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.