Thrombosis-on-a-Chip: A New Way to Model a Complex Process

In the last decade, advances in microchip-based technologies have provided useful, inexpensive, and easily reproducible microfluidic devices for conducting microscale biological and biochemical experiments. Similar to a computer chip but with plumbing, a microfluidic chip comprises a set of microchannels etched or molded into a material (typically, a polymer such as polydimethylsiloxane) into which fluids can be perfused via syringe pumps or hydrostatic pressure. The ability to tightly control biological conditions and the dynamic shear environment within these devices have enabled microfluidics to be ideal tools for quantitatively analyzing hematologic and vascular processes such as thrombosis and hemostasis. To that end, several groups including our own have recently incorporated the live culture of endothelial cells into these devices, thereby developing microfluidic systems that accurately recapitulate and integrate the myriad of interactions among blood cells, endothelial cells, and soluble factors that occur in vivo and are vital in studying hemostasis and thrombosis. As such, these "endothelialized" microfluidic devices hold several novel and key advantages as research-enabling systems, including the capabilities to: include or subtract different blood cell subpopulations (platelets, red cells, leukocyte subsets) and/or soluble factors (coagulation proteins, inflammatory mediators, etc.) for mechanistic studies, tightly control shear conditions, incorporate human and patient blood samples, modulate endothelial function and activation, culture different endothelial cell phenotypes (e.g., various anatomic beds, different species), and directly visualize clot formation in real time via brightfield and fluorescence videomicroscopy. With these physiologically relevant features not commonly found in existing in vitro assays of clot formation, endothelialized microfluidics therefore aptly complement in vivo studies of thrombosis and under certain circumstances, may even serve as alternatives for murine thrombosis models. Indeed, recently published seminal studies that leveraged endothelialized microfluidics studies have quantitatively demonstrated the relationship between shear rate and thrombosis via endothelial secretion of von Willebrand factor as well as answered questions related to clot formation that were technologically infeasible to resolve with existing in vivo and in vitro methods. Moreover, these thrombosis-on-a-chip systems have also been recently utilized as physiologic in vitro drug discovery platforms for novel antithrombotic therapeutics or novel applications of existing pharmacologic agents. In addition, numerous efforts to apply thrombosis-on-a-chip systems as point-of-care diagnostics to determine thrombosis risk in patients are also currently underway. However, as no in vitro device can fully recapitulate all in vivo conditions, thrombosis-on-a-chip systems are not without limitations and shortcomings, which often involve the material and geometric properties of these devices. To address those issues, more recent studies have further advanced the endothelialized microfluidic technology to comprise microdevices that are either hydrogel-based or incorporate complex vascular geometries that occur in vivo . With mounting evidence that murine models do not exactly recapitulate all aspects of thrombosis in humans, thrombosis-on-a-chip technologies provide ample opportunities to apply the benefits of microfluidic technology to investigate, diagnose, and treat clotting disorders.