Focusing of blood cells
Blood is a non-Newtonian fluid. The viscosity of the blood is dependent
on hematocrit, plasma protein concentration and blood cell count. In
flow through spirals, the blood rheology will affect the Dean and lift
forces differently. In addition, the visco-elastic nature of the PEO
will interact with the blood cells, as well. Initially, we found that
whole blood needs to be diluted in order to remain focused. Three
different dilutions of blood (1:2, 1:5 and 1:10) were tested. Notably,
all the diluted blood cells are focused at the outer wall and
effectively collected through the outer outlet channel (Fig.5A). The 1:2
diluted blood (~25% hematocrit) sample is broader,
indicating more particle-particle interaction. To investigate the
optimal dilution , we mixed 1 µm particles with diluted blood and
processed through the spiral device. As shown in Fig.5B, for 1:10
diluted blood (~5% hematocrit), 96% of 1 µm particles
were collected at the inner outlet (O1), 93% for 1:5 diluted blood
(~10% hematocrit), and 82% for 1:2 diluted blood,
respectively. On the other hand, 100% of blood cells were collected in
O2 for all three cases. The results indicate the blood cells are fully
focused throughout the channel length and as the concentration of blood
cells increases, the separation efficiency of 1 µm particles reduces
indicating cell-particle interaction. Consequently, there is a need to
dilute the blood sample, and an increase in dilution will result in
improved separation efficiency of 1 µm particles. It is possible that
some particles get stuck to blood cells due to high solid content. In
addition, high solid content may ultimately affect how the local forces
interact on neighboring particles. Although more work is needed to
effectively decipher the effect of solid content, it is evident that
high solid content prevents smaller particles from being effectively
carried by the Dean vortices toward the inner wall and thus reducing the
efficiency of separation. The challenges from particle-particle
interaction due to high hematocrit content and the requirement of blood
dilution is also reported previously by Shen et al., and Zhou et al.,
respectively 54, 55.