Blood plasma separation (BPS) is a prerequisite in numerous biomedical assays involving low abundance plasma-borne biomarkers and thus is the fundamental step before many bioanalytical steps. The interest in microscale BPS solutions has emerged with the development of microfluidic technologies and has continued in recent years as few solutions have so far achieved both high yield and high purity without sample dilution, in volumes compatible with current clinical assays. In this project, we have investigated some key parameters affecting the BPS efficiency and the yield of BPS devices.  

Hydrodynamic or acoustic BPS microdevices have attracted considerable attention from the microfluidic community in the continuous separation of  blood samples with a volume of a few mL due to their high throughput and insensitivity to clogging [1-2]. However, obtaining a high yield from whole blood is challenging because the volume of red blood cells or hematocrit (HCT) typically rises above physiological levels after each separation region, following plasma extraction. In the first part of this project, we sought to establish experimentally, for the first time, the maximum HCT level and flow rate achievable in a microchannel, without hemolysis. We have demonstrated that blood can be flowed in microchannels at very high HCT (≥ 90%) without damaging blood cells but for a very short resident time (typically <0.5s) (MicroTAS2020). The calculated viscosity of different HCT samples were stable across almost all pressure-flow rate couples and was found to be comparable to data obtained on viscometers [3–5].

Secondly, we have sought to investigate the effect of syringe pump-induced fluctuations in hydrodynamic BPS structures. Syringe pumps are widely used for flow rate control in biomedical microfluidic experiments because of their wide availability, user-friendliness and cost-effectiveness. However, stepper motors inside syringe pumps can be an unwanted source of flow fluctuations [6-7]. Using a low-cost coded compressive rotating mirror camera, we have uncovered for the first time that the relative pressure fluctuations reach a plateau when the flow rate exceeds 5 mL/h. Additionally, pressure fluctuations directly correlate with cell-free zone fluctuation in frequency and amplitude, and degrades the separation efficiency of the device [8].

This study not only reveals the reasons behind the slow progress in the development of high-throughput BPS devices capable of handling whole blood samples, but also provides a framework for the design optimisation of future BPS devices.