Red Blood Cell Sickling in Microdroplet Arrays

Abstract

We have developed a microfluidic approach to study the sickling of red blood cells associated with sickle cell anemia by varying the oxygen partial pressure within arrays of microdroplets in flowing oil. By etching holes into the channel's top surface, droplets can be held stationary while maintaining an external flow of oil. Such control is possible for drops that are squeezed by the channel roof, by allowing them to reduce their surface energy as they enter into a local depression. By using the perfluorinated carrier oil as a sink or source of oxygen, the oxygen level within the water droplets equilibrates through exchange with the surrounding oil. This provides control over the oxygen partial pressure within an aqueous microdroplet ranging from 1 kPa to ambient partial pressure, i.e. 21 kPa. The controlled deoxygenation is used to trigger the polymerization of hemoglobin within sickle red blood cells, encapsulated within drops. This process is observed using polarization microscopy, which yields a robust criterion to detect polymerization based on transmitted light intensity through crossed polarizers (Abbyad et al., Lab Chip, 2010, 10, 2513). We demonstrate in particular how the oxygen levels within the drops can be controlled spatially and temporally, either by exposing rows of drops to two streams of oil at different gas concentrations or by periodically switching oil inputs to vary the gas concentration of drops as a function of time. Cycles of oxygenation and deoxygenation of anchored droplets induce depolymerization and polymerization of the hemoglobin, thus providing a method to simulate the cycling that takes place in physiological blood flow. Droplet content is varied to study the effect of different biochemical or therapeutic agents on sickling.