## 9 Conclusions and Applications  
Microfluidic devices allow for precise control over flow rate, shear stress, and chemical environment. They mimic capillary-size vessels, making the approach ideal for studying RBC behavior in conditions similar to those in vivo. They enable real-time imaging and measurement of aggregation dynamics, including formation and disaggregation.  
It has been shown that microfluidics, when directed to the conditions of capillary flow, can provide essential information about the mechanical properties of blood cells. A comparison of microfluidic assays with studies utilizing micropipettes, ektacytometry, and micropore cell-transit times shows that the microfluidics-based devices offer a low-cost, high throughput alternative to these previous, and now rather ancient, technologies. Bycombining the unique features of hemorheology and microfluidic technology for single cell analysis, valuable advances in personalized medicine can be achieved towards new treatments and diagnostic approaches. It could be expected that the characterization of the physical properties of blood and blood cells in a microfluidic environment would be considered a standard blood test in the future.  
Microfluidic platforms are ideal for assessing deformability in a quantitative, reproducible way. Examples include constriction channels measuring transit time as cells squeeze through narrow passages and pillar arrays, and observing how cells navigate around obstacles, revealing the stiffness, hydrodynamic stretching, and droplet-based sorting for diagnostics. Other possible applications of microfluidics for the characterization of RBC aggregation concern disease diagnostics--altered RBC aggregation is linked with diseases like diabetes, cardiovascular disease and infections, and could be relevant in drug testing, etc. The microfluidic approach could be very useful in investigating the effects of drugs on RBC behavior, and in the development of blood analogues by designing synthetic fluids that mimic blood flow.  
The study of the mechanical properties of leukocytes is important for understanding their function in the circulatory system. A rheological model of leukocytes is necessary for the analysis of almost any physiological problem involving leukocytes, such as their movement and deformation in blood vessels, leukocyte rolling along the venous endothelium, their migration across the capillary wall into tissues, and other phenomena. The rheological models can be used to analyze any of these phenomena, as well as to further develop the functions of leukocytes.  
Writing--original draft preparation, N.A. and K.K.; writing--review and editing, N.A. and K.K. All authors have read and agreed to the published version of the manuscript.  
This study was funded by the Bulgarian National Science Fund--grant number N\({}^{\circ}\) KII-06-H57/14 from 2021.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} WBV & Whole-blood viscosity \\ PV & Plasma viscosity \\ RBC & Red blood cell \\ Fagg & Aggregation force \\ CDL & Cell depletion layer \\ G\({}^{\star}\) & Complex modulus \\ IAE & Aggregation index \\ PDE & Phosphodiesterase \\ \(\alpha\) & Relative leukocyte adhesion index \\ CVD & Cerebrovascular disease \\ PAD & Peripheral arterial disease \\ WBC & White blood cell \\ RBCD & Red blood cell deformability \\ RBCA & Red blood cell aggregation \\ T2 DM & Diabetes mellitus type 2 \\ PAOD & Peripheral arterial occlusive disease \\ LT & Laser tweezers \\ Ht & Hematocrit \\ EI & Elongation index \\ O & Osmolality \\ \end{tabular}