  Flexible strain gauges with 88% optical transmittance, of reduced graphene
oxide (rGO) on poly dimethylsiloxne membranes, are produced form monolayers of
graphene oxide assembled into densely packed sheets at an immiscible
hexane/water interface and subsequently reduced in HI vapor to increase
electrical conductivity. Pre-straining and relaxing the membranes introduces a
population of cracks into the rGO film. Subsequent straining opens these
cracks, inducing piezoresistivity. Reduction for 30 s forms an array of
parallel cracks that do not individually span the membrane and results in a
strain gauge with a usable strain range > 0.2 and gauge factor of 20 - 100 at
low strain levels that increases with increasing pre-strain. In all cases the
gauge facto decreases with increasing applied strain and asymptotes to a value
of about 3, as it approaches the pre-strain value. If the rGO is reduced for 60
s, the cracks fully span the width of the membrane, leading to an increased
gauge resistance but a much more sensitive strain gauge with GF ranging from
1000 - 16000. However, the usable strain range reduces to < 0.01. A simple
equivalent resistor model is proposed to describe the behaviour of both gauge
types. The gauges show a repeatable and stable response with loading
frequencies up to 1 kHz and have been used to detect human body motion in a
simple e-skin demonstration.
