Flexible and Transparent Reduced Graphene Oxide Strain Gauges with Tuneable Piezoresistivity for Wearable Sensing Applications

Flexible and transparent strain gauges are of interest for applications in human motion sensing, enabling future applications in human-machine interfacing and healthcare. Development of flexible and transparent sensors with high sensitivity and working strain over the range typical of skin in human motion (≈ 30 %) remains a challenge. Here, we present the simple and efficient assembly of highly sensitive, flexible and transparent resistive strain sensors formed from sheets of reduced graphene oxide (rGO) assembled at the interface between immiscible solvents deposited on the biocompatible elastomer PDMS. These rGO films demonstrate initial optoelectronic properties which are around the highest observed for conductive transparent electrodes of rGO (transmittance = 88 %, sheet resistance = 850 Ohm/sq).<br/><br/>Crack-based strain sensors based on metal thin films on flexible substrates have previously demonstrated ultrasensitive resistance response to strain, analogous to the slit sensory organs of arachnids.¹ Crack-based strain sensors fabricated from nanomaterials films on flexible substrates have been reported previously.² These crack-based strain gauges have previously demonstrated a characteristic high sensitivity and repeatable resistance response over strain. However, another characteristic of crack based strain gauges is a small strain sensing range, typically &lt; 6 %.^3,4 Thus making them incompatible with some strain sensing application areas such as human motion sensing.<br/><br/>The formation of parallel cracks in these devices under uniaxial strain is afforded by the mismatched mechanical properties of the conductive film and the elastomeric substrate.⁵In our devices, the deposition of a monolayer of rGO with predominantly edge-to-edge contact leads to short cracks which nucleate at junctions between nanoflakes. This leads to kirigami-like structures which offer reasonable sensitivity (gauge factor = 280) and reversible resistance response over a high working strain range (≤ 30 %). However, the formation of a thin silica-like layer on the PDMS surface during fabrication of the devices leads to long and straight cracks which span the entire device. An extremely high gauge factor of around 10,000 is observed in this case, as conductive pathways are completely ‘cut’ with minimal strain (0.6 %). Crucially, we find that these devices can be tuned between kirigami-like cracks and long cracks by controlling the duration of the rGO reduction step. This allows for a wide variety of strain sensing applications from a single production scheme.<br/><br/><u>References</u><br/>1. Kang, D. <i>et al.</i> Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system. <i>Nature</i> 516, 222–226 (2014).<br/>2. Song, J. <i>et al.</i> Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors. <i>ACS Appl. Mater. Interfaces</i> 11, 1283–1293 (2019).<br/>3. Lee, W. S. <i>et al.</i> Multiaxial and Transparent Strain Sensors Based on Synergetically Reinforced and Orthogonally Cracked Hetero-Nanocrystal Solids. <i>Adv. Funct. Mater.</i> 29, 1–12 (2019).<br/>4. Park, B. <i>et al.</i> Strain-Visualization with Ultrasensitive Nanoscale Crack-Based Sensor Assembled with Hierarchical Thermochromic Membrane. <i>Adv. Funct. Mater.</i> 29, 1–7 (2019).<br/>5. Thouless, M. D., Olsson, E. & Gupta, A. Cracking of brittle films on elastic substrates. <i>Acta Metall. Mater.</i> 40, 1287–1292 (1992).