Plasma separation by recycled squeeze flow of a single blood droplet

Emerging diagnostic protocols impose constraints on blood sample volume, necessitating efficient microscale separation of plasma. We present a novel method for extracting plasma from a single blood droplet of only 25 μL (with and without dilution), using a variable-gap lifted Hele-Shaw cell. The device consists of (i) an elastomeric top surface cyclically deformed by a linear actuator to form a continuously varying spherical cap and (ii) a flat, stationary bottom chip containing a microfilter interfaced with a microfluidic channel outlet. The blood droplet is confined within a periodically varying micro-gap between these two surfaces and is recirculated by radial stretching and contraction of the film on the filter surface. Dead-end and tangential filtration are, thus, coupled to mitigate membrane blocking by cells: a critical bottleneck of blood microfiltration. Controlled shearing of the squeezed blood film induces cell migration away from the filter surface, thereby depleting the filter cake layer. The filtrate plasma is directed from the sub-membrane space to the outlet channel by capillary action. The imposed oscillatory shear field responsible for cell redistribution is tuned to maintain high plasma flux. Free hemoglobin levels in plasma were monitored through spectrophotometry. An optimal operating condition is found, under which the device extracted a plasma yield of 60.51%±0.05%. Protein recovery in separated plasma was assessed against centrifuged plasma using a C-reactive protein assay. The proposed approach demonstrates the utility of dynamically sheared blood flow in a micro-confinement, either toward a standalone plasma separator or in integration with biosensors.