A Review of Flexible Strain Sensors Based on Natural Fiber Materials

Abstract

strain sensors have been favored by many researchers for their good flexibility, small size, light weight, and high conformability and have been initially explored for human health monitoring, human-computer interaction, and other fields. [7][8][9][10] The design and development of strain sensing materials are the core of flexible strain sensors. [11][12][13] With the concept of environmental protection and sustainable development deeply rooted, renewable, degradable, and natural fiber materials have become the more competitive materials to manufacture flexible strain sensors due to their low price, easy acquisition, and easy combination with textiles. [14][15][16] Natural fiber materials are mainly divided into cellulose fiber (cotton, fibrilia, and viscose) and protein fiber (wool and silk), which all show excellent softness and can be woven into yarn, nonwoven film, and 2D or 3D fabric with different structural characteristics. [17][18][19][20] The above materials can be directly carbonized or mixed with other conductive nanomaterials to construct flexible conductive networks. These green manufacturing materials have become an important trend to develop flexible strain sensors. [21][22][23] The carbonization temperature and time are the decisive factors for balancing the electrical conductivity and strain sensing performance of carbonized natural fiber materials. [24][25][26] Many studies found that although carbonized fibers exhibit excellent conductivity, they have a small strain range and high brittleness. Thus, they need to be combined with elastic polymers to improve their flexibility and strain capacity. [27][28][29] Original natural fiber materials can maintain excellent softness after they are combined with conductive nanomaterials, but they still exhibit major challenges, such as poor resilience, small strain range, poor wear resistance, and others. Therefore, they need to be combined with elastic polymers to improve the sensing performance. [30][31][32] The main polymer materials are polyurethane (TPU), polydimethylsiloxane (PDMS), and Ecoflex. The breaking elongation of the polymer determines the upper limit of the sensor strain range. The resilience, viscoelasticity, and fatigue resistance of the polymer are very important to the hysteresis, durability, and response speed of the sensor. The interfacial interaction between the polymer and conductive material affects the stability and sensitivity of the sensor. Conductive nanomaterials are diversiform, including carbon black (CB), carbon nanotubes (CNTs), graphene (GR), silver nanoparticles (AgNP), silver nanowires (AgNW), copper nanoparticles Flexible strain sensors with outstanding stretchability and sensitivity can be widely used in medical health, smart robot, intelligent garment, man-machine interaction, and other fields. The use of natural fibers enables a green manufacturing pathway to design strain sensors within the context of increasingly serious environmental pollution. Commercialized natural fibers, including cellulose fibers (cotton and ...

Multiple-Language-Responsive Conductive Hydrogel Composites for Flexible Strain and Epidermis Sensors

“…Further, the low detectable range and poor sensing also hindered the use of these materials. 14 Therefore, it is important to design highly stretchable and sensitive to long strain range sensors with high fatigue performance as smart wearable sensors.…”

Liquid Metal and Carbon Nanofiber-Based Strain Sensor for Monitoring Gesture, Voice, and Physiological Signals

“…Soft and stretchable strain sensors, as one of the important elements in the family of wearable sensors, , have undergone development from traditional wire and/or foil strain gauges − to ultrathin-film-state strain sensors, for the purpose of making them respond simultaneously to the epidermic changes , and realizing precise detection . In addition, numerous contributions have been made to strain sensors by the use of newly developed materials, such as low-dimensional carbon materials, , biomass, , metal nanowires, , MXene fabric, , as well as hydrogels , and composites loaded with nanoconductive fillers, as well as the recently proposed crack-based strain sensors by imitating slit sensilla of arthropods . The optimizations of stretchable electronic materials and their layout to construct elastic and conductive networks are the key points in the studies of soft strain sensors. , For example, the micro crack-junctions’ disconnection–reconnection of a 20 nm film on a viscoelastic polymer can contribute to the ultrahigh gauge factor (GF = 2000); meanwhile, the zigzag crack structure on flexible and interlaced graphene ribbons was demonstrated to improve sensitivity. , The main idea in crack-based strain sensors is focused on how to make the conductive layer split and coalesce by integrating ductile metals (such as Au, Ag, and Cu) and viscoelastic polymers [such as poly­(urethane acrylate) (PUA) and poly­(ethylene terephthalate) (PET)].…”

“…1 Soft and stretchable strain sensors, as one of the important elements in the family of wearable sensors, 2,3 have undergone development from traditional wire and/or foil strain gauges 4−6 to ultrathin-film-state strain sensors, 7 for the purpose of making them respond simultaneously to the epidermic changes 8,9 and realizing precise detection. 10 In addition, numerous contributions have been made to strain sensors by the use of newly developed materials, such as lowdimensional carbon materials, 11,12 biomass, 13,14 metal nanowires, 15,16 MXene fabric, 17,18 as well as hydrogels 19,20 and composites 21 loaded with nanoconductive fillers, as well as the recently proposed crack-based strain sensors by imitating slit sensilla of arthropods. 22 The optimizations of stretchable electronic materials and their layout to construct elastic and conductive networks are the key points in the studies of soft strain sensors.…”

Design and performance of stretchable resistive sensor based on knitted loop structures for motion detection

“…[4][5][6][7] Notably, resistive strain sensors have been extensively studied due to their simplicity, reliability, wide detection range, high sensitivity, and ease of integration. [8][9][10] Fabric-based flexible sensors depend heavily on the fabric structure and the conductive material for optimal performance. Knitted fabrics, distinguished by their unique loop structure, offer excellent softness and a wide stretch range compared to other fabric types like woven and nonwoven fabrics.…”

“…4–7 Notably, resistive strain sensors have been extensively studied due to their simplicity, reliability, wide detection range, high sensitivity, and ease of integration. 8–10…”