Transparent Flexible Strain Gauges for Wearable Devices based on Tiled Graphene Films

Here we present a novel transparent strain gauge, with a large gauge factor (GF) and usable strain range, based on 2D materials confined to the surface of an elastomeric substrate. These have good optical transmittance and demonstrate repeatability, under cyclic straining at frequencies &gt; 1 kHz, Gauges have also been mounted on a thin polymer glove to demonstrate its capability as a transparent e-skin strain sensor.<br/>During manufacture, closely packed films of graphene oxide are confined to a 2D plane, through assembly at an immiscible H₂O/hexane interface. These films have predominantly edge-to-edge nanosheet contact and show little overlap after deposition onto a polydimethylsiloxane (PDMS) substrate. The films are treated in a hydroiodic acid (HI) reducing atmosphere to introduce electrical conductivity, with the consequent reduced graphene oxide (rGO) films showing &lt; 900 Ω/sq sheet resistance, and &gt; 88% optical transmittance. Piezoresistivity is generated through a pre-straining process that introduces a population of cracks, normal to the straining direction, into the conducting film. After relaxation the cracked films show a repeatable change in resistance when strained to values smaller than the initial pre-strain. Through careful control of the HI chemical reduction phase, it is possible to generate strain gauging devices with different operating ranges and sensitivity. Films pre-strained after reduction for 30 s form an array of parallel cracks that do not individually span the membrane. Subsequent straining of the cracked membrane does not further extend these cracks and the sheet extends in a kirigami manner as the cracks open. This cracking behaviour results in a strain gauge with a usable strain range &gt; 0.2 and GF of 20 - 100 at low strains, however, this GF increases with increasing pre-strain. In all cases, GF decreases with increasing applied strain and asymptotes to a value of about 3 as it approaches the pre-strain value. However, If the rGO film is reduced for a longer time of 60 s, the cracks extend during pre-straining to fully span the width of the PDMS membrane, leading to a much more sensitive strain gauge with GF ranging from 1000 - 16000. However, in this case, the usable strain range reduces to &lt; 0.01. We present a simplified, equivalent resistor model, that can be used to interpret the behaviour of both gauge cracking configurations.