Computational and functional evaluation of a microfluidic blood flow device

Revisiting hemodynamics and blood oxygenation in a microfluidic microvasculature replica

The complexity of microvascular circulation has led to the development of advanced imaging techniques and biomimetic models. This study developed a multifaceted microfluidic-based microdevice as an in vitro model of microvasculature to replicate important geometric and functional features of in vivo perfusion in mice. The microfluidic device consisted of a microchannel for blood perfusion, mirroring the natural hierarchical branching vascular structures found in mice. Additionally, the device incorporated a steady gradient of oxygen (O2) which diffused through the polydimethylsiloxane (PDMS) layer, allowing for dynamic blood oxygenation. The assembled multi-layered microdevice was accompanied by a dual-modal imaging system that combined laser speckle contrast imaging (LSCI) and intrinsic signal optical imaging (ISOI) to visualize full-field blood flow distributions and blood O2 profiles. By closely reproducing in vivo blood perfusion and oxygenation conditions, this microvasculature model, in conjunction with numerical simulation results, can provide quantitative information on physiologically relevant hemodynamics and key O2 transport parameters that are not directly measurable in traditional animal studies.

Research and Development of Microphysiological Systems in Japan Supported by the AMED-MPS Project

Microphysiological systems (MPS) have been actively developed as a new technology for in vitro toxicity testing platforms in recent years. MPS are culture techniques for the reconstruction of the specific functions of human organs or tissues in a limited space to create miniaturized human test systems. MPS have great promise as next-generation in vitro toxicity assessment systems. Here, I will review the current status of MPS and discuss the requirements that must be met in order for MPS to be implemented in the field of drug discovery, presenting the example of an in vitro cell assay system for drug-induced liver injury, which is the research subject in our laboratory. Projects aimed at the development of MPS were implemented early in Europe and the United States, and the AMED-MPS project was launched in Japan in 2017. The AMED-MPS project involves industry, government, and academia. Researchers in the field of drug discovery in the pharmaceutical industry also participate in the project. Based on the discussions made in the project, I will introduce the requirements that need to be met by liver-MPS as in vitro toxicity test platforms.

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.

Assessing and improving the biocompatibility of microfluidic artificial lungs

Microfluidic artificial lungs (µALs) have the potential to improve the treatment and quality of life for patients with acute or chronic lung injury. In order to realize the full potential of this technology (including as a destination therapy), the biocompatibility of these devices needs to be improved to produce long-lasting devices that are safe for patient use with minimal or no systemic anticoagulation. Many studies exist which probe coagulation and thrombosis on polydimethyl siloxane (PDMS) surfaces, and many strategies have been explored to improve surface biocompatibility. As the field of µALs is young, there are few studies which investigate biocompatibility of functioning µALs; and even fewer which were performed in vivo. Here, we use both in vitro and in vivo models to investigate two strategies to improve µAL biocompatibility: 1) a hydrophilic surface coating (polyethylene glycol, PEG) to prevent surface fouling, and 2) the addition of nitric oxide (NO) to the sweep gas to inhibit platelet activation locally within the µAL. In this study, we challenge µALs with clottable blood or platelet-rich plasma (PRP) and monitor the resistance to blood flow over time. Device lifetime (the amount of time the µAL remains patent and unobstructed by clot) is used as the primary indicator of biocompatibility. This study is the first study to: 1) investigate the effect of NO release on biocompatibility in a microfluidic network; 2) combine a hydrophilic PEG coating with NO release to improve blood compatibility; and 3) perform extended in vivo biocompatibility testing of a µAL. We found that µALs challenged in vitro with PRP remained patent significantly longer when the sweep gas contained NO than without NO. In the in vivo rabbit model, neither approach alone (PEG coating nor NO sweep gas) significantly improved biocompatibility compared to controls (though with larger sample size significance may become apparent); while the combination of a PEG coating with NO sweep gas resulted in significant improvement of device lifetime. STATEMENT OF SIGNIFICANCE: The development of microfluidic artificial lungs (µALs) can potentially have a massive impact on the treatment of patients with acute and chronic lung impairments. Before these devices can be deployed clinically, the biocompatibility of µALs must be improved and more comprehensively understood. This work explores two strategies for improving biocompatibility, a hydrophilic surface coating (polyethylene glycol) for general surface passivation and the addition of nitric oxide (NO) to the sweep gas to quell platelet and leukocyte activation. These two strategies are investigated separately and as a combined device treatment. Devices are challenged with clottable blood using in vitro testing and in vivo testing in rabbits. This is the first study to our knowledge that allows statistical comparisons of biocompatible µALs in animals, a key step towards eventual clinical use.

Elastomeric microvalve geometry affects haemocompatibility

This paper reports on the parameters that determine the haemocompatibility of elastomeric microvalves for blood handling in microfluidic systems. Using a comprehensive investigation of blood function, we describe a hierarchy of haemocompatibility as a function of microvalve geometry and identify a "normally-closed" v-gate pneumatic microvalve design that minimally affects blood plasma fibrinogen and von Willebrand factor composition, minimises effects on erythrocyte structure and function, and limits effects on platelet activation and aggregation, while facilitating rapid switching control for blood sample delivery. We propose that the haemodynamic profile of valve gate geometries is a significant determinant of platelet-dependent biofouling and haemocompatibility. Overall our findings suggest that modification of microvalve gate geometry and consequently haemodynamic profile can improve haemocompatibility, while minimising the requirement for chemical or protein modification of microfluidic surfaces. This biological insight and approach may be harnessed to inform future haemocompatible microfluidic valve and component design, and is an advance towards lab-on-chip automation for blood based diagnostic systems.

Numerical design and optimization of hydraulic resistance and wall shear stress inside pressure-driven microfluidic networks

Microfluidic networks represent the milestone of microfluidic devices. Recent advancements in microfluidic technologies mandate complex designs where both hydraulic resistance and pressure drop across the microfluidic network are minimized, while wall shear stress is precisely mapped throughout the network. In this work, a combination of theoretical and modeling techniques is used to construct a microfluidic network that operates under minimum hydraulic resistance and minimum pressure drop while constraining wall shear stress throughout the network. The results show that in order to minimize the hydraulic resistance and pressure drop throughout the network while maintaining constant wall shear stress throughout the network, geometric and shape conditions related to the compactness and aspect ratio of the parent and daughter branches must be followed. Also, results suggest that while a "local" minimum hydraulic resistance can be achieved for a geometry with an arbitrary aspect ratio, a "global" minimum hydraulic resistance occurs only when the aspect ratio of that geometry is set to unity. Thus, it is concluded that square and equilateral triangular cross-sectional area microfluidic networks have the least resistance compared to all rectangular and isosceles triangular cross-sectional microfluidic networks, respectively. Precise control over wall shear stress through the bifurcations of the microfluidic network is demonstrated in this work. Three multi-generation microfluidic network designs are considered. In these three designs, wall shear stress in the microfluidic network is successfully kept constant, increased in the daughter-branch direction, or decreased in the daughter-branch direction, respectively. For the multi-generation microfluidic network with constant wall shear stress, the design guidelines presented in this work result in identical profiles of wall shear stresses not only within a single generation but also through all the generations of the microfluidic network under investigation. The results obtained in this work are consistent with previously reported data and suitable for a wide range of lab-on-chip applications.

Lung assist device technology with physiologic blood flow developed on a tissue engineered scaffold platform

There is no technology available to support failing lung function for patients outside the hospital. An implantable lung assist device would augment lung function as a bridge to transplant or possible destination therapy. Utilizing biomimetic design principles, a microfluidic vascular network was developed for blood inflow from the pulmonary artery and blood return to the left atrium. Computational fluid dynamics analysis was used to optimize blood flow within the vascular network. A micro milled variable depth mold with 3D features was created to achieve both physiologic blood flow and shear stress. Gas exchange occurs across a thin silicone membrane between the vascular network and adjacent alveolar chamber with flowing oxygen. The device had a surface area of 23.1 cm(2) and respiratory membrane thickness of 8.7 ± 1.2 μm. Carbon dioxide transfer within the device was 156 ml min(-1) m(-2) and the oxygen transfer was 34 ml min(-1) m(-2). A lung assist device based on tissue engineering architecture achieves gas exchange comparable to hollow fiber oxygenators yet does so while maintaining physiologic blood flow. This device may be scaled up to create an implantable ambulatory lung assist device.

Realization and efficiency evaluation of a micro-photocatalytic cell prototype for real-time blood oxygenation

A novel, miniaturized, high-efficiency photocatalytic cell, able to work in dynamic conditions, has been designed and validated in this study. Microfluidic channels were molded out of polydimethylsiloxane (PDMS) by means of standard soft lithography techniques, so as to work as photocatalytic cells, where the coupling of anatase titanium dioxide thin films and platinum electrodes, allows an electrically assisted photocatalytic reaction to produce dissolved oxygen gas from the water content of flowing fluid (e.g. blood). The thin films were deposited onto quartz glass substrates at room temperature (300 K) using reactive radio-frequency sputtering with a titanium metal target. The photocatalytic activity was evaluated through reduction rate of methylene blue solution. The results of the current study, as a proof of concept, have shown that the device can generate oxygen at a rate of 4.06 μM O(2)/(cm(2)min), thus extending its possible application range to the full oxygenation of flowing venous blood.

Principles of biomimetic vascular network design applied to a tissue-engineered liver scaffold

Branched vascular networks are a central component of scaffold architecture for solid organ tissue engineering. In this work, seven biomimetic principles were established as the major guiding technical design considerations of a branched vascular network for a tissue-engineered scaffold. These biomimetic design principles were applied to a branched radial architecture to develop a liver-specific vascular network. Iterative design changes and computational fluid dynamic analysis were used to optimize the network before mold manufacturing. The vascular network mold was created using a new mold technique that achieves a 1:1 aspect ratio for all channels. In vitro blood flow testing confirmed the physiologic hemodynamics of the network as predicted by computational fluid dynamic analysis. These results indicate that this biomimetic liver vascular network design will provide a foundation for developing complex vascular networks for solid organ tissue engineering that achieve physiologic blood flow.

Microchannel technologies for artificial lungs: (3) open rectangular channels

Lithographic techniques were used to develop patterned silicone rubber membranes that provide 15 microm high microchannels for artificial lungs. Two types of devices were fabricated as a proof-of-concept: one has a series of parallel, straight, open rectangular channels that are each 300 microm wide, separated by 200-microm walls, and 3-mm long and the other is a wide rectangular channel with support posts, also 3- mm long. Experiments with 30% hematocrit, venous, bovine blood showed average oxygen fluxes ranging from 11 x 10(-7) moles/(min x cm(2)) at a residence time of 0.04 sec to 6.5 x 10(-7) moles/(min x cm(2)) at a residence time of 0.20 sec. The average oxygen flux vs. residence time, which is due to transverse molecular diffusion, follows the same relation for all membranes tested. The corresponding increase in hemoglobin saturation ranged from 9% at the residence time of 0.04 sec to 24% at the residence time of 0.20 sec. The support-post channel membranes are attractive for designers because they can be arbitrarily wide and would be less prone to blockage.