  All-electronic interrogation of biofluid flow velocity by sensors
incorporated in ultra-low-power or self-sustained systems offers the promise of
enabling multifarious emerging research and applications. Electrical sensors
based on nanomaterials are of high spatiotemporal resolution and exceptional
sensitivity to external flow stimulus and easily integrated and fabricated
using scalable techniques. But existing nano-based electrical flow-sensing
technologies remain lacking in precision and stability and are typically only
applicable to simple aqueous solutions or liquid/gas dual-phase mixtures,
making them unsuitable for monitoring low-flow (~micrometer/second) yet
important characteristics of continuous biofluids (e.g., hemorheological
behaviors in microcirculation). Here we show that monolayer-graphene single
microelectrodes harvesting charge from continuous aqueous flow provide an ideal
flow sensing strategy: Our devices deliver over six months stability and
sub-micrometer/second resolution in real-time quantification of whole-blood
flows with multiscale amplitude-temporal characteristics in a microfluidic
chip. The flow transduction is enabled by low-noise charge transfer at
graphene/water interface in response to flow-sensitive rearrangement of the
interfacial electrical double layer. Our results demonstrate the feasibility of
using a graphene-based self-powered strategy for monitoring biofluid flow
velocity with key performance metrics orders of magnitude higher than other
electrical approaches.
