The unique selective ion-transport characteristics of nanofluids make them applicable in energy harvesting and sensing. However, developing scalable, self-powered nanofluidic devices remains challenging due to high cost, processing complexity, and reliance on external power sources. In this work, surface-twisted, internally aligned algae fibers (twisted fibers) are fabricated using an asymmetric flow field to regulate the assembly process of the algae cellulose nanofibers. Unlike aligned fibers from the symmetrical process, asymmetric flow-mediated twisted fibers exhibit a significantly reduced diameter (33.6-20.4 mu m), increased packing density (0.87-1.47 g cm-3), superior fractured stress (249.4-468.5 MPa), and an enhanced Herman's orientation parameter (from 0.77 to 0.89). Importantly, twisted fibers demonstrate energy-harvesting up to 12.87 W m-2 under a 50-fold salinity gradient and can serve as self-powered urine monitors, effectively distinguishing infants' urination from motility behaviors and alerting urine saturation due to high ionic conductivity (7.8 mS cm-1) at dilute electrolyte concentrations. This study provides a novel design concept for a self-powered biomass-based nanofluidic health sensing system.