Printing and Mechanism Modelling of Nanocomposites Strain Sensors

Conductive nanocomposites are often piezoresistive, displaying significant changes in resistance upon deformation, making them ideal for use as strain and pressure sensors. Such nanocomposites typically consist of ductile polymers filled with conductive nanomaterials, such as graphene nanosheets or carbon nanotubes, and can display sensitivities, or gauge factors, which are much higher than those of traditional metal strain gauges.<br/>Visco-elastic polymer nanocomposites have shown particular promise in the area of strain sensing due to their large piezoresistive response and self-healing properties [1,2]. However, this comes a cost of large hysteretic and rate/frequency dependent effects. Minimizing these effects, while developing sensors that offer additional variety in terms of printability and functionality will be crucial to the development of mature tactile sensing technologies. We develop a versatile method for converting a viscoelastic nanocomposite, known as G-putty [1], into inks which can be tailored to suit a range of commercial printing processes, for patterned strain sensing on flexible substrates [3]. The mechanical effects of the substrate largely suppress hysteresis and almost completely remove strain rate and frequency dependence. This allows the fabrication of practical, high-gauge-factor, wearable sensors for pulse measurements as well as patterned sensors for low-signal vibrational and dynamic sensing.<br/>While innumerable combinations of polymer, nanomaterials and synthesis techniques have been explored, comparatively little attention has been given to the mechanisms underlying piezoresistive sensitivity. We develop a simple model which relates nanocomposite gauge factors to both filler loading composite conductivity [3]. These equations can be used to fit experimental data, outputting figures of merit or can be used predict experimental data once certain physical parameters are known. We have found these equations to match extremely well with both our own experimental and literature data. Importantly, the model shows the response of composite strain sensors to be more complex than previously thought and shows factors other than the effect of strain on the interparticle resistance to be performance limiting. These findings will facilitate our future development of state-of-the-art printed flexible sensors.<br/><br/><br/>[1] Boland, C. S.; Khan, U.; Ryan, G.; Barwich, S.; Charifou, R.; Harvey, A.; Backes, C.; Li, Z.; Ferreira, M. S.; Mobius, M. E.; Young, R. J.; Coleman, J. N., <i>Science </i><b>2016,</b> <i>354</i> (6317), 1257-1260.<br/>[2] Zhouyue Lei, Peiyi Wu, <i>Nature Communications </i><b>2018,</b> 9,1134<br/>[3] O’Driscoll, D. P.; McMahon, S.; Garcia, J.; Biccai, S.; Kelly, A. G.; Barwich, S.; Moebius, M.; Boland, C. S.; Coleman, J. N., <i>Small</i><b> 2021</b>, 17, 23, 2006542.<br/>[4] Garcia, J. R.; O’Suilleabhain, D.; Kaur, H.; Coleman, J. N., <i>ACS Applied Nano Materials </i><b>2021</b>, 4, 3, 2876–2886.