Bifurcations: focal points of particle adhesion in microvascular networks

Technology for the formation of engineered microvascular network models and their biomedical applications

Tissue engineering and regenerative medicine have made great progress in recent decades, as the fields of bioengineering, materials science, and stem cell biology have converged, allowing tissue engineers to replicate the structure and function of various levels of the vascular tree. Nonetheless, the lack of a fully functional vascular system to efficiently supply oxygen and nutrients has hindered the clinical application of bioengineered tissues for transplantation. To investigate vascular biology, drug transport, disease progression, and vascularization of engineered tissues for regenerative medicine, we have analyzed different approaches for designing microvascular networks to create models. This review discusses recent advances in the field of microvascular tissue engineering, explores potential future challenges, and offers methodological recommendations.

Emerging Approaches to Understanding Microvascular Endothelial Heterogeneity: A Roadmap for Developing Anti-Inflammatory Therapeutics

The endothelium is the inner layer of all blood vessels and it regulates hemostasis. It also plays an active role in the regulation of the systemic inflammatory response. Systemic inflammatory disease often results in alterations in vascular endothelium barrier function, increased permeability, excessive leukocyte trafficking, and reactive oxygen species production, leading to organ damage. Therapeutics targeting endothelium inflammation are urgently needed, but strong concerns regarding the level of phenotypic heterogeneity of microvascular endothelial cells between different organs and species have been expressed. Microvascular endothelial cell heterogeneity in different organs and organ-specific variations in endothelial cell structure and function are regulated by intrinsic signals that are differentially expressed across organs and species; a result of this is that neutrophil recruitment to discrete organs may be regulated differently. In this review, we will discuss the morphological and functional variations in differently originated microvascular endothelia and discuss how these variances affect systemic function in response to inflammation. We will review emerging in vivo and in vitro models and techniques, including microphysiological devices, proteomics, and RNA sequencing used to study the cellular and molecular heterogeneity of endothelia from different organs. A better understanding of microvascular endothelial cell heterogeneity will provide a roadmap for developing novel therapeutics to target the endothelium.

Experimental Approaches to Evaluate Leukocyte-Endothelial Cell Interactions in Sepsis and Inflammation

Sepsis is a life-threatening syndrome of organ dysfunction caused by a dysregulated host response to infection characterized by excessive neutrophil infiltration into vital organs. In sepsis, patients often die of organ failure and therapies directed against endothelial cell dysfunction and tissue damage are important targets for treatment of this disease. Novel approaches are required to understand the underlying pathophysiology of neutrophil dysregulation and neutrophil-endothelial cell interactions that play a critical role in the early course of organ damage and disruption of endothelial protective barrier. Here, we review methodologies that our laboratories have employed to study neutrophil-endothelial interaction and endothelial barrier function in in vivo and in vitro models of sepsis. We will focus on in vivo rodent models of sepsis and in vitro tools that use human cell culture models under static conditions and the more physiologically relevant biomimetic microfluidic assays. This Methods paper is based on our presentation in the Master Class Symposium at the 41st Annual Conference on Shock 2018.

A converging artery-sized model for shear adhesion mapping of particles

Drug carriers for targeting cardiovascular diseases have been gaining a respectable attention, however, designing such carriers is challenging due to the biophysical complexity of the vascular system. Wall shear stress (WSS), exerted by blood flow on the endothelium surface, is a crucial factor in the circulatory system. WSS affects the adhesion and preferential accumulation of drug carriers. Here, we propose, an innovative approach to investigate particle adhesion in a converging artery-sized model, lined with human endothelial cells. Unlike widely used microfluidic and in vivo setups, our model enables to investigate particle accumulation in a continuous WSS range, performed in a single experiment, and at the right scale relevant for human arteries. First, we characterized the flow and the WSS map along the designed model, which can span along the entire arterial WSS range. We then used the model to examine the effect of particle size and the suspension buffer on particle adhesion distribution. The results demonstrated the role of particle size, where the same particles with a diameter of 2 µm exhibit shear-decreased adhesion while 500 nm particles exhibit shear-enhanced adhesion. Furthermore, under the same WSS, particles show a similar behavior when suspended in a Dextran buffer, having a viscosity analogous to blood, compared to a phosphate buffer solution without Dextran. Moreover, experiments with RBCs in the phosphate buffer, at a 40% physiological hematocrit, decreased particle adhesion and affected their deposition pattern. Altogether, our study suggests an original platform for investigating and optimizing intravascular drug carriers and their targeting properties.

Mechanical Stimulation: A Crucial Element of Organ-on-Chip Models

Organ-on-chip (OOC) systems recapitulate key biological processes and responses in vitro exhibited by cells, tissues, and organs in vivo. Accordingly, these models of both health and disease hold great promise for improving fundamental research, drug development, personalized medicine, and testing of pharmaceuticals, food substances, pollutants etc. Cells within the body are exposed to biomechanical stimuli, the nature of which is tissue specific and may change with disease or injury. These biomechanical stimuli regulate cell behavior and can amplify, annul, or even reverse the response to a given biochemical cue or drug candidate. As such, the application of an appropriate physiological or pathological biomechanical environment is essential for the successful recapitulation of in vivo behavior in OOC models. Here we review the current range of commercially available OOC platforms which incorporate active biomechanical stimulation. We highlight recent findings demonstrating the importance of including mechanical stimuli in models used for drug development and outline emerging factors which regulate the cellular response to the biomechanical environment. We explore the incorporation of mechanical stimuli in different organ models and identify areas where further research and development is required. Challenges associated with the integration of mechanics alongside other OOC requirements including scaling to increase throughput and diagnostic imaging are discussed. In summary, compelling evidence demonstrates that the incorporation of biomechanical stimuli in these OOC or microphysiological systems is key to fully replicating in vivo physiology in health and disease.

Cellular Interactions of Liposomes and PISA Nanoparticles during Human Blood Flow in a Microvascular Network

A key concept in nanomedicine is encapsulating therapeutic or diagnostic agents inside nanoparticles to prolong blood circulation time and to enhance interactions with targeted cells. During circulation and depending on the selected application (e.g., cancer drug delivery or immune modulators), nanoparticles are required to possess low or high interactions with cells in human blood and blood vessels to minimize side effects or maximize delivery efficiency. However, analysis of cellular interactions in blood vessels is challenging and is not yet realized due to the diverse components of human blood and hemodynamic flow in blood vessels. Here, the first comprehensive method to analyze cellular interactions of both synthetic and commercially available nanoparticles under human blood flow conditions in a microvascular network is developed. Importantly, this method allows to unravel the complex interplay of size, charge, and type of nanoparticles on their cellular associations under the dynamic flow of human blood. This method offers a unique platform to study complex interactions of any type of nanoparticles in human blood flow conditions and serves as a useful guideline for the rational design of liposomes and polymer nanoparticles for diverse applications in nanomedicine.

Computational analysis of adhesion between a cancer cell and a white blood cell in a bifurcated microvessel

BACKGROUND AND OBJECTIVE

Cancer is one of the diseases caused by irregular and uncontrolled growth of cells and their propagation into various parts of the body. The motion and adhesion of cancer cells in a blood vessel is a critical step in tumor progression that depends on some vascular parameters such as vessel branching. In this study, effect of microvessel branching on the bonds between a cancer cell and a white blood has been investigated as compared to an analogous problem in a straight vessel.

METHODS

The analysis is performed using finite elements and fluid-structure interaction methods. Moreover, the equations for adhesion of the cancer cell to white blood cell are coded in MATLAB for calculating forces between them and the code is coupled directly and simultaneously with the COMSOL software. For fluid-structure interaction analysis, it is assumed that the properties of the blood and the cells are homogeneous and the fluid is incompressible and Newtonian. Cancer cell is modeled as a rigid body and white blood cell is assumed as linear elastic.

RESULTS

The results show that although the geometry of the vessel does not affect on the separation distance of cancer cell considerably, but at the area near a bifurcation, high fluctuations in cancer cell velocity is occurred due to increasing in number of bond formation between the cancer cell and the white blood cell. Accordingly, it can be predicted that higher concentration of adhered particles occurs near the bifurcations. Moreover, shear stress at the point of contact of the cancer cell and the white blood cell in the branched vessel is greater than that in the straight path. In addition to, the probability of breaking of the bond between the cancer cell and the white blood cell increases in the branched vessel.

CONCLUSIONS

Through consideration in the adhesion charts of this study along with observations from medical interventions such as drug delivery to cancer patients, considerable developments on the treatment or prevention of cancer metastasis may be achieved.

Bubbles Moving in Blood Flow in a Microchannel Network: The Effect on the Local Hematocrit

Air inside of blood vessels is a phenomenon known as gas embolism. During the past years, studies have been performed to assess the influence of air bubbles in microcirculation. In this study, we investigated the flow of bubbles in a microchannel network with several bifurcations, mimicking part of a capillary system. Thus, two working fluids were used, composed by sheep red blood cells (RBCs) suspended in a Dextran 40 solution with different hematocrits (5% and 10%). The experiments were carried out in a polydimethylsiloxane (PDMS) microchannel network fabricated by a soft lithography. A high-speed video microscopy system was used to obtain the results for a blood flow rate of 10 µL/min. This system enables the visualization of bubble formation and flow along the network. The results showed that the passage of air bubbles strongly influences the cell's local concentration, since a higher concentration of cells was observed upstream of the bubble, whereas a lower local hematocrit was visualized at the region downstream of the bubble. In bifurcations, bubbles may split asymmetrically, leading to an uneven distribution of RBCs between the outflow branches.

Neutrophil-endothelial interactions of murine cells is not a good predictor of their interactions in human cells

All drugs recently developed in rodent models to treat inflammatory disease have failed in clinical trials. We therefore used our novel biomimetic microfluidic assay (bMFA) to determine whether the response of murine cells to inflammatory activation or anti-inflammatory treatment is predictive of the response in human cells. Under physiologically relevant flow conditions, permeability and transendothelial electrical resistance (TEER) of human or mouse lung microvascular endothelial cells (HLMVEC or MLMVEC), and neutrophil-endothelial cell interaction was measured. The differential impact of a protein kinase C-delta TAT peptide inhibitor (PKCδ-i) was also quantified. Permeability of HLMVEC and MLMVEC was similar under control conditions but tumor necrosis factor α (TNF-α) and PKCδ-i had a significantly higher impact on permeability of HLMVEC. TEER across HLMVEC was significantly higher than MLMVEC, but PKCδ-i returned TEER to background levels only in human cells. The kinetics of N-formylmethionyl-leucyl-phenylalanine (fMLP)-mediated neutrophil migration was significantly different between the two species and PKCδ-i was significantly more effective in attenuating human neutrophil migration. However, human and mouse neutrophil adhesion patterns to microvascular endothelium were not significantly different. Surprisingly, while intercellular adhesion molecule 1 (ICAM-1) was significantly upregulated on activated HLMVEC, it was not significantly upregulated on activated MLMVEC. Responses to activation and anti-inflammatory treatment in mice may not always be predictive of their response in humans.

Co-cultured microfluidic model of the airway optimized for microscopy and micro-optical coherence tomography imaging

We have developed a human bronchial epithelial (HBE) cell and endothelial cell co-cultured microfluidic model to mimic the in vivo human airway. This airway-on-a-chip was designed with a central epithelial channel and two flanking endothelial channels, with a three-dimensional monolayers of cells growing along the four walls of the channel, forming central clear lumens. These cultures mimic airways and microvasculature in vivo. The central channel cells are grown at air-liquid interface and show features of airway differentiation including tight-junction formation, mucus production, and ciliated cells. Combined with novel micro-optical coherence tomography, this chip enables functional imaging of the interior of the lumen, which includes quantitation of cilia motion including beat frequency and mucociliary transport. This airway-on-a chip is a significant step forward in the development of microfluidics models for functional imaging.

Evaluating Nanoparticles in Preclinical Research Using Microfluidic Systems

Nanoparticles (NPs) have found a wide range of applications in clinical therapeutic and diagnostic fields. However, currently most NPs are still in the preclinical evaluation phase with few approved for clinical use. Microfluidic systems can simulate dynamic fluid flows, chemical gradients, partitioning of multi-organs as well as local microenvironment controls, offering an efficient and cost-effective opportunity to fast screen NPs in physiologically relevant conditions. Here, in this review, we are focusing on summarizing key microfluidic platforms promising to mimic in vivo situations and test the performance of fabricated nanoparticles. Firstly, we summarize the key evaluation parameters of NPs which can affect their delivery efficacy, followed by highlighting the importance of microfluidic-based NP evaluation. Next, we will summarize main microfluidic systems effective in evaluating NP haemocompatibility, transport, uptake and toxicity, targeted accumulation and general efficacy respectively, and discuss the future directions for NP evaluation in microfluidic systems. The combination of nanoparticles and microfluidic technologies could greatly facilitate the development of drug delivery strategies and provide novel treatments and diagnostic techniques for clinically challenging diseases.

The Role of Tyrosine Phosphorylation of Protein Kinase C Delta in Infection and Inflammation

Protein Kinase C (PKC) is a family composed of phospholipid-dependent serine/threonine kinases that are master regulators of inflammatory signaling. The activity of different PKCs is context-sensitive and these kinases can be positive or negative regulators of signaling pathways. The delta isoform (PKCδ) is a critical regulator of the inflammatory response in cancer, diabetes, ischemic heart disease, and neurodegenerative diseases. Recent studies implicate PKCδ as an important regulator of the inflammatory response in sepsis. PKCδ, unlike other members of the PKC family, is unique in its regulation by tyrosine phosphorylation, activation mechanisms, and multiple subcellular targets. Inhibition of PKCδ may offer a unique therapeutic approach in sepsis by targeting neutrophil-endothelial cell interactions. In this review, we will describe the overall structure and function of PKCs, with a focus on the specific phosphorylation sites of PKCδ that determine its critical role in cell signaling in inflammatory diseases such as sepsis. Current genetic and pharmacological tools, as well as in vivo models, that are used to examine the role of PKCδ in inflammation and sepsis are presented and the current state of emerging tools such as microfluidic assays in these studies is described.

PKCδ inhibition as a novel medical countermeasure for radiation-induced vascular damage

In the event of a radiologic catastrophe, endothelial cell and neutrophil dysfunction play important roles in tissue injury. Clinically available therapeutics for radiation-induced vascular injury are largely supportive. PKCδ was identified as a critical regulator of the inflammatory response, and its inhibition was shown to protect critical organs during sepsis. We used a novel biomimetic microfluidic assay (bMFA) to interrogate the role of PKCδ in radiation-induced neutrophil-endothelial cell interaction and endothelial cell function. HUVECs formed a complete lumen in bMFA and were treated with 0.5, 2, or 5 Gy ionizing radiation (IR). At 24 h post-IR, the cells were treated with a PKCδ inhibitor for an additional 24 h. Under physiologic shear flow, the role of PKCδ on endothelium function and neutrophil adherence/migration was determined. PKCδ inhibition dramatically attenuated IR-induced endothelium permeability increase and significantly decreased neutrophil migration across IR-treated endothelial cells. Moreover, neutrophil adhesion to irradiated endothelial cells was significantly decreased after PKCδ inhibition in a flow-dependent manner. PKCδ inhibition downregulated IR-induced P-selectin, intercellular adhesion molecule 1, and VCAM-1 but not E-selectin overexpression. PKCδ is an important regulator of neutrophil-endothelial cell interaction post-IR, and its inhibition can serve as a potential radiation medical countermeasure.-Soroush, F., Tang, Y., Zaidi, H. M., Sheffield, J. B., Kilpatrick, L. E., Kiani, M. F. PKCδ inhibition as a novel medical countermeasure for radiation-induced vascular damage.

Isolating the influences of fluid dynamics on selectin-mediated particle rolling at venular junctional regions

The objective of this study was to isolate the impact of hydrodynamics on selectin-mediated cell rolling in branched microvessels. Significant advancements have been made in furthering the understanding of complex interactions between biochemical and physical factors in the inflammatory cascade in simplified planar geometries. However, few studies have sought to quantify the effects of branched configurations and to isolate the effects of associated fluid forces. Experimental techniques were developed to perform in vitro adhesion experiments in Y-shaped micro-slides. The micro-slides were coated with P-selectin and microspheres coated with Sialyl-Lewis^x were observed as they rolled in the chambers at different wall shear stresses. Study results revealed that microsphere rolling velocities and rolling flux were lowest in regions closest to the apex of a junctional region and were dependent on both branch angle and wall shear stress. The regions closest to the junctional region were shown to have low bulk flow velocities and shear stresses using computational fluid dynamics (CFD) modeling. Collectively, the study demonstrates that despite the presence of a uniform coating of P-selectin, hydrodynamic factors associated with the chamber geometry yield non-uniform effects on particle behavior. These findings could explain why cells have been observed to preferentially adhere or transmigrate near junctional regions. Future characterization of inflammatory processes in microvascular network configurations is therefore crucial for furthering our fundamental understanding of inflammation.

Microfluidics in nanoparticle drug delivery; From synthesis to pre-clinical screening

Microfluidic technologies employ nano and microscale fabrication techniques to develop highly controllable and reproducible fluidic microenvironments. Utilizing microfluidics, lead compounds can be produced with the controlled physicochemical properties, characterized in a high-throughput fashion, and evaluated in in vitro biomimetic models of human organs; organ-on-a-chip. As a step forward from conventional in vitro culture methods, microfluidics shows promise in effective preclinical testing of nanoparticle-based drug delivery. This review presents a curated selection of state-of-the-art microfluidic platforms focusing on the fabrication, characterization, and assessment of nanoparticles for drug delivery applications. We also discuss the current challenges and future prospects of nanoparticle drug delivery development using microfluidics.

Organ-on-a-chip devices advance to market

To curb the high cost of drug development, there is an urgent need to develop more predictive tissue models using human cells to determine drug efficacy and safety in advance of clinical testing. Recent insights gained through fundamental biological studies have validated the importance of dynamic cell environments and cellular communication to the expression of high fidelity organ function. Building on this knowledge, emerging organ-on-a-chip technology is poised to fill the gaps in drug screening by offering predictive human tissue models with methods of sophisticated tissue assembly. Organ-on-a-chip start-ups have begun to spawn from academic research to fill this commercial space and are attracting investment to transform the drug discovery industry. This review traces the history, examines the scientific foundation and envisages the prospect of these renowned organ-on-a-chip technologies. It serves as a guide for new members of this dynamic field to navigate the existing scientific and market space.

A novel microfluidic assay reveals a key role for protein kinase C δ in regulating human neutrophil-endothelium interaction

A key step in neutrophil-mediated tissue damage is the migration of activated neutrophils across the vascular endothelium. Previously, we identified protein kinase C δ as a critical regulator of neutrophil migration in sepsis but did not identify specific steps in migration. In this study, we used our novel biomimetic microfluidic assay to delineate systematically the mechanism by which protein kinase C δ regulates individual steps in human neutrophil-endothelial interaction during inflammation. The biomimetic microfluidic assay includes a network of vascular channels, produced from in vivo images connected to a tissue compartment through a porous barrier. HUVECs cultured in vascular channels formed a complete lumen under physiologic shear flow. HUVECs were pretreated with TNF-α ± a protein kinase C δ inhibitor, and the tissue compartment was filled with a chemoattractant (fMLP or IL-8). Under physiologic shear flow, the role of protein kinase C δ on spatial and temporal neutrophil adherence/migration was quantified. Protein kinase C δ inhibition significantly reduced neutrophil adhesion in response to fMLP and IL-8 only under low shear rate and near bifurcations. Protein kinase C δ inhibition also decreased adherence to nonactivated HUVECs in response to fMLP or IL-8. Protein kinase C δ inhibition reduced neutrophil migration into the tissue compartment in response to fMLP and to a lesser degree, to IL-8. Antibody-coated microparticles demonstrated that protein kinase C δ inhibition down-regulated E-selectin and ICAM-1 but not VCAM-1 expression. With the use of a physiologically relevant in vitro model system, we demonstrate that protein kinase C δ plays an important role in the regulation of neutrophil adherence/migration during inflammation and identifies key steps regulated by protein kinase C δ in neutrophil-endothelial interactions.

Expanding imaging capabilities for microfluidics: applicability of darkfield internal reflection illumination (DIRI) to observations in microfluidics

Microfluidics is used increasingly for engineering and biomedical applications due to recent advances in microfabrication technologies. Visualization of bubbles, tracer particles, and cells in a microfluidic device is important for designing a device and analyzing results. However, with conventional methods, it is difficult to observe the channel geometry and such particles simultaneously. To overcome this limitation, we developed a Darkfield Internal Reflection Illumination (DIRI) system that improved the drawbacks of a conventional darkfield illuminator. This study was performed to investigate its utility in the field of microfluidics. The results showed that the developed system could clearly visualize both microbubbles and the channel wall by utilizing brightfield and DIRI illumination simultaneously. The methodology is useful not only for static phenomena, such as clogging, but also for dynamic phenomena, such as the detection of bubbles flowing in a channel. The system was also applied to simultaneous fluorescence and DIRI imaging. Fluorescent tracer beads and channel walls were observed clearly, which may be an advantage for future microparticle image velocimetry (μPIV) analysis, especially near a wall. Two types of cell stained with different colors, and the channel wall, can be recognized using the combined confocal and DIRI system. Whole-slide imaging was also conducted successfully using this system. The tiling function significantly expands the observing area of microfluidics. The developed system will be useful for a wide variety of engineering and biomedical applications for the growing field of microfluidics.

Adhesion patterns in the microvasculature are dependent on bifurcation angle

Particle adhesion in vivo is highly dependent on the microvascular environment comprising of unique anatomical, geometrical, physiological fluid flow conditions and cell-particle and cell-cell interactions. Hence, proper design of vascular-targeted drug carriers that efficiently deliver therapeutics to the targeted cells or tissue at effective concentrations must account for these complex conditions observed in vivo. In this study, we build upon our previous results with the goal of characterizing the effects of bifurcations and their corresponding angle on adhesion of functionalized particles and neutrophils to activated endothelium. Our hypothesis is that adhesion is significantly affected by the type of biochemical interactions between particles and vessel wall as well as the presence of bifurcations and their corresponding angle. Here, we investigate adhesion of functionalized particles (2 μm and 7 μm microparticles) to protein coated channels as well as adhesion of human neutrophils to human endothelial cells under various physiological flow conditions in microfluidic bifurcating channels comprising of different contained angles (30°, 60°, 90°, or 120°). Our findings indicate that both functionalized particle and neutrophil adhesion propensity increase with a larger bifurcation angle. Moreover, the difference in the adhesion patterns of neutrophils and rigid, similar sized (7 μm) particles is more apparent in the junction regions with a larger contained angle. By selecting the right particle size range, enhanced targeted binding of vascular drug carriers can be achieved along with a higher efficacy at optimal drug dosage. Hence, vascular drug particle design needs to be tailored to account for higher binding propensity at larger bifurcation angles.

Techniques and assays for the study of angiogenesis

The importance of studying angiogenesis, the formation of new blood vessels from pre-existing vessels, is underscored by its involvement in both normal physiology, such as embryonic growth and wound healing, and pathologies, such as diabetes and cancer. Treatments targeting the molecular drive of angiogenesis have been developed, but many of the molecular mechanisms that mediate vascularization, as well as how these mechanisms can be targeted in therapy, remain poorly understood. The limited capacity to quantify angiogenesis properly curtails our molecular understanding and development of new drugs and therapies. Although there are a number of assays for angiogenesis, many of them strip away its important components and/or limit control of the variables that direct this highly cooperative and complex process. Here we review assays commonly used in endothelial cell biology and describe the progress toward development of a physiologically realistic platform that will enable a better understanding of the molecular and physical mechanisms that govern angiogenesis.

Bioinspired microfluidic assay for in vitro modeling of leukocyte-endothelium interactions

Current in vitro models of the leukocyte adhesion cascade cannot be used for real-time studies of the entire leukocyte adhesion cascade, including rolling, adhesion, and migration in a single assay. In this study, we have developed and validated a novel bioinspired microfluidic assay (bMFA) and used it to test the hypothesis that blocking of specific steps in the adhesion/migration cascade significantly affects other steps of the cascade. The bMFA consists of an endothelialized microvascular network in communication with a tissue compartment via a 3 μm porous barrier. Human neutrophils in bMFA preferentially adhered to activated human endothelial cells near bifurcations with rolling and adhesion patterns in close agreement with in vivo observations. Treating endothelial cells with monoclonal antibodies to E-selectin or ICAM-1 or treating neutrophils with wortmannin reduced rolling, adhesion, and migration of neutrophils to 60%, 20%, and 18% of their respective control values. Antibody blocking of specific steps in the adhesion/migration cascade (e.g., mAb to E-selectin) significantly downregulated other steps of the cascade (e.g., migration). This novel in vitro assay provides a realistic human cell based model for basic science studies, identification of new treatment targets, selection of pathways to target validation, and rapid screening of candidate agents.

Particle margination and its implications on intravenous anticancer drug delivery

"Margination" refers to the movement of particles in flow toward the walls of a channel. The term was first coined in physiology for describing the behavior of white blood cells (WBCs) and platelets in blood flow. The margination of particles is desirable for anticancer drug delivery because it results in the close proximity of drug-carrying particles to the endothelium, where they can easily diffuse into cancerous tumors through the leaky vasculature. Understanding the fundamentals of margination may further lead to the rational design of particles and allow for more specific delivery of anticancer drugs into tumors, thereby increasing patient comfort during cancer treatment. This paper reviews existing theoretical and experimental studies that focus on understanding margination. Margination is a complex phenomenon that depends on the interplay between inertial, hydrodynamic, electrostatic, lift, van der Waals, and Brownian forces. Parameters that have been explored thus far include the particle size, shape, density, stiffness, shear rate, and the concentration and aggregation state of red blood cells (RBCs). Many studies suggested that there exists an optimal particle size for margination to occur, and that nonspherical particles tend to marginate better than spherical particles. There are, however, conflicting views on the effects of particle density, stiffness, shear rate, and RBCs. The limitations of using the adhesion of particles to the channel walls in order to quantify margination propensity are explained, and some outstanding questions for future research are highlighted.

Generation of shear adhesion map using SynVivo synthetic microvascular networks

Cell/particle adhesion assays are critical to understanding the biochemical interactions involved in disease pathophysiology and have important applications in the quest for the development of novel therapeutics. Assays using static conditions fail to capture the dependence of adhesion on shear, limiting their correlation with in vivo environment. Parallel plate flow chambers that quantify adhesion under physiological fluid flow need multiple experiments for the generation of a shear adhesion map. In addition, they do not represent the in vivo scale and morphology and require large volumes (~ml) of reagents for experiments. In this study, we demonstrate the generation of shear adhesion map from a single experiment using a microvascular network based microfluidic device, SynVivo-SMN. This device recreates the complex in vivo vasculature including geometric scale, morphological elements, flow features and cellular interactions in an in vitro format, thereby providing a biologically realistic environment for basic and applied research in cellular behavior, drug delivery, and drug discovery. The assay was demonstrated by studying the interaction of the 2 µm biotin-coated particles with avidin-coated surfaces of the microchip. The entire range of shear observed in the microvasculature is obtained in a single assay enabling adhesion vs. shear map for the particles under physiological conditions.

Characterization of nanoparticle delivery in microcirculation using a microfluidic device

This work focuses on the characterization of particle delivery in microcirculation through a microfluidic device. In microvasculature the vessel size is comparable to that of red blood cells (RBCs) and the existence of blood cells largely influences the dispersion and binding distribution of drug loaded particles. The geometry of the microvasculature leads to non-uniform particle distribution and affects the particle binding characteristics. We perform an in vitro study in a microfluidic chip with micro vessel mimicking channels having a rectangular cross section. Various factors that influence particle distribution and delivery such as the vessel geometry, shear rate, blood cells, particle size, particle antibody density are considered in this study. Around 10% higher particle binding density is observed at bifurcation regions of the mimetic microvasculature geometry compared to straight regions. Particle binding density is found to decrease with increased shear rates. RBCs enhance particle binding for both 210 nm and 2 μm particles for shear rates between 200-1600 s(-1) studied. The particle binding density increases about 2-3 times and 6-10 times when flowing in whole blood at 25% RBC concentration compared to the pure particle case, for 210 nm and 2 μm particles respectively. With RBCs, the binding enhancement is more significant for 2 μm particles than that for 210 nm particles, which indicates an enhanced size dependent exclusion of 2 μm particles from the channel centre to the cell free layer (CFL). Increased particle antibody coating density leads to higher particle binding density for both 210 nm and 2 μm particles.

Scaling and systems biology for integrating multiple organs-on-a-chip

Coupled systems of in vitro microfabricated organs-on-a-chip containing small populations of human cells are being developed to address the formidable pharmacological and physiological gaps between monolayer cell cultures, animal models, and humans that severely limit the speed and efficiency of drug development. These gaps present challenges not only in tissue and microfluidic engineering, but also in systems biology: how does one model, test, and learn about the communication and control of biological systems with individual organs-on-chips that are one-thousandth or one-millionth of the size of adult organs, or even smaller, i.e., organs for a milliHuman (mHu) or microHuman (μHu)? Allometric scaling that describes inter-species variation of organ size and properties provides some guidance, but given the desire to utilize these systems to extend and validate human pharmacokinetic and pharmacodynamic (PK/PD) models in support of drug discovery and development, it is more appropriate to scale each organ functionally to ensure that it makes the suitable physiological contribution to the coupled system. The desire to recapitulate the complex organ-organ interactions that result from factors in the blood and lymph places a severe constraint on the total circulating fluid (~5 mL for a mHu and ~5 μL for a μHu) and hence on the pumps, valves, and analytical instruments required to maintain and study these systems. Scaling arguments also provide guidance on the design of a universal cell-culture medium, typically without red blood cells. This review presents several examples of scaling arguments and discusses steps that should ensure the success of this endeavour.

Adhesive interaction of functionalized particles and endothelium in idealized microvascular networks

OBJECTIVE

Leukocytes play a key role in the early response to tissue injury/infection resulting from physical, chemical or biological stimuli. This process involves the initiation of the leukocyte adhesion cascade mediated by a series of interactions between receptors and ligands on the endothelium and the leukocytes. Here, we characterize the adhesion profile of functionalized particles under physiological flow conditions in an idealized synthetic microvascular network (SMN) characterized by a bifurcation. We hypothesize that differences in the level of adhesion of functionalized particles in bifurcating SMNs are dependent on the ratio of adhesion molecules on the particles as well as geometric features of the in vitro networks.

METHODS

Functionalized particles were prepared by coating their surfaces with different ratios of antibodies against ICAM-1 and E-selectin (aICAM-1:aE-selectin=100:0, 70:30, 50:50, 30:70, and 0:100). The adhesion of functionalized particles to 4h TNF-α activated human umbilical vein endothelial cells under shear flow (0.5, 2, and 4dyn/cm(2)) in bifurcating SMNs and in a parallel plate flow chamber was then quantified.

RESULTS

The level of adhesion of 50:50 aICAM-1:aE-selectin particles was significantly higher compared to other particles in the bifurcating SMNs (~1.5-4 fold higher). However, in the parallel plate flow chamber 70:30 aICAM-1:aE-selectin particles exhibited a significantly higher level of adhesion (~1.5-2.5 fold higher). Furthermore, the adhesion of particles in junction regions was about 3-18 fold higher than that in straight sections of the SMNs. As expected, in straight sections of the SMNs and in the parallel plate flow chamber particle adhesion increased with decreasing shear. However, particle adhesion did not change significantly with decreasing shear at the junction regions of SMNs for all functionalized particles.

CONCLUSION

Adhesion efficiency of functionalized particles is significantly affected by cell-adhesion molecule ratio density as well as geometric features of the vessels. Moreover, the differential adhesion patterns of particles between straight sections of bifurcating SMNs and parallel plate flow chamber, as well as straight sections and junction regions of bifurcating SMNs, indicates that adhesion profile of particles is highly dependent on the vascular geometry of the system used.

Microfluidic system for facilitated quantification of nanoparticle accumulation to cells under laminar flow

The identification of novel, synthetic targeting ligands to endothelial receptors has led to the rapid development of targeted nanoparticles for drug, gene and imaging probe delivery. Central to development and optimization are effective models for assessing particle binding in vitro. Here, we developed a simple and cost effective method to quantitatively assess nanoparticle accumulation under physiologically-relevant laminar flow. We designed reversibly vacuum-sealed PDMS microfluidic chambers compatible with 35 mm petri dishes, which deliver uniform or gradient shear stress. These chambers have sufficient surface area for facile cell collection for particle accumulation quantitation through FACS. We tested this model by synthesizing and flowing liposomes coated with APN (K (D) ~ 300 μM) and VCAM-1-targeting (K (D) ~ 30 μM) peptides over HUVEC. Particle binding significantly increased with ligand concentration (up to 6 mol%) and decreased with excess PEG. While the accumulation of particles with the lower affinity ligand decreased with shear, accumulation of those with the higher affinity ligand was highest in a low shear environment (2.4 dyne/cm(2)), as compared with greater shear or the absence of shear. We describe here a robust flow chamber model that is applied to optimize the properties of 100 nm liposomes targeted to inflamed endothelium.

Microfluidic devices for modeling cell-cell and particle-cell interactions in the microvasculature

Cell-fluid and cell-cell interactions are critical components of many physiological and pathological conditions in the microvasculature. Similarly, particle-cell interactions play an important role in targeted delivery of therapeutics to tissue. Development of in vitro fluidic devices to mimic these microcirculatory processes has been a critical step forward in our understanding of the inflammatory process, developing of nano-particulate drug carriers, and developing realistic in vitro models of the microvasculature and its surrounding tissue. However, widely used parallel plate flow based devices and assays have a number of important limitations for studying the physiological conditions in vivo. In addition, these devices are resource hungry and time consuming for performing various assays. Recently developed, more realistic, microfluidic based devices have been able to overcome many of these limitations. In this review, an overview of the fluidic devices and their use in studying the effects of shear forces on cell-cell and cell-particle interactions is presented. In addition, use of mathematical models and computational fluid dynamics (CFD) based models for interpreting the complex flow patterns in the microvasculature is highlighted. Finally, the potential of 3D microfluidic devices and imaging for better representing in vivo conditions under which cell-cell and cell-particle interactions take place is discussed.