Graphene-Based Transparent Flexible Strain Gauges with Tunable Sensitivity and Strain Range

Monolayers of graphene oxide, assembled into densely packed sheets at an immiscible hexane/water interface, form transparent conducting films on polydimethylsiloxane membranes after reduction in hydroiodic acid (HI) vapor to reduced graphene oxide (rGO). Prestraining and relaxing the membranes introduces cracks in the rGO film. Subsequent straining opens these cracks and induces piezoresistivity, enabling their application as transparent strain gauges. The sensitivity and strain range of these gauges is controlled by the cracked film structure that is determined by the reducing conditions used in manufacture. Reduction for 30 s in HI vapor leads to an array of parallel cracks that do not individually span the membrane. These cracks do not extend on subsequent straining, leading to a gauge with a usable strain range >0.2 and gauge factor (GF) at low strains ranging from 20 to 100, depending on the prestrain applied. The GF reduces with increasing applied strain and asymptotes to about 3, for all prestrains. Reduction for 60 s leads to cracks spanning the entire membrane and an increased film resistance but a highly sensitive strain gauge, with GF ranging from 800 to 16,000. However, the usable strain range reduces to <0.01. A simple equivalent resistor model is proposed to describe the behavior of both gauge types. The gauges show a repeatable and stable response with loading frequencies >1 kHz and have been used to detect human body strains in a simple e-skin demonstration.