Polymer Nanocomposite Materials. Группа авторов
2.5i.
2.4 Application in Sensors
CPCs have been widely used for different sensors, due to its light weight, portability, flexibility, and low cost [90, 119]. The sensing mechanism of CPC sensors is based on the resistance variation of CPCs when experiencing external stimulus like mechanical deformation [120, 121], temperature variation [122, 123], and adsorption of vapors or organic solvent [124, 125]. The resistance of CPCs is affected by the evolution of conductive network under external stimulus [5]. These transient resistance change can be detected as a signal for sensing purpose.
2.4.1 Strain Sensor
As a device for detecting object deformation, CPC based strain sensors have been widely used for health monitoring, electronic skin, wearable electronics, etc. For the purpose of monitoring, the resistance (conductive networks) should response upon external strain/stress [119]. Thus, CPCs should possess excellent resilience such that the material could be elongated under external strain (destruction of conductive networks) and recovered immediately after the removal of the stress (reconstruction of conductive networks).
Conductive natural rubber/elastomer composites decorated with different conductive nanofillers are explored as promising candidates of wearable strain sensors [110, 126, 127]. The sensors show broad workable sensing range, high sensitivity, and excellent recoverability. Among these materials, conductive fabric composite strain sensors have shown promising applications in wearable electronics, thanks to their breathability, skin affinity, and so on [128, 129]. For example, they can be incorporated into clothing or gloves to detect human body motions [111, 114] and detect the motions of human body as shown in Figure 2.6. Figure 2.6a–d demonstrate that the fiber strain sensor could be used for human–machine interfaces by simply incorporating the fiber composites onto a glove, demonstrating the potential application in remote control of hand robot. Figure 2.6e–j show sensing performance of the smart integrated glove in monitoring motions like head-forward, shoulder imbalance, kyphosis, etc. In order to improve the environmental suitability of the sensors, self-protective CPCs with superhydrophobicity have attracted great interests [128, 130] from the academia. This superhydrophobicity endows the sensor with self-cleaning behavior and can prevent the water or even corrosive solution diffusion into the materials, which can extend their applications in some harsh conditions. To reveal the strain sensing mechanism, the evolution of conductive network of the CPC under different strains is observed by using the SEM [126].
Hydrogel as a new type of conductive polymer composite demonstrating preferable bio-compatibility, self-healing, and self-adhesiveness is an ideal material for strain sensing usage [131, 132]. Zhang et al. [133] developed a MXene (Ti3C2Tx)/PVA hydrogel sensor with outstanding sensing performance by mixing MXene nanosheet with PVA hydrogel. The hydrogel composite shows outstanding stretchability, self-healing property, and strong adhesiveness to skin, which can adhere to human skin without the assistance of bonding materials to detect subtle motions including facial expression, vocal signals, handwriting, and finger bending, demonstrating high accuracy and sensitivity.
2.4.2 Piezoresistive Sensor
In recent years, CPC based pressure sensors as a branch of smart material, which could respond to compressive deformation and transform mechanical forces into electrical signals, have been extensively developed [134]. To meet the demands of different applications, innovated material structures and fabrication strategies are developed to prepare flexible and wearable pressure sensors with excellent sensing performance. CPCs containing an elastic matrix and conductive nanofillers are the most popular candidates of the piezoresistive sensor [135]. Textiles with fiber networks are often used as flexible scaffolds of piezoresistive sensors by carbonization or coating conductive fillers onto the fiber surface [136, 137]. Li et al. [137] fabricated flexible and electrically conductive carbon cotton (CC)/PDMS composites by infiltrating PDMS glue into the CC scaffold. The CC/PDMS demonstrates a sensitivity of 6.04 kPa−1, a working pressure of 700 kPa and durability over 1000 cycles. CPCs with a porous structure (foam, aerogels) are another ideal active materials of piezoresistive sensors because of their highly reversibility and hence reusability after large-deformation cycles [138, 139]. Zhong and coworkers [140] successfully fabricated lightweight MXene (Ti3C2) aerogel with ultra-stable lamellar structure by freeze drying the mixture of bacterial cellulose fiber and MXene nanosheets. The carbon aerogel used for pressure sensor demonstrates ultrahigh compressibility and superelasticity, a wide linear range and low detection limits.
Figure 2.6 (a) Photograph showing the smart glove integrated with the fiber composites. (b, c) Strain sensing behavior of the smart glove for (b) fingers bending and (c) a full bending stimulation of each finger. (d) Photographs showing the hand robot operated by the smart glove. Source: (a)–(d) Reproduced with permission. [111] Copyright 2018, American Chemical Society. Relative resistance change of the sensor responding to various postures: (e) head-forward, (f) shoulder imbalance, and (g) kyphosis, respectively. (h) Photos showing a smart glove incorporated with fiber composite. Strain sensing behavior of the smart glove for (i) different gestures of the five fingers and (j) different bending angels of the wrist. Source: (e)–(j) Reproduced with permission. [114] Copyright 2019, The Royal Society of Chemistry.
To enhance the response intensity and low detection limit, CPCs with delicate structures like fingerprint pattern [141, 142], microdome or micropillar array [143, 144], hair of human skin [145], plant leaves [146, 147], hollow spheres [148], etc. were studied. Park et al. [149] have reported flexible piezoresistive sensor by building interlocked microdome arrays on the elastomer composite film. The flexible film with regular microdome arrays can be observed by SEM as shown in Figure 2.7a. The inset picture demonstrates its excellent flexibility. The schematic diagram (top) and SEM image (bottom) of the fracture surface for the composite films are shown in Figure 2.7b, and an interlocked structure can be observed. The operating principle of the sensing device is also schematically illustrated in Figure 2.7c. Under the stress of external pressure, the microdomes would deform and the contact area between interlocked microdomes was increased, which in turn influenced the tunneling resistance of the sensor. Figure 2.7d shows the pressure sensing performance (relative electrical resistance (R/R0)) of composite film with different structures. The film with interlocked microdomes demonstrates a sharp decrease in R/R0 when the applied pressure increases from 0 to 10 kPa, while the sensing signals for films with single microdome arrays and planar structure are much weaker. Also, the sensing curve (log–log plot) of R/R0 vs. pressure demonstrates a high linearity (Figure 2.7e), indicating an exponential dependence of resistance on the applied pressure.