  Realizing flexible strain sensor with high sensitivity and tunable gauge
factor is a challenge. To meet this challenge, we report an ionic liquid gated
three-dimensional graphene field effect strain sensor. The charge carrier
concentration in this 3D graphene is modulated by applied electric field
through an all-around self-assembled electrical double layer capacitance formed
at the interface of graphene with ionic liquid. Strain causes folding and
unfolding of microscopic wrinkles and formation of cracks in the graphene
network altering transistor behavior. Mechanical deformation of graphene also
alters its bandgap providing inherent strain sensitivity. Use of 3D network
results in robust operation since there exists multiple paths for the charge
carriers to flow between source and drain terminals. Interestingly, changing
the applied bias allows one to tune the gauge factor of this graphene
transistor based strain sensor. The current-voltage characteristics of the
sensor were measured for different tensile strain values of 0.5% to 35%. Our
results show that the sensor maintains its field effect characteristics over a
large strain range; moreover it enables up to 68% tunability in the strain
gauge factor. We also report cyclic measurements with varied magnitude and
frequency showing repeatability and robustness as a highly sensitive strain
sensor.
