A microfluidic flow-stretch chip for investigating blood vessel biomechanics

Progress of organoid platform in cardiovascular research

Cardiovascular disease is a significant cause of death in humans. Various models are necessary for the study of cardiovascular diseases, but once cellular and animal models have some defects, such as insufficient fidelity. As a new technology, organoid has certain advantages and has been used in many applications in the study of cardiovascular diseases. This article aims to summarize the application of organoid platforms in cardiovascular diseases, including organoid construction schemes, modeling, and application of cardiovascular organoids. Advances in cardiovascular organoid research have provided many models for different cardiovascular diseases in a variety of areas, including myocardium, blood vessels, and valves. Physiological and pathological models of different diseases, drug research models, and methods for evaluating and promoting the maturation of different kinds of organ tissues are provided for various cardiovascular diseases, including cardiomyopathy, myocardial infarction, and atherosclerosis. This article provides a comprehensive overview of the latest research progress in cardiovascular organ tissues, including construction protocols for cardiovascular organoid tissues and their evaluation system, different types of disease models, and applications of cardiovascular organoid models in various studies. The problems and possible solutions in organoid development are summarized.

Strain sensor on a chip for quantifying the magnitudes of tensile stress on cells

During cardiac development, mechanotransduction from the in vivo microenvironment modulates cardiomyocyte growth in terms of the number, area, and arrangement heterogeneity. However, the response of cells to different degrees of mechanical stimuli is unclear. Organ-on-a-chip, as a platform for investigating mechanical stress stimuli in cellular mimicry of the in vivo microenvironment, is limited by the lack of ability to accurately quantify externally induced stimuli. However, previous technology lacks the integration of external stimuli and feedback sensors in microfluidic platforms to obtain and apply precise amounts of external stimuli. Here, we designed a cell stretching platform with an in-situ sensor. The in-situ liquid metal sensors can accurately measure the mechanical stimulation caused by the deformation of the vacuum cavity exerted on cells. The platform was applied to human cardiomyocytes (AC16) under cyclic strain (5%, 10%, 15%, 20 and 25%), and we found that cyclic strain promoted cell growth induced the arrangement of cells on the membrane to gradually unify, and stabilized the cells at 15% amplitude, which was even more effective after 3 days of culture. The platform's precise control and measurement of mechanical forces can be used to establish more accurate in vitro microenvironmental models for disease modeling and therapeutic research.

Microfluidic technology for cell biology-related applications: a review

Fluid flow at the microscale level exhibits a unique phenomenon that can be explored to fabricate microfluidic devices integrated with components that can perform various biological functions. In this manuscript, the importance of physics for microscale fluid dynamics using microfluidic devices has been reviewed. Microfluidic devices provide new opportunities with regard to spatial and temporal control over cell growth. Furthermore, the manuscript presents an overview of cellular stimuli observed by combining surfaces that mimic the complex biochemistries and different geometries of the extracellular matrix, with microfluidic channels regulating the transport of fluids, soluble factors, etc. We have also explained the concept of mechanotransduction, which defines the relation between mechanical force and biological response. Furthermore, the manipulation of cellular microenvironments by the use of microfluidic systems has been highlighted as a useful device for basic cell biology research activities. Finally, the article focuses on highly integrated microfluidic platforms that exhibit immense potential for biomedical and pharmaceutical research as robust and portable point-of-care diagnostic devices for the assessment of clinical samples.

Development of a multifunctional bioreactor to evaluate the promotion effects of cyclic stretching and electrical stimulation on muscle differentiation

A multifunctional bioreactor was fabricated in this study to investigate the facilitation efficiency of electrical and mechanical stimulations on myogenic differentiation. This bioreactor consisted of a highly stretchable conductive membrane prepared by depositing polypyrrole (PPy) on a flexible polydimethylsiloxane (PDMS) film. The tensile deformation of the PPy/PDMS membrane can be tuned by adjusting the channel depth. In addition, PPy/PDMS maintained its electrical conductivity under continuous cyclic stretching in the strain range of 6.5%-13% for 24 h. This device can be used to individually or simultaneously perform cyclic stretching and electrical stimulation. The results of single stimulation showed that either cyclic stretching or electrical stimulation upregulated myogenic gene expression and promoted myotube formation, where electrical stimulation improved better than cyclic stretching. However, only cyclic stretching can align C2C12 cells perpendicular to the stretching direction, and electrical stimulation did not affect cell morphology. Myosin heavy chain (MHC) immunostaining demonstrated that oriented cells under cyclic stretching resulted in parallel myotubes. The combination of these two stimuli exhibited synergetic effects on both myogenic gene regulation and myotube formation, and the incorporated electrical field did not affect the orientation effect of the cyclic stretching. These results suggested that these two treatments likely influenced cells through different pathways. Overall, the simultaneous application of cyclic stretching and electrical stimulation preserved both stimuli's advantages, so myo-differentiation can be highly improved to obtain abundant parallel myotubes, suggesting that our developed multifunctional bioreactor should benefit muscle tissue engineering applications.

Highly Accurate Pneumatically Tunable Optofluidic Distributed Feedback Dye Lasers

Optofluidic dye lasers integrated into microfluidic chips are promising miniature coherent light sources for biosensing. However, achieving the accurate and efficient tuning of lasers remains challenging. This study introduces a novel pneumatically tunable optofluidic distributed feedback (DFB) dye laser in a multilayer microfluidic chip. The dye laser device integrates microfluidic channels, grating structures, and vacuum chambers. A second-order DFB grating configuration is utilized to ensure single-mode lasing. The application of vacuum pressure to the chambers stretches the soft grating layer, enabling the sensitive tuning of the lasing wavelength at a high resolution of 0.25 nm within a 7.84 nm range. The precise control of pressure and laser tuning is achieved through an electronic regulator. Additionally, the integrated microfluidic channels and optimized waveguide structure facilitate efficient dye excitation, resulting in a low pump threshold of 164 nJ/pulse. This pneumatically tunable optofluidic DFB laser, with its high-resolution wavelength tuning range, offers new possibilities for the development of integrated portable devices for biosensing and spectroscopy.

A microfluidic organ-on-a-chip: into the next decade of bone tissue engineering applied in dentistry

A comprehensive understanding of the complex physiological and pathological processes associated with alveolar bones, their responses to different therapeutics strategies, and cell interactions with biomaterial becomes necessary in precisely treating patients with severe progressive periodontitis, as a bone-related issue in dentistry. However, existing monolayer cell culture or pre-clinical models have been unable to mimic the complex physiological, pathological and regeneration processes in the bone microenvironment in response to different therapeutic strategies. In this point, 'organ-on-a-chip' (OOAC) technology, specifically 'alveolar-bone-on-a-chip', is expected to resolve the problems by better imitating infection site microenvironment and microphysiology within the oral tissues. The OOAC technology is assessed in this study toward better approaches in disease modeling and better therapeutics strategy for bone tissue engineering applied in dentistry.

Engineering Organ-on-a-Chip Systems for Vascular Diseases

Vascular diseases, such as atherosclerosis and thrombosis, are major causes of morbidity and mortality worldwide. Traditional in vitro models for studying vascular diseases have limitations, as they do not fully recapitulate the complexity of the in vivo microenvironment. Organ-on-a-chip systems have emerged as a promising approach for modeling vascular diseases by incorporating multiple cell types, mechanical and biochemical cues, and fluid flow in a microscale platform. This review provides an overview of recent advancements in engineering organ-on-a-chip systems for modeling vascular diseases, including the use of microfluidic channels, ECM (extracellular matrix) scaffolds, and patient-specific cells. We also discuss the limitations and future perspectives of organ-on-a-chip for modeling vascular diseases.

Bio-inspired microfluidics: A review

Biomicrofluidics, a subdomain of microfluidics, has been inspired by several ideas from nature. However, while the basic inspiration for the same may be drawn from the living world, the translation of all relevant essential functionalities to an artificially engineered framework does not remain trivial. Here, we review the recent progress in bio-inspired microfluidic systems via harnessing the integration of experimental and simulation tools delving into the interface of engineering and biology. Development of "on-chip" technologies as well as their multifarious applications is subsequently discussed, accompanying the relevant advancements in materials and fabrication technology. Pointers toward new directions in research, including an amalgamated fusion of data-driven modeling (such as artificial intelligence and machine learning) and physics-based paradigm, to come up with a human physiological replica on a synthetic bio-chip with due accounting of personalized features, are suggested. These are likely to facilitate physiologically replicating disease modeling on an artificially engineered biochip as well as advance drug development and screening in an expedited route with the minimization of animal and human trials.

Microfluidic organoids-on-a-chip: The future of human models

Microfluidics opened the possibility to model the physiological environment by controlling fluids flows, and therefore nutrients supply. It allows to integrate external stimuli such as electricals or mechanicals and in situ monitoring important parameters such as pH, oxygen and metabolite concentrations. Organoids are self-organized 3D organ-like clusters, which allow to closely model original organ functionalities. Applying microfluidics to organoids allows to generate powerful human models for studying organ development, diseases, and drug testing. In this review, after a brief introduction on microfluidics, organoids and organoids-on-a-chip are described by organs (brain, heart, gastrointestinal tract, liver, pancreas) highlighting the microfluidic approaches since this point of view was overlooked in previously published reviews. Indeed, the review aims to discuss from a different point of view, primary microfluidics, the available literature on organoids-on-a-chip, standing out from the published literature by focusing on each specific organ.

Development of vessel mimicking microfluidic device for studying mechano-response of endothelial cells

The objective of this study is to develop a device to mimic a microfluidic system of human arterial blood vessels. The device combines fluid shear stress (FSS) and cyclic stretch (CS), which are resulting from blood flow and blood pressure, respectively. The device can reveal real-time observation of dynamic morphological change of cells in different flow fields (continuous flow, reciprocating flow and pulsatile flow) and stretch. We observe the effects of FSS and CS on endothelial cells (ECs), including ECs align their cytoskeleton proteins with the fluid flow direction and paxillin redistribution to the cell periphery or the end of stress fibers. Thus, understanding the morphological and functional changes of endothelial cells on physical stimuli can help us to prevent and improve the treatment of cardiovascular diseases.

Platelet accumulation in an endothelium-coated elastic vein valve model of deep vein thrombosis is mediated by GPIbα-VWF interaction

Deep vein thrombosis is a life-threatening disease that takes millions of people's lives worldwide. Given both technical and ethical issues of using animals in research, it is necessary to develop an appropriate in vitro model that would recapitulate the conditions of venous thrombus development. We present here a novel microfluidics vein-on-a-chip with moving valve leaflets to mimic the hydrodynamics in a vein, and Human Umbilical Vein Endothelial Cell (HUVEC) monolayer. A pulsatile flow pattern, typical for veins, was used in the experiments. Unstimulated human platelets, reconstituted with the whole blood, accumulated at the luminal side of the leaflet tips proportionally to the leaflet flexibility. Platelet activation by thrombin induced robust platelet accrual at the leaflet tips. Inhibition of glycoprotein (GP) IIb-IIIa did not decrease but, paradoxically, slightly increased platelet accumulation. In contrast, blockade of the interaction between platelet GPIbα and A1 domain of von Willebrand factor completely abolished platelet deposition. Stimulation of the endothelium with histamine, a known secretagogue of Weibel-Palade bodies, promoted platelet accrual at the basal side of the leaflets, where human thrombi are usually observed. Thus, platelet deposition depends on the leaflet flexibility, and accumulation of activated platelets at the valve leaflets is mediated by GPIbα-VWF interaction.

Sensors-integrated organ-on-a-chip for biomedical applications

As a promising new micro-physiological system, organ-on-a-chip has been widely utilized for in vitro pharmaceutical study and tissues engineering based on the three-dimensional constructions of tissues/organs and delicate replication of in vivo-like microenvironment. To better observe the biological processes, a variety of sensors have been integrated to realize in-situ, real-time, and sensitive monitoring of critical signals for organs development and disease modeling. Herein, we discuss the recent research advances made with respect to sensors-integrated organ-on-a-chip in this overall review. Firstly, we briefly explore the underlying fabrication procedures of sensors within microfluidic platforms and several classifications of sensory principles. Then, emphasis is put on the highlighted applications of different types of organ-on-a-chip incorporated with various sensors. Last but not least, perspective on the remaining challenges and future development of sensors-integrated organ-on-a-chip are presented.

Development of a Pneumatic-Driven Fiber-Shaped Robot Scaffold for Use as a Complex 3D Dynamic Culture System

Cells can sense and respond to different kinds of continuous mechanical strain in the human body. Mechanical stimulation needs to be included within the in vitro culture system to better mimic the existing complexity of in vivo biological systems. Existing commercial dynamic culture systems are generally two-dimensional (2D) which fail to mimic the three-dimensional (3D) native microenvironment. In this study, a pneumatically driven fiber robot has been developed as a platform for 3D dynamic cell culture. The fiber robot can generate tunable contractions upon stimulation. The surface of the fiber robot is formed by a braiding structure, which provides promising surface contact and adequate space for cell culture. An in-house dynamic stimulation using the fiber robot was set up to maintain NIH3T3 cells in a controlled environment. The biocompatibility of the developed dynamic culture systems was analyzed using LIVE/DEAD™ and alamarBlue™ assays. The results showed that the dynamic culture system was able to support cell proliferation with minimal cytotoxicity similar to static cultures. However, we observed a decrease in cell viability in the case of a high strain rate in dynamic cultures. Differences in cell arrangement and proliferation were observed between braided sleeves made of different materials (nylon and ultra-high molecular weight polyethylene). In summary, a simple and cost-effective 3D dynamic culture system has been proposed, which can be easily implemented to study complex biological phenomena in vitro.

Realizations of vascularized tissues: From in vitro platforms to in vivo grafts

Vascularization is essential for realizing thick and functional tissue constructs that can be utilized for in vitro study platforms and in vivo grafts. The vasculature enables the transport of nutrients, oxygen, and wastes and is also indispensable to organ functional units such as the nephron filtration unit, the blood-air barrier, and the blood-brain barrier. This review aims to discuss the latest progress of organ-like vascularized constructs with specific functionalities and realizations even though they are not yet ready to be used as organ substitutes. First, the human vascular system is briefly introduced and related design considerations for engineering vascularized tissues are discussed. Second, up-to-date creation technologies for vascularized tissues are summarized and classified into the engineering and cellular self-assembly approaches. Third, recent applications ranging from in vitro tissue models, including generic vessel models, tumor models, and different human organ models such as heart, kidneys, liver, lungs, and brain, to prevascularized in vivo grafts for implantation and anastomosis are discussed in detail. The specific design considerations for the aforementioned applications are summarized and future perspectives regarding future clinical applications and commercialization are provided.

Organ-on-a-chip technologies for biomedical research and drug development: A focus on the vasculature

Current biomedical models fail to replicate the complexity of human biology. Consequently, almost 90% of drug candidates fail during clinical trials after decades of research and billions of investments in drug development. Despite their physiological similarities, animal models often misrepresent human responses, and instead, trigger ethical and societal debates regarding their use. The overall aim across regulatory entities worldwide is to replace, reduce, and refine the use of animal experimentation, a concept known as the Three Rs principle. In response, researchers develop experimental alternatives to improve the biological relevance of in vitro models through interdisciplinary approaches. This article highlights the emerging organ-on-a-chip technologies, also known as microphysiological systems, with a focus on models of the vasculature. The cardiovascular system transports all necessary substances, including drugs, throughout the body while in charge of thermal regulation and communication between other organ systems. In addition, we discuss the benefits, limitations, and challenges in the widespread use of new biomedical models. Coupled with patient-derived induced pluripotent stem cells, organ-on-a-chip technologies are the future of drug discovery, development, and personalized medicine.

Atherosclerosis and endothelial mechanotransduction: current knowledge and models for future research

Endothelium health is essential to the regulation of physiological vascular functions. Because of the critical capability of endothelial cells (ECs) to sense and transduce chemical and mechanical signals in the local vascular environment, their dysfunction is associated with a vast variety of vascular diseases and injuries, especially atherosclerosis and subsequent cardiovascular diseases. This review describes the mechanotransduction events that are mediated through ECs, the EC subcellular components involved, and the pathways reported to be potentially involved. Up-to-date research efforts involving in vivo animal models and in vitro biomimetic models are also discussed, including their advantages and drawbacks, with recommendations on future modeling approaches to aid the development of novel therapies targeting atherosclerosis and related cardiovascular diseases.

Organs-on-Chips Platforms Are Everywhere: A Zoom on Biomedical Investigation

Over the decades, conventional in vitro culture systems and animal models have been used to study physiology, nutrient or drug metabolisms including mechanical and physiopathological aspects. However, there is an urgent need for Integrated Testing Strategies (ITS) and more sophisticated platforms and devices to approach the real complexity of human physiology and provide reliable extrapolations for clinical investigations and personalized medicine. Organ-on-a-chip (OOC), also known as a microphysiological system, is a state-of-the-art microfluidic cell culture technology that sums up cells or tissue-to-tissue interfaces, fluid flows, mechanical cues, and organ-level physiology, and it has been developed to fill the gap between in vitro experimental models and human pathophysiology. The wide range of OOC platforms involves the miniaturization of cell culture systems and enables a variety of novel experimental techniques. These range from modeling the independent effects of biophysical forces on cells to screening novel drugs in multi-organ microphysiological systems, all within microscale devices. As in living biosystems, the development of vascular structure is the salient feature common to almost all organ-on-a-chip platforms. Herein, we provide a snapshot of this fast-evolving sophisticated technology. We will review cutting-edge developments and advances in the OOC realm, discussing current applications in the biomedical field with a detailed description of how this technology has enabled the reconstruction of complex multi-scale and multifunctional matrices and platforms (at the cellular and tissular levels) leading to an acute understanding of the physiopathological features of human ailments and infections in vitro.

Zyxin and actin structure confer anisotropic YAP mechanotransduction

Tissues and the embedded cells experience anisotropic deformations due to their functions and anatomical locations. The resident cells, such as tenocytes and muscle cells, are often restricted by their extracellular matrix and organize parallel to their major loading direction, yet most studies on cellular responses to strains use isotropic substrates without predetermined organizations. To understand how confined cells sense and respond to anisotropic loading, we combine cell patterning and uniaxial stretch to have precise geometric control. Dynamic stretch parallel to the long axis of the cell activates YAP nuclear translocation, but not when stretched in the perpendicular direction. Looking at the initial cytoskeleton response, parallel stretch leads to actin breakage and repair within the first minute, mediated by zyxin, the focal adhesion protein. In addition, this zyxin-mediated repair response is controlled by focal adhesion kinase (FAK) and leads to YAP signaling. As these factors are intimately involved in a wide range of mechanical regulation, our findings point to new roles of zyxin and YAP in anisotropic mechanotransduction, and may provide additional perspectives in cellular adaptive responses and tissue homeostasis. STATEMENT OF SIGNIFICANCE: Structure and deformation of tissues control gene expression, migration, and proliferation of the resident cells. In an effort to understand the underlying mechanisms, we find that the transcription cofactor YAP respond to mechanical stretch in a direction-dependent manner. We demonstrate that parallel stretch induces actin cytoskeleton damage, focal adhesion kinase (FAK) activation, and zyxin relocation, which are involved in the anisotropic YAP signaling. Our findings provide additional perspectives in the interactions of tissue structure and cell mechanotransduction.

Cardiovascular consequences of sickle cell disease

Sickle cell disease (SCD) is an inherited blood disorder caused by a single point mutation within the beta globin gene. As a result of this mutation, hemoglobin polymerizes under low oxygen conditions causing red blood cells to deform, become more adhesive, and increase in rigidity, which affects blood flow dynamics. This process leads to enhanced red blood cell interactions with the endothelium and contributes to vaso-occlusion formation. Although traditionally defined as a red blood cell disorder, individuals with SCD are affected by numerous clinical consequences including stroke, painful crisis episodes, bone infarctions, and several organ-specific complications. Elevated cardiac output, endothelium activation along with the sickling process, and the vaso-occlusion events pose strains on the cardiovascular system. We will present a review of the cardiovascular consequences of sickle cell disease and show connections with the vasculopathy related to SCD. We will also highlight biophysical properties and engineering tools that have been used to characterize the disease. Finally, we will discuss therapies for SCD and potential implications on SCD cardiomyopathy.

Pneumatic Cell Stretching Chip to Generate Uniaxial Strain Using an Elastomeric Membrane with Ridge Structure

Cyclic mechanical stretching, including uniaxial strain, has been manifested to regulate the cell morphology and functions directly. In recent years, many techniques have been developed to apply cyclic mechanical stretching to cells in vitro. Pneumatically actuated stretching is one of the extensively used methods owing to its advantages of integration, miniaturization, and long-term stretching. However, the intrinsic difficulty in fabrication and adjusting the strain mode also impedes its development and application. In this study, inspired by the topological defects principle, we incorporated a ridge structure into the membrane surface of a traditional pneumatic cavity stretching chip to regulate the strain mode. Our results showed that the surface ridge structure can directly change the equiaxial stretching mode to the standard uniaxial strain, and it is ridge width-independent. The uniaxial strain mode was further proved by the cell orientation behavior under cyclic stretching stimulation. Moreover, it is easy to realize the multimodal strain fields by controlling the width and height of the ridge and to achieve high-throughput testing by creating a cavity array using microfabrication. Together, we propose a smart method to change the surface strain field and introduce a simple, yet effective, high-throughput pneumatically actuated uniaxial stretching platform, which can not only realize the multimodal mechanical stimulation but also achieve multiscale mechanosensing behaviors of single-cell or multi-cell (tissue and/or organoid) mechanobiology applications.

Automated measurement of cell mechanical properties using an integrated dielectrophoretic microfluidic device

Cell mechanics is closely related to and interacts with cellular functions, which has the potential to be an effective biomarker to indicate disease onset and progression. Although several techniques have been developed for measuring cell mechanical properties, the issues of limited measurement data and biological significance because of complex and labor-intensive manipulation remain to be addressed, especially for the dielectrophoresis-based approach that is difficult to utilize flow measurement techniques. In this work, a dielectrophoresis-based solution is proposed to automatically obtain mass cellular mechanical data by combining a designed microfluidic device integrated the functions of cell capture, dielectrophoretic stretching, and cell release and an automatic control scheme. Experiments using human umbilical vein endothelial cells and breast cells revealed the automation capability of this device. The proposed method provides an effective way to address the low-throughput problem of dielectrophoresis-based cell mechanical property measurements, which enhance the biostatistical significance for cellular mechanism studies.

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Cell capture, dielectrophoretic stretching, and release in one microfluidic chip

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Automatic measurement scheme to realize circularly measurement

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Automatic acquisition of large amounts of cell mechanical properties data

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Significant advances in dielectrophoretic measurement of cell mechanical properties

In Vitro Flow Chamber Design for the Study of Endothelial Cell (Patho)Physiology

In the native vasculature, flowing blood produces a frictional force on vessel walls that affects endothelial cell function and phenotype. In the arterial system, the vasculature's local geometry directly influences variations in flow profiles and shear stress magnitudes. Straight arterial sections with pulsatile shear stress have been shown to promote an athero-protective endothelial phenotype. Conversely, areas with more complex geometry, such as arterial bifurcations and branch points with disturbed flow patterns and lower, oscillatory shear stress, typically lead to endothelial dysfunction and the pathogenesis of cardiovascular diseases. Many studies have investigated the regulation of endothelial responses to various shear stress environments. Importantly, the accurate in vitro simulation of in vivo hemodynamics is critical to the deeper understanding of mechanotransduction through the proper design and use of flow chamber devices. In this review, we describe several flow chamber apparatuses and their fluid mechanics design parameters, including parallel-plate flow chambers, cone-and-plate devices, and microfluidic devices. In addition, chamber-specific design criteria and relevant equations are defined in detail for the accurate simulation of shear stress environments to study endothelial cell responses.

Blood vessel-on-a-chip examines the biomechanics of microvasculature

We use a three-dimensional (3D) microvascular platform to measure the elasticity and membrane permeability of the endothelial cell layer. The microfluidic platform is connected with a pneumatic pressure controller to apply hydrostatic pressure. The deformation is measured by tracking the mean vessel diameter under varying pressures up to 300 Pa. We obtain a value for the Young's modulus of the cell layer in low strain where a linear elastic response is observed and use a hyperelastic model that describes the strain hardening observed at larger strains (pressure). A fluorescent dye is used to track the flow through the cell layer to determine the membrane flow resistance as a function of applied pressure. Finally, we track the 3D positions of cell nuclei while the vessel is pressurized to observe local deformation and correlate inter-cell deformation with the local structure of the cell layer. This approach is able to probe the mechanical properties of blood vessels in vitro and provides a methodology for investigating microvascular related diseases.

Therapeutic Effect of Platinum Nanoparticles on Atherosclerosis Research Model Based on Microfluidics and Determination of Injury

The atherosclerosis (AS) microenvironment plays an important role in pathogenicity, including blood flow and blood pressure, high cholesterol, high blood sugar, angiotensin II, tumor necrosis factor, and the like. The AS microfluidic model was established, and the fluid shear stress and cyclic stretching were set to 5.07 Pa and 1.17 Hz to simulate normal blood flow, respectively. The effects of different biochemical environments on endothelial cells (ECs) and cardiomyocytes were analyzed. The results confirmed that different biochemical environments had significant damage to ECs and cardiomyocytes. Subsequently, the further effect of ECs on cardiomyocytes in AS microenvironment was studied, and the results proved that ECs caused further damage to cardiomyocytes under AS biochemical factors. We used Pt nanoparticles (Pt NPs) to study the anti-AS efficiency. The results showed that the addition of Pt NPs played a particular role in the AS treatment of ECs in the AS microenvironment, and the protection for myocardial cells was achieved.

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.

Luminal flow actuation generates coupled shear and strain in a microvessel-on-chip

In the microvasculature, blood flow-derived forces are key regulators of vascular structure and function. Consequently, the development of hydrogel-based microvessel-on-chip systems that strive to mimic thein vivocellular organization and mechanical environment has received great attention in recent years. However, despite intensive efforts, current microvessel-on-chip systems suffer from several limitations, most notably failure to produce physiologically relevant wall strain levels. In this study, a novel microvessel-on-chip based on the templating technique and using luminal flow actuation to generate physiologically relevant levels of wall shear stress and circumferential stretch is presented. Normal forces induced by the luminal pressure compress the surrounding soft collagen hydrogel, dilate the channel, and create large circumferential strain. The fluid pressure gradient in the system drives flow forward and generates realistic pulsatile wall shear stresses. Rigorous characterization of the system reveals the crucial role played by the poroelastic behavior of the hydrogel in determining the magnitudes of the wall shear stress and strain. The experimental measurements are combined with an analytical model of flow in both the lumen and the porous hydrogel to provide an exceptionally versatile user manual for an application-based choice of parameters in microvessels-on-chip. This unique strategy of flow actuation adds a dimension to the capabilities of microvessel-on-chip systems and provides a more general framework for improving hydrogel-basedin vitroengineered platforms.

Patterned vascularization in a directional ice-templated scaffold of decellularized matrix

Vascularization is fundamental for large-scale tissue engineering. Most of the current vascularization strategies including microfluidics and three-dimensional (3D) printing aim to precisely fabricate microchannels for individual microvessels. However, few studies have examined the remodeling capacity of the microvessels in the engineered constructs, which is important for transplantation in vivo. Here we present a method for patterning microvessels in a directional ice-templated scaffold of decellularized porcine kidney extracellular matrix. The aligned microchannels made by directional ice templating allowed for fast and efficient cell seeding. The pure decellularized matrix without any fixatives or cross-linkers maximized the potential of tissue remodeling. Dramatical microvascular remodeling happened in the scaffold in 2 weeks, from small primary microvessel segments to long patterned microvessels. The majority of the microvessels were aligned in parallel and interconnected with each other to form a network. This method is compatible with other engineering techniques, such as microfluidics and 3D printing, and multiple cell types can be co-cultured to make complex vascularized tissue and organ models.

Liver microphysiological platforms for drug metabolism applications

Drug development is a costly and lengthy process with low success rates. To improve the efficiency of drug development, there has been an increasing need in developing alternative methods able to eliminate toxic compounds early in the drug development pipeline. Drug metabolism plays a key role in determining the efficacy of a drug and its potential side effects. Since drug metabolism occurs mainly in the liver, liver cell‐based alternative engineering platforms have been growing in the last decade. Microphysiological liver cell‐based systems called liver‐on‐a‐chip platforms can better recapitulate the environment for human liver cells in laboratory settings and have the potential to reduce the number of animal models used in drug development by predicting the response of the liver to a drug in vitro. In this review, we discuss the liver microphysiological platforms from the perspective of drug metabolism studies. We highlight the stand‐alone liver‐on‐a‐chip platforms and multi‐organ systems integrating liver‐on‐a‐chip devices used for drug metabolism mimicry in vitro and review the state‐of‐the‐art platforms reported in the last few years. With the development of more robust and reproducible liver cell‐based microphysiological platforms, the drug development field has the potential of reducing the costs and lengths associated with currently existing drug testing methods.

Mechanical Strain-Enabled Reconstitution of Dynamic Environment in Organ-on-a-Chip Platforms: A Review

Organ-on-a-chip (OOC) uses the microfluidic 3D cell culture principle to reproduce organ- or tissue-level functionality at a small scale instead of replicating the entire human organ. This provides an alternative to animal models for drug development and environmental toxicology screening. In addition to the biomimetic 3D microarchitecture and cell-cell interactions, it has been demonstrated that mechanical stimuli such as shear stress and mechanical strain significantly influence cell behavior and their response to pharmaceuticals. Microfluidics is capable of precisely manipulating the fluid of a microenvironment within a 3D cell culture platform. As a result, many OOC prototypes leverage microfluidic technology to reproduce the mechanically dynamic microenvironment on-chip and achieve enhanced in vitro functional organ models. Unlike shear stress that can be readily generated and precisely controlled using commercial pumping systems, dynamic systems for generating proper levels of mechanical strains are more complicated, and often require miniaturization and specialized designs. As such, this review proposes to summarize innovative microfluidic OOC platforms utilizing mechanical actuators that induce deflection of cultured cells/tissues for replicating the dynamic microenvironment of human organs.

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Go with the flow: modeling unique biological flows in engineered in vitro platforms

Interest in recapitulating in vivo phenomena in vitro using organ-on-a-chip technology has grown rapidly and with it, attention to the types of fluid flow experienced in the body has followed suit. These platforms offer distinct advantages over in vivo models with regards to human relevance, cost, and control of inputs (e.g., controlled manipulation of biomechanical cues from fluid perfusion). Given the critical role biophysical forces play in several tissues and organs, it is therefore imperative that engineered in vitro platforms capture the complex, unique flow profiles experienced in the body that are intimately tied with organ function. In this review, we outline the complex and unique flow regimes experienced by three different organ systems: blood vasculature, lymphatic vasculature, and the intestinal system. We highlight current state-of-the-art platforms that strive to replicate physiological flows within engineered tissues while introducing potential limitations in current approaches.

Remodeling of an in vitro microvessel exposed to cyclic mechanical stretch

In the lungs, vascular endothelial cells experience cyclic mechanical strain resulting from rhythmic breathing motions and intraluminal blood pressure. Mechanical stress creates evident physiological, morphological, biochemical, and gene expression changes in vascular endothelial cells. However, the exact mechanisms of the mechanical signal transduction into biological responses remain to be clarified. Besides, the level of mechanical stress is difficult to determine due to the complexity of the local distension patterns in the lungs and thus assumed to be the same as the one acting on the alveolar epithelium. Existing in vitro models used to investigate the effect of mechanical stretch on endothelial cells are usually limited to two-dimensional (2D) cell culture platforms, which poorly mimic the typical three-dimensional structure of the vessels. Therefore, the development of an advanced in vitro vasculature model that closely mimics the dynamic of the human lung vasculatures is highly needed. Here, we present the first study that investigates the interplay of the three-dimensional (3D) mechanical cyclic stretch and its magnitude with vascular endothelial growth factor (VEGF) stimulation on a 3D perfusable vasculature in vitro. We studied the effects of the cyclic strain on a perfusable 3D vasculature, made of either human lung microvascular endothelial cells or human umbilical vein endothelial cells embedded in a gel layer. The in vitro 3D vessels underwent both in vivo-like longitudinal and circumferential deformations, simultaneously. Our results showed that the responses of the human lung microvascular endothelial cells and human umbilical vein endothelial cells to cyclic stretch were in good agreement. Although our 3D model was in agreement with the 2D model in predicting a cytoskeletal remodeling in response to different magnitudes of cyclic stretch, however, we observed several phenomena in the 3D model that the 2D model was unable to predict. Angiogenic sprouting induced by VEGF decreased significantly in the presence of cyclic stretch. Similarly, while treatment with VEGF increased vascular permeability, the cyclic stretch restored vascular barrier tightness and significantly decreased vascular permeability. One of the major findings of this study was that a 3D microvasculature can be exposed to a much higher mechanical cyclic stress level than reported in the literature without any dysfunction of its barrier. For higher magnitudes of the cyclic stretch, the applied longitudinal strain level was 14% and the associated circumferential strain reached the equivalent of 63%. In sharp contrast to our findings, such strain typically leads to the disruption of the endothelial barrier in a 2D stretching assay and is considered pathological. This highlights the importance of 3D modeling to investigate mechanobiology effects rather than using a simple endothelial monolayer, which truly recapitulates the in vivo situation.

Critical Frequency and Critical Stretching Rate for Reorientation of Cells on a Cyclically Stretched Polymer in a Microfluidic Chip

The ability of cells to sense and respond to mechanical signals from their surrounding microenvironments is one of the key issues in tissue engineering and regeneration, yet a fundamental study of cells with both cell observation and mechanical stimulus is challenging and should be based upon an appropriate microdevice. Herein we designed and fabricated a two-layer microfluidic chip to enable simultaneous observation of live cells and cyclic stretching of an elastic polymer, polydimethylsiloxane (PDMS), with a modified surface for enhanced cell adhesion. Human mesenchymal stem cells (hMSCs) were examined with a series of frequencies from 0.00003 to 2 Hz and varied amplitudes of 2%, 5%, or 10%. The cells with an initial random orientation were confirmed to be reoriented perpendicular to the stretching direction at frequencies greater than a threshold value, which we term critical frequency (f_c); additionally, the critical frequency f_c was amplitude-dependent. We further introduced the concept of critical stretching rate (R_c) and found that this quantity can unify both frequency and amplitude dependences. The reciprocal value of R_c in this study reads 8.3 min, which is consistent with the turnover time of actin filaments reported in the literature, suggesting that the supramolecular relaxation in the cytoskeleton within a cell might be responsible for the underlying cell mechanotransduction. The theoretical calculation of cell reorientation based on a two-dimensional tensegrity model under uniaxial cyclic stretching is well consistent with our experiments. The above findings provide new insight into the crucial role of critical frequency and critical stretching rate in regulating cells on biomaterials under biomechanical stimuli.

Healthy and diseased in vitro models of vascular systems

Irregular hemodynamics affects the progression of various vascular diseases, such atherosclerosis or aneurysms. Despite the extensive hemodynamics studies on animal models, the inter-species differences between humans and animals hamper the translation of such findings. Recent advances in vascular tissue engineering and the suitability of in vitro models for interim analysis have increased the use of in vitro human vascular tissue models. Although the effect of flow on endothelial cell (EC) pathophysiology and EC-flow interactions have been vastly studied in two-dimensional systems, they cannot be used to understand the effect of other micro- and macro-environmental parameters associated with vessel wall diseases. To generate an ideal in vitro model of the vascular system, essential criteria should be included: 1) the presence of smooth muscle cells or perivascular cells underneath an EC monolayer, 2) an elastic mechanical response of tissue to pulsatile flow pressure, 3) flow conditions that accurately mimic the hemodynamics of diseases, and 4) geometrical features required for pathophysiological flow. In this paper, we review currently available in vitro models that include flow dynamics and discuss studies that have tried to address the criteria mentioned above. Finally, we critically review in vitro fluidic models of atherosclerosis, aneurysm, and thrombosis.

Microfluidics and organ-on-a-chip technologies: A systematic review of the methods used to mimic bone marrow

Bone marrow (BM) is an organ responsible for crucial processes in living organs, e. g., hematopoiesis. In recent years, Organ-on-a-Chip (OoC) devices have been used to satisfy the need for in vitro systems that better mimic the phenomena occurring in the BM microenvironment. Given the growing interest in these systems and the diversity of developed devices, an integrative systematic literature review is required. We have performed this review, following the PRISMA method aiming to identify the main characteristics and assess the effectiveness of the devices that were developed to represent the BM. A search was performed in the Scopus, PubMed, Web of Science and Science Direct databases using the keywords (("bone marrow" OR "hematopoietic stem cells" OR "haematopoietic stem cells") AND ("organ in a" OR "lab on a chip" OR "microfluidic" OR "microfluidic*" OR ("bioreactor" AND "microfluidic*"))). Original research articles published between 2009 and 2020 were included in the review, giving a total of 21 papers. The analysis of these papers showed that their main purpose was to study BM cells biology, mimic BM niches, model pathological BM, and run drug assays. Regarding the fabrication protocols, we have observed that polydimethylsiloxane (PDMS) material and soft lithography method were the most commonly used. To reproduce the microenvironment of BM, most devices used the type I collagen and alginate. Peristaltic and syringe pumps were mostly used for device perfusion. Regarding the advantages compared to conventional methods, there were identified three groups of OoC devices: perfused 3D BM; co-cultured 3D BM; and perfused co-cultured 3D BM. Cellular behavior and mimicking their processes and responses were the mostly commonly studied parameters. The results have demonstrated the effectiveness of OoC devices for research purposes compared to conventional cell cultures. Furthermore, the devices have a wide range of applicability and the potential to be explored.

Endothelial cell polarization and orientation to flow in a novel microfluidic multimodal shear stress generator

Endothelial cells (EC) respond to shear stress to maintain vascular homeostasis, and a disrupted response is associated with cardiovascular diseases. To understand how different shear stress modalities affect EC morphology and behavior, we developed a microfluidic device that concurrently generates three different levels of uniform wall shear stress (WSS) and six different WSS gradients (WSSG). In this device, human umbilical vein endothelial cells (HUVECs) exhibited a rapid and robust response to WSS, with the relative positioning of the Golgi and nucleus transitioning from a non-polarized to polarized state in a WSS magnitude- and gradient-dependent manner. By contrast, polarized HUVECs oriented their Golgi and nucleus polarity to the flow vector in a WSS magnitude-dependent manner, with positive WSSG inhibiting and negative WSSG promoting upstream orientation. Having validated this device, this chip can now be used to dissect the mechanisms underlying EC responses to different WSS modalities, including shear stress gradients, and to investigate the influence of flow on a diverse range of cells during development, homeostasis and disease.

Organ-on-a-chip platforms for accelerating the evaluation of nanomedicine

Nanomedicine involves the use of engineered nanoscale materials in an extensive range of diagnostic and therapeutic applications and can be applied to the treatment of many diseases. Despite the rapid progress and tremendous potential of nanomedicine in the past decades, the clinical translational process is still quite slow, owing to the difficulty in understanding, evaluating, and predicting nanomaterial behaviors within the complex environment of human beings. Microfluidics-based organ-on-a-chip (Organ Chip) techniques offer a promising way to resolve these challenges. Sophisticatedly designed Organ Chip enable in vitro simulation of the in vivo microenvironments, thus providing robust platforms for evaluating nanomedicine. Herein, we review recent developments and achievements in Organ Chip models for nanomedicine evaluations, categorized into seven broad sections based on the target organ systems: respiratory, digestive, lymphatic, excretory, nervous, and vascular, as well as coverage on applications relating to cancer. We conclude by providing our perspectives on the challenges and potential future directions for applications of Organ Chip in nanomedicine.

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Microfluidics-based organ-on-a-chip (Organ Chip) techniques offer a promising way to understand, evaluate, and predict nanomedicine behaviors within the complex environment.

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Organ Chip models for nanomedicine evaluations are categorized into seven broad sections based on the targeted body systems.

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Limitations, challenges, and perspectives of Organ Chip for accelerating the assessment of nanomedicine are discussed, respectively.

Microfluidics by Additive Manufacturing for Wearable Biosensors: A Review

Wearable devices are nowadays at the edge-front in both academic research as well as in industry, and several wearable devices have been already introduced in the market. One of the most recent advancements in wearable technologies for biosensing is in the area of the remote monitoring of human health by detection on-the-skin. However, almost all the wearable devices present in the market nowadays are still providing information not related to human ‘metabolites and/or disease’ biomarkers, excluding the well-known case of the continuous monitoring of glucose in diabetic patients. Moreover, even in this last case, the glycaemic level is acquired under-the-skin and not on-the-skin. On the other hand, it has been proven that human sweat is very rich in molecules and other biomarkers (e.g., ions), which makes sweat a quite interesting human liquid with regards to gathering medical information at the molecular level in a totally non-invasive manner. Of course, a proper collection of sweat as it is emerging on top of the skin is required to correctly convey such liquid to the molecular biosensors on board of the wearable system. Microfluidic systems have efficiently come to the aid of wearable sensors, in this case. These devices were originally built using methods such as photolithographic and chemical etching techniques with rigid materials. Nowadays, fabrication methods of microfluidic systems are moving towards three-dimensional (3D) printing methods. These methods overcome some of the limitations of the previous method, including expensiveness and non-flexibility. The 3D printing methods have a high speed and according to the application, can control the textures and mechanical properties of an object by using multiple materials in a cheaper way. Therefore, the aim of this paper is to review all the most recent advancements in the methods for 3D printing to fabricate wearable fluidics and provide a critical frame for the future developments of a wearable device for the remote monitoring of the human metabolism directly on-the-skin.

Microphysiological Systems: Design, Fabrication, and Applications

Microphysiological systems, including organoids, 3-D printed tissue constructs and organ-on-a-chips (organ chips), are physiologically relevant in vitro models and have experienced explosive growth in the past decades. Different from conventional, tissue culture plastic-based in vitro models or animal models, microphysiological systems recapitulate key microenvironmental characteristics of human organs and mimic their primary functions. The advent of microphysiological systems is attributed to evolving biomaterials, micro-/nanotechnologies and stem cell biology, which enable the precise control over the matrix properties and the interactions between cells, tissues and organs in physiological conditions. As such, microphysiological systems have been developed to model a broad spectrum of organs from microvasculature, eye, to lung and many others to understand human organ development and disease pathology and facilitate drug discovery. Multiorgans-on-a-chip systems have also been developed by integrating multiple associated organ chips in a single platform, which allows to study and employ the organ function in a systematic approach. Here we first discuss the design principles of microphysiological systems with a focus on the anatomy and physiology of organs, and then review the commonly used fabrication techniques and biomaterials for microphysiological systems. Subsequently, we discuss the recent development of microphysiological systems, and provide our perspectives on advancing microphysiological systems for preclinical investigation and drug discovery of human disease.

ECM-based microchannel for culturing in vitro vascular tissues with simultaneous perfusion and stretch

We present an extracellular matrix (ECM)-based stretchable microfluidic system for culturing in vitro three-dimensional (3D) vascular tissues, which mimics in vivo blood vessels. Human umbilical vein endothelial cells (HUVECs) can be cultured under perfusion and stretch simultaneously with real-time imaging by our proposed system. Our ECM (transglutaminase (TG) cross-linked gelatin)-based microchannel was fabricated by dissolving water-soluble sacrificial polyvinyl alcohol (PVA) molds printed with a 3D printer. Flows in the microchannel were analyzed under perfusion and stretch. We demonstrated simultaneous perfusion and stretch of TG gelatin-based microchannels culturing HUVECs. We suggest that our TG gelatin-based stretchable microfluidic system proves to be a useful tool for understanding the mechanisms of vascular tissue formation and mechanotransduction.

Current challenges and future trends in manufacturing small diameter artificial vascular grafts in bioreactors

Cardiovascular diseases are a leading cause of death. Vascular surgery is mainly used to solve this problem. However, the generation of a functional and suitable substitute for small diameter (< 6 mm) displacement is challengeable. Moreover, synthetic prostheses, made of polyethylene terephthalate and extended polytetrafluoroethylene show have shown insufficient performance. Therefore, the challenges dominating the use of autografts have prevented their efficient use. Tissue engineering is highlighted in regenerative medicine perhaps in aiming to address the issue of end-stage organ failure. While organs and complex tissues require the vascular supply to support the graft survival and render the bioartificial organ role, vascular tissue engineering has shown to be a hopeful method for cell implantation by the production of tissues in vitro. Bioreactors are a salient point in vascular tissue engineering due to the capability for reproducible and controlled variations showing a new horizon in blood vessel substitution. This review strives to display the overview of current concepts in the development of small-diameter by using bioreactors. In this work, we show a critical look at different factors for developing small-diameter and give suggestions for future studies.

Pulsatile Flow in Microfluidic Systems

This review describes the current knowledge and applications of pulsatile flow in microfluidic systems. Elements of fluid dynamics at low Reynolds number are first described in the context of pulsatile flow. Then the practical applications in microfluidic processes are presented: the methods to generate a pulsatile flow, the generation of emulsion droplets through harmonic flow rate perturbation, the applications in mixing and particle separation, and the benefits of pulsatile flow for clog mitigation. The second part of the review is devoted to pulsatile flow in biological applications. Pulsatile flows can be used for mimicking physiological systems, to alter or enhance cell cultures, and for bioassay automation. Pulsatile flows offer unique advantages over a steady flow, especially in microfluidic systems, but also require some new physical insights and more rigorous investigation to fully benefit future applications.

Lung Microvascular Niche, Repair, and Engineering

Biomaterials have been used for a long time in the field of medicine. Since the success of "tissue engineering" pioneered by Langer and Vacanti in 1993, tissue engineering studies have advanced from simple tissue generation to whole organ generation with three-dimensional reconstruction. Decellularized scaffolds have been widely used in the field of reconstructive surgery because the tissues used to generate decellularized scaffolds can be easily harvested from animals or humans. When a patient's own cells can be seeded onto decellularized biomaterials, theoretically this will create immunocompatible organs generated from allo- or xeno-organs. The most important aspect of lung tissue engineering is that the delicate three-dimensional structure of the organ is maintained during the tissue engineering process. Therefore, organ decellularization has special advantages for lung tissue engineering where it is essential to maintain the extremely thin basement membrane in the alveoli. Since 2010, there have been many methodological developments in the decellularization and recellularization of lung scaffolds, which includes improvements in the decellularization protocols and the selection and preparation of seeding cells. However, early transplanted engineered lungs terminated in organ failure in a short period. Immature vasculature reconstruction is considered to be the main cause of engineered organ failure. Immature vasculature causes thrombus formation in the engineered lung. Successful reconstruction of a mature vasculature network would be a major breakthrough in achieving success in lung engineering. In order to regenerate the mature vasculature network, we need to remodel the vascular niche, especially the microvasculature, in the organ scaffold. This review highlights the reconstruction of the vascular niche in a decellularized lung scaffold. Because the vascular niche consists of endothelial cells (ECs), pericytes, extracellular matrix (ECM), and the epithelial-endothelial interface, all of which might affect the vascular tight junction (TJ), we discuss ECM composition and reconstruction, the contribution of ECs and perivascular cells, the air-blood barrier (ABB) function, and the effects of physiological factors during the lung microvasculature repair and engineering process. The goal of the present review is to confirm the possibility of success in lung microvascular engineering in whole organ engineering and explore the future direction of the current methodology.

Bioprinting a 3D vascular construct for engineering a vessel-on-a-chip

The organ-on-a-chip model mimics the structural and functional features of human tissues or organs and has great importance in translational research. For vessel-on-a-chip model, conventional fabrication techniques are unable to efficiently imitate the intimal-medial unit of the vessel wall. Bioprinting technology, which can precisely control the organization of cells, biomolecules, and the extracellular matrix, has the potential to fabricate three-dimensional (3D) tissue constructs with spatial heterogeneity. In this study, we applied the gelatin-methacryloyl-based bioprinting technology to print 3D construct containing endothelial cells (ECs) and smooth muscle cells (SMCs) on a microfluidic chip. Compared with traditional culture system, EC-SMC coculturing chip model upregulated αSMA and SM22 protein expression of the SMC to a greater degree and maintains the contractile phenotype of the SMC, which mimics the natural vascular microenvironment. This strategy enabled us to establish an in vitro vascular model for studies of the physiologic and pathologic process in vascular wall.

Microfluidics-Based Biomaterials and Biodevices

The rapid development of microfluidics technology has promoted new innovations in materials science, particularly by interacting with biological systems, based on precise manipulation of fluids and cells within microscale confinements. This article reviews the latest advances in microfluidics-based biomaterials and biodevices, highlighting some burgeoning areas such as functional biomaterials, cell manipulations, and flexible biodevices. These areas are interconnected not only in their basic principles, in that they all employ microfluidics to control the makeup and morphology of materials, but also unify at the ultimate goals in human healthcare. The challenges and future development trends in biological application are also presented.

Microfluidic-Based Platform for the Evaluation of Nanomaterial-Mediated Drug Delivery: From High-Throughput Screening to Dynamic Monitoring

Nanomaterial-based drug delivery holds tremendous promise for improving targeting capacity, biodistribution, and performance of therapeutic/diagnostic agents. Accelerating the clinical translation of current nanomedicine requires an in-depth understanding of the mechanism underlying the dynamic interaction between nanomaterials and cells in a physiological/pathophysiological-relevant condition. The introduction of the advanced microfluidic platform with miniaturized, well-controlled, and high-throughput features opens new investigation and application opportunities for nanomedicine evaluation. This review highlights the current state-of-theart in the field of 1) microfluidic-assisted in vitro assays that are capable of providing physiological-relevant flow conditions and performing high-throughput drug screening, 2) advanced organ-on-a-chip technology with the combination of microfabrication and tissue engineering techniques for mimicking microenvironment and better predicting in vivo response of nanomedicine, and 3) the integration of microdevice with various detection techniques that can monitor cell-nanoparticle interaction with high spatiotemporal resolution. Future perspectives regarding optimized on-chip disease modeling and personalized nanomedicine screening are discussed towards further expanding the utilization of the microfluidic-based platform in assessing the biological behavior of nanomaterials.

Microfluidic Fabrication of Biomimetic Helical Hydrogel Microfibers for Blood-Vessel-on-a-Chip Applications

Nature has created many perfect helical microstructures, including DNA, collagen fibrils, and helical blood vessels, to achieve unique physiological functions. While previous studies have developed a number of microfabrication strategies, the preparation of complex helical structures and cell-laden helical structures for biomimetic applications remains challenging. In this study, a one-step microfluidics-based methodology is presented for preparing complex helical hydrogel microfibers and cell-laden helical hydrogel microfibers. Several types of complex helical structures, including multilayer helical microfibers and superhelical hollow microfibers with helical channels, are prepared by simply tuning the flow rates or modifying the geometry of microfluidic device. With the decent perfusability, the hollow microfibers may simulate the structural characteristics of helical blood vessels and create swirling blood flow in a blood-vessel-on-chip setup. Such hydrogel-based helical microstructures may potentially be used in areas such as blood vessel tissue engineering, organ-on-chips, drug screening, and biological actuators.

Cell-Based Assays on Microfluidics for Drug Screening

Microfluidics is an appealing platform for drug screening and discovery. Compared with the conventional drug screening methods based on Petri dishes and experimental animals, microfluidic devices have many advantages including miniaturized size, ease-to-use, high sensitivity, and high throughput. More importantly, bioassays on microfluidics can avoid ethical issues which can be a big obstacle hindering the performance of the experiments on animals or human being. Furthermore, three-dimensional (3D) microchips can recapitulate various biochemical and biophysical conditions in vivo and mimic the natural microenvironment of the tissues/organs, providing versatile in vitro models for biomedical applications. In this Perspective, we will focus on the cell-based microfluidic assays for drug screening. Meanwhile, we also propose potential solutions for the difficulties in this field and discuss the prospects of microfluidics-based technologies for drug screening.