Blood Diagnostic Kits

Blood cells migrate to the middle of a micro-channel due to the enhanced shear-induced migration rate at small scales. At the middle, the blood cells travel at a speed higher than the average plasma velocity. If a blood sample enters a channel by wetting, this excess speed beyond the plasma velocity will cause the blood cells to pack onto the meniscus and arrest further sample transport. This represents the major obstacle in scaling down blood diagnostic kits. Shown in the first frame of blood-cell migration to the middle of a capillary axis and onto a wetting meniscus in a 100 micron capillary containing diluted bovine blood.The advancing meniscus moves to the left. The packing also reduces the blood cell concentration far upstream of the meniscus, as shown in subsequent frames. This migration phenomenon is shown to be much more pronounced for elastic blood cells than rigidified ones.

We have exploited such blood cell segregation phenomena to separate plasma from blood cells, to develop plug-free micro-needles and to identify sickle cells with lower elasticity.

Relevant publications:

Zhou, R. and Chang, H.-C., "Capillary Penetration Failure of Blood Suspensions", J. of Colloid and Interfacial Sci., 267, 147(2005).

 Mensicus Packing by blood cells in the top frame. The bottom two frames are upstream of the meniscus. The blood cells migrate to the middle of the capillary and hence travel at a velocity faster than the average liquid velocity--the meniscus velocity. As a result, packing at the meniscus occurs.