently used to separate islets from acinar tissue causes islets to be sparsely distributed over the centrifuged bins, so that the transplant sample obtained by combining multiple bins also contains significant acinar tissue levels. We show that in comparison to the significant size and density overlaps between the islet and acinar tissue populations post-organ digestion, their deformability overlaps are minimal. This feature is utilized to design a microfluidic separation strategy, wherein tangential flows enable selective deformation of acinar populations towards the bifurcating waste stream and sequential switching of hydrodynamic resistance enables the collection of rigid islets. Using 25 bifurcating daughter channels, a throughput of ∼300 islets per hour per device is obtained for enabling islet enrichment from relatively dilute starting levels to purity levels that meet the transplant criteria, as well as to further enhance islet purity from samples following density gradient enrichment. Based on confirmation of viability and functionality of the microfluidic-isolated islets using insulin secretion analysis and an angiogenesis assay, we envision utilizing this strategy to generate small-volume transplant plugs with high islet purity and significantly reduced acinar levels for minimizing immune responses after transplantation.

In situ generation of human brain organoids on a micropillar array

Brain organoids derived from human induced pluripotent stem cells can recapitulate the early stages of brain development, representing a powerful in vitro system for modeling brain development and diseases. However, the existing methods for brain organoid formation often require time-consuming procedures, including the initial formation of embryoid bodies (EBs) from hiPSCs, and subsequent neural induction and differentiation companied by multi-steps of cell transfer and encapsulation in a 3D matrix. Herein, we propose a simple strategy to enable in situ formation of massive brain organoids from hiPSCs on a micropillar array without tedious manual procedures. The optimized micropillar configurations allow for controlled EB formation, neural induction and differentiation, and generation of functional human brain organoids in 3D culture on a single device. The generated brain organoids were examined to imitate brain organogenesis in vivo at early stages of gestation with specific features of neuronal differentiation, brain regionalization, and cortical organization. By combining microfabrication techniques with stem cells and developmental biology principles, the proposed method can greatly simplify brain organoid formation protocols as compared to conventional methods, overcoming the potential limitations of cell contamination, lower throughput and variance of organoid morphology. It can also provide a useful platform for the engineering of stem cell organoids with improved functions and extending their applications in developmental biology, drug testing and disease modeling.

Automated Cell Culture Systems and Their Applications to Human Pluripotent Stem Cell Studies

Pluripotent stem cells are an extremely powerful tool in modeling human diseases and hold much promise for personalized regenerative or cell replacement therapies. There is an increasing need for reproducible large-scale stem cell and differentiated progeny production, with minimal variation, rendering manual approaches impracticable. Here, we provide an overview of systems currently available for automated stem cell culture, and undertake a review of their capacities, capabilities, and relative limitations. With the merging of modern technology and stem cell biology, an increased demand and implementation of automated platforms for stem cell studies is anticipated.

Stem cell culture and differentiation in microfluidic devices toward organ-on-a-chip

Microfluidic lab-on-a-chip provides a new platform with unique advantages to mimic complex physiological microenvironments in vivo and has been increasingly exploited to stem cell research. In this review, we highlight recent advances of microfluidic devices for stem cell culture and differentiation toward the development of organ-on-a-chip, especially with an emphasis on vital innovations within the last 2 years. Various aspects for improving on-chip stem-cell culture and differentiation, particularly toward organ-on-a-chip, are discussed, along with microenvironment control, surface modification, extracellular scaffolds, high throughput and stimuli. The combination of microfluidic technologies and stem cells hold great potential toward versatile systems of ‘organ-on-a-chip’ as desired.

Adapted with permission from [1–8].

Stem cells, capable of self-renewing and differentiating into cells of various tissue types, are drawing more and more attention for their enormous potential in many clinically associated applications that include drug screening, disease modeling and regenerative medicine. Conventional cell culture methods, however, have proven to be difficult to mimic in vivo like microenvironments and to provide a number of well-controlled stimuli that are critical for stem cell culture and differentiation. In contrast, microfluidic devices offer new capacities and unique advantages to mimic complex physiological microenvironments in vivo, and has been increasingly applied to stem cell research.

High-throughput screening of microchip-synthesized genes in programmable double-emulsion droplets

The rapid advances in synthetic biology and biotechnology are increasingly demanding high-throughput screening technology, such as screening of the functionalities of synthetic genes for optimization of protein expression. Compartmentalization of single cells in water-in-oil (W/O) emulsion droplets allows screening of a vast number of individualized assays, and recent advances in automated microfluidic devices further help realize the potential of droplet technology for high-throughput screening. However these single-emulsion droplets are incompatible with aqueous phase analysis and the inner droplet environment cannot easily communicate with the external phase. We present a high-throughput, miniaturized screening platform for microchip-synthesized genes using microfluidics-generated water-in-oil-in-water (W/O/W) double emulsion (DE) droplets that overcome these limitations. Synthetic gene variants of fluorescent proteins are synthesized with a custom-built microarray inkjet synthesizer, which are then screened for expression in Escherichia coli (E. coli) cells. Bacteria bearing individual fluorescent gene variants are encapsulated as single cells into DE droplets where fluorescence signals are enhanced by 100 times within 24 h of proliferation. Enrichment of functionally-correct genes by employing an error correction method is demonstrated by screening DE droplets containing fluorescent clones of bacteria with the red fluorescent protein (rfp) gene. Permeation of isopropyl β-d-1-thiogalactopyranoside (IPTG) through the thin oil layer from the external solution initiates target gene expression. The induced expression of the synthetic fluorescent proteins from at least ∼100 bacteria per droplet generates detectable fluorescence signals to enable fluorescence-activated cell sorting (FACS) of the intact droplets. This technology obviates time- and labor-intensive cell culture typically required in conventional bulk experiment.

Microfluidic Platform for the Continuous Production and Characterization of Multilamellar Vesicles: A Synchrotron Small-Angle X-ray Scattering (SAXS) Study

A microfluidic platform combined with synchrotron small-angle X-ray scattering (SAXS) was used for monitoring the continuous production of multilamellar vesicles (MLVs). Their production was fast and started to evolve within less than 0.43 s of contact between the lipids and the aqueous phase. To obtain nanoparticles with a narrow size distribution, it was important to use a modified hydrodynamic flow focusing (HFF) microfluidic device with narrower microchannels than those normally used for SAXS experiments. Monodispersed MLVs as small as 160 nm in size, with a polydispersity index (PDI) of approximately 0.15 were achieved. The nanoparticles produced were smaller and had a narrower size distribution than those obtained via conventional bulk mixing methods. This microfluidic platform therefore has a great potential for the continuous production of monodispersed NPs.

Microfluidic engineering of neural stem cell niches for fate determination

Neural stem cell (NSC) transplantation has great therapeutic potential for neurodegenerative diseases and central nervous system injuries. Successful NSC replacement therapy requires precise control over the cellular behaviors. However, the regulation of NSC fate is largely unclear, which severely restricts the potential clinical applications. To develop an effective model, we designed an assembled microfluidic system to engineer NSC niches and assessed the effects of various culture conditions on NSC fate determination. Five types of NSC microenvironments, including two-dimensional (2D) cellular monolayer culture, 2D cellular monolayer culture on the extracellular matrix (ECM), dispersed cells in the ECM, three-dimensional (3D) spheroid aggregates, and 3D spheroids cultured in the ECM, were constructed within an integrated microfluidic chip simultaneously. In addition, we evaluated the influence of static and perfusion culture on NSCs. The efficiency of this approach was evaluated comprehensively by characterization of NSC viability, self-renewal, proliferation, and differentiation into neurons, astrocytes, or oligodendrocytes. Differences in the status and fate of NSCs governed by the culture modes and micro-niches were analyzed. NSCs in the microfluidic device demonstrated good viability, the 3D culture in the ECM facilitated NSC self-renewal and proliferation, and 2D culture in the static state and spheroid culture under perfusion conditions benefited NSC differentiation. Regulation of NSC self-renewal and differentiation on this microfluidic device could provide NSC-based medicinal products and references for distinct nerve disease therapy.

Efficient generation of hepatic cells from mesenchymal stromal cells by an innovative bio-microfluidic cell culture device

Background

Mesenchymal stromal cells (MSCs) are multipotent and have great potential in cell therapy. Previously we reported the differentiation potential of human MSCs into hepatocytes in vitro and that these cells can rescue fulminant hepatic failure. However, the conventional static culture method neither maintains growth factors at an optimal level constantly nor removes cellular waste efficiently. In addition, not only is the duration of differentiating hepatocyte lineage cells from MSCs required to improve, but also the need for a large number of hepatocytes for cell therapy has not to date been addressed fully. The purpose of this study is to design and develop an innovative microfluidic device to overcome these shortcomings.

Methods

We designed and fabricated a microfluidic device and a culture system for hepatic differentiation of MSCs using our protocol reported previously. The microfluidic device contains a large culture chamber with a stable uniform flow to allow homogeneous distribution and expansion as well as efficient induction of hepatic differentiation for MSCs.

Results

The device enables real-time observation under light microscopy and exhibits a better differentiation efficiency for MSCs compared with conventional static culture. MSCs grown in the microfluidic device showed a higher level of hepatocyte marker gene expression under hepatic induction. Functional analysis of hepatic differentiation demonstrated significantly higher urea production in the microfluidic device after 21 days of hepatic differentiation.

Conclusions

The microfluidic device allows the generation of a large number of MSCs and induces hepatic differentiation of MSCs efficiently. The device can be adapted for scale-up production of hepatic cells from MSCs for cellular therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0371-7) contains supplementary material, which is available to authorized users.

Morphological plasticity of bacteria-Open questions

Morphological plasticity of bacteria is a cryptic phenomenon, by which bacteria acquire adaptive benefits for coping with changing environments. Some environmental cues were identified to induce morphological plasticity, but the underlying molecular mechanisms remain largely unknown. Physical and chemical factors causing morphological changes in bacteria have been investigated and mostly associated with potential pathways linked to the cell wall synthetic machinery. These include starvation, oxidative stresses, predation effectors, antimicrobial agents, temperature stresses, osmotic shock, and mechanical constraints. In an extreme scenario of morphological plasticity, bacteria can be induced to be shapeshifters when the cell walls are defective or deficient. They follow distinct developmental pathways and transform into assorted morphological variants, and most of them would eventually revert to typical cell morphology. It is suggested that phenotypic heterogeneity might play a functional role in the development of morphological diversity and/or plasticity within an isogenic population. Accordingly, phenotypic heterogeneity and inherited morphological plasticity are found to be survival strategies adopted by bacteria in response to environmental stresses. Here, microfluidic and nanofabrication technology is considered to provide versatile solutions to induce morphological plasticity, sort and isolate morphological variants, and perform single-cell analysis including transcriptional and epigenetic profiling. Questions such as how morphogenesis network is modulated or rewired (if epigenetic controls of cell morphogenesis apply) to induce bacterial morphological plasticity could be resolved with the aid of micro-nanofluidic platforms and optimization algorithms, such as feedback system control.

Simple surface engineering of polydimethylsiloxane with polydopamine for stabilized mesenchymal stem cell adhesion and multipotency

Polydimethylsiloxane (PDMS) has been extensively exploited to study stem cell physiology in the field of mechanobiology and microfluidic chips due to their transparency, low cost and ease of fabrication. However, its intrinsic high hydrophobicity renders a surface incompatible for prolonged cell adhesion and proliferation. Plasma-treated or protein-coated PDMS shows some improvement but these strategies are often short-lived with either cell aggregates formation or cell sheet dissociation. Recently, chemical functionalization of PDMS surfaces has proved to be able to stabilize long-term culture but the chemicals and procedures involved are not user- and eco-friendly. Herein, we aim to tailor greener and biocompatible PDMS surfaces by developing a one-step bio-inspired polydopamine coating strategy to stabilize long-term bone marrow stromal cell culture on PDMS substrates. Characterization of the polydopamine-coated PDMS surfaces has revealed changes in surface wettability and presence of hydroxyl and secondary amines as compared to uncoated surfaces. These changes in PDMS surface profile contribute to the stability in BMSCs adhesion, proliferation and multipotency. This simple methodology can significantly enhance the biocompatibility of PDMS-based microfluidic devices for long-term cell analysis or mechanobiological studies.

Sorting of human mesenchymal stem cells by applying optimally designed microfluidic chip filtration

Human bone marrow-derived mesenchymal stem cells (hMSCs) consist of heterogeneous subpopulations with different multipotent properties: small and large cells with high and low multipotency, respectively. Accordingly, sorting out a target subpopulation from the others is very important to increase the effectiveness of cell-based therapy. We performed flow-based sorting of hMSCs by using optimally designed microfluidic chips based on the hydrodynamic filtration (HDF) principle. The chip was designed with the parameters rigorously determined by the complete analysis of laminar flow for flow fraction and complicated networks of main and multi-branched channels for hMSCs sorting into three subpopulations: small (<25 μm), medium (25-40 μm), and large (>40 μm) cells. By focusing with a proper ratio between main and side flows, cells migrate toward the sidewall due to a virtual boundary of fluid layers and enter the branch channels. This opens the possibility of sorting stem cells rapidly without damage. Over 86% recovery was achieved for each population of cells with complete purity in small cells, but the sorting efficiency of cells is slightly lower than that of rigid model particles, due to the effect of cell deformation. Finally, we confirmed that our method could successfully fractionate the three subpopulations of hMSCs by analyzing the surface marker expressions of cells from each outlet.

Controlled electromechanical cell stimulation on-a-chip

Stem cell research has yielded promising advances in regenerative medicine, but standard assays generally lack the ability to combine different cell stimulations with rapid sample processing and precise fluid control. In this work, we describe the design and fabrication of a micro-scale cell stimulator capable of simultaneously providing mechanical, electrical, and biochemical stimulation, and subsequently extracting detailed morphological and gene-expression analysis on the cellular response. This micro-device offers the opportunity to overcome previous limitations and recreate critical elements of the in vivo microenvironment in order to investigate cellular responses to three different stimulations. The platform was validated in experiments using human bone marrow mesenchymal stem cells. These experiments demonstrated the ability for inducing changes in cell morphology, cytoskeletal fiber orientation and changes in gene expression under physiological stimuli. This novel bioengineering approach can be readily applied to various studies, especially in the fields of stem cell biology and regenerative medicine.

Measurement of single leukemia cell's density and mass using optically induced electric field in a microfluidics chip

We present a method capable of rapidly (∼20 s) determining the density and mass of a single leukemic cell using an optically induced electrokinetics (OEK) platform. Our team had reported recently on a technique that combines sedimentation theory, computer vision, and micro particle manipulation techniques on an OEK microfluidic platform to determine the mass and density of micron-scale entities in a fluidic medium; the mass and density of yeast cells were accurately determined in that prior work. In the work reported in this paper, we further refined the technique by performing significantly more experiments to determine a universal correction factor to Stokes' equation in expressing the drag force on a microparticle as it falls towards an infinite plane. Specifically, a theoretical model for micron-sized spheres settling towards an infinite plane in a microfluidic environment is presented, and which was validated experimentally using five different sizes of micro polystyrene beads. The same sedimentation process was applied to two kinds of leukemic cancer cells with similar sizes in an OEK platform, and their density and mass were determined accordingly. Our tests on mouse lymphocytic leukemia cells (L1210) and human leukemic cells (HL-60) have verified the practical viability of this method. Potentially, this new method provides a new way of measuring the volume, density, and mass of a single cell in an accurate, selective, and repeatable manner.

Characterizing the dielectric properties of human mesenchymal stem cells and the effects of charged elastin-like polypeptide copolymer treatment

HUMAN MESENCHYMAL STEM CELLS (HMSCS) HAVE THREE KEY PROPERTIES THAT MAKE THEM DESIRABLE FOR STEM CELL THERAPEUTICS: differentiation capacity, trophic activity, and ability to self-renew. However, current separation techniques are inefficient, time consuming, expensive, and, in some cases, alter hMSCs cellular function and viability. Dielectrophoresis (DEP) is a technique that uses alternating current electric fields to spatially separate biological cells based on the dielectric properties of their membrane and cytoplasm. This work implements the first steps toward the development of a continuous cell sorting microfluidic device by characterizing native hMSCs dielectric signatures and comparing them to hMSCs morphologically standardized with a polymer. A quadrapole Ti-Au electrode microdevice was used to observe hMSC DEP behaviors, and quantify frequency spectra and cross-over frequency of hMSCs from 0.010-35 MHz in dextrose buffer solutions (0.030 S/m and 0.10 S/m). This combined approach included a systematic parametric study to fit a core-shell model to the DEP spectra over the entire tested frequency range, adding robustness to the analysis technique. The membrane capacitance and permittivity were found to be 2.2 pF and 2.0 in 0.030 S/m and 4.5 pF and 4.1 in 0.10 S/m, respectively. Elastin-like polypeptide (ELP-) polyethyleneimine (PEI) copolymer was used to control hMSCs morphology to spheroidal cells and aggregates. Results demonstrated that ELP-PEI treatment controlled hMSCs morphology, increased experiment reproducibility, and concurrently increased hMSCs membrane permittivity to shift the cross-over frequency above 35 MHz. Therefore, ELP-PEI treatment may serve as a tool for the eventual determination of biosurface marker-dependent DEP signatures and hMSCs purification.

Advantages and challenges of microfluidic cell culture in polydimethylsiloxane devices

Culture of cells using various microfluidic devices is becoming more common within experimental cell biology. At the same time, a technological radiation of microfluidic cell culture device designs is currently in progress. Ultimately, the utility of microfluidic cell culture will be determined by its capacity to permit new insights into cellular function. Especially insights that would otherwise be difficult or impossible to obtain with macroscopic cell culture in traditional polystyrene dishes, flasks or well-plates. Many decades of heuristic optimization have gone into perfecting conventional cell culture devices and protocols. In comparison, even for the most commonly used microfluidic cell culture devices, such as those fabricated from polydimethylsiloxane (PDMS), collective understanding of the differences in cellular behavior between microfluidic and macroscopic culture is still developing. Moving in vitro culture from macroscopic culture to PDMS based devices can come with unforeseen challenges. Changes in device material, surface coating, cell number per unit surface area or per unit media volume may all affect the outcome of otherwise standard protocols. In this review, we outline some of the advantages and challenges that may accompany a transition from macroscopic to microfluidic cell culture. We focus on decisive factors that distinguish macroscopic from microfluidic cell culture to encourage a reconsideration of how macroscopic cell culture principles might apply to microfluidic cell culture.

Lab-on-a-chip technologies for stem cell analysis

The combination of microfabrication-based technologies with cell biology has laid the foundation for the development of advanced in vitro diagnostic systems capable of analyzing cell cultures under physiologically relevant conditions. In the present review, we address recent lab-on-a-chip developments for stem cell analysis. We highlight in particular the tangible advantages of microfluidic devices to overcome most of the challenges associated with stem cell identification, expansion and differentiation, with the greatest advantage being that lab-on-a-chip technology allows for the precise regulation of culturing conditions, while simultaneously monitoring relevant parameters using embedded sensory systems. State-of-the-art lab-on-a-chip platforms for in vitro assessment of stem cell cultures are presented and their potential future applications discussed.

Rapid formation of multicellular spheroids in double-emulsion droplets with controllable microenvironment

An attractive option for tissue engineering is to use of multicellular spheroids as microtissues, particularly with stem cell spheroids. Conventional approaches of fabricating spheroids suffer from low throughput and polydispersity in size, and fail to supplement cues from extracellular matrix (ECM) for enhanced differentiation. In this study, we report the application of microfluidics-generated water-in-oil-in-water (w/o/w) double-emulsion (DE) droplets as pico-liter sized bioreactor for rapid cell assembly and well-controlled microenvironment for spheroid culture. Cells aggregated to form size-controllable (30–80 μm) spheroids in DE droplets within 150 min and could be retrieved via a droplet-releasing agent. Moreover, precursor hydrogel solution can be adopted as the inner phase to produce spheroid-encapsulated microgels after spheroid formation. As an example, the encapsulation of human mesenchymal stem cells (hMSC) spheroids in alginate and alginate-arginine-glycine-aspartic acid (-RGD) microgel was demonstrated, with enhanced osteogenic differentiation further exhibited in the latter case.

Three-dimensional perfused cell culture

Compelling evidence suggests the limitation and shortcomings of the current and well established cell culture method using multi-well plates, flasks and Petri dishes. These are particularly important when cell functions are sensitive to the local microenvironment, cell-cell and cell-extracellular matrix interactions. There is a clear need for advanced cell culture systems which mimic in vivo and more physiological conditions. This review summarises and analyses recent progress in three dimensional (3D) cell culture with perfusion as the next generation cell culture tools, while excluding engineered tissue culture where three dimensional scaffold has to be used for structural support and perfusion for overcoming mass transfer control. Apart from research activities in academic community, product development in industry is also included in this review.

Probing cell-cell communication with microfluidic devices

Intercellular communication is a mechanism that regulates critical events during embryogenesis and coordinates signalling within differentiated tissues, such as the nervous and cardiovascular systems. To perform specialized activities, these tissues utilize the rapid exchange of signals among networks that, while are composed of different cell types, are nevertheless functionally coupled. Errors in cellular communication can lead to varied deleterious effects such as degenerative and autoimmune diseases. However, the intercellular communication network is extremely complex in multicellular organisms making isolation of the functional unit and study of basic mechanisms technically challenging. New experimental methods to examine mechanisms of intercellular communication among cultured cells could provide insight into physiological and pathological processes alike. Recent developments in microfluidic technology allow miniaturized and integrated devices to perform intercellular communication experiments on-chip. Microfluidics have many advantages, including the ability to replicate in vitro the chemical, mechanical, and physical cellular microenvironment of tissues with precise spatial and temporal control combined with dynamic characterization, high throughput, scalability and reproducibility. In this Focus article, we highlight some of the recent work and advances in the application of microfluidics to the study of mammalian intercellular communication with particular emphasis on cell contact and soluble factor mediated communication. In addition, we provide some insights into likely direction of the future developments in this field.

A High-Throughput Device for Patterned Differentiation of Embryoid Bodies

Although ontogenesis in vivo may proceed in a spatiotemporally heterogeneous environment, in vitro differentiation of an embryoid body (EB) has been carried out in uniform conditions using conventional culture methods at low throughput. In the present study, a microfluidic device with multiple culture chambers for simultaneous patterned differentiation of multiple EBs of pluripotent stem cells is newly developed. Theoretical simulation and experiments using a suspension of fluorescent particles or fluorescent solution show that proper chemical gradients can be formed with almost no flow in the chambers. After multiple EBs are introduced into the device, these EBs move along the flow channel and into trapping cups. The EBs are pushed by air bubbles into the culture chambers. These multiple EBs can be cultured within the culture chambers after flowing culture medium removes the air bubble from the device. In our experiment, differentiation and proliferation of these multiple EBs are studied by exposing them to two different media for 6 days: one to induce differentiation and the other to keep the pluripotent and self-renewing state of the cells. It is shown that patterned differentiation of the multiple EBs is successfully conducted simultaneously in the device when these two media are perfused into the device. The results suggest that differentiation and proliferation of multiple EBs can be analyzed by applying chemical gradients in the present microfluidic device. This will be a helpful tool in a wide variety of experiments involving EBs or spheroids.

Microfluidic devices for cell cultivation and proliferation

Microfluidic technology provides precise, controlled-environment, cost-effective, compact, integrated, and high-throughput microsystems that are promising substitutes for conventional biological laboratory methods. In recent years, microfluidic cell culture devices have been used for applications such as tissue engineering, diagnostics, drug screening, immunology, cancer studies, stem cell proliferation and differentiation, and neurite guidance. Microfluidic technology allows dynamic cell culture in microperfusion systems to deliver continuous nutrient supplies for long term cell culture. It offers many opportunities to mimic the cell-cell and cell-extracellular matrix interactions of tissues by creating gradient concentrations of biochemical signals such as growth factors, chemokines, and hormones. Other applications of cell cultivation in microfluidic systems include high resolution cell patterning on a modified substrate with adhesive patterns and the reconstruction of complicated tissue architectures. In this review, recent advances in microfluidic platforms for cell culturing and proliferation, for both simple monolayer (2D) cell seeding processes and 3D configurations as accurate models of in vivo conditions, are examined.

Macro and microfluidic flows for skeletal regenerative medicine

Fluid flow has a great potential as a cell stimulatory tool for skeletal regenerative medicine, because fluid flow-induced bone cell mechanotransduction in vivo plays a critical role in maintaining healthy bone homeostasis. Applications of fluid flow for skeletal regenerative medicine are reviewed at macro and microscale. Macroflow in two dimensions (2D), in which flow velocity varies along the normal direction to the flow, has explored molecular mechanisms of bone forming cell mechanotransduction responsible for flow-regulated differentiation, mineralized matrix deposition, and stem cell osteogenesis. Though 2D flow set-ups are useful for mechanistic studies due to easiness in in situ and post-flow assays, engineering skeletal tissue constructs should involve three dimensional (3D) flows, e.g., flow through porous scaffolds. Skeletal tissue engineering using 3D flows has produced promising outcomes, but 3D flow conditions (e.g., shear stress vs. chemotransport) and scaffold characteristics should further be tailored. Ideally, data gained from 2D flows may be utilized to engineer improved 3D bone tissue constructs. Recent microfluidics approaches suggest a strong potential to mimic in vivo microscale interstitial flows in bone. Though there have been few microfluidics studies on bone cells, it was demonstrated that microfluidic platform can be used to conduct high throughput screening of bone cell mechanotransduction behavior under biomimicking flow conditions.

Microfluidic systems: a new toolbox for pluripotent stem cells

Conventional culture systems are often limited in their ability to regulate the growth and differentiation of pluripotent stem cells. Microfluidic systems can overcome some of these limitations by providing defined growth conditions with user-controlled spatiotemporal cues. Microfluidic systems allow researchers to modulate pluripotent stem cell renewal and differentiation through biochemical and mechanical stimulation, as well as through microscale patterning and organization of cells and extracellular materials. Essentially, microfluidic tools are reducing the gap between in vitro cell culture environments and the complex and dynamic features of the in vivo stem cell niche. These microfluidic culture systems can also be integrated with microanalytical tools to assess the health and molecular status of pluripotent stem cells. The ability to control biochemical and mechanical input to cells, as well as rapidly and efficiently analyze the biological output from cells, will further our understanding of stem cells and help translate them into clinical use. This review provides a comprehensive insignt into the implications of microfluidics on pluripotent stem cell research.

The effect of red blood cell aggregation on velocity and cell-depleted layer characteristics of blood in a bifurcating microchannel

Red blood cell (RBC) aggregation is a multifaceted phenomenon, and whether it is generally beneficial or deleterious remains unclear. In order to better understand its effect on microvascular blood flow, the phenomenon must be studied in complex geometries, as it is strongly dependent on time, flow, and geometry. The cell-depleted layer (CDL) which forms at the walls of microvessels has been observed to be enhanced by aggregation; however, details of the characteristics of the CDL in complex regions, such as bifurcations, require further investigation. In the present study, a microchannel with a T-junction was used to analyze the influence of aggregation on the flow field and the CDL. Micro-PIV using RBCs as tracers provided high resolution cell velocity data. CDL characteristics were measured from the same data using a newly developed technique based on motion detection. Skewed and sharpened velocity profiles in the daughter branches were observed, contrary to the behavior of a continuous Newtonian fluid. RBC aggregation was observed to increase the skewness, but decrease the sharpening, of the velocity profiles in the daughter branches. The CDL width was found to be significantly greater, with a wider distribution, in the presence of aggregation and the mean width increased proportionally with the reciprocal of the fraction of flow entering the daughter branch. Aggregation also significantly increased the roughness of the interface between the CDL and the RBC core. The present results provide further insight into how RBC aggregation may affect the flow in complex geometries, which is of importance in both understanding its functions invivo, and utilizing it as a tool in microfluidic devices.

Formation of embryoid bodies using dielectrophoresis

Embryoid body (EB) formation forms an important step in embryonic stem cell differentiation invivo. In murine embryonic stem cell (mESC) cultures EB formation is inhibited by the inclusion of leukaemic inhibitory factor (LIF) in the medium. Assembly of mESCs into aggregates by positive dielectrophoresis (DEP) in high field regions between interdigitated oppositely castellated electrodes was found to initiate EB formation. Embryoid body formation in aggregates formed with DEP occurred at a more rapid rate-in fact faster compared to conventional methods-in medium without LIF. However, EB formation also occurred in medium in which LIF was present when the cells were aggregated with DEP. The optimum characteristic size for the electrodes for EB formation with DEP was found to be 75-100 microns; aggregates smaller than this tended to merge, whilst aggregates larger than this tended to split to form multiple EBs. Experiments with ESCs in which green fluorescent protein (GFP) production was targeted to the mesodermal gene brachyury indicated that differentiation within embryoid bodies of this size may preferentially occur along the mesoderm lineage. As hematopoietic lineages during normal development derive from mesoderm, the finding points to a possible application of DEP formed EBs in the production of blood-based products from ESCs.

Preface to Special Topic: Microfluidics in cell biology and tissue engineering

In this special issue of Biomicrofluidics, a wide variety of applications of microfluidics to tissue engineering and cell biology are presented. The articles illustrate the benefits of using microfluidics for controlling the cellular environment in a precise yet high rate manner using minimum reagents. The topic is very timely and takes a stab at portraying a glimpse of what is to come in this exciting and emerging field of research.

Preface to Special Topic: Biological microfluidics in tissue engineering and regenerative medicine

In this special issue of Biomicrofluidics, many manifestations of biological microfluidics have been highlighted that have significance to regenerative biology and medicine. The collated articles demonstrate the applicability of these biological microfluidics for studying a wide range of biomedical problems most useful for understanding and shining light on basic biology to those applications relevant to clinical medicine.