High‐Resolution Hydrophilic Patterning in Hydrophobic Nonwoven Substrates for Sub‐3 µL Biofluid Viscosity Measurement

ABSTRACT Plasma and saliva viscosity are clinically important indicators of inflammation, metabolic abnormalities, and early disease; however, conventional viscometry requires large sample volumes and external pumping, limiting applicability in point‐of‐care settings. This study presents a paper‐based microfluidic device that can measure the viscosity of biofluids using an ultralow sample volume of less than 3 µL. Vacuum ultraviolet irradiation patterns hydrophilic microchannels within a highly hydrophobic nonwoven substrate, creating well‐defined capillary pathways for passive liquid transport. By analyzing the capillary wetting dynamics of bovine serum albumin, blood analog, and mucin–saline solutions, viscosity is determined without involving any pumping mechanism. The device produces linear Lucas–Washburn behavior and viscosity values in close agreement with standard glass‐capillary measurements, demonstrating reliable performance despite ultralow sample usage. These results establish stable hydrophilic patterning in hydrophobic nonwoven media as a versatile platform for ultralow‐volume capillary transport, viscosity estimation, particle filtration, and future paper‐like microfluidic systems.