Continuous multidisciplinary advancements in medicine, science and engineering have led to the rise of biomedical microfluidic devices for clinical diagnoses, laboratory research for modeling and screening of drugs or disease states, and implantable organs such as artificial kidneys. Blood is often the biological fluid of choice for these purposes. However, unique hemodynamic properties observed only in microscale channels complicate experimental repeatability and reliability.
For vessels with 10-300μm diameters, red blood cell properties such as deformability have a significant impact on hemorheology, and the blood can no longer be considered as a homogeneous fluid. The flowing blood segregates into a red blood cell rich core bounded by a cell-free layer composed almost entirely of plasma. Viscous forces dominate flow behavior, and shear rates at the wall are much higher than in arteries and veins. The overall viscosity becomes dependent on vessel diameter. These unique characteristics are interesting from a biophysics perspective, but the value of biomedical microfluidic technologies makes research in this regime even more critical. Accordingly, this work focuses on experimental comparison of the microfluidic flow properties of red blood cells with varying physical characteristics.
Blood viscosity in microscale tubes was investigated experimentally for 6 blood types (goat, sheep, pig, llama, chicken and turkey) at a range of hematocrits (0-50%). The selected blood types represented a small sample of the wide-ranging red blood cell characteristics found in mammals, birds, reptiles, amphibians and fish. These red blood cells vary in size over an order of magnitude, represent shapes ranging from biconcave to ellipsoidal, and include both nucleated cell types found in birds, amphibians and reptiles and denucleated mammalian cells. Pressure drop experiments at physiologically relevant flow rates were carried out for rigid tubing diameters ranging from 73μm -161μm. The resulting viscosities were normalized relative to the measurements made of the homologous plasma for each species. The viscosity of blood in this regime is much different than in larger vessels (>500μm) or in small capillaries (<10μm), but existing studies in this size range focus only on human blood.
The results analyzed in the context of four primary variables: hematocrit, red blood cell size, red blood cell shape, and red blood cell deformability. The aggregation of the porcine blood complicated the data collection process, resulting in only a few usable data points.
Examination of the role of hematocrit yielded results which aligned well with existing hemorheology research: viscosity increases with hematocrit. After applying an existing fitting equation the collected data, three primary trends were observed. First, the chicken blood had the highest viscosity at every hematocrit regardless of tubing diameter. Secondly, the viscosities of the goat and sheep blood were very similar at all hematocrits, and ultimately had the lowest viscosity of all samples at the highest measured hematocrit values. Finally, the turkey and llama blood generally had the lowest viscosity at low hematocrit, with a deflection point around 30% hematocrit where viscosity began to increase much more sharply.
The role of cell size was considered in the context of both mean cell volume and major cell axis relative to vessel diameter, to account for the elongated shape of the llama, chicken and turkey red blood cells. The results indicated that cell major axis is better correlated with viscosity than cell volume, suggesting the potential importance of cell shape. The red blood cells were then characterized as either oblate or prolate to further investigate the importance of shape. The results further supported the idea that overall blood viscosity in small vessels depends on both cell size and shape. However, as with the hematocrit analysis, the chicken blood was an outlier. The chicken red blood cells are quite similar to turkey red blood cells in both size and shape, yet the chicken blood was consistently far more viscous than turkey blood. A comparison with theoretical rigid particles suggested that the chicken red blood cells may be the least deformable of the sampled blood types.
Two additional experiments were performed to assess the potential importance of deformability. Additional pressure drop measurements with chemically-hardened red blood cells demonstrated that the measurement system is quite sensitive to changes in cell deformability. Flow visualization in a microfluidic contraction indicated that the high viscosity of the chicken blood relative to the turkey blood could be attributed to differences in deformability.
Blood viscosity is influenced by multiple cell characteristics, including size, shape and deformability. The role of these parameters is worthy of further investigation alongside ongoing research in the rheology of human blood. The impact of red blood cell deformability on viscosity in small vessels is particularly interesting. The described experimental apparatus is easily replicable and highly customizable, and may serve as a helpful tool to analyze blood parameters in biomedical microfluidic device research and development. The collected data sets are available to interested researchers, and can currently be obtained by direct request. Ultimately, an online database will be made available via the Liepmann lab website.