## 4 Discussion  
The main objective of this study was to show the feasibility of using an advanced lab-on-chip microfluidic platform to visualize, quantify and characterize properties of RBCs relevant to SCD complications, especially VOC. This objective was met, and for the few patients sampled, the presented results even agree with the previously known heterogeneity of SCD properties and also point to new directions for SCD research. Although we have presented results about the morpho-rheological properties of RBCs from SCD patients, we are careful not to generalize our findings owing to the limited number of patient samples. Interestingly, the MMM readouts for rheological (transit time) properties of the s RBCs showed significant variability (\(p<0.001\)) among all patients tested. Changes in transit time between normoxic and hypoxic conditions ex vivo were diverse and patient-specific, including both increases and decreases in transit time between normoxic and hypoxic conditions. Furthermore, changes in transit time following ex vivo treatment with HU were also patient-specific. The morphological parameters (circularity and roundness) we measured suggest dynamic changes in the percentage of sickled RBCs in individual patients' blood, amplifying the call for personalized measurements using point-of-care technologies [23, 31, 32, 33]. Combining these parameters during patient monitoring should enable the quantitative tracking of morphological parameters for correlation with clinical parameters, thereby potentially informing personalized clinical care. Our work shows the feasibility and potential utility of personalized clinical monitoring of SCD patients using an ex vivo real-time microfluidic lab-on-chip device even in resource-challenged settings, where they are most needed. Recent computational models that use patient-specific data have also led to patient specificity in the morpho-rheological properties of RBCs of SCD patients even at the meso  
Figure 7: Non-significant change in transit time following induced hypoxia. **(a)** Histogram distribution for patient 4 (P4, \(n=60\)) and patient 4 treated with SM (P4 + SM, \(n=43\)). **(b)** Box plot of two-sample t-test, showing significant non-significant (NS) change in transit time from 0.45 \(\pm\) 0.01 s for P4 to 0.47 \(\pm\) 0.02 s for P4 + SM. scopic level [35]. [PERSON] et al. further suggest, based on their in silico work, that treatment with HU improves or worsens the rheological characteristics of blood in SCD depending on the degree of hypoxia [35]. Beyond SCD, [PERSON] et al. have reviewed a vast body of evidence demonstrating the central role of microcirculation in inflammation, hyperviscosity, cell-cell interactions, endothelial dysfunction, tissue edema and hemodynamic regulation, with hypoxia being the leading feature in health and disease states [36]. Our morphometric results, which capture longitudinal heterogeneity within a specific patient as well as changes based on hypoxia or treatments, using shape descriptors such as roundness, circularity, skewness and kurtosis, will likely be useful in further unraveling components of VOC for SCD management as well as exploring the impact of hypoxia on cell components of the microcirculation in other hematological disorders. Our measurement of transit times for single RBCs using the MMM provides surrogate rheological characterization that also reveals heterogeneity within the same patient and between patients. This result agrees with [PERSON] et al. 's report of distinct biophysical RBC subpopulations with high interpatient variability in SCD [23]. Our work illustrates the feasibility and potential utility of deploying the MMM along with high-throughput multimodal microfluidic platforms, such as the integrated automated particle tracking microfluidic [23] and real-time deformability cytometer [22], for patient-specific high-content cell analysis in studies involving patients with SCD and other blood disorders. The feasibility of ex vivo active monitoring of RBCs of SCD patients is demonstrated in this work.