  Humidity sensing is important to a variety of technologies and industries,
ranging from environmental and industrial monitoring to medical applications.
Although humidity sensors abound, few available solutions are thin,
transparent, compatible with large-area sensor production and flexible, and
almost none are fast enough to perform human respiration monitoring through
breath detection or real-time finger proximity monitoring via skin humidity
sensing. This work describes chemiresistive graphene-based humidity sensors
produced in few steps with facile liquid phase exfoliation (LPE) followed by
Langmuir-Blodgett assembly that enables active areas of practically any size.
The graphene sensors provide a unique mix of performance parameters, exhibiting
resistance changes up to 10% with varying humidity, linear performance over
relative humidity (RH) levels between 8% and 95%, weak response to other
constituents of air, flexibility, transparency of nearly 80%, and response
times of 30 ms. The fast response to humidity is shown to be useful for
respiration monitoring and real-time finger proximity detection, with potential
applications in flexible touchless interactive panels.
