The probe and widespread use of rheological parameters of bodily fluids such as blood as potential label-free biomarkers for cardiovascular disease, COVID-19, and diabetes, among other conditions, is currently hindered by the complexity associated with conventional rheological instruments. As an alternative, the rheological analysis of complex fluids at the point-of-need (PON) can be enabled by microfluidic platforms that process small sample volumes (microliters) in portable easy-to-operate and low-cost set-ups. A convenient pump-free realization of such systems uses a smartphone to monitor self-driven capillary flow in large enough microchannels (~400-800 micrometers in diameter), where open/closed microchannel configurations allow modulation of the shear rate range during capillary filling. However, despite the advantages of relying on self-driven capillary flow, such approaches have not been widely adopted because degradation of the microchannel’s inner surfaces can affect their equilibrium contact angle (ECA). This degradation can introduce spatiotemporal variations in ECAs during long-term storage, leading to inconsistent flow rates and poor repeatability and reliability at the PON, a current challenge in capillary-driven microfluidics. Here we present our efforts to ensure long-term consistent surface properties in capillary-driven glass microfluidic devices for rheological analysis at the PON. To do this, clean glass capillary tubes (GCTs) were coated with a silane-coupling agent (2% weight aqueous solution), 2-[acetoxy (polyethyleneoxy) propyl] triethoxysilane (SIA0078.0, Gelest, USA) (SIA for short), and their ECAs were tracked over several months. Our results show that SIA coating of GCTs successfully reduces spatiotemporal variability in ECAs, with SIA-coated GCTs exhibiting consistent ECAs with <15% coefficient of variation (CV) for at least 200 days, relative to CVs as high as ~47% observed in clean GCTs during the same period of time. The PON performance of these SIA-coated GCTs is further evaluated in the rheological analysis of known fluids used as references, such as Newtonian water-glycerol solutions and shear-thinning water-xanthan gum solutions, where the latter exhibit comparable rheological behavior to blood. Our findings have important implications for the long-term storage and deployment of capillary-driven microfluidic platforms at the PON, where consistent and reliable performance must be ensured. More specifically, our smartphone-enabled capillary-based rheological analysis platform facilitates the rheology of complex fluids to be easily measured in PON and resource-limited settings, potentially enabling probing blood rheology as a label-free biomarker for various diseases.
