Multiaxial and Transparent Strain Sensors Based on Synergetically Reinforced and Orthogonally Cracked Hetero‐Nanocrystal Solids

Abstract

Wearable strain sensors are widely researched as core components in electronic skin. However, their limited capability of detecting only a single axial strain, and their low sensitivity, stability, opacity, and high production costs hinder their use in advanced applications. Herein, multiaxially highly sensitive, optically transparent, chemically stable, and solution‐processed strain sensors are demonstrated. Transparent indium tin oxide and zinc oxide nanocrystals serve as metallic and insulating components in a metal–insulator matrix and as active materials for strain gauges. Synergetic sensitivity‐ and stability‐reinforcing agents are developed using a transparent SU‐8 polymer to enhance the sensitivity and encapsulate the devices, elevating the gauge factor up to over 3000 by blocking the reconnection of cracks caused by the Poisson effect. Cross‐shaped patterns with an orthogonal crack strategy are developed to detect a complex multiaxial strain, efficiently distinguishing strains applied in various directions with high sensitivity and selectivity. Finally, all‐transparent wearable strain sensors with Ag nanowire electrodes are fabricated using an all‐solution process, which effectively measure not only the human motion or emotion, but also the multiaxial strains occurring during human motion in real time. The strategies can provide a pathway to realize cost‐effective and high‐performance wearable sensors for advanced applications such as bio‐integrated devices.

Quantifying the Contributing Factors toward Signal Fatigue in Nanocomposite Strain Sensors

“…From the model presented, we can now understand on a fundamental level not only what causes conditioning in nanocomposite strain sensors but also why some materials, though not presenting any obvious signs of materials degradation, never report a steady signal. However, it can also be said that there are several examples of nanocomposites reporting little to no discernible tread in (R/R0)R,C with C. From this work, this can also now be attributed to their superior mechanical hysteresis as a result of fibre formation, [63][64][65][66] use of lament layers [63][64][65][66] and micro-structuring [67][68][69][70] which appears to quench effects of stress softening on electromechanical response. However, these morphologies, in particular the fibres, generally have larger values for Young's modulus which has been noted by the author in prior work as being a vital parameter of nanocomposites that must be minimised to facilitate applications as bodily sensors.…”

Thin Film Protected Flexible Nanoparticle Strain Sensors: Experiments and Modeling

“…Strain sensors based on NP films, in particular, have been of growing interest due to their increased sensitivity [19][20][21] when compared to existing metal strain sensors that incorporate thin film technology [23]. In addition, the low processing temperatures required in the case of NP-based strain sensing devices, render them fully compatible with flexible substrate technology [24].…”

“…Particularly in the field of flexible electronics, strain sensors are of great importance in emerging technologies such as the internet of things. Over the last decade, many novel nanomaterials have been used in strain sensing applications such as carbon nanotubes [ 11 , 12 , 13 ], nanowires [ 5 , 14 , 15 ], MoS 2 [ 16 ], graphene [ 17 ], and nanoparticles (NPs) [ 6 , 18 , 19 , 20 , 21 , 22 ].…”

“…Such conductivity mechanisms are governed by the quantum tunneling effect which relates exponentially to inter-NP distance [ 28 ]. Many research groups focus their interest on increasing the sensitivity of NP-based sensors, and usually, this can be achieved by incorporating NP films with varying conductivities, so as to manipulate the charge transport mechanisms of the device [ 19 , 20 , 22 , 29 ]. Lee et al [ 19 ] studied the combination of metallic and insulated NPs as sensitive materials, and thereby combining Au NPs, CdSe NPs, and nanocracks, a gauge factor of up to 5045 was achieved.…”

Wearable sensors based on colloidal nanocrystals

“…First, NCs can be synthesized at a large scale using wet chemical methods, and the resulting NC inks can be deposited onto various substrates in a large area under room-temperature and in an atmospheric environment using a solution based process such as roll-to-roll printing, drop casting, spin-coating, and inkjet printing [23–30]. Second, the electronic, optical, and magnetic properties of NCs can be easily controlled by adjusting their size, shape, composition, and surface state; thus enabling them to demonstrate application-specific functionality [31–37]. Based on these advantages, significant research effort has been directed toward the realization of high performance NC-based strain, pressure, and temperature sensors by the control of the interparticle distance between the NCs, or by the design of new NC structures [38–47].…”