A hollow sphere soft lithography approach for long-term hanging drop methods

Reflow-molded deep concave microwell arrays for robust and large-scale production of embryoid bodies

Embryonic stem cell (ESC)-derived aggregates, called embryoid bodies (EBs), are powerful in vitro models used to study human development and disease. However, the cost-effective and large-scale production of homogeneous EBs still remains a challenge. Here, we report a rapid, straightforward method for fabricating closely arrayed deep concave microwells, enabling the mass production of uniform EBs from single cell suspensions. By simply combining micromilling, caramel replica molding, and thermal reflow, we generate convex micromolds with high aspect ratios and excellent surface smoothness. Benefitting from the nature of reflow, this method can produce rounded bottom polydimethylsiloxane (PDMS) microwells, which are not easily achieved with standard soft lithography techniques but critical to producing spherical EBs. To achieve optimal concave microwells, we investigated the effect of thermal reflow temperature and time on the surface smoothness and roundness of the finished microwells. In addition, to further improve the utility of this method, we also investigated the effect of microwell aspect ratio (AR) on the loss of EBs during medium manipulation. The capability of this deep concave microwell system was validated by rapidly generating a large number of human embryonic stem cell (hESC)-derived EBs and then efficiently differentiating them into a cardiac lineage. The proposed fabrication method and deep concave microwell platform are highly practical, and thus will benefit the mass production of EBs for potential tissue regeneration and cell therapy applications.

Bench to Bedside: New Therapeutic Approaches with Extracellular Vesicles and Engineered Biomaterials for Targeting Therapeutic Resistance of Cancer Stem Cells

Cancer has recently been the second leading cause of death worldwide, trailing only cardiovascular disease. Cancer stem cells (CSCs), represented as tumor-initiating cells (TICs), are mainly liable for chemoresistance and disease relapse due to their self-renewal capability and differentiating capacity into different types of tumor cells. The intricate molecular mechanism is necessary to elucidate CSC's chemoresistance properties and cancer recurrence. Establishing efficient strategies for CSC maintenance and enrichment is essential to elucidate the mechanisms and properties of CSCs and CSC-related therapeutic measures. Current approaches are insufficient to mimic the in vivo chemical and physical conditions for the maintenance and growth of CSC and yield unreliable research results. Biomaterials are now widely used for simulating the bone marrow microenvironment. Biomaterial-based three-dimensional (3D) approaches for the enrichment of CSC provide an excellent promise for future drug discovery and elucidation of molecular mechanisms. In the future, the biomaterial-based model will contribute to a more operative and predictive CSC model for cancer therapy. Design strategies for materials, physicochemical cues, and morphology will offer a new direction for future modification and new methods for studying the CSC microenvironment and its chemoresistance property. This review highlights the critical roles of the microenvironmental cues that regulate CSC function and endow them with drug resistance properties. This review also explores the latest advancement and challenges in biomaterial-based scaffold structure for therapeutic approaches against CSC chemoresistance. Since the recent entry of extracellular vesicles (EVs), cell-derived nanostructures, have opened new avenues of investigation into this field, which, together with other more conventionally studied signaling pathways, play an important role in cell-to-cell communication. Thus, this review further explores the subject of EVs in-depth. This review also discusses possible future biomaterial and biomaterial-EV-based models that could be used to study the tumor microenvironment (TME) and will provide possible therapeutic approaches. Finally, this review concludes with potential perspectives and conclusions in this area.

Flipped Well-Plate Hanging-Drop Technique for Growing Three-Dimensional Tumors

Three-dimensional (3D) tumor culture techniques are gaining popularity as in vitro models of tumoral tissue analogues. Despite the widespread interest, need, and present-day effort, most of the 3D tumor culturing methodologies have not gone beyond the inventors’ laboratories. This, in turn, limits their applicability and standardization. In this study, we introduce a straightforward and user-friendly approach based on standard 96-well plates with basic amenities for growing 3D tumors in a scaffold-free/scaffold-based format. Hanging drop preparation can be easily employed by flipping a universal 96-well plate. The droplets of the medium generated by the well-plate flip (WPF) method can be easily modified to address various mechanisms and processes in cell biology, including cancer. To demonstrate the applicability and practicality of the conceived approach, we utilized human colorectal carcinoma cells (HCT116) to first show the generation of large scaffold-free 3D tumor spheroids over 1.5 mm in diameter in single-well plates. As a proof-of-concept, we also demonstrate matrix-assisted tumor culture techniques in advancing the broader use of 3D culture systems. The conceptualized WPF approach can be adapted for a range of applications in both basic and applied biological/engineering research.

Development of 3D cultures of zebrafish liver and embryo cell lines: a comparison of different spheroid formation methods

Fish cell spheroids are promising 3D culture models for vertebrate replacement in ecotoxicology. However, new alternative ecotoxicological methods must be adapted for applications in industry and for regulatory purposes; such methods must be cost-effective, simple to manipulate and provide rapid results. Therefore, we compared the effectiveness of the traditional hanging drop (HD), orbital shaking (OS), and HD combined with OS (HD+OS) methods on the formation of zebrafish cell line spheroids (ZFL and ZEM2S). Time in HD (3-5 days) and different 96-well plates [flat-bottom or ultra-low attachment of round-bottom (ULA-plates)] in OS were evaluated. Easy handling, rapid spheroid formation, uniform-sized spheroids, and circularity were assessed to identify the best spheroid protocol. Traditional HD alone did not result in ZFL spheroid formation, whereas HD (5 days)+OS did. When using the OS, spheroids only formed on the ULA-plate. Both HD+OS and OS were reproducible in size (177.50 ± 2.81 µm and 225.62 ± 19.20 µm, respectively) and circularity (0.83 ± 0.02 and 0.80 ± 0.01, respectively) of ZFL spheroids. Nevertheless, HD+OS required a considerable time to completely form spheroids (10 days) and intensive handling, whereas the OS was fast (5 days of incubation) and simple. OS also yielded reproducible ZEM2S spheroids in 1 day (226.23 ± 0.57 µm diameter and 0.80 ± 0.01 circularity). In conclusion, OS in ULA-plate is an effective and simple spheroid protocol for high-throughput ecotoxicity testing. This study contributes to identify a fast, reproducible, and simple protocol of single piscine spheroid formation in 96-well plates and supports the application of fish 3D model in industry and academia.

A superhydrophobic chip integrated with an array of medium reservoirs for long-term hanging drop spheroid culture

Hanging drop (HD) is one of the most popular methods used for forming three-dimensional (3D) cell spheroids. However, conventional hanging drop systems are only applicable for short-term spheroid culture due to their inconvenience in exchanging cell culture media. Here we present a medium-reservoir-integrated superhydrophobic (MRI-SH) chip for long-term HD spheroid cultures. The device consists of two main components: i) a patterned superhydrophobic (SH) surface containing an array of wettable spots which anchor arrays of droplets of cell suspension, and ii) an array of chambers that serve as medium reservoirs, both interconnected via an array of thru-holes. This configuration provides two distinct advantages over conventional HD configurations: i) the high wettability contrast of the SH pattern on the chip leads to the formation and adhesion of nearly spherical hanging droplets on its surface, which minimizes interactions between the liquid and the substrate; ii) the integrated chambers provide large volumes of medium to maintain longer culture durations. Using this device, spheroids of MHCC97H cells were successfully formed, and the cultured spheroids could maintain high viability for up to 30 days and exhibited enhanced spheroid morphology compared to those cultured in the conventional HD systems. STATEMENT OF SIGNIFICANCE: This paper presents a medium-reservoir-integrated superhydrophobic hanging drop (HD) platform for the long-term culture of spheroids with enhanced morphology. By monolithically integrating medium reservoirs and a patterned SH surface into a single device, this HD platform can not only produce high-quality spheroids, but also permit them to sustain high viability for up to 30 days without the need for tedious medium replenishment. We believe that such a platform will be valuable in a wide range of biological or biomedical applications, including tissue engineering, regenerative medicine, and drug discovery.

Scaffold-free generation of heterotypic cell spheroids using acoustofluidics

3D cell cultures such as cell spheroids are widely used for tissue engineering, regenerative medicine, and translational medicine, but challenges remain in recapitulating the architectural complexity and spatiotemporal heterogeneity of tissues. Thus, we developed a scaffold-free and versatile acoustofluidic device to fabricate heterotypic cell spheroids with complexity over cell architectures and components. By varying the concentrations of cell suspension, we can precisely control the size of spheroids aggregated by a contact-free acoustic radiation force. By tuning the cell components including tumor cells, fibroblasts, and endothelial cells, heterotypic spheroids were controllably fabricated. These heterotypic spheroids can be used as a proof-of concept to model the spatial organization of tumor tissues. We demonstrated that the assembled components can self-assemble into layered structures as instructed by their cadherin expression. Finally, we demonstrated the acoustic assembly of mouse mammary gland components into spheroids and observed their maturation in culture. To conclude, we developed an acoustofluidic platform to fabricate complex spheroids with multiple components. We envision that this platform will pave the way for the high accuracy of spheroid fabrication and offer broad applications in numerous areas, such as tumor research, tissue engineering, developmental biology, and drug discovery.

Chick fetal organ spheroids as a model to study development and disease

BACKGROUND

Organ culture models have been used over the past few decades to study development and disease. The in vitro three-dimensional (3D) culture system of organoids is well known, however, these 3D systems are both costly and difficult to culture and maintain. As such, less expensive, faster and less complex methods to maintain 3D cell culture models would complement the use of organoids. Chick embryos have been used as a model to study human biology for centuries, with many fundamental discoveries as a result. These include cell type induction, cell competence, plasticity and contact inhibition, which indicates the relevance of using chick embryos when studying developmental biology and disease mechanisms.

RESULTS

Here, we present an updated protocol that enables time efficient, cost effective and long-term expansion of fetal organ spheroids (FOSs) from chick embryos. Utilizing this protocol, we generated FOSs in an anchorage-independent growth pattern from seven different organs, including brain, lung, heart, liver, stomach, intestine and epidermis. These three-dimensional (3D) structures recapitulate many cellular and structural aspects of their in vivo counterpart organs and serve as a useful developmental model. In addition, we show a functional application of FOSs to analyze cell-cell interaction and cell invasion patterns as observed in cancer.

CONCLUSION

The establishment of a broad ranging and highly effective method to generate FOSs from different organs was successful in terms of the formation of healthy, proliferating 3D organ spheroids that exhibited organ-like characteristics. Potential applications of chick FOSs are their use in studies of cell-to-cell contact, cell fusion and tumor invasion under defined conditions. Future studies will reveal whether chick FOSs also can be applicable in scientific areas such as viral infections, drug screening, cancer diagnostics and/or tissue engineering.

Engineered Microsystems for Spheroid and Organoid Studies

3D in vitro model systems such as spheroids and organoids provide an opportunity to extend the physiological understanding using recapitulated tissues that mimic physiological characteristics of in vivo microenvironments. Unlike 2D systems, 3D in vitro systems can bridge the gap between inadequate 2D cultures and the in vivo environments, providing novel insights on complex physiological mechanisms at various scales of organization, ranging from the cellular, tissue-, to organ-levels. To satisfy the ever-increasing need for highly complex and sophisticated systems, many 3D in vitro models with advanced microengineering techniques have been developed to answer diverse physiological questions. This review summarizes recent advances in engineered microsystems for the development of 3D in vitro model systems. The relationship between the underlying physics behind the microengineering techniques, and their ability to recapitulate distinct 3D cellular structures and functions of diverse types of tissues and organs are highlighted and discussed in detail. A number of 3D in vitro models and their engineering principles are also introduced. Finally, current limitations are summarized, and perspectives for future directions in guiding the development of 3D in vitro model systems using microengineering techniques are provided.

3D culture technologies of cancer stem cells: promising ex vivo tumor models

Cancer stem cells have been shown to be important in tumorigenesis processes, such as tumor growth, metastasis, and recurrence. As such, many three-dimensional models have been developed to establish an ex vivo microenvironment that cancer stem cells experience under in vivo conditions. Cancer stem cells propagating in three-dimensional culture systems show physiologically related signaling pathway profiles, gene expression, cell-matrix and cell-cell interactions, and drug resistance that reflect at least some of the tumor properties seen in vivo. Herein, we discussed the presently available Cancer stem cell three-dimensional culture models that use biomaterials and engineering tools and the biological implications of these models compared to the conventional ones.

The Tumor-on-Chip: Recent Advances in the Development of Microfluidic Systems to Recapitulate the Physiology of Solid Tumors

The ideal in vitro recreation of the micro-tumor niche-although much needed for a better understanding of cancer etiology and development of better anticancer therapies-is highly challenging. Tumors are complex three-dimensional (3D) tissues that establish a dynamic cross-talk with the surrounding tissues through complex chemical signaling. An extensive body of experimental evidence has established that 3D culture systems more closely recapitulate the architecture and the physiology of human solid tumors when compared with traditional 2D systems. Moreover, conventional 3D culture systems fail to recreate the dynamics of the tumor niche. Tumor-on-chip systems, which are microfluidic devices that aim to recreate relevant features of the tumor physiology, have recently emerged as powerful tools in cancer research. In tumor-on-chip systems, the use of microfluidics adds another dimension of physiological mimicry by allowing a continuous feed of nutrients (and pharmaceutical compounds). Here, we discuss recently published literature related to the culture of solid tumor-like tissues in microfluidic systems (tumor-on-chip devices). Our aim is to provide the readers with an overview of the state of the art on this particular theme and to illustrate the toolbox available today for engineering tumor-like structures (and their environments) in microfluidic devices. The suitability of tumor-on-chip devices is increasing in many areas of cancer research, including the study of the physiology of solid tumors, the screening of novel anticancer pharmaceutical compounds before resourcing to animal models, and the development of personalized treatments. In the years to come, additive manufacturing (3D bioprinting and 3D printing), computational fluid dynamics, and medium- to high-throughput omics will become powerful enablers of a new wave of more sophisticated and effective tumor-on-chip devices.

Fabrication of Human Keratinocyte Cell Clusters for Skin Graft Applications by Templating Water-in-Water Pickering Emulsions

Most current methods for the preparation of tissue spheroids require complex materials, involve tedious physical steps and are generally not scalable. We report a novel alternative, which is both inexpensive and up-scalable, to produce large quantities of viable human keratinocyte cell clusters (clusteroids). The method is based on a two-phase aqueous system of incompatible polymers forming a stable water-in-water (w/w) emulsion, which enabled us to rapidly fabricate cell clusteroids from HaCaT cells. We used w/w Pickering emulsion from aqueous solutions of the polymers dextran (DEX) and polyethylene oxide (PEO) and a particle stabilizer based on whey protein (WP). The HaCaT cells clearly preferred to distribute into the DEX-rich phase and this property was utilized to encapsulate them in the water-in-water (DEX-in-PEO) emulsion drops then osmotically shrank to compress them into clusters. Prepared formulations of HaCaT keratinocyte clusteroids in alginate hydrogel were grown where the cells percolated to mimic 3D tissue. The HaCaT cell clusteroids grew faster in the alginate film compared to the individual cells formulated in the same matrix. This methodology could potentially be utilised in biomedical applications.

Bioengineered in vitro Vascular Models for Applications in Interventional Radiology

The role of endovascular interventions has progressed rapidly over the past several decades. While animal models have long-served as the mainstay for the advancement of this field, the use of in vitro models has become increasingly widely adopted with recent advances in engineering technologies. Here, we review the strategies, mainly including bioprinting and microfabrication, which allow for fabrication of biomimetic vascular models that will potentially serve to supplement the conventional animal models for convenient investigations of endovascular interventions. Besides normal blood vessels, those in diseased states, such as thrombosis, may also be modeled by integrating cues that simulate the microenvironment of vascular disorders. These novel engineering strategies for the development of biomimetic in vitro vascular structures will possibly enable unconventional means of studying complex endovascular intervention problems that are otherwise hard to address using existing models.

Application of 3D cultured multicellular spheroid tumor models in tumor-targeted drug delivery system research

Tumor-targeted drug delivery systems are promising for their advantages in enhanced tumor accumulation and reduced toxicity towards normal organs. However, few nanomedicines have been successfully translated into clinical application. One reason is the gap between current pre-clinical and clinical studies. The prevalent in vitro models utilized in pre-clinical phase are mainly based on the two-dimensional (2D) cell culture and are limited by the difficulty of simulating three-dimensional physiological conditions in human body, such as three-dimensional (3D) architecture, cell heterogeneity, nutrient gradients and the interaction between cells and the extracellular matrix (ECM). In addition, traditional animal models have drawbacks such as high-cost, long periods and physiological differences between animal and human. On the other hand, the employment of 3D tumor cell culture models, especially multicellular tumor spheroids (MCTS), has increased significantly in recent decades. These models have been shown to simulate 3D structures of tumors in vitro with relatively low cost and simple protocols. Currently, MCTS have also been widely exploited in drug delivery system research for comprehensive study of drug efficacy, drug penetration, receptor targeting, and cell recruitment abilities. This review summarizes the delivery barriers for nano-carriers presented in tumor microenvironment, the characteristics and formation methods for applicable multicellular tumor spheroid culture models and recent studies related to their applications in tumor-targeted drug delivery system research.

A microfluidic trap array for longitudinal monitoring and multi-modal phenotypic analysis of individual stem cell aggregates

Three-dimensional pluripotent stem cell (PSC) cultures have the ability to undergo differentiation, self-organization, and morphogenesis to yield complex, in vitro tissue models that recapitulate key elements of native tissues. These tissue models offer a system for studying mechanisms of tissue development, investigating disease mechanisms, and performing drug screening. It remains challenging, however, to standardize PSC aggregate differentiation and morphogenesis methods due to heterogeneity stemming from biological and environmental sources. It is also difficult to monitor and assess large numbers of individual samples longitudinally throughout culture using typical batch-based culture methods. To address these challenges, we have developed a microfluidic platform for culture, longitudinal monitoring, and phenotypic analysis of individual stem cell aggregates. This platform uses a hydrodynamic loading principle to capture pre-formed stem cell aggregates in independent traps. We demonstrated that multi-day culture of aggregates in this platform reduces heterogeneity in phenotypic parameters such as size and morphology. Additionally, we showed that culture and analysis steps can be performed sequentially in the same platform, enabling correlation of multiple modes of analysis for individual samples. We anticipate this platform being applied to improve abilities for phenotypic analysis of PSC aggregate tissues and to facilitate research in standardizing culture systems in order to dually increase the yield and reduce the heterogeneity of PSC-derived tissues.

Controlled Self-assembly of Stem Cell Aggregates Instructs Pluripotency and Lineage Bias

Stem cell-derived organoids and other 3D microtissues offer enormous potential as models for drug screening, disease modeling, and regenerative medicine. Formation of stem/progenitor cell aggregates is common in biomanufacturing processes and critical to many organoid approaches. However, reproducibility of current protocols is limited by reliance on poorly controlled processes (e.g., spontaneous aggregation). Little is known about the effects of aggregation parameters on cell behavior, which may have implications for the production of cell aggregates and organoids. Here we introduce a bioengineered platform of labile substrate arrays that enable simple, scalable generation of cell aggregates via a controllable 2D-to-3D "self-assembly". As a proof-of-concept, we show that labile substrates generate size- and shape-controlled embryoid bodies (EBs) and can be easily modified to control EB self-assembly kinetics. We show that aggregation method instructs EB lineage bias, with faster aggregation promoting pluripotency loss and ectoderm, and slower aggregation favoring mesoderm and endoderm. We also find that aggregation kinetics of EBs markedly influence EB structure, with slower kinetics resulting in increased EB porosity and growth factor signaling. Our findings suggest that controlling internal structure of cell aggregates by modifying aggregation kinetics is a potential strategy for improving 3D microtissue models for research and translational applications.

Quantitative image analysis of the shape and size of circular wound sites generated by vertically stamped scratches

A protocol for quantitative image analysis of wound generation is important to better understand the integrative process of wound healing and the closure mechanism. Here, we present a method for quantitative analysis of microscopic images of circular wound sites generated by vertically stamped scratches. To demonstrate proof-of-concept validation, we used two types of mechanical stamping tools, a mechanical pencil lead (type 1; brittle) and polydimethylsiloxane (PDMS) pillars (type 2; ductile), to create circular wound sites. We also present a method for analysis of microscopic images of the generated wound sites by suggesting new parameters, such as controlled area transfer ratio, modified shape factor, and roundness index, specifically to investigate the shape and size of wounds via house-coded image processing. We believe that this approach can be potentially useful by providing a better way of studying vertical wound generation for future skin wound generation and care applications compared with its counterpart, conventional horizontal wound generation.

A PDMS-Based Microfluidic Hanging Drop Chip for Embryoid Body Formation

The conventional hanging drop technique is the most widely used method for embryoid body (EB) formation. However, this method is labor intensive and limited by the difficulty in exchanging the medium. Here, we report a microfluidic chip-based approach for high-throughput formation of EBs. The device consists of microfluidic channels with 6 × 12 opening wells in PDMS supported by a glass substrate. The PDMS channels were fabricated by replicating polydimethyl-siloxane (PDMS) from SU-8 mold. The droplet formation in the chip was tested with different hydrostatic pressures to obtain optimal operation pressures for the wells with 1000 μm diameter openings. The droplets formed at the opening wells were used to culture mouse embryonic stem cells which could subsequently developed into EBs in the hanging droplets. This device also allows for medium exchange of the hanging droplets making it possible to perform immunochemistry staining and characterize EBs on chip.

High-Throughput Single-Cell Derived Sphere Formation for Cancer Stem-Like Cell Identification and Analysis

Considerable evidence suggests that many malignancies are driven by a cellular compartment that displays stem cell properties. Cancer stem-like cells (CSCs) can be identified by expression of cell surface markers or enzymatic activity, but these methods are limited by phenotypic heterogeneity and plasticity of CSCs. An alternative phenotypic methodology based on in-vitro sphere formation has been developed, but it is typically labor-intensive and low-throughput. In this work, we present a 1,024-microchamber microfluidic platform for single-cell derived sphere formation. Utilizing a hydrodynamic capturing scheme, more than 70% of the microchambers capture only one cell, allowing for monitoring of sphere formation from heterogeneous cancer cell populations for identification of CSCs. Single-cell derived spheres can be retrieved and dissociated for single-cell analysis using a custom 96-gene panel to probe heterogeneity within the clonal CSC spheres. This microfluidic platform provides reliable and high-throughput sphere formation for CSC identification and downstream clonal analysis.

A scaffoldless technique for self-generation of three-dimensional keratinospheroids on liquid crystal surfaces

We describe a new scaffold-free three-dimensional (3D) cell culture model using cholesteryl ester based lyotropic liquid crystal (LC) substrates. Keratinocytes were deposited randomly on the LC surface where they self-assembled into 3D microtissues or keratinospheroids. The cell density required to form spheroids was optimized. We investigated cell viability using dead/live cell assays. The adhesion characteristics of cells within the microtissues were determined using histological sectioning and immunofluorescence staining. Fourier transform infrared spectroscopy (FTIR) was used to characterize the biochemistry of the keratinospheroids. We found that both cells and microtissues could migrate on the LC surface. The viability study indicated approximately 80% viability of cells in the microtissues up to 20 days of culture. Strong intercellular adhesion was observed in the stratification of the multi-layered microspheroids using field emission-scanning electron microscopy (FE-SEM) and histochemical staining. The cytoskeleton and vinculins of the cells in the microtissues were expressed diffusely, but the microtissues were enriched with lipids and nucleic acids, which indicates close resemblance to the conditions in vivo. The basic 3D culture model based on LC may be used for cell and microtissue migration studies in response to cytochemical treatment.

Point-, line-, and plane-shaped cellular constructs for 3D tissue assembly

Microsized cellular constructs such as cellular aggregates and cell-laden hydrogel blocks are attractive cellular building blocks to reconstruct 3D macroscopic tissues with spatially ordered cells in bottom-up tissue engineering. In this regard, microfluidic techniques are remarkable methods to form microsized cellular constructs with high production rate and control of their shapes such as point, line, and plane. The fundamental shapes of the cellular constructs allow for the fabrication of larger arbitrary-shaped tissues by assembling them. This review introduces microfluidic formation methods of microsized cellular constructs and manipulation techniques to assemble them with control of their arrangements. Additionally, we show applications of the cellular constructs to biological studies and clinical treatments and discuss future trends as their potential applications.

Engineering cancer microenvironments for in vitro 3-D tumor models

The natural microenvironment of tumors is composed of extracellular matrix (ECM), blood vasculature, and supporting stromal cells. The physical characteristics of ECM as well as the cellular components play a vital role in controlling cancer cell proliferation, apoptosis, metabolism, and differentiation. To mimic the tumor microenvironment outside the human body for drug testing, two-dimensional (2-D) and murine tumor models are routinely used. Although these conventional approaches are employed in preclinical studies, they still present challenges. For example, murine tumor models are expensive and difficult to adopt for routine drug screening. On the other hand, 2-D in vitro models are simple to perform, but they do not recapitulate natural tumor microenvironment, because they do not capture important three-dimensional (3-D) cell-cell, cell-matrix signaling pathways, and multi-cellular heterogeneous components of the tumor microenvironment such as stromal and immune cells. The three-dimensional (3-D) in vitro tumor models aim to closely mimic cancer microenvironments and have emerged as an alternative to routinely used methods for drug screening. Herein, we review recent advances in 3-D tumor model generation and highlight directions for future applications in drug testing.

Flip channel: A microfluidic device for uniform-sized embryoid body formation and differentiation

This paper reports a two-layered polydimethylsiloxane microfluidic device-Flip channel, capable of forming uniform-sized embryoid bodies (EBs) and performing stem cell differentiation within the same device after flipping the microfluidic channel. The size of EBs can be well controlled by designing the device geometries, and EBs with multiple sizes can be formed within a single device to study EB size-dependent stem cell differentiation. During operation of the device, cells are positioned in the designed positions. As a result, observation and monitoring specific population of cells can be achieved for further analysis. In addition, after flipping the microfluidic channel, stem cell differentiation from the EBs can be performed on an unconfined flat surface that is desired for various differentiation processes. In the experiments, murine embryonic stem cells (ES-D3) are cultured and formed EBs inside the developed device. The size of EBs is well controlled inside the device, and the neural differentiation is performed on the formed EBs after flipping the channel. The EB size-dependent stem cell differentiation is studied using the device to demonstrate its functions. The device provides a useful tool to study stem cell differentiation without complicated device fabrication and tedious cell handling under better-controlled microenvironments.

High-Throughput Cancer Cell Sphere Formation for Characterizing the Efficacy of Photo Dynamic Therapy in 3D Cell Cultures

Photodynamic therapy (PDT), wherein light sensitive non-toxic agents are locally and selectively activated using light, has emerged as an appealing alternative to traditional cancer chemotherapy. Yet to date, PDT efficacy has been mostly characterized using 2D cultures. Compared to 2D cultures, 3D sphere culture generates unique spatial distributions of nutrients and oxygen for the cells that better mimics the in-vivo conditions. Using a novel polyHEMA (non-adherent polymer) fabrication process, we developed a microfluidic sphere formation platform that can (1) generate 1,024 uniform (size variation <10%) cancer spheres within a 2 cm by 2 cm core area, (2) culture spheres for more than 2 weeks, and (3) allow the retrieval of spheres. Using the presented platform, we have successfully characterized the different responses in 2D and 3D cell culture to PDT. Furthermore, we investigated the treatment resistance effect in cancer cells induced by tumor associated fibroblasts (CAF). Although the CAFs can enhance the resistance to traditional chemotherapy agents, no significant difference in PDT was observed. The preliminary results suggest that the PDT can be an attractive alternative cancer therapy, which is less affected by the therapeutic resistance induced by cancer associated cells.

The effects analysis of two neonicotinoid insecticides on in vitro maturation of porcine oocytes using hanging drop monoculture method

Acetamiprid (ACE) and imidacroprid (IMI) are known neonicotinoid insecticides with strong affinities for the insect-selective nicotinic acetylcholine receptor. These provide insect control by hyperstimulating insect nerves and are used for agricultural pest management. However, it has also been reported that ACE and IMI affect mammalian reproductive function. We determined the effects of ACE and IMI on the in vitro maturation of porcine oocytes. Significant decreases in nuclear maturation rates were observed in the ACE or IMI-exposed groups. Also, in matured oocytes from the ACE or IMI-exposed groups, irregular chromosomes were observed. Our results suggest that ACE and IMI exposure was detrimental to porcine oocytes and the extent of the effects depends on the concentration of exposure.

In vitro spatially organizing the differentiation in individual multicellular stem cell aggregates

With significant potential as a robust source to produce specific somatic cells for regenerative medicine, stem cells have attracted increasing attention from both academia and government. In vivo, stem cell differentiation is a process under complicated regulations to precisely build tissue with unique spatial structures. Since multicellular spheroidal aggregates of stem cells, commonly called as embryoid bodies (EBs), are considered to be capable of recapitulating the events in early stage of embryonic development, a variety of methods have been developed to form EBs in vitro for studying differentiation of embryonic stem cells. The regulation of stem cell differentiation is crucial in directing stem cells to build tissue with the correct spatial architecture for specific functions. However, stem cells within the three-dimensional multicellular aggregates undergo differentiation in a less unpredictable and spatially controlled manner in vitro than in vivo. Recently, various microengineering technologies have been developed to manipulate stem cells in vitro in a spatially controlled manner. Herein, we take the spotlight on these technologies and researches that bring us the new potential for manipulation of stem cells for specific purposes.

Critical analysis of 3-D organoid in vitro cell culture models for high-throughput drug candidate toxicity assessments

Drug failure due to toxicity indicators remains among the primary reasons for staggering drug attrition rates during clinical studies and post-marketing surveillance. Broader validation and use of next-generation 3-D improved cell culture models are expected to improve predictive power and effectiveness of drug toxicological predictions. However, after decades of promising research significant gaps remain in our collective ability to extract quality human toxicity information from in vitro data using 3-D cell and tissue models. Issues, challenges and future directions for the field to improve drug assay predictive power and reliability of 3-D models are reviewed.

Manipulating magnetic 3D spheroids in hanging drops for applications in tissue engineering and drug screening

Magnetic spheroid manipulation can be carried out in hanging drops to generate distinctly structured heterotypic microtissues through sequential addition of cells or spheroid to homotypic spheroids. These spheroids can also be incorporated in a droplet-based assay to screen for therapeutic efficacy in prolonged studies. This simple and versatile technique can offer potential benefits in tissue engineering and drug screening applications.

Three-dimensional cell culture based on microfluidic techniques to mimic living tissues

This mini-review consists of microfluidic fabrication methods of cellular spheroids and cell-laden hydrogels, and their applications for tissue engineering. Using microfluidic devices, cellular spheroids and cell-laden hydrogels with controllable design are formed reproducibly. Owing to their size uniformity, they are used as building blocks for bottom-up tissue engineering to construct uniform and arbitrarily shaped tissues. Thus, cellular spheroids and cell-laden hydrogels based on microfluidic techniques are powerful tools to create tissues for human implantation and the treatment of diseases.

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.

Controlling embryonic stem cell growth and differentiation by automation: enhanced and more reliable differentiation for drug discovery

Despite significant use in basic research, embryonic stem cells have just begun to be used in the drug discovery process. Barriers to the adoption of embryonic stem cells in drug discovery include the difficulty in growing cells and inconsistent differentiation to the desired cellular phenotype. Embryonic stem cell cultures require consistent and frequent handling to maintain the cells in a pluripotent state. In addition, the preferred hanging drop method of embryoid body (EB) differentiation is not amenable to high-throughput methods, and suspension cultures of EBs show a high degree of variability. Murine embryonic stem cells passaged on an automated platform maintained ≥ 90% viability and pluripotency. We also developed a method of EB formation using 384-well microplates that form a single EB per well, with excellent uniformity across EBs. This format facilitated high-throughput differentiation and enabled screens to optimize directed differentiation into a desired cell type. Using this approach, we identified conditions that enhanced cardiomyocyte differentiation sevenfold. This optimized differentiation method showed excellent consistency for such a complex biological process. This automated approach to embryonic stem cell handling and differentiation can provide the high and consistent yields of differentiated cell types required for basic research, compound screens, and toxicity studies.

Micro-ring structures stabilize microdroplets to enable long term spheroid culture in 384 hanging drop array plates

Using stereolithography, 20 different structural variations comprised of millimeter diameter holes surrounded by trenches, plateaus, or micro-ring structures were prepared and tested for their ability to stably hold arrays of microliter sized droplets within the structures over an extended period of time. The micro-ring structures were the most effective in stabilizing droplets against mechanical and chemical perturbations. After confirming the importance of micro-ring structures using rapid prototyping, we developed an injection molding tool for mass production of polystyrene 3D cell culture plates with an array of 384 such micro-ring surrounded through-hole structures. These newly designed and injection molded polystyrene 384 hanging drop array plates with micro-rings were stable and robust against mechanical perturbations as well as surface fouling-facilitated droplet spreading making them capable of long term cell spheroid culture of up to 22 days within the droplet array. This is a significant improvement over previously reported 384 hanging drop array plates which are susceptible to small mechanical shocks and could not reliably maintain hanging drops for longer than a few days. With enhanced droplet stability, the hanging drop array plates with micro-ring structures provide better platforms and open up new opportunities for high-throughput preparation of microscale 3D cell constructs for drug screening and cell analysis.

384 hanging drop arrays give excellent Z-factors and allow versatile formation of co-culture spheroids

We previously reported the development of a simple, user-friendly, and versatile 384 hanging drop array plate for 3D spheroid culture and the importance of utilizing 3D cellular models in anti-cancer drug sensitivity testing. The 384 hanging drop array plate allows for high-throughput capabilities and offers significant improvements over existing 3D spheroid culture methods. To allow for practical 3D cell-based high-throughput screening and enable broader use of the plate, we characterize the robustness of the 384 hanging drop array plate in terms of assay performance and demonstrate the versatility of the plate. We find that the 384 hanging drop array plate performance is robust in fluorescence- and colorimetric-based assays through Z-factor calculations. Finally, we demonstrate different plate capabilities and applications, including: spheroid transfer and retrieval for Janus spheroid formation, sequential addition of cells for concentric layer patterning of different cell types, and culture of a wide variety of cell types.

Embryonic stem cell bioprinting for uniform and controlled size embryoid body formation

Embryonic stem cells (ESCs) are pluripotent with multilineage potential to differentiate into virtually all cell types in the organism and thus hold a great promise for cell therapy and regenerative medicine. In vitro differentiation of ESCs starts with a phase known as embryoid body (EB) formation. EB mimics the early stages of embryogenesis and plays an essential role in ESC differentiation in vitro. EB uniformity and size are critical parameters that directly influence the phenotype expression of ESCs. Various methods have been developed to form EBs, which involve natural aggregation of cells. However, challenges persist to form EBs with controlled size, shape, and uniformity in a reproducible manner. The current hanging-drop methods are labor intensive and time consuming. In this study, we report an approach to form controllable, uniform-sized EBs by integrating bioprinting technologies with the existing hanging-drop method. The approach presented here is simple, robust, and rapid. We present significantly enhanced EB size uniformity compared to the conventional manual hanging-drop method.

A three-dimensional in vitro ovarian cancer coculture model using a high-throughput cell patterning platform

In vitro 3D cancer models that provide a more accurate representation of disease in vivo are urgently needed to improve our understanding of cancer pathology and to develop better cancer therapies. However, development of 3D models that are based on manual ejection of cells from micropipettes suffer from inherent limitations such as poor control over cell density, limited repeatability, low throughput, and, in the case of coculture models, lack of reproducible control over spatial distance between cell types (e.g., cancer and stromal cells). In this study, we build on a recently introduced 3D model in which human ovarian cancer (OVCAR-5) cells overlaid on Matrigel™ spontaneously form multicellular acini. We introduce a high-throughput automated cell printing system to bioprint a 3D coculture model using cancer cells and normal fi broblasts micropatterned on Matrigel™ . Two cell types were patterned within a spatially controlled microenvironment (e.g., cell density, cell-cell distance) in a high-throughput and reproducible manner; both cell types remained viable during printing and continued to proliferate following patterning. This approach enables the miniaturization of an established macro-scale 3D culture model and would allow systematic investigation into the multiple unknown regulatory feedback mechanisms between tumor and stromal cells and provide a tool for high-throughput drug screening.

High-throughput 3D spheroid culture and drug testing using a 384 hanging drop array

Culture of cells as three-dimensional (3D) aggregates can enhance in vitro tests for basic biological research as well as for therapeutics development. Such 3D culture models, however, are often more complicated, cumbersome, and expensive than two-dimensional (2D) cultures. This paper describes a 384-well format hanging drop culture plate that makes spheroid formation, culture, and subsequent drug testing on the obtained 3D cellular constructs as straightforward to perform and adapt to existing high-throughput screening (HTS) instruments as conventional 2D cultures. Using this platform, we show that drugs with different modes of action produce distinct responses in the physiological 3D cell spheroids compared to conventional 2D cell monolayers. Specifically, the anticancer drug 5-fluorouracil (5-FU) has higher anti-proliferative effects on 2D cultures whereas the hypoxia activated drug commonly referred to as tirapazamine (TPZ) are more effective against 3D cultures. The multiplexed 3D hanging drop culture and testing plate provides an efficient way to obtain biological insights that are often lost in 2D platforms.

Controlled-size embryoid body formation in concave microwell arrays

Embryonic stem (ES) cells hold great potential as a renewable cell source for regenerative medicine and cell-based therapy. Despite the potential of ES cells, conventional stem cell culture methods do not enable the control of the microenvironment. A number of microscale engineering approaches have been recently developed to control the extracellular microenvironment and to direct embryonic stem cell fate. Here, we used engineered concave microwell arrays to regulate the size and shape of embryoid bodies (EBs)-cell aggregate intermediates derived from ES cells. Murine ES cells were aggregated within concave microwells, and their aggregate sizes were controlled by varying the microwell widths (200, 500, and 1000 mum). Differentiation of murine ES cells into three germ layers was assessed by analyzing gene expression. We found that ES cell-derived cardiogenesis and neurogenesis were strongly regulated by the EB size, showing that larger concave microwell arrays induced more neuronal and cardiomyocyte differentiation than did smaller microwell arrays. Therefore, this engineered concave microwell array could be a potentially useful tool for controlling ES cell behavior.

Tunable shrink-induced honeycomb microwell arrays for uniform embryoid bodies

Embryoid body (EB) formation closely recapitulates early embryonic development with respect to lineage commitment. Because it is greatly affected by cell-cell and cell-substrate interactions, the ability to control the initial number of cells in the aggregates and to provide an appropriate substrate are crucial parameters for uniform EB formation. Here we report of an ultra-rapid fabrication and culture method utilizing a laser-jet printer to generate closely arrayed honeycomb microwells of tunable sizes for the induction of uniform EBs from single cell suspension. By printing various microwell patterns onto pre-stressed polystyrene sheets, and through heat induced shrinking, high aspect micromolds are generated. Notably, we achieve rounded bottom polydimethylsiloxane (PDMS) wells not easily achievable with standard microfabrication methods, but critical to achieve spherical EBs. Furthermore, by simply controlling the size of the microwells and the concentration of the cell suspension we can control the initial size of the cell aggregate, thus influencing lineage commitment. In addition, these microwells are easily adaptable and scalable to most standard well plates and easily integrated into commercial liquid handling systems to provide an inexpensive and easy high throughput compound screening platform.