High-Fidelity, Low-Hysteresis Bionic Flexible Strain Sensors for Soft Machines.

Stretchable flexible strain sensors based on conductive elastomers are rapidly emerging as a highly promising candidate for popular wearable flexible electronic and soft-mechanical sensing devices. However, due to the intrinsic limitations of low fidelity and high hysteresis, existing flexible strain sensors are unable to exploit their full application potential. Herein, a design strategy for a successive three-dimensional crack conductive network is proposed to cope with the uncoordinated variation of the output resistance signal arising from the conductive elastomer. The electrical characteristics of the sensor are dominated by the successive crack conductive network through a greater resistance variation and a concise sensing mechanism. As a result, the developed elastomer bionic strain sensors exhibit excellent sensing performance in terms of a smaller overshoot response, a lower hysteresis (∼2.9%), and an ultralow detection limit (0.00179%). What's more, the proposed strategy is universal and applicable to many conductive elastomers with different conductive fillers (including 0-D, 1-D, and 2-D conductive fillers). This approach improves the sensing signal accuracy and reliability of conductive elastomer strain sensors and holds promising potential for various applications in the fields of e-skin and soft robotic systems.

A broad range and piezoresistive flexible pressure sensor based on carbon nanotube network dip-coated porous elastomer sponge.

Flexible pressure sensors have provided an attractive option for potential applications in wearable fields like human motion monitoring or human-machine interfaces. For the development of flexible pressure sensors, achieving high performance or multifunctions are popular research tendencies in recent years, such as improving their sensitivity, working range, or stability. Sponge materials with porous structures have been demonstrated that they are one of the potential substrates for developing novel and excellent flexible pressure sensors. However, for sponge-based pressure sensors, it is still a great challenge to realize a wide range of pressures from Pa level to hundreds kPa level. And how to achieve mechanical robustness remains unsolved. Here, we develop a flexible pressure sensor based on multicarbon nanotubes (MWCNTs) network-coated porous elastomer sponge with a broad range and robust features for use in wearable applications. Specifically, polyurethane (PU) sponge is used as the substrate matrix while dip-coated PU/MWCNTs composites as a conductive layer, achieving a highly bonding effect between the substrate and the conductive material, hence a great mechanical robust advantage is obtained and the working range also is improved. The pressure sensor show range of up to 350 kPa, while the minimum detection threshold is as low as 150 Pa. And before and after rolling by a bicycle or electric motorcycle, the sensor has the almost same responses, exhibiting great robustness.

Flexible Equivalent Strain Sensor with Ordered Concentric Circular Curved Cracks Inspired by Scorpion.

Slit sensillum, a unique sensing organ on the scorpion's legs, is composed of several cracks with curved shapes. In fact, it is just its particular morphological distribution and structure that endows the scorpions with ultrasensitive sensing capacity. Here, a scorpion-inspired flexible strain sensor with an ordered concentric circular curved crack array (CCA) was designed and fabricated by using an optimized solvent-induced and template transfer combined method. The morphology of the cracks can be effectively controlled by the heating temperature and the lasting time. Instead of the nonuniform stress distribution induced by disordered cracks, ordered concentric circle curved structures are introduced to generate a uniform stress distribution and larger deformation, which can significantly improve the performance of the strain sensor. Thus, the CCA sensor exhibits ultrahigh sensitivity (GF ∼ 7878.6), excellent stability (over 16 000 cycles), and fast response time (110 ms). Furthermore, the CCA sensor was demonstrated to be feasible for monitoring human motions and detecting noncontact vibration signals, indicating its great potential in human-health monitoring and vibration signal detection applications.

Bioinspired, Omnidirectional, and Hypersensitive Flexible Strain Sensors.

Sensors are widely used in various fields, among which flexible strain sensors that can sense minuscule mechanical signals and are easy to adapt to many irregular surfaces are attractive for structure health monitoring, early detection, and failure prevention in humans, machines, or buildings. In practical applications, subtle and abnormal vibrations generated from any direction are highly desired to detect and even orientate their directions initially to eliminate potential hazards. However, it is challenging for flexible strain sensors to achieve hypersensitivity and omnidirectionality simultaneously due to the restrictions of many materials with anisotropic mechanical/electrical properties and some micro/nanostructures they employed. Herein, it is revealed that the vision-degraded scorpion detects subtle vibrations spatially and omnidirectionally using a slit sensillum with fan-shaped grooves. A bioinspired flexible strain sensor consisting of curved microgrooves arranged around a central circle is devised, exhibiting an unprecedented gauge factor of over 18 000 and stability over 7000 cycles. It can sense and recognize vibrations of diverse input waveforms at different locations, bouncing behaviors of a free-falling bead, and human wrist pulses regardless of sensor installation angles. The geometric designs can be translated to other material systems for potential applications including human health monitoring and engineering failure detection.

Ultrasensitive, Highly Stable, and Flexible Strain Sensor Inspired by Nature.

For advanced flexible strain sensors, it is not difficult to achieve high sensitivity only. However, integrating high sensitivity, high stability, and high durability into one sensor still remains a great challenge. Fortunately, natural creatures with diversified excellent performances have given us a lot of ready-made solutions. Here, scorpion and spiderweb are selected as coupling bionic prototypes, which are famous for their ultrasensitive sensing capacity and excellent structural durability, respectively. Based on that, a bioinspired strain sensor is successfully fabricated. The results demonstrate that the bioinspired strain sensor has a sensitivity of 940.5 in the strain range of 0-1.5% and a sensitivity of 2742.3 between 1.5 and 2.5%. Meantime, this sensor with a spiderweb-like reticular structure has a great improvement in stability and durability. Specifically, the sensor exhibits excellent stability during bending and stretching cycles over 80,000 times. Moreover, the response time and recovery time of the sensor are 169 and 195 ms, respectively. Besides, the sensor also has functions such as vibrating frequency identification due to its low hysteresis. Based on the excellent performance, the sensor can be applied to monitor human body motions serving as wearable electronics.

A Selective-Response Bioinspired Strain Sensor Using Viscoelastic Material as Middle Layer.

Flexible strain sensors have an irreplaceable role in critical and emerging fields, such as electronic skins, flexible robots, and prosthetics. Although numerous efforts have been made to improve sensor sensitivity to meet specific application scenarios, the signal-to-noise ratio (SNR) is an extremely critical and non-negligible indicator, which takes into account higher sensitivity, meaning that they can also detect the noise signals with high sensitivity. Coincidentally, scorpions with ultrasensitive vibration sensilla also face such a dilemma. Here, it is found that the scorpion ingeniously uses the viscoelastic material in front of its slit sensilla to realize efficient preprocessing of the signal. Its mechanism is that the loss factor of materials changes with frequency, affecting energy storage and transmission. Inspired by this ingenious strategy, a bioinspired strain sensor insensitive to a low strain rate was designed using a two-step template transfer method. As a result, its relative change in resistance reached 110% under the same strain (0.3197%) but with different strain rates (0.1 Hz and ∼20 Hz). The noncontact vibration experiments also show different responses to low-frequency vibration and high-frequency impact. Moreover, it can also be used as a typical flexible strain sensor. Under the tensile state, it has a gauge factor (GF) as high as 4596 upon 0.6% strain, and the response time is 140 ms. Therefore, it is expected that this strain sensor will be used in many important ultraprecision measurement fields, especially when the measured signal is small.

Bioinspired, Superhydrophobic, and Paper-Based Strain Sensors for Wearable and Underwater Applications.

There is currently a growing demand for flexible strain sensors with high performance and water repellency for various applications such as human motion monitoring, sweat or humidity detection, and certain underwater tests. Among these strain sensors, paper-based ones have attracted increasing attention because they coincide with the future development trend of environment-friendly electronic products. However, paper-based electronics are easy to fail when they encounter water and are thus unable to be applied to humid or underwater circumstances. Herein, based on a strategy of coupling bionics inspired by lotus leaf and scorpion, which exhibit superhydrophobic characteristics and ultrasensitive vibration-sensing capacity, respectively, a paper-based strain sensor with high sensitivity and water repellency is successfully fabricated. As a result, the strain sensor exhibits a gauge factor of 263.34, a high strain resolution (0.098%), a fast response time (78 ms), excellent stability over 12,000 cycles, and a water contact angle of 164°. Owing to the bioinspired structures and function mechanisms, the paper-based strain sensor is suitable to not only serve as regular wearable electronics to monitor human motions in real-time but also to detect subtle underwater vibrations, demonstrating its great potential for numerous applications like wearable electronics, water environmental protection, and underwater robots.