3D cardiac μtissues within a microfluidic device with real-time contractile stress readout

Microphysiological systems for human aging research

Recent advances in microphysiological systems (MPS), also known as organs-on-a-chip (OoC), enable the recapitulation of more complex organ and tissue functions on a smaller scale in vitro. MPS therefore provide the potential to better understand human diseases and physiology. To date, numerous MPS platforms have been developed for various tissues and organs, including the heart, liver, kidney, blood vessels, muscle, and adipose tissue. However, only a few studies have explored using MPS platforms to unravel the effects of aging on human physiology and the pathogenesis of age-related diseases. Age is one of the risk factors for many diseases, and enormous interest has been devoted to aging research. As such, a human MPS aging model could provide a more predictive tool to understand the molecular and cellular mechanisms underlying human aging and age-related diseases. These models can also be used to evaluate preclinical drugs for age-related diseases and translate them into clinical settings. Here, we provide a review on the application of MPS in aging research. First, we offer an overview of the molecular, cellular, and physiological changes with age in several tissues or organs. Next, we discuss previous aging models and the current state of MPS for studying human aging and age-related conditions. Lastly, we address the limitations of current MPS and present future directions on the potential of MPS platforms for human aging research.

Step-by-step fabrication of heart-on-chip systems as models for cardiac disease modeling and drug screening

Cardiovascular diseases are caused by hereditary factors, environmental conditions, and medication-related issues. On the other hand, the cardiotoxicity of drugs should be thoroughly examined before entering the market. In this regard, heart-on-chip (HOC) systems have been developed as a more efficient and cost-effective solution than traditional methods, such as 2D cell culture and animal models. HOCs must replicate the biology, physiology, and pathology of human heart tissue to be considered a reliable platform for heart disease modeling and drug testing. Therefore, many efforts have been made to find the best methods to fabricate different parts of HOCs and to improve the bio-mimicry of the systems in the last decade. Beating HOCs with different platforms have been developed and techniques, such as fabricating pumpless HOCs, have been used to make HOCs more user-friendly systems. Recent HOC platforms have the ability to simultaneously induce and record electrophysiological stimuli. Additionally, systems including both heart and cancer tissue have been developed to investigate tissue-tissue interactions' effect on cardiac tissue response to cancer drugs. In this review, all steps needed to be considered to fabricate a HOC were introduced, including the choice of cellular resources, biomaterials, fabrication techniques, biomarkers, and corresponding biosensors. Moreover, the current HOCs used for modeling cardiac diseases and testing the drugs are discussed. We finally introduced some suggestions for fabricating relatively more user-friendly HOCs and facilitating the commercialization process.

Recent developments in organ-on-a-chip technology for cardiovascular disease research

Cardiovascular diseases are a group of heart and blood vessel disorders which remain a leading cause of morbidity and mortality worldwide. Currently, cardiovascular disease research commonly depends on in vivo rodent models and in vitro human cell culture models. Despite their widespread use in cardiovascular disease research, there are some long-standing limitations: animal models often fail to faithfully mimic human response, while traditional cell models ignore the in vivo microenvironment, intercellular communications, and tissue-tissue interactions. The convergence of microfabrication and tissue engineering has given rise to organ-on-a-chip technologies. The organ-on-a-chip is a microdevice containing microfluidic chips, cells, and extracellular matrix to reproduce the physiological processes of a certain part of the human body, and is nowadays considered a promising bridge between in vivo models and in vitro 2D or 3D cell culture models. Considering the difficulty in obtaining human vessel and heart samples, the development of vessel-on-a-chip and heart-on-a-chip systems can guide cardiovascular disease research in the future. In this review, we elaborate methods and materials to fabricate organ-on-a-chip systems and summarize the construction of vessel and heart chips. The construction of vessels-on-a-chip must consider the cyclic mechanical stretch and fluid shear stress, while hemodynamic forces and cardiomyocyte maturation are key factors in building hearts-on-a-chip. We also introduce the application of organs-on-a-chip in cardiovascular disease study.

Microfluidic techniques for mechanical measurements of biological samples

The use of microfluidics to make mechanical property measurements is increasingly common. Fabrication of microfluidic devices has enabled various types of flow control and sensor integration at micrometer length scales to interrogate biological materials. For rheological measurements of biofluids, the small length scales are well suited to reach high rates, and measurements can be made on droplet-sized samples. The control of flow fields, constrictions, and external fields can be used in microfluidics to make mechanical measurements of individual bioparticle properties, often at high sampling rates for high-throughput measurements. Microfluidics also enables the measurement of bio-surfaces, such as the elasticity and permeability properties of layers of cells cultured in microfluidic devices. Recent progress on these topics is reviewed, and future directions are discussed.

Human Heart Anoxia and Reperfusion Tissue (HEART) Model for the Rapid Study of Exosome Bound miRNA Expression As Biomarkers for Myocardial Infarction

Current biomarkers for myocardial infarction (MI) diagnosis are typically late markers released upon cell death, incapable of distinguishing between ischemic and reperfusion injury and can be symptoms of other pathologies. Circulating microRNAs (miRNAs) have recently been proposed as alternative biomarkers for MI diagnosis; however, detecting the changes in the human cardiac miRNA profile during MI is extremely difficult. Here, to study the changes in miRNA levels during acute MI, a heart-on-chip model with a cardiac channel, containing human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes in human heart decellularized matrix and collagen, and a vascular channel, containing hiPSC-derived endothelial cells, is developed. This model is exposed to anoxia followed by normoxia to mimic ischemia and reperfusion, respectively. Using a highly sensitive miRNA biosensor that the authors developed, the exact same increase in miR-1, miR-208b, and miR-499 levels in the MI-on-chip and the time-matched human blood plasma samples collected before and after ischemia and reperfusion, is shown. That the surface marker profile of exosomes in the engineered model changes in response to ischemic and reperfusion injury, which can be used as biomarkers to detect MI, is also shown. Hence, the MI-on-chip model developed here can be used in biomarker discovery.

An In Vitro Microfluidic Alveolus Model to Study Lung Biomechanics

The gas exchange units of the lung, the alveoli, are mechanically active and undergo cyclic deformation during breathing. The epithelial cells that line the alveoli contribute to lung function by reducing surface tension via surfactant secretion, which is highly influenced by the breathing-associated mechanical cues. These spatially heterogeneous mechanical cues have been linked to several physiological and pathophysiological states. Here, we describe the development of a microfluidically assisted lung cell culture model that incorporates heterogeneous cyclic stretching to mimic alveolar respiratory motions. Employing this device, we have examined the effects of respiratory biomechanics (associated with breathing-like movements) and strain heterogeneity on alveolar epithelial cell functions. Furthermore, we have assessed the potential application of this platform to model altered matrix compliance associated with lung pathogenesis and ventilator-induced lung injury. Lung microphysiological platforms incorporating human cells and dynamic biomechanics could serve as an important tool to delineate the role of alveolar micromechanics in physiological and pathological outcomes in the lung.

Human Organ-on-a-Chip Microphysiological Systems to Model Musculoskeletal Pathologies and Accelerate Therapeutic Discovery

Human Microphysiological Systems (hMPS), otherwise known as organ- and tissue-on-a-chip models, are an emerging technology with the potential to replace in vivo animal studies with in vitro models that emulate human physiology at basic levels. hMPS platforms are designed to overcome limitations of two-dimensional (2D) cell culture systems by mimicking 3D tissue organization and microenvironmental cues that are physiologically and clinically relevant. Unlike animal studies, hMPS models can be configured for high content or high throughput screening in preclinical drug development. Applications in modeling acute and chronic injuries in the musculoskeletal system are slowly developing. However, the complexity and load bearing nature of musculoskeletal tissues and joints present unique challenges related to our limited understanding of disease mechanisms and the lack of consensus biomarkers to guide biological therapy development. With emphasis on examples of modeling musculoskeletal tissues, joints on chips, and organoids, this review highlights current trends of microphysiological systems technology. The review surveys state-of-the-art design and fabrication considerations inspired by lessons from bioreactors and biological variables emphasizing the role of induced pluripotent stem cells and genetic engineering in creating isogenic, patient-specific multicellular hMPS. The major challenges in modeling musculoskeletal tissues using hMPS chips are identified, including incorporating biological barriers, simulating joint compartments and heterogenous tissue interfaces, simulating immune interactions and inflammatory factors, simulating effects of in vivo loading, recording nociceptors responses as surrogates for pain outcomes, modeling the dynamic injury and healing responses by monitoring secreted proteins in real time, and creating arrayed formats for robotic high throughput screens. Overcoming these barriers will revolutionize musculoskeletal research by enabling physiologically relevant, predictive models of human tissues and joint diseases to accelerate and de-risk therapeutic discovery and translation to the clinic.

Microfluidic models of the human circulatory system: versatile platforms for exploring mechanobiology and disease modeling

The human circulatory system is a marvelous fluidic system, which is very sensitive to biophysical and biochemical cues. The current animal and cell culture models do not recapitulate the functional properties of the human circulatory system, limiting our ability to fully understand the complex biological processes underlying the dysfunction of this multifaceted system. In this review, we discuss the unique ability of microfluidic systems to recapitulate the biophysical, biochemical, and functional properties of the human circulatory system. We also describe the remarkable capacity of microfluidic technologies for exploring the complex mechanobiology of the cardiovascular system, mechanistic studying of cardiovascular diseases, and screening cardiovascular drugs with the additional benefit of reducing the need for animal models. We also discuss opportunities for further advancement in this exciting field.

Recent Advances in Cardiac Tissue Engineering for the Management of Myocardium Infarction

Myocardium Infarction (MI) is one of the foremost cardiovascular diseases (CVDs) causing death worldwide, and its case numbers are expected to continuously increase in the coming years. Pharmacological interventions have not been at the forefront in ameliorating MI-related morbidity and mortality. Stem cell-based tissue engineering approaches have been extensively explored for their regenerative potential in the infarcted myocardium. Recent studies on microfluidic devices employing stem cells under laboratory set-up have revealed meticulous events pertaining to the pathophysiology of MI occurring at the infarcted site. This discovery also underpins the appropriate conditions in the niche for differentiating stem cells into mature cardiomyocyte-like cells and leads to engineering of the scaffold via mimicking of native cardiac physiological conditions. However, the mode of stem cell-loaded engineered scaffolds delivered to the site of infarction is still a challenging mission, and yet to be translated to the clinical setting. In this review, we have elucidated the various strategies developed using a hydrogel-based system both as encapsulated stem cells and as biocompatible patches loaded with cells and applied at the site of infarction.

Fabrication and Biomedical Applications of Heart-on-a-chip

Heart diseases have become the main killer threatening human health, and various methods have been developed to study heart disease. Among them, heart-on-a-chip has emerged in recent years as a method for constructing disease (or normal) models in vitro and is considered as a promising tool to study heart diseases. Compared with other methods, the advantages of heart-on-a-chip include the high portability, high throughput, and the capability to mimic microenvironments in vivo. It has shown a great potential in disease mechanism study and drug screening. In this paper, we review the recent advances in heart-on-a-chip, including the fabrication methods (e.g., 3D bioprinting) and biomedical applications. By analyzing the structure of the existing heart-on-a-chip, we proposed that a highly integrated heart-on-a-chip includes four elements: Microfluidic chips, cells/microtissues, microactuators to construct the microenvironment, and microsensors for results readout. Finally, the current challenges and future directions of heart-on-a-chip are discussed.

Tumor-on-a-chip: from bioinspired design to biomedical application

Cancer is one of the leading causes of human death, despite enormous efforts to explore cancer biology and develop anticancer therapies. The main challenges in cancer research are establishing an efficient tumor microenvironment in vitro and exploring efficient means for screening anticancer drugs to reveal the nature of cancer and develop treatments. The tumor microenvironment possesses human-specific biophysical and biochemical factors that are difficult to recapitulate in conventional in vitro planar cell models and in vivo animal models. Therefore, model limitations have hindered the translation of basic research findings to clinical applications. In this review, we introduce the recent progress in tumor-on-a-chip devices for cancer biology research, medicine assessment, and biomedical applications in detail. The emerging tumor-on-a-chip platforms integrating 3D cell culture, microfluidic technology, and tissue engineering have successfully mimicked the pivotal structural and functional characteristics of the in vivo tumor microenvironment. The recent advances in tumor-on-a-chip platforms for cancer biology studies and biomedical applications are detailed and analyzed in this review. This review should be valuable for further understanding the mechanisms of the tumor evolution process, screening anticancer drugs, and developing cancer therapies, and it addresses the challenges and potential opportunities in predicting drug screening and cancer treatment.

Microfluidics in cardiovascular disease research: state of the art and future outlook

Due to extremely severe morbidity and mortality worldwide, it is worth achieving a more in-depth and comprehensive understanding of cardiovascular diseases. Tremendous effort has been made to replicate the cardiovascular system and investigate the pathogenesis, diagnosis and treatment of cardiovascular diseases. Microfluidics can be used as a versatile primary strategy to achieve a holistic picture of cardiovascular disease. Here, a brief review of the application of microfluidics in comprehensive cardiovascular disease research is presented, with specific discussions of the characteristics of microfluidics for investigating cardiovascular diseases integrally, including the study of pathogenetic mechanisms, the development of accurate diagnostic methods and the establishment of therapeutic treatments. Investigations of critical pathogenetic mechanisms for typical cardiovascular diseases by microfluidic-based organ-on-a-chip are categorized and reviewed, followed by a detailed summary of microfluidic-based accurate diagnostic methods. Microfluidic-assisted cardiovascular drug evaluation and screening as well as the fabrication of novel delivery vehicles are also reviewed. Finally, the challenges with and outlook on further advancing the use of microfluidics technology in cardiovascular disease research are highlighted and discussed.

Cardiovascular microphysiological systems (CVMPS) for safety studies - a pharma perspective

The integrative responses of the cardiovascular (CV) system are essential for maintaining blood flow to provide oxygenation, nutrients, and waste removal for the entire body. Progress has been made in independently developing simple in vitro models of two primary components of the CV system, namely the heart (using induced pluripotent stem-cell derived cardiomyocytes) and the vasculature (using endothelial cells and smooth muscle cells). These two in vitro biomimics are often described as immature and simplistic, and typically lack the structural complexity of native tissues. Despite these limitations, they have proven useful for specific "fit for purpose" applications, including early safety screening. More complex in vitro models offer the tantalizing prospect of greater refinement in risk assessments. To this end, efforts to physically link cardiac and vascular components to mimic a true CV microphysiological system (CVMPS) are ongoing, with the goal of providing a more holistic and integrated CV response model. The challenges of building and implementing CVMPS in future pharmacological safety studies are many, and include a) the need for more complex (and hence mature) cell types and tissues, b) the need for more realistic vasculature (within and across co-modeled tissues), and c) the need to meaningfully couple these two components to allow for integrated CV responses. Initial success will likely come with simple, bioengineered tissue models coupled with fluidics intended to mirror a vascular component. While the development of more complex integrated CVMPS models that are capable of differentiating safe compounds and providing mechanistic evaluations of CV liabilities may be feasible, adoption by pharma will ultimately hinge on model efficiency, experimental reproducibility, and added value above current strategies.

Microfluidic and Organ-on-a-Chip Approaches to Investigate Cellular and Microenvironmental Contributions to Cardiovascular Function and Pathology

Over the past decade, advances in microfabrication and biomaterials have facilitated the development of microfluidic tissue and organ models to address challenges with conventional animal and cell culture systems. These systems have largely been developed for human disease modeling and preclinical drug development and have been increasingly used to understand cellular and molecular mechanisms, particularly in the cardiovascular system where the characteristic mechanics and architecture are difficult to recapitulate in traditional systems. Here, we review recent microfluidic approaches to model the cardiovascular system and novel insights provided by these systems. Key features of microfluidic approaches include the ability to pattern cells and extracellular matrix (ECM) at cellular length scales and the ability to use patient-derived cells. We focus the review on approaches that have leveraged these features to explore the relationship between genetic mutations and the microenvironment in cardiovascular disease progression. Additionally, we discuss limitations and benefits of the various approaches, and conclude by considering the role further advances in microfabrication technology and biochemistry techniques play in establishing microfluidic cardiovascular disease models as central tools for understanding biological mechanisms and for developing interventional strategies.

Tissue Chips and Microphysiological Systems for Disease Modeling and Drug Testing

Tissue chips (TCs) and microphysiological systems (MPSs) that incorporate human cells are novel platforms to model disease and screen drugs and provide an alternative to traditional animal studies. This review highlights the basic definitions of TCs and MPSs, examines four major organs/tissues, identifies critical parameters for organization and function (tissue organization, blood flow, and physical stresses), reviews current microfluidic approaches to recreate tissues, and discusses current shortcomings and future directions for the development and application of these technologies. The organs emphasized are those involved in the metabolism or excretion of drugs (hepatic and renal systems) and organs sensitive to drug toxicity (cardiovascular system). This article examines the microfluidic/microfabrication approaches for each organ individually and identifies specific examples of TCs. This review will provide an excellent starting point for understanding, designing, and constructing novel TCs for possible integration within MPS.

Advances in Multidimensional Cardiac Biosensing Technologies: From Electrophysiology to Mechanical Motion and Contractile Force

Cardiovascular disease is currently a leading killer to human, while drug-induced cardiotoxicity remains the main cause of the withdrawal and attrition of drugs. Taking clinical correlation and throughput into account, cardiomyocyte is perfect as in vitro cardiac model for heart disease modeling, drug discovery, and cardiotoxicity assessment by accurately measuring the physiological multiparameters of cardiomyocytes. Remarkably, cardiomyocytes present both electrophysiological and biomechanical characteristics due to the unique excitation-contraction coupling, which plays a significant role in studying the cardiomyocytes. This review mainly focuses on the recent advances of biosensing technologies for the 2D and 3D cardiac models with three special properties: electrophysiology, mechanical motion, and contractile force. These high-performance multidimensional cardiac models are popular and effective to rebuild and mimic the heart in vitro. To help understand the high-quality and accurate physiologies, related detection techniques are highly demanded, from microtechnology to nanotechnology, from extracellular to intracellular recording, from multiple cells to single cell, and from planar to 3D models. Furthermore, the characteristics, advantages, limitations, and applications of these cardiac biosensing technologies, as well as the future development prospects should contribute to the systematization and expansion of knowledge.

Instrumented Microphysiological Systems for Real-Time Measurement and Manipulation of Cellular Electrochemical Processes

Recent advancements in electronic materials and subsequent surface modifications have facilitated real-time measurements of cellular processes far beyond traditional passive recordings of neurons and muscle cells. Specifically, the functionalization of conductive materials with ligand-binding aptamers has permitted the utilization of traditional electronic materials for bioelectronic sensing. Further, microfabrication techniques have better allowed microfluidic devices to recapitulate the physiological and pathological conditions of complex tissues and organs in vitro or microphysiological systems (MPS). The convergence of these models with advances in biological/biomedical microelectromechanical systems (BioMEMS) instrumentation has rapidly bolstered a wide array of bioelectronic platforms for real-time cellular analytics. In this review, we provide an overview of the sensing techniques that are relevant to MPS development and highlight the different organ systems to integrate instrumentation for measurement and manipulation of cellular function. Special attention is given to how instrumented MPS can disrupt the drug development and fundamental mechanistic discovery processes.

In vitro aged, hiPSC-origin engineered heart tissue models with age-dependent functional deterioration to study myocardial infarction

Deaths attributed to ischemic heart disease increased by 41.7% from 1990 to 2013. This is primarily due to an increase in the aged population, however, research on cardiovascular disease (CVD) has been overlooking aging, a well-documented contributor to CVD. The use of young animals is heavily preferred due to lower costs and ready availability, despite the prominent differences between young and aged heart structure and function. Here we present the first human induced pluripotent stem cell (hiPSC)-derived cardiomyocyte (iCM)-based, in vitro aged myocardial tissue model as an alternative research platform. Within 4 months, iCMs go through accelerated senescence and show cellular characteristics of aging. Furthermore, the model tissues fabricated using aged iCMs, with stiffness resembling that of aged human heart, show functional and pharmacological deterioration specific to aged myocardium. Our novel tissue model with age-appropriate physiology and pathology presents a promising new platform for investigating CVD or other age-related diseases. STATEMENT OF SIGNIFICANCE: In vitro and in vivo models of cardiovascular disease are aimed to provide crucial insight on the pathology and treatment of these diseases. However, the contribution of age-dependent cardiovascular changes is greatly underestimated through the use of young animals and premature cardiomyocytes. Here, we developed in vitro aged cardiac tissue models that mimic the aged heart tissue microenvironment and cellular phenotype and present the first evidence that age-appropriate in vitro disease models can be developed to gain more physiologically-relevant insight on development, progression, and amelioration of cardiovascular diseases.

Tissue engineered bone mimetics to study bone disorders ex vivo: Role of bioinspired materials

Recent advances in materials development and tissue engineering has resulted in a substantial number of bioinspired materials that recapitulate cardinal features of bone extracellular matrix (ECM) such as dynamic inorganic and organic environment(s), hierarchical organization, and topographical features. Bone mimicking materials, as defined by its self-explanatory term, are developed based on the current understandings of the natural bone ECM during development, remodeling, and fracture repair. Compared to conventional plastic cultures, biomaterials that resemble some aspects of the native environment could elicit a more natural molecular and cellular response relevant to the bone tissue. Although current bioinspired materials are mainly developed to assist tissue repair or engineer bone tissues, such materials could nevertheless be applied to model various skeletal diseases in vitro. This review summarizes the use of bioinspired materials for bone tissue engineering, and their potential to model diseases of bone development and remodeling ex vivo. We largely focus on biomaterials, designed to re-create different aspects of the chemical and physical cues of native bone ECM. Employing these bone-inspired materials and tissue engineered bone surrogates to study bone diseases has tremendous potential and will provide a closer portrayal of disease progression and maintenance, both at the cellular and tissue level. We also briefly touch upon the application of patient-derived stem cells and introduce emerging technologies such as organ-on-chip in disease modeling. Faithful recapitulation of disease pathologies will not only offer novel insights into diseases, but also lead to enabling technologies for drug discovery and new approaches for cell-based therapies.

Ex Vivo Tumor-on-a-Chip Platforms to Study Intercellular Interactions within the Tumor Microenvironment

The emergence of immunotherapies and recent FDA approval of several of them makes them a promising therapeutic strategy for cancer. While these advancements underscore the potential of engaging the immune system to target tumors, this approach has so far been efficient only for certain cancers. Extending immunotherapy as a widely acceptable treatment for various cancers requires a deeper understanding of the interactions of tumor cells within the tumor microenvironment (TME). The immune cells are a key component of the TME, which also includes other stromal cells, soluble factors, and extracellular matrix-based cues. While in vivo studies function as a gold standard, tissue-engineered microphysiological tumor models can offer patient-specific insights into cancer-immune interactions. These platforms, which recapitulate cellular and non-cellular components of the TME, enable a systematic understanding of the contribution of each component toward disease progression in isolation and in concert. Microfluidic-based microphysiological platforms recreating these environments, also known as "tumor-on-a-chip," are increasingly being utilized to study the effect of various elements of TME on tumor development. Herein are reviewed advancements in tumor-on-a-chip technology that are developed and used to understand the interaction of tumor cells with other surrounding cells, including immune cells, in the TME.

Bioinspired Engineering of Organ-on-Chip Devices

The human body can be viewed as an organism consisting of a variety of cellular and non-cellular materials interacting in a highly ordered manner. Its complex and hierarchical nature inspires the multi-level recapitulation of the human body in order to gain insights into the inner workings of life. While traditional cell culture models have led to new insights into the cellular microenvironment and biological control in vivo, deeper understanding of biological systems and human pathophysiology requires the development of novel model systems that allow for analysis of complex internal and external interactions within the cellular microenvironment in a more relevant organ context. Engineering organ-on-chip systems offers an unprecedented opportunity to unravel the complex and hierarchical nature of human organs. In this chapter, we first highlight the advances in microfluidic platforms that enable engineering of the cellular microenvironment and the transition from cells-on-chips to organs-on-chips. Then, we introduce the key features of the emerging organs-on-chips and their proof-of-concept applications in biomedical research. We also discuss the challenges and future outlooks of this state-of-the-art technology.

High-precision modular microfluidics by micromilling of interlocking injection-molded blocks

Wider use and adaptation of microfluidics is hindered by the infrastructure, knowledge, and time required to build prototype systems, especially when multiple fluid operations and measurements are required. As a result, 3D printing of microfluidics is attracting interest, yet cannot readily achieve the feature size, smoothness, and optical transparency needed for many standard microfluidic systems. Herein we present a new approach to the design and construction of high-precision modular microfluidics, using standard injection-molded blocks that are modified using micromilling and assembled via elastically averaged contacts. Desktop micromilling achieves channel dimensions as small as 50 μm depth and 150 μm width and adhesive films seal channels to allow internal fluid pressure of >400 kPa. Elastically averaged connections between bricks result in a mechanical locating repeatability of ∼1 μm, enabling fluid to pass between bricks via an O-ring seal with >99.9% reliability. We demonstrated and tested block-based systems for generating droplets at rates above 9000 min^-1 and COV <3%, and integrated optical sensors. We also show how blocks can be used to build easily reconfigurable interfaces with glass microfluidic devices and imaging hardware. Microfluidic bricks fabricated by FDM and SLA 3D printing cannot achieve the dimensional quality of molded bricks, yet 3D printing allows customized bricks to be integrated with standard LEGOs. Our approach enables a wide variety of modular microfluidic units to be built using a widely available, cost-effective platform, encouraging use in both research and education.

Organ-On-A-Chip Platforms: A Convergence of Advanced Materials, Cells, and Microscale Technologies

Significant advances in biomaterials, stem cell biology, and microscale technologies have enabled the fabrication of biologically relevant tissues and organs. Such tissues and organs, referred to as organ-on-a-chip (OOC) platforms, have emerged as a powerful tool in tissue analysis and disease modeling for biological and pharmacological applications. A variety of biomaterials are used in tissue fabrication providing multiple biological, structural, and mechanical cues in the regulation of cell behavior and tissue morphogenesis. Cells derived from humans enable the fabrication of personalized OOC platforms. Microscale technologies are specifically helpful in providing physiological microenvironments for tissues and organs. In this review, biomaterials, cells, and microscale technologies are described as essential components to construct OOC platforms. The latest developments in OOC platforms (e.g., liver, skeletal muscle, cardiac, cancer, lung, skin, bone, and brain) are then discussed as functional tools in simulating human physiology and metabolism. Future perspectives and major challenges in the development of OOC platforms toward accelerating clinical studies of drug discovery are finally highlighted.

Functional and Biomimetic Materials for Engineering of the Three-Dimensional Cell Microenvironment

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology. The extracellular matrix (ECM) within the cell microenvironment serves not only as a structural foundation for cells but also as a source of three-dimensional (3D) biochemical and biophysical cues that trigger and regulate cell behaviors. Increasing evidence suggests that the 3D character of the microenvironment is required for development of many critical cell responses observed in vivo, fueling a surge in the development of functional and biomimetic materials for engineering the 3D cell microenvironment. Progress in the design of such materials has improved control of cell behaviors in 3D and advanced the fields of tissue regeneration, in vitro tissue models, large-scale cell differentiation, immunotherapy, and gene therapy. However, the field is still in its infancy, and discoveries about the nature of cell-microenvironment interactions continue to overturn much early progress in the field. Key challenges continue to be dissecting the roles of chemistry, structure, mechanics, and electrophysiology in the cell microenvironment, and understanding and harnessing the roles of periodicity and drift in these factors. This review encapsulates where recent advances appear to leave the ever-shifting state of the art, and it highlights areas in which substantial potential and uncertainty remain.

Skeletal muscle-on-a-chip: an in vitro model to evaluate tissue formation and injury

Engineered skeletal muscle tissues can be used for in vitro studies that require physiologically relevant models of native tissues. Herein, we describe the development of a three-dimensional (3D) skeletal muscle tissue that recapitulates the architectural and structural complexities of muscle within a microfluidic device. Using a 3D photo-patterning approach, we spatially confined a cell-laden gelatin network around two bio-inert hydrogel pillars, which induce uniaxial alignment of the cells and serve as anchoring sites for the encapsulated cells and muscle tissues as they form and mature. We have characterized the tissue morphology and strain profile during differentiation of the cells and skeletal muscle tissue formation by using a combination of fluorescence microscopy and computational tools. The time-dependent strain profile suggests the existence of individual cells within the gelatin matrix, which differentiated to form a multinucleated skeletal muscle tissue bundle as a function of culture time. We have also developed a method to calculate the passive tension generated by the engineered muscle tissue bundles suspended between two pillars. Finally, as a proof-of-concept we have examined the applicability of the skeletal muscle-on-chip system as a screening platform and in vitro muscle injury model. We studied the dose-dependent effect of cardiotoxin on the engineered muscle tissue architecture and its subsequent effect on the passive tension. This simple yet effective tool can be appealing for studies that necessitate the analysis of skeletal muscle structure and function, including preclinical drug discovery and development.

Biophysical Regulation of Cancer Stem/Initiating Cells: Implications for Disease Mechanisms and Translation

Cancer stem/initiating cells (CSCs) are a subset of tumor cells proposed to play privileged roles in seeding tumors and driving metastasis. CSCs have emerged as an increasingly important target of interest in cancer biology and therapy. Recent work has suggested that CSC maintenance and metastatic potential may be modulated by physical inputs within the tissue microenvironment, including interstitial pressure and extracellular matrix stiffness. Here we review recent progress in our understanding of CSC regulation by biophysical signals within the tumor microenvironment. While the mechanistic basis of this signaling remains incompletely understood, we discuss emerging evidence that mechanical inputs can epigenetically regulate CSC behavior and that some CSCs can evade mechanotransductive signals to more efficiently infiltrate tissue. We also describe efforts to leverage these findings to engineer culture platforms for the characterization of CSC mechanics for discovery and screening.

Human iPSC-derived myocardium-on-chip with capillary-like flow for personalized medicine

The heart wall tissue, or the myocardium, is one of the main targets in cardiovascular disease prevention and treatment. Animal models have not been sufficient in mimicking the human myocardium as evident by the very low clinical translation rates of cardiovascular drugs. Additionally, current in vitro models of the human myocardium possess several shortcomings such as lack of physiologically relevant co-culture of myocardial cells, lack of a 3D biomimetic environment, and the use of non-human cells. In this study, we address these shortcomings through the design and manufacture of a myocardium-on-chip (MOC) using 3D cell-laden hydrogel constructs and human induced pluripotent stem cell (hiPSC) derived myocardial cells. The MOC utilizes 3D spatially controlled co-culture of hiPSC derived cardiomyocytes (iCMs) and hiPSC derived endothelial cells (iECs) integrated among iCMs as well as in capillary-like side channels, to better mimic the microvasculature seen in native myocardium. We first fully characterized iCMs using immunostaining, genetic, and electrochemical analysis and iECs through immunostaining and alignment analysis to ensure their functionality, and then seeded these cells sequentially into the MOC device. We showed that iECs could be cultured within the microfluidic device without losing their phenotypic lineage commitment, and align with the flow upon physiological level shear stresses. We were able to incorporate iCMs within the device in a spatially controlled manner with the help of photocrosslinkable polymers. The iCMs were shown to be viable and functional within the device up to 7 days, and were integrated with the iECs. The iCMs and iECs in this study were derived from the same hiPSC cell line, essentially mimicking the myocardium of an individual human patient. Such devices are essential for personalized medicine studies where the individual drug response of patients with different genetic backgrounds can be tested in a physiologically relevant manner.

Droplet-based microtumor model to assess cell-ECM interactions and drug resistance of gastric cancer cells

Gastric cancer (GC) is a common aggressive malignant tumor with high incidence and mortality worldwide. GC is classified into intestinal and diffuse types according to the histo-morphological features. Because of distinctly different clinico-pathological features, new cancer therapy strategies and in vitro preclinical models for the two pathological variants of GC is necessary. Since extracellular matrix (ECM) influence the biological behavior of tumor cells, we hypothesized that GC might be more similarly modeled in 3D with matrix rather than in 2D. Herein, we developed a microfluidic-based a three-dimensional (3D) in vitro gastric cancer model, with subsequent drug resistance assay. AGS (intestinal type) and Hs746T (diffuse type) gastric cancer cell lines were encapsulated in collagen beads with high cellular viability. AGS exhibited an aggregation pattern with expansive growth, whereas Hs746T showed single-cell-level infiltration. Importantly, in microtumor models, epithelial-mesenchymal transition (EMT) and metastatic genes were upregulated, whereas E-cadherin was downregulated. Expression of ß-catenin was decreased in drug-resistant cells, and chemosensitivity toward the anticancer drug (5-FU) was observed in microtumors. These results suggest that in vitro microtumor models may represent a biologically relevant platform for studying gastric cancer cell biology and tumorigenesis, and for accelerating the development of novel therapeutic targets.

Real-Time Force and Frequency Analysis of Engineered Human Heart Tissue Derived from Induced Pluripotent Stem Cells Using Magnetic Sensing

Engineered heart tissues made from human pluripotent stem cell-derived cardiomyocytes have been used for modeling cardiac pathologies, screening new therapeutics, and providing replacement cardiac tissue. Current methods measure the functional performance of engineered heart tissue by their twitch force and beating frequency, typically obtained by optical measurements. In this article, we describe a novel method for assessing twitch force and beating frequency of engineered heart tissue using magnetic field sensing, which enables multiple tissues to be measured simultaneously. The tissues are formed as thin structures suspended between two silicone posts, where one post is rigid and another is flexible and contains an embedded magnet. When the tissue contracts it causes the flexible post to bend in proportion to its twitch force. We measured the bending of the post using giant magnetoresistive (GMR) sensors located underneath a 24-well plate containing the tissues. We validated the accuracy of the readings from the GMR sensors against optical measurements. We demonstrated the utility and sensitivity of our approach by testing the effects of three concentrations of isoproterenol and verapamil on twitch force and beating frequency in real-time, parallel experiments. This system should be scalable beyond the 24-well format, enabling greater automation in assessing the contractile function of cardiomyocytes in a tissue-engineered environment.

Controlled evacuation using the biocompatible and energy efficient microfluidic ejector

Development of controlled vacuum is having many applications in the realm of biotechnology, cell transfer, gene therapy, biomedical engineering and other engineering activities involving separation or chemical reactions. Here we show the controlled vacuum generation through a biocompatible, energy efficient, low-cost and flexible miniature device. We have designed and fabricated microfluidic devices from polydimethylsiloxane which are capable of producing vacuum at a highly controlled rate by using water as a motive fluid. Scrupulous removal of infected fluid/body fluid from the internal hemorrhage affected parts during surgical operations, gene manipulation, cell sorting, and other biomedical activities require complete isolation of the delicate cells or tissues adjacent to the targeted location. We demonstrate the potential of the miniature device to obtain controlled evacuation without the use of highly pressurized motive fluids. Water has been used as a motive liquid to eject vapor and liquid at ambient conditions through the microfluidic devices prepared using a low-cost fabrication method. The proposed miniature device may find applications in vacuum generation especially where the controlled rate of evacuation, and limited vacuum generation are of utmost importance in order to precisely protect the cells in the nearby region of the targeted evacuated area.

Cardiac Meets Skeletal: What's New in Microfluidic Models for Muscle Tissue Engineering

In the last few years microfluidics and microfabrication technique principles have been extensively exploited for biomedical applications. In this framework, organs-on-a-chip represent promising tools to reproduce key features of functional tissue units within microscale culture chambers. These systems offer the possibility to investigate the effects of biochemical, mechanical, and electrical stimulations, which are usually applied to enhance the functionality of the engineered tissues. Since the functionality of muscle tissues relies on the 3D organization and on the perfect coupling between electrochemical stimulation and mechanical contraction, great efforts have been devoted to generate biomimetic skeletal and cardiac systems to allow high-throughput pathophysiological studies and drug screening. This review critically analyzes microfluidic platforms that were designed for skeletal and cardiac muscle tissue engineering. Our aim is to highlight which specific features of the engineered systems promoted a typical reorganization of the engineered construct and to discuss how promising design solutions exploited for skeletal muscle models could be applied to improve cardiac tissue models and vice versa.

A Bioprinted Liver-on-a-Chip for Drug Screening Applications. Trends Biotechnol. 2016;34:681-682. [PMID: 27291461 DOI: 10.1016/j.tibtech.2016.05.014]

The need for a liver-on-a-chip tissue model for drug screening is particularly important in tissue engineering because of the high frequency of drug-induced liver injury. Recently, a liver tissue model conducive to hepatotoxicity testing was developed by bioprinting hepatic spheroids encapsulated in a hydrogel scaffold into a microfluidic device.

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.

Chemotaxis-driven assembly of endothelial barrier in a tumor-on-a-chip platform

The integration of three-dimensional micropatterning with microfluidics provides a unique opportunity to create perfusable tissue constructs in vitro. Herein, we have used this approach to create a tumor-on-a-chip with an endothelial barrier. Specifically, we photopatterned a mixture of endothelial cells and cancer spheroids within a gelatin methacrylate (GelMA) hydrogel inside a microfluidic device. The differential motility of endothelial and cancer cells in response to a controlled morphogen gradient across the cell-laden network drove the migration of endothelial cells to the periphery while maintaining the cancer cells within the interior of the hydrogel. The resultant endothelial cell layer forming cell-cell contact via VE-cadherin junctions was found to encompass the entire GelMA hydrogel structure. Furthermore, we have also examined the potential of such a tumor-on-a-chip system as a drug screening platform using doxorubicin, a model cancer drug.

Myocyte-fibroblast communication in cardiac fibrosis and arrhythmias: Mechanisms and model systems

Development of cardiac fibrosis and arrhythmias is controlled by the activity of and communication between cardiomyocytes and fibroblasts in the heart. Myocyte-fibroblast interactions occur via both direct and indirect means including paracrine mediators, extracellular matrix interactions, electrical modulators, mechanical junctions, and membrane nanotubes. In the diseased heart, cardiomyocyte and fibroblast ratios and activity, and thus myocyte-fibroblast interactions, change and are thought to contribute to the course of disease including development of fibrosis and arrhythmogenic activity. Fibroblasts have a developing role in modulating cardiomyocyte electrical and hypertrophic activity, however gaps in knowledge regarding these interactions still exist. Research in this field has necessitated the development of unique approaches to isolate and control myocyte-fibroblast interactions. Numerous methods for 2D and 3D co-culture systems have been developed, while a growing part of this field is in the use of better tools for in vivo systems including cardiomyocyte and fibroblast specific Cre mouse lines for cell type specific genetic ablation. This review will focus on (i) mechanisms of myocyte-fibroblast communication and their effects on disease features such as cardiac fibrosis and arrhythmias as well as (ii) methods being used and currently developed in this field.