Partitioning microfluidic channels with hydrogel to construct tunable 3-D cellular microenvironments

Zonal patterning of extracellular matrix and stromal cell populations along a perfusable cellular microchannel

The spatial organization of biophysical and biochemical cues in the extracellular matrix (ECM) in concert with reciprocal cell-cell signaling is vital to tissue patterning during development. However, elucidating the role an individual microenvironmental factor plays using existing in vivo models is difficult due to their inherent complexity. In this work, we have developed a microphysiological system to spatially pattern the biochemical, biophysical, and stromal cell composition of the ECM along an epithelialized 3D microchannel. This technique is adaptable to multiple hydrogel compositions and scalable to the number of zones patterned. We confirmed that the methodology to create distinct zones resulted in a continuous, annealed hydrogel with regional interfaces that did not hinder the transport of soluble molecules. Further, the interface between hydrogel regions did not disrupt microchannel structure, epithelial lumen formation, or media perfusion through an acellular or cellularized microchannel. Finally, we demonstrated spatially patterned tubulogenic sprouting of a continuous epithelial tube into the surrounding hydrogel confined to local regions with stromal cell populations, illustrating spatial control of cell-cell interactions and signaling gradients. This easy-to-use system has wide utility for modeling three-dimensional epithelial and endothelial tissue interactions with heterogeneous hydrogel compositions and/or stromal cell populations to investigate their mechanistic roles during development, homeostasis, or disease.

Zonal Patterning of Extracellular Matrix and Stromal Cell Populations Along a Perfusable Cellular Microchannel

The spatial organization of biophysical and biochemical cues in the extracellular matrix (ECM) in concert with reciprocal cell-cell signaling is vital to tissue patterning during development. However, elucidating the role an individual microenvironmental factor plays using existing in vivo models is difficult due to their inherent complexity. In this work, we have developed a microphysiological system to spatially pattern the biochemical, biophysical, and stromal cell composition of the ECM along an epithelialized 3D microchannel. This technique is adaptable to multiple hydrogel compositions and scalable to the number of zones patterned. We confirmed that the methodology to create distinct zones resulted in a continuous, annealed hydrogel with regional interfaces that did not hinder the transport of soluble molecules. Further, the interface between hydrogel regions did not disrupt microchannel structure, epithelial lumen formation, or media perfusion through an acellular or cellularized microchannel. Finally, we demonstrated spatially patterned tubulogenic sprouting of a continuous epithelial tube into the surrounding hydrogel confined to local regions with stromal cell populations, illustrating spatial control of cell-cell interactions and signaling gradients. This easy-to-use system has wide utility for modeling three-dimensional epithelial and endothelial tissue interactions with heterogeneous hydrogel compositions and/or stromal cell populations to investigate their mechanistic roles during development, homeostasis, or disease.

A Modular Microfluidic Organoid Platform Using LEGO-Like Bricks

The convergence of organoid and organ-on-a-chip (OoC) technologies is urgently needed to overcome limitations of current 3D in vitro models. However, integrating organoids in standard OoCs faces several technical challenges, as it is typically laborious, lacks flexibility, and often results in even more complex and less-efficient cell culture protocols. Therefore, specifically adapted and more flexible microfluidic platforms need to be developed to facilitate the incorporation of complex 3D in vitro models. Here, a modular, tubeless fluidic circuit board (FCB) coupled with reversibly sealed cell culture bricks for dynamic culture of embryonic stem cell-derived thyroid follicles is developed. The FCB is fabricated by milling channels in a polycarbonate (PC) plate followed by thermal bonding against another PC plate. LEGO-like fluidic interconnectors allow plug-and-play connection between a variety of cell culture bricks and the FCB. Lock-and-play clamps are integrated in the organoid brick to enable easy (un)loading of organoids. A multiplexed perfusion experiment is conducted with six FCBs, where thyroid organoids are transferred on-chip within minutes and cultured up to 10 d without losing their structure and functionality, thus validating this system as a flexible, easy-to-use platform, capable of synergistically combining organoids with advanced microfluidic platforms.

Superhydrophilic–superhydrophobic patterned surfaces: From simplified fabrication to emerging applications

Superhydrophilic–superhydrophobic patterned surfaces constitute a branch of surface chemistry involving the two extreme states of superhydrophilicity and superhydrophobicity combined on the same surface in precise patterns. Such surfaces have many advantages, including controllable wettability, enrichment ability, accessibility, and the ability to manipulate and pattern water droplets, and they offer new functionalities and possibilities for a wide variety of emerging applications, such as microarrays, biomedical assays, microfluidics, and environmental protection. This review presents the basic theory, simplified fabrication, and emerging applications of superhydrophilic–superhydrophobic patterned surfaces. First, the fundamental theories of wettability that explain the spreading of a droplet on a solid surface are described. Then, the fabrication methods for preparing superhydrophilic–superhydrophobic patterned surfaces are introduced, and the emerging applications of such surfaces that are currently being explored are highlighted. Finally, the remaining challenges of constructing such surfaces and future applications that would benefit from their use are discussed.

Advances in Organ-on-a-Chip Materials and Devices

The organ-on-a-chip (OoC) paves a way for biomedical applications ranging from preclinical to clinical translational precision. The current trends in the in vitro modeling is to reduce the complexity of human organ anatomy to the fundamental cellular microanatomy as an alternative of recreating the entire cell milieu that allows systematic analysis of medicinal absorption of compounds, metabolism, and mechanistic investigation. The OoC devices accurately represent human physiology in vitro; however, it is vital to choose the correct chip materials. The potential chip materials include inorganic, elastomeric, thermoplastic, natural, and hybrid materials. Despite the fact that polydimethylsiloxane is the most commonly utilized polymer for OoC and microphysiological systems, substitute materials have been continuously developed for its advanced applications. The evaluation of human physiological status can help to demonstrate using noninvasive OoC materials in real-time procedures. Therefore, this Review examines the materials used for fabricating OoC devices, the application-oriented pros and cons, possessions for device fabrication and biocompatibility, as well as their potential for downstream biochemical surface alteration and commercialization. The convergence of emerging approaches, such as advanced materials, artificial intelligence, machine learning, three-dimensional (3D) bioprinting, and genomics, have the potential to perform OoC technology at next generation. Thus, OoC technologies provide easy and precise methodologies in cost-effective clinical monitoring and treatment using standardized protocols, at even personalized levels. Because of the inherent utilization of the integrated materials, employing the OoC with biomedical approaches will be a promising methodology in the healthcare industry.

Construction of in vitro 3-D model for lung cancer-cell metastasis study

Background

Cancer metastasis is the main cause of mortality in cancer patients. However, the drugs targeting metastasis processes are still lacking, which is partially due to the short of effective in vitro model for cell invasion studies. The traditional 2-D culture method cannot reveal the interaction between cells and the surrounding extracellular matrix during invasion process, while the animal models usually are too complex to explain mechanisms in detail. Therefore, a precise and efficient 3-D in vitro model is highly desirable for cell invasion studies and drug screening tests.

Methods

Precise micro-fabrication techniques are developed and integrated with soft hydrogels for constructing of 3-D lung-cancer micro-environment, mimicking the pulmonary gland or alveoli as in vivo.

Results

A 3-D in vitro model for cancer cell culture and metastasis studies is developed with advanced micro-fabrication technique, combining microfluidic system with soft hydrogel. The constructed microfluidic platform can provide nutrition and bio-chemical factors in a continuous transportation mode and has the potential to form stable chemical gradient for cancer invasion research. Hundreds of micro-chamber arrays are constructed within the collagen gel, ensuring that all surrounding substrates for tumor cells are composed of natural collagen hydrogel, like the in vivo micro-environment. The 3-D in vitro model can also provide a fully transparent platform for the visual observation of the cell morphology, proliferation, invasion, cell-assembly, and even the protein expression by immune-fluorescent tests if needed.

The lung-cancer cells A549 and normal lung epithelial cells (HPAEpiCs) have been seeded into the 3-D system. It is found out that cells can normally proliferate in the microwells for a long period. Moreover, although the cancer cells A549 and alveolar epithelial cells HPAEpiCs have the similar morphology on 2-D solid substrate, in the 3-D system the cancer cells A549 distributed sparsely as single round cells on the extracellular matrix (ECM) when they attached to the substrate, while the normal lung epithelial cells can form cell aggregates, like the structure of normal tissue. Importantly, cancer cells cultured in the 3-D in vitro model can exhibit the interaction between cells and extracellular matrix. As shown in the confocal microscope images, the A549 cells present round and isolated morphology without much invasion into ECM, while starting from around Day 5, cells changed their shape to be spindle-like, as in mesenchymal morphology, and then started to destroy the surrounding ECM and invade out of the micro-chambers.

Conclusions

A 3-D in vitro model is constructed for cancer cell invasion studies, combining the microfluidic system and micro-chamber structures within hydrogel. To show the invasion process of lung cancer cells, the cell morphology, proliferation, and invasion process are all analyzed. The results confirmed that the micro-environment in the 3-D model is vital for revealing the lung cancer cell invasion as in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09546-9.

Stiff and strong hydrogel tube with great mechanical properties and high stability in various solutions

Hydrogel tubes are widely used in fields such as artificial blood vessels, drug delivery, biomedical scaffolds and cell adhesion, yet their application is often limited by unsatisfactory mechanical properties and poor stability in various solutions. Herein, a novel hydrogel tube exhibiting a remarkable mechanical performance and stability in various solutions is prepared by introducing a dual physically cross-linked double network (DN) hydrogel matrix. The obtained hydrogel tube can withstand ∼60 N load without fracture and be stretched to over twice its original length before and after immersing in various solutions. The great mechanical properties and stability in various solutions of hydrogel tubes are due to the introduction of a dual physically cross-linked poly(acrylamide-co-acrylic acid)/carboxymethylcellulose sodium/Fe³⁺ DN hydrogel, which possesses high elastic modulus (3.71 MPa), fracture energy (15.4 kJ m^-2), and great stability in various solutions. In addition, the hydrogel tubes with different thickness, diameters, shapes and the multiple branched hydrogel tubes can also be fabricated to enable further functionalization for application requirements. Therefore, this new type of hydrogel tube presents tremendous potential for applications in biomedical and engineering fields.

Engineered neural circuits for modeling brain physiology and neuropathology

The neural circuits of the central nervous system are the regulatory pathways for feeling, motion control, learning, and memory, and their dysfunction is closely related to various neurodegenerative diseases. Despite the growing demand for the unraveling of the physiology and functional connectivity of the neural circuits, their fundamental investigation is hampered because of the inability to access the components of neural circuits and the complex microenvironment. As an alternative approach, in vitro human neural circuits show principles of in vivo human neuronal circuit function. They allow access to the cellular compartment and permit real-time monitoring of neural circuits. In this review, we summarize recent advances in reconstituted in vitro neural circuits using engineering techniques. To this end, we provide an overview of the fabrication techniques and methods for stimulation and measurement of in vitro neural circuits. Subsequently, representative examples of in vitro neural circuits are reviewed with a particular focus on the recapitulation of structures and functions observed in vivo, and we summarize their application in the study of various brain diseases. We believe that the in vitro neural circuits can help neuroscience and the neuropharmacology. STATEMENT OF SIGNIFICANCE: Despite the growing demand to unravel the physiology and functional connectivity of the neural circuits, the studies on the in vivo neural circuits are frequently limited due to the poor accessibility. Furthermore, single neuron-based analysis has an inherent limitation in that it does not reflect the full spectrum of the neural circuit physiology. As an alternative approach, in vitro engineered neural circuit models have arisen because they can recapitulate the structural and functional characteristics of in vivo neural circuits. These in vitro neural circuits allow the mimicking of dysregulation of the neural circuits, including neurodegenerative diseases and traumatic brain injury. Emerging in vitro engineered neural circuits will provide a better understanding of the (patho-)physiology of neural circuits.

Coupling fluid flow to hydrogel fluidic devices with reversible "pop-it" connections

Hydrogels are soft, water-based polymer gels that are increasingly used to fabricate free-standing fluidic devices for tissue and biological engineering applications. For many of these applications, pressurized liquid must be driven through the hydrogel device. To couple pressurized liquid to a hydrogel device, a common approach is to insert tubing into a hole in the gel; however, this usually results in leakage and expulsion of the tubing, and other options for coupling pressurized liquid to hydrogels remain limited. Here, we describe a simple coupling approach where microfluidic tubing is inserted into a plastic, 3D-printed bulb-shaped connector, which "pops" into a 3D-printed socket in the gel. By systematically varying the dimensions of the connector relative to those of the socket entrance, we find an optimal head-socket ratio that provides maximum resistance to leakage and expulsion. The resulting connection can withstand liquid pressures on the order of several kilopascals, three orders of magnitude greater than traditional, connector-free approaches. We also show that two-sided connectors can be used to link multiple hydrogels to one another to build complex, reconfigurable hydrogel systems from modular components. We demonstrate the potential usefulness of these connectors by established long-term nutrient flow through a 3D-printed hydrogel device containing bacteria. The simple coupling approach outlined here will enable a variety of applications in hydrogel fluidics.

Oncoimmunology Meets Organs-on-Chip

Oncoimmunology represents a biomedical research discipline coined to study the roles of immune system in cancer progression with the aim of discovering novel strategies to arm it against the malignancy. Infiltration of immune cells within the tumor microenvironment is an early event that results in the establishment of a dynamic cross-talk. Here, immune cells sense antigenic cues to mount a specific anti-tumor response while cancer cells emanate inhibitory signals to dampen it. Animals models have led to giant steps in this research context, and several tools to investigate the effect of immune infiltration in the tumor microenvironment are currently available. However, the use of animals represents a challenge due to ethical issues and long duration of experiments. Organs-on-chip are innovative tools not only to study how cells derived from different organs interact with each other, but also to investigate on the crosstalk between immune cells and different types of cancer cells. In this review, we describe the state-of-the-art of microfluidics and the impact of OOC in the field of oncoimmunology underlining the importance of this system in the advancements on the complexity of tumor microenvironment.

Engineered Tools to Study Intercellular Communication

All multicellular organisms rely on intercellular communication networks to coordinate physiological functions. As members of a dynamic social network, each cell receives, processes, and redistributes biological information to define and maintain tissue homeostasis. Uncovering the molecular programs underlying these processes is critical for prevention of disease and aging and development of therapeutics. The study of intercellular communication requires techniques that reduce the scale and complexity of in vivo biological networks while resolving the molecular heterogeneity in "omic" layers that contribute to cell state and function. Recent advances in microengineering and high-throughput genomics offer unprecedented spatiotemporal control over cellular interactions and the ability to study intercellular communication in a high-throughput and mechanistic manner. Herein, this review discusses how salient engineered approaches and sequencing techniques can be applied to understand collective cell behavior and tissue functions.

3D Printed Hydrogels with Aligned Microchannels to Guide Neural Stem Cell Migration

Following traumatic or ischemic brain injury, rapid cell death and extracellular matrix degradation lead to the formation of a cavity at the brain lesion site, which is responsible for prolonged neurological deficits and permanent disability. Transplantation of neural stem/progenitor cells (NSCs) represents a promising strategy for reconstructing the lesion cavity and promoting tissue regeneration. In particular, the promotion of neuronal migration, organization, and integration of transplanted NSCs is critical to the success of stem cell-based therapy. This is particularly important for the cerebral cortex, the most common area involved in brain injuries, because the highly organized structure of the cerebral cortex is essential to its function. Biomaterials-based strategies show some promise for conditioning the lesion site microenvironment to support transplanted stem cells, but the progress in demonstrating organized cell engraftment and integration into the brain is very limited. An effective approach to sufficiently address these challenges has not yet been developed. Here, we have implemented a digital light-processing-based 3D printer and printed hydrogel scaffolds with a designed shape, uniaxially aligned microchannels, and tunable mechanical properties. We demonstrated the capacity to achieve high shape precision to the lesion site with brain tissue-matching mechanical properties. We also established spatial control of bioactive molecule distribution within 3D printed hydrogel scaffolds. These printed hydrogel scaffolds have shown high neuro-compatibility with aligned neuronal outgrowth along with the microchannels. This study will provide a biomaterial-based approach that can serve as a protective and guidance vehicle for transplanted NSC organization and integration for brain tissue regeneration after injuries.

3D Culture Systems for Exploring Cancer Immunology

Simple Summary

To study any disease, researchers need convenient and relevant disease models. In cancer, the most commonly used models are two-dimensional (2D) culture models, which grow cells on hard, rigid, plastic surfaces, and mouse models. Cancer immunology is especially difficult to model because the immune system is exceedingly complex; it contains multiple types of cells, and each cell type has several subtypes and a spectrum of activation states. These many immune cell types interact with cancer cells and other components of the tumor, ultimately influencing disease outcomes. 2D culture methods fail to recapitulate these complex cellular interactions. Mouse models also suffer because the murine and human immune systems vary significantly. Three-dimensional (3D) culture systems therefore provide an alternative method to study cancer immunology and can fill the current gaps in available models. This review will describe common 3D culture models and how those models have been used to advance our understanding of cancer immunology.

Abstract

Cancer immunotherapy has revolutionized cancer treatment, spurring extensive investigation into cancer immunology and how to exploit this biology for therapeutic benefit. Current methods to investigate cancer-immune cell interactions and develop novel drug therapies rely on either two-dimensional (2D) culture systems or murine models. However, three-dimensional (3D) culture systems provide a potentially superior alternative model to both 2D and murine approaches. As opposed to 2D models, 3D models are more physiologically relevant and better replicate tumor complexities. Compared to murine models, 3D models are cheaper, faster, and can study the human immune system. In this review, we discuss the most common 3D culture systems—spheroids, organoids, and microfluidic chips—and detail how these systems have advanced our understanding of cancer immunology.

Enhanced In Vivo Delivery of Stem Cells using Microporous Annealed Particle Scaffolds

Delivery to the proper tissue compartment is a major obstacle hampering the potential of cellular therapeutics for medical conditions. Delivery of cells within biomaterials may improve localization, but traditional and newer void-forming hydrogels must be made in advance with cells being added into the scaffold during the manufacturing process. Injectable, in situ cross-linking microporous scaffolds are recently developed that demonstrate a remarkable ability to provide a matrix for cellular proliferation and growth in vitro in three dimensions. The ability of these scaffolds to deliver cells in vivo is currently unknown. Herein, it is shown that mesenchymal stem cells (MSCs) can be co-injected locally with microparticle scaffolds assembled in situ immediately following injection. MSC delivery within a microporous scaffold enhances MSC retention subcutaneously when compared to cell delivery alone or delivery within traditional in situ cross-linked nanoporous hydrogels. After two weeks, endothelial cells forming blood vessels are recruited to the scaffold and cells retaining the MSC marker CD29 remain viable within the scaffold. These findings highlight the utility of this approach in achieving localized delivery of stem cells through an injectable porous matrix while limiting obstacles of introducing cells within the scaffold manufacturing process.

The Need for Physiological Micro-Nanofluidic Systems of the Brain

In this article, we review brain-on-a-chip models and associated underlying technologies. Micro-nanofluidic systems of the brain can utilize the entire spectrum of organoid technology. Notably, there is an urgent clinical need for a physiologically relevant microfluidic platform that can mimic the brain. Brain diseases affect millions of people worldwide, and this number will grow as the size of elderly population increases, thus making brain disease a serious public health problem. Brain disease modeling typically involves the use of in vivo rodent models, which is time consuming, resource intensive, and arguably unethical because many animals are required for a single study. Moreover, rodent models may not accurately predict human diseases, leading to erroneous results, thus rendering animal models poor predictors of human responses to treatment. Various clinical researchers have highlighted this issue, showing that initial physiological descriptions of animal models rarely encompass all the desired human features, including how closely the model captures what is observed in patients. Consequently, such animal models only mimic certain disease aspects, and they are often inadequate for studying how a certain molecule affects various aspects of a disease. Thus, there is a great need for the development of the brain-on-a-chip technology based on which a human brain model can be engineered by assembling cell lines to generate an organ-level model. To produce such a brain-on-a-chip device, selection of appropriate cells lines is critical because brain tissue consists of many different neuronal subtypes, including a plethora of supporting glial cell types. Additionally, cellular network bio-architecture significantly varies throughout different brain regions, forming complex structures and circuitries; this needs to be accounted for in the chip design process. Compartmentalized microenvironments can also be designed within the microphysiological cell culture system to fulfill advanced requirements of a given application. On-chip integration methods have already enabled advances in Parkinson's disease, Alzheimer's disease, and epilepsy modeling, which are discussed herein. In conclusion, for the brain model to be functional, combining engineered microsystems with stem cell (hiPSC) technology is specifically beneficial because hiPSCs can contribute to the complexity of tissue architecture based on their level of differentiation and thereby, biology itself.

Quantitative Label-Free Imaging of 3D Vascular Networks Self-Assembled in Synthetic Hydrogels

Vascularization is an important strategy to overcome diffusion limits and enable the formation of complex, physiologically relevant engineered tissues and organoids. Self-assembly is a technique to generate in vitro vascular networks, but engineering the necessary network morphology and function remains challenging. Here, autofluorescence multiphoton microscopy (aMPM), a label-free imaging technique, is used to quantitatively evaluate in vitro vascular network morphology. Vascular networks are generated using human embryonic stem cell-derived endothelial cells and primary human pericytes encapsulated in synthetic poly(ethylene glycol)-based hydrogels. Two custom-built bioreactors are used to generate distinct fluid flow patterns during vascular network formation: recirculating flow or continuous flow. aMPM is used to image these 3D vascular networks without the need for fixation, labels, or dyes. Image processing and analysis algorithms are developed to extract quantitative morphological parameters from these label-free images. It is observed with aMPM that both bioreactors promote formation of vascular networks with lower network anisotropy compared to static conditions, and the continuous flow bioreactor induces more branch points compared to static conditions. Importantly, these results agree with trends observed with immunocytochemistry. These studies demonstrate that aMPM allows label-free monitoring of vascular network morphology to streamline optimization of growth conditions and provide quality control of engineered tissues.

An optimized procedure to develop a 3-dimensional microfluidic hydrogel with parallel transport networks

The development of microfluidic hydrogels is an attractive method to generate continuous perfusion, induce vascularization, increase solute delivery, and ultimately improve cell viability. However, the transport processes in many in vitro studies still have not been realized completely. To address this problem, we have developed a microchanneled hydrogel with different collagen type I concentrations of 1, 2, and 3 wt% and assessed its physical properties and obtained diffusion coefficient of nutrient within the hydrogel. It is well known that microchannel geometry has critical role in maintaining stable perfusion rate. Therefore, in this study, a computational modeling was applied to simulate the 3D microfluidic hydrogel and study the effect of geometric parameters such as microchannel diameters and their distance on the nutrient diffusion. The simulation results showed that the sample with 3 channels with a diameter of 300 μm has adequate diffusion rates and efficiency (56%). Moreover, this system provides easy control and continuous perfusion rate during 5 days of cell culturing. The simulation results were compared with experimental data, and a good correlation was observed for nutrient profiles and cell viability across the hydrogel.

Detection and Automation Technologies for the Mass Production of Droplet Biomicrofluidics

Droplet microfluidics utilizes two immiscible flows to generate small droplets with the diameter of a few to a few hundred micrometers. These droplets are promising tools for biomedical engineering because of the high throughput and the ease to finely tune the microenvironments. In addition to the great success of droplet biomicrofluidics in the proof-of-concept biosensing, regenerative medicine, and drug delivery, few droplet biomicrofluidic devices have a transformative impact on the industrial and clinical applications. The main issues are the low volume throughput and the lack of proper methods for quality control and automation. This review covers the methodologies for the mass production, detection, and automation of droplet generators. Recent advances in droplet mass production using parallelized devices and modified junction structures are discussed. Detection techniques, including optical and electrical detection methods, are comprehensively reviewed in detail. Newly emerged droplet closed-loop control systems are surveyed to highlight the progress in system integration and automation. Overall, with the advances in parallel droplet generation, highly sensitive detection, and robust closed-loop regulation, it is anticipated that the productivity and reliability of droplet biomicrofluidics will be significantly improved to meet the industrial and clinical needs.

Thermal Conductivity of Polyacrylamide Hydrogels at the Nanoscale

A polymer network can imbibe copious amounts of water and swell, and the resulting state is known as a hydrogel. In many potential applications of hydrogels, such as stretchable conductors, ionic cables, and neuroprostheses, the thermal conductivities of hydrogels should be understood clearly. In the present work, we build molecular dynamics (MD) models of random cross-linked polyacrylamide hydrogels with different water volume fractions through a reaction method. On the basis of these models, thermal conductivities of hydrogels at the nanoscale are investigated by a none-equilibrium MD method. This work reveals that when the water fraction of hydrogels is under 85%, the thermal conductivity increases with the water fraction, and can be even higher than the thermal conductivities of both pure polymer networks and pure water because of the influence of the interface between polymer networks and water. However, when the water fraction in hydrogels is bigger than 85%, its thermal conductivity will decrease and get close to the water's conductivity. Accordingly, to explain this abnormal phenomenon, a 2-order-3-phase theoretical model is proposed by considering hydrogel as a 3-phase composite. It can be found that the proposed theory can predict results which agree quite well with our simulated results.

Tuning Enzymatically Crosslinked Silk Fibroin Hydrogel Properties for the Development of a Colorectal Cancer Extravasation 3D Model on a Chip

Microfluidic devices are now the most promising tool to mimic in vivo like scenarios such as tumorigenesis and metastasis due to its ability to more closely mimic cell's natural microenvironment (such as 3D environment and continuous perfusion of nutrients). In this study, the ability of 2% and 3% enzymatically crosslinked silk fibroin hydrogels with different mechanical properties are tested in terms of colorectal cancer cell migration, under different microenvironments in a 3D dynamic model. Matrigel is used as control. Moreover, a comprehensive comparison between the traditional Boyden chamber assay and the 3D dynamic microfluidic model in terms of colorectal cancer cell migration is presented. The results show profound differences between the two used biomaterials and the two migration models, which are explored in terms of mechanical properties of the hydrogels as well as the intrinsic characteristics of the models. Moreover, the developed 3D dynamic model is validated by demonstrating that hVCAM-1 plays a major role in the extravasation process, influencing extravasation rate and traveled distance. Furthermore, the developed model enables precise visualization of cancer cell migration within a 3D matrix in response to microenvironmental cues, shedding light on the importance of biophysical properties in cell behavior.

Microfabrication-Based Three-Dimensional (3-D) Extracellular Matrix Microenvironments for Cancer and Other Diseases

Exploring the complicated development of tumors and metastases needs a deep understanding of the physical and biological interactions between cancer cells and their surrounding microenvironments. One of the major challenges is the ability to mimic the complex 3-D tissue microenvironment that particularly influences cell proliferation, migration, invasion, and apoptosis in relation to the extracellular matrix (ECM). Traditional cell culture is unable to create 3-D cell scaffolds resembling tissue complexity and functions, and, in the past, many efforts were made to realize the goal of obtaining cell clusters in hydrogels. However, the available methods still lack a precise control of cell external microenvironments. Recently, the rapid development of microfabrication techniques, such as 3-D printing, microfluidics, and photochemistry, has offered great advantages in reconstructing 3-D controllable cancer cell microenvironments in vitro. Consequently, various biofunctionalized hydrogels have become the ideal candidates to help the researchers acquire some new insights into various diseases. Our review will discuss some important studies and the latest progress regarding the above approaches for the production of 3-D ECM structures for cancer and other diseases. Especially, we will focus on new discoveries regarding the impact of the ECM on different aspects of cancer metastasis, e.g., collective invasion, enhanced intravasation by stress and aligned collagen fibers, angiogenesis regulation, as well as on drug screening.

Upgrading well plates using open microfluidic patterning

Cellular communication between multiple cell types is a ubiquitous process that is responsible for vital physiological responses observed in vivo (e.g., immune response, organ function). Many in vitro coculture strategies have been developed, both in traditional culture and microscale systems, and have shown the potential to recreate some of the physiological behaviors of organs or groups of cells. A fundamental limitation of current systems is the difficulty of reconciling the additional engineering requirements for creating soluble factor signaling systems (e.g., segregated cell culture) with the use of well-characterized materials and platforms that have demonstrated successful results and biocompatibility in assays. We present a new open-microfluidic platform, the Monorail Device, that is placed in any existing well plate or Petri dish and enables patterning of segregated coculture regions, thereby allowing the direct upgrade of monoculture experiments into multiculture assays. Our platform patterns biocompatible hydrogel walls via microfluidic spontaneous capillary flow (SCF) along a rail insert set inside commercially available cultureware, creating customized pipette-accessible cell culture chambers that require fewer cells than standard macroscale culture. Importantly, the device allows the use of native surfaces without additional modification or treatments, while creating permeable dividers for the diffusion of soluble factors. Additionally, the ease of patterning afforded by our platform makes reconfiguration of the culture region as simple as changing the rail insert. We demonstrate the ability of the device to pattern flows on a variety of cell culture surfaces and create hydrogel walls in complex and precise shapes. We characterize the physical parameters that enable a reproducible SCF-driven flow and highlight specialized design features that increase the ease of use of the device and control of the open microfluidic flow. Further, we present the performance of our platform according to useful coculture criteria, including permeability and integrity of our hydrogel walls and surface-sensitive cell culture. Lastly, we show the potential of this type of platform to create modular multikingdom culture systems that can be used to study soluble factor signaling between mammalian cells, bacteria, and fungi, as well as the potential for adaptation of this technology by researchers across multiple fields.

A tuneable microfluidic system for long duration chemotaxis experiments in a 3D collagen matrix

In many cell types, migration can be oriented towards a chemical stimulus. In mammals, for example, embryonic cells migrate to follow developmental cues, immune cells migrate toward sites of inflammation, and cancer cells migrate away from the primary tumour and toward blood vessels during metastasis. Understanding how cells migrate in 3D environments in response to chemical cues is thus crucial to understanding directed migration in normal and disease states. To date, chemotaxis in mammalian cells has been primarily studied using 2D migration models. However, it is becoming increasingly clear that the mechanisms by which cells migrate in 2D and 3D environments dramatically differ, and cells in their native environments are confronted with a complex chemical milieu. To address these issues, we developed a microfluidic device to monitor the behaviour of cells embedded in a 3D collagen matrix in the presence of complex concentration fields of chemoattractants. This tuneable microsystem enables the generation of (1) homogeneous, stationary gradients set by a purely diffusive mechanism, or (2) spatially evolving, stationary gradients, set by a convection-diffusion mechanism. The device allows for stable gradients over several days and is large enough to study the behaviour of large cell aggregates. We observe that primary mature dendritic cells respond uniformly to homogeneous diffusion gradients, while cell behaviour is highly position-dependent in spatially variable convection-diffusion gradients. In addition, we demonstrate a directed response of cancer cells migrating away from tumour-like aggregates in the presence of soluble chemokine gradients. Together, this microfluidic device is a powerful system to observe the response of different cells and aggregates to tuneable chemical gradients.

A novel 3-D bio-microfluidic system mimicking in vivo heterogeneous tumour microstructures reveals complex tumour-stroma interactions

A 3-D microfluidic system consisting of microchamber arrays embedded in a collagen hydrogel with tuneable biochemical gradients that mimics the tumour microenvironment of mammary glands was constructed for the investigation on the interactions between invasive breast cancer cells and stromal cells. The hollow microchambers in collagen provide a very similar 3-D environment to that in vivo that regulates collective cellular dynamics and behaviour, while the microfluidic channels surrounding the collagen microchamber arrays allow one to impose complex concentration gradients of specific biological molecules or drugs. We found that breast epithelial cells (MCF-10A) seeded in the microchambers formed lumen-like structures similar to those in epithelial layers. When MCF-10A cells were co-cultured with invasive breast cancer cells (MDA-MB-231), the formation of lumen-like structures in the microchambers was inhibited, indicating the capability of cancer cells to disrupt the structures formed by surrounding cells for further invasion and metastasis. Subsequent mechanism studies showed that down regulation of E-cad expression due to MMPs produced by the cancer cells plays a dominant role in determining the cellular behaviour. Our microfluidic system offers a robust platform for high throughput studies that aim to understand combinatorial effects of multiple biochemical and microenvironmental factors.

Conceptual and Experimental Tools to Understand Spatial Effects and Transport Phenomena in Nonlinear Biochemical Networks Illustrated with Patchy Switching

Many biochemical systems are spatially heterogeneous and exhibit nonlinear behaviors, such as state switching in response to small changes in the local concentration of diffusible molecules. Systems as varied as blood clotting, intracellular calcium signaling, and tissue inflammation are all heavily influenced by the balance of rates of reaction and mass transport phenomena including flow and diffusion. Transport of signaling molecules is also affected by geometry and chemoselective confinement via matrix binding. In this review, we use a phenomenon referred to as patchy switching to illustrate the interplay of nonlinearities, transport phenomena, and spatial effects. Patchy switching describes a change in the state of a network when the local concentration of a diffusible molecule surpasses a critical threshold. Using patchy switching as an example, we describe conceptual tools from nonlinear dynamics and chemical engineering that make testable predictions and provide a unifying description of the myriad possible experimental observations. We describe experimental microfluidic and biochemical tools emerging to test conceptual predictions by controlling transport phenomena and spatial distribution of diffusible signals, and we highlight the unmet need for in vivo tools.

Reconfigurable microfluidic device with discretized sidewall

Various microfluidic features, such as traps, have been used to manipulate flows, cells, and other particles within microfluidic systems. However, these features often become undesirable in subsequent steps requiring different fluidic configurations. To meet the changing needs of various microfluidic configurations, we developed a reconfigurable microfluidic channel with movable sidewalls using mechanically discretized sidewalls of laterally aligned rectangular pins. The user can deform the channel sidewall at any time after fabrication by sliding the pins. We confirmed that the flow resistance of the straight microchannel could be reversibly adjusted in the range of 10¹-10⁵Pa s/μl by manually displacing one of the pins comprising the microchannel sidewall. The reconfigurable microchannel also made it possible to manipulate flows and cells by creating a segmented patterned culture of COS-7 cells and a coculture of human umbilical vein endothelial cells (HUVECs) and human lung fibroblasts (hLFs) inside the microchannel. The reconfigurable microfluidic device successfully maintained a culture of COS-7 cells in a log phase throughout the entire period of 216 h. Furthermore, we performed a migration assay of cocultured HUVEC and hLF spheroids within one microchannel and observed their migration toward each other.

Generating Multicompartmental 3D Biological Constructs Interfaced through Sequential Injections in Microfluidic Devices

A novel technique is presented for molding and culturing composite 3D cellular constructs within microfluidic channels. The method is based on the use of removable molding polydimethylsiloxane (PDMS) inserts, which allow to selectively and incrementally generate composite 3D constructs featuring different cell types and/or biomaterials, with a high spatial control. The authors generate constructs made of either stacked hydrogels, with uniform horizontal interfaces, or flanked hydrogels with vertical interfaces. The authors also show how this technique can be employed to create custom-shaped endothelial barriers and monolayers directly interfaced with 3D cellular constructs. This method dramatically improves the significance of in vitro 3D biological models, enhancing mimicry and enabling for controlled studies of complex biological districts.

Bridging the gap: microfluidic devices for short and long distance cell-cell communication

Cell-cell communication is a crucial component of many biological functions. For example, understanding how immune cells and cancer cells interact, both at the immunological synapse and through cytokine secretion, can help us understand and improve cancer immunotherapy. The study of how cells communicate and form synaptic connections is important in neuroscience, ophthalmology, and cancer research. But in order to increase our understanding of these cellular phenomena, better tools need to be developed that allow us to study cell-cell communication in a highly controlled manner. Some technical requirements for better communication studies include manipulating cells spatiotemporally, high resolution imaging, and integrating sensors. Microfluidics is a powerful platform that has the ability to address these requirements and other current limitations. In this review, we describe some new advances in microfluidic technologies that have provided researchers with novel methods to study intercellular communication. The advantages of microfluidics have allowed for new capabilities in both single cell-cell communication and population-based communication. This review highlights microfluidic communication devices categorized as "short distance", or primarily at the single cell level, and "long distance", which mostly encompasses population level studies. Future directions and translation/commercialization will also be discussed.

Cytotoxic responses of carnosic acid and doxorubicin on breast cancer cells in butterfly-shaped microchips in comparison to 2D and 3D culture

Two dimensional (2D) cell culture systems lack the ability to mimic in vivo conditions resulting in limitations for preclinical cell-based drug and toxicity screening assays and modelling tumor biology. Alternatively, 3D cell culture systems mimic the specificity of native tissue with better physiological integrity. In this regard, microfluidic chips have gained wide applicability for in vitro 3D cancer cell studies. The aim of this research was to develop a 3D biomimetic model comprising culture of breast cancer cells in butterfly-shaped microchip to determine the cytotoxicity of carnosic acid and doxorubicin on both estrogen dependent (MCF-7) and independent (MDA-MB231) breast cancer cells along with healthy mammary epithelial cells (MCF-10A) in 2D, 3D Matrigel™ and butterfly-shaped microchip environment. According to the developed mimetic model, carnosic acid exhibited a higher cytotoxicity towards MDA-MB 231, while doxorubicin was more effective against MCF-7. Although the cell viabilities were higher in comparison to 2D and 3D cell culture systems, the responses of the investigated molecules were different in the microchips based on the molecular weight and structural complexity indicating the importance of biomimicry in a physiologically relevant matrix.

Detecting cell-secreted growth factors in microfluidic devices using bead-based biosensors

Microfluidic systems provide an interesting alternative to standard macroscale cell cultures due to the decrease in the number of cells and reagents as well as the improved physiology of cells confined to small volumes. However, the tools available for cell-secreted molecules inside microfluidic devices remain limited. In this paper, we describe an integrated microsystem composed of a microfluidic device and a fluorescent microbead-based assay for the detection of the hepatocyte growth factor (HGF) and the transforming growth factor (TGF)-β1 secreted by primary hepatocytes. This microfluidic system is designed to separate a cell culture chamber from sensing chambers using a permeable hydrogel barrier. Cell-secreted HGF and TGF-β1 diffuse through the hydrogel barrier into adjacent sensing channels and are detected using fluorescent microbead-based sensors. The specificity of sensing microbeads is defined by the choice of antibodies; therefore, our microfluidic culture system and sensing microbeads may be applied to a variety of cells and cell-secreted factors.

Transient microfluidic compartmentalization using actionable microfilaments for biochemical assays, cell culture and organs-on-chip

We report here a simple yet robust transient compartmentalization system for microfluidic platforms. Cylindrical microfilaments made of commercially available fishing lines are embedded in a microfluidic chamber and employed as removable walls, dividing the chamber into several compartments. These partitions allow tight sealing for hours, and can be removed at any time by longitudinal sliding with minimal hydrodynamic perturbation. This allows the easy implementation of various functions, previously impossible or requiring more complex instrumentation. In this study, we demonstrate the applications of our strategy, firstly to trigger chemical diffusion, then to make surface co-coating or cell co-culture on a two-dimensional substrate, and finally to form multiple cell-laden hydrogel compartments for three-dimensional cell co-culture in a microfluidic device. This technology provides easy and low-cost solutions, without the use of pneumatic valves or external equipment, for constructing well-controlled microenvironments for biochemical and cellular assays.

Evaluating Biomaterial- and Microfluidic-Based 3D Tumor Models

Cancer is a major cause of morbidity and mortality worldwide, with a disease burden estimated to increase over the coming decades. Disease heterogeneity and limited information on cancer biology and disease mechanisms are aspects that 2D cell cultures fail to address. Here, we review the current ‘state-of-the-art’ in 3D tissue-engineering (TE) models developed for, and used in, cancer research. We assess the potential for scaffold-based TE models and microfluidics to fill the gap between 2D models and clinical application. We also discuss recent advances in combining the principles of 3D TE models and microfluidics, with a special focus on biomaterials and the most promising chip-based 3D models.

Combinatorial Method/High Throughput Strategies for Hydrogel Optimization in Tissue Engineering Applications

Combinatorial method/high throughput strategies, which have long been used in the pharmaceutical industry, have recently been applied to hydrogel optimization for tissue engineering applications. Although many combinatorial methods have been developed, few are suitable for use in tissue engineering hydrogel optimization. Currently, only three approaches (design of experiment, arrays and continuous gradients) have been utilized. This review highlights recent work with each approach. The benefits and disadvantages of design of experiment, array and continuous gradient approaches depending on study objectives and the general advantages of using combinatorial methods for hydrogel optimization over traditional optimization strategies will be discussed. Fabrication considerations for combinatorial method/high throughput samples will additionally be addressed to provide an assessment of the current state of the field, and potential future contributions to expedited material optimization and design.

Imaging live cells at high spatiotemporal resolution for lab-on-a-chip applications

Conventional optical imaging techniques are limited by the diffraction limit and difficult-to-image biomolecular and sub-cellular processes in living specimens. Novel optical imaging techniques are constantly evolving with the desire to innovate an imaging tool that is capable of seeing sub-cellular processes in a biological system, especially in three dimensions (3D) over time, i.e. 4D imaging. For fluorescence imaging on live cells, the trade-offs among imaging depth, spatial resolution, temporal resolution and photo-damage are constrained based on the limited photons of the emitters. The fundamental solution to solve this dilemma is to enlarge the photon bank such as the development of photostable and bright fluorophores, leading to the innovation in optical imaging techniques such as super-resolution microscopy and light sheet microscopy. With the synergy of microfluidic technology that is capable of manipulating biological cells and controlling their microenvironments to mimic in vivo physiological environments, studies of sub-cellular processes in various biological systems can be simplified and investigated systematically. In this review, we provide an overview of current state-of-the-art super-resolution and 3D live cell imaging techniques and their lab-on-a-chip applications, and finally discuss future research trends in new and breakthrough research areas of live specimen 4D imaging in controlled 3D microenvironments.

Gel integration for microfluidic applications

Molecular diffusive membranes or materials are important for biological applications in microfluidic systems. Hydrogels are typical materials that offer several advantages, such as free diffusion for small molecules, biocompatibility with most cells, temperature sensitivity, relatively low cost, and ease of production. With the development of microfluidic applications, hydrogels can be integrated into microfluidic systems by soft lithography, flow-solid processes or UV cure methods. Due to their special properties, hydrogels are widely used as fluid control modules, biochemical reaction modules or biological application modules in different applications. Although hydrogels have been used in microfluidic systems for more than ten years, many hydrogels' properties and integrated techniques have not been carefully elaborated. Here, we systematically review the physical properties of hydrogels, general methods for gel-microfluidics integration and applications of this field. Advanced topics and the outlook of hydrogel fabrication and applications are also discussed. We hope this review can help researchers choose suitable methods for their applications using hydrogels.

Interplay between flow and diffusion in capillary alginate hydrogels

Alginate gels with naturally occurring macroscopic capillaries have been used as a model system to study the interplay between laminar flow and diffusion of nanometer-sized solutes in real time. Calcium alginate gels that contain homogeneously distributed parallel-aligned capillary structures were formed by external addition of crosslinking ions to an alginate sol. The effects of different flow rates (0, 1, 10, 50 and 100 μl min(-1)) and three different probes (fluorescein, 10 kDa and 500 kDa fluorescein isothiocyanate-dextran) on the diffusion rates of the solutes across the capillary wall and in the bulk gel in between the capillaries were investigated using confocal laser scanning microscopy. The flow in the capillaries was produced using a syringe pump that was connected to the capillaries via a tube. Transmission electron microscopy revealed an open aggregated structure close to the capillary wall, followed by an aligned network layer and the isotropic network of the bulk gel. The most pronounced effect was observed for the 1 nm-diameter fluorescein probe, for which an increase in flow rate increased the mobility of the probe in the gel. Fluorescence recovery after photobleaching confirmed increased mobility close to the channel, with increasing flow rate. Mobility maps derived using raster image correlation spectroscopy showed that the layer with the lowest mobility corresponded to the anisotropic layer of ordered network chains. The combination of microscopy techniques used in the present study elucidates the flow and diffusion behaviors visually, qualitatively and quantitatively, and represents a promising tool for future studies of mass transport in non-equilibrium systems.

3D Bioprinting of Tissue/Organ Models

In vitro tissue/organ models are useful platforms that can facilitate systematic, repetitive, and quantitative investigations of drugs/chemicals. The primary objective when developing tissue/organ models is to reproduce physiologically relevant functions that typically require complex culture systems. Bioprinting offers exciting prospects for constructing 3D tissue/organ models, as it enables the reproducible, automated production of complex living tissues. Bioprinted tissues/organs may prove useful for screening novel compounds or predicting toxicity, as the spatial and chemical complexity inherent to native tissues/organs can be recreated. In this Review, we highlight the importance of developing 3D in vitro tissue/organ models by 3D bioprinting techniques, characterization of these models for evaluating their resemblance to native tissue, and their application in the prioritization of lead candidates, toxicity testing, and as disease/tumor models.

Fetal brain extracellular matrix boosts neuronal network formation in 3D bioengineered model of cortical brain tissue

The extracellular matrix (ECM) constituting up to 20% of the organ volume is a significant component of the brain due to its instructive role in the compartmentalization of functional microdomains in every brain structure. The composition, quantity and structure of ECM changes dramatically during the development of an organism greatly contributing to the remarkably sophisticated architecture and function of the brain. Since fetal brain is highly plastic, we hypothesize that the fetal brain ECM may contain cues promoting neural growth and differentiation, highly desired in regenerative medicine. Thus, we studied the effect of brain-derived fetal and adult ECM complemented with matricellular proteins on cortical neurons using in vitro 3D bioengineered model of cortical brain tissue. The tested parameters included neuronal network density, cell viability, calcium signaling and electrophysiology. Both, adult and fetal brain ECM as well as matricellular proteins significantly improved neural network formation as compared to single component, collagen I matrix. Additionally, the brain ECM improved cell viability and lowered glutamate release. The fetal brain ECM induced superior neural network formation, calcium signaling and spontaneous spiking activity over adult brain ECM. This study highlights the difference in the neuroinductive properties of fetal and adult brain ECM and suggests that delineating the basis for this divergence may have implications for regenerative medicine.

Overexpression of monocarboxylate anion transporter 1 and 4 in T24-induced cancer-associated fibroblasts regulates the progression of bladder cancer cells in a 3D microfluidic device

Stromal fibroblasts are essential for tumor proliferation and invasion. Here we presented a 3-dimensional (3D) microfluidic co-culture device to reconstruct an in vivo-like tumor microenvironment for investigation of the interactions of cancer-associated fibroblasts (CAFs) and bladder cancer cells. With this device, we verified that the cytokines secreted by bladder cancer cells T24 effectively transform the fibroblasts into CAFs. Compared to fibroblasts, the CAFs, which undergo the aerobic glycolysis, showed higher ability to produce lactate and provide energy for bladder cancer cell proliferation and invasion. We also demonstrated that this kind of tumor-promoting effect was associated with the upregulation of monocarboxylate anion transporter 1 (MCT1) and MCT4 expression in CAFs. We concluded that MCT1 and MCT4 are involved in bladder cancer cell proliferation and invasiveness. Moreover, this 3D microfluidic co-culture device allows for the assay to characterize various cellular events in a single device sequentially, facilitating a better understanding of the interactions among heterotypic cells in a sophisticated microenvironment.