Abstract Fiber materials are highly desirable for wearable electronics that are expected to be flexible and stretchable. Compared with rigid and planar electronic devices, fiber-based wearable electronics provide significant advantages in terms of flexibility, stretchability and breathability, and they are considered as the pioneers in the new generation of soft wearables. The convergence of textile science, electronic engineering and nanotechnology has made it feasible to build electronic functions on fibers and maintain them during wear. Over the last few years, fiber-shaped wearable electronics with desired designability and integration features have been intensively explored and developed. As an indispensable part and cornerstone of flexible wearable devices, fibers are of great significance. Herein, the research progress of advanced fiber materials is reviewed, which mainly includes various material preparations, fabrication technologies and representative studies on different wearable applications. Finally, key challenges and future directions of fiber materials and wearable electronics are examined along with an analysis of possible solutions. Graphical abstract 

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Advanced Fiber Materials for Wearable Electronics

Yarn-based wearable sensors have attracted attention, but it is difficult to fabricate yarn strain sensors with large-scale production, weavability, dyeability and durability toward real applications as wearable electronics and smart textiles. Herein, we report a facile core spun strategy to fabricated spandex/CNT cotton yarns (SCCY) on an industrial spinning machine. For the first time, wearable sensor could be dyed through a simple dip dyeing process with different colors (red, yellow, blue and black). The unique core-sheath structure endowed the yarn with good sensitivity (gauge factor of 21.85 at 300% strain), broad sensing range (0%-300% strain), excellent durability (against dip dyeing, washing, acid and alkali) and abrasion resistance (resistance change 5% after rubber 100 cycles). The sensor yarn was also successfully woven/knitted into textiles and it could be sewn into a fabric. The weavable and dyeable core-spun yarn in the present work provide a feasible way for industrial-scale manufacture wearable sensors.

Large-scale production of weavable, dyeable and durable spandex/CNT/cotton core-sheath yarn for wearable strain sensors

Flexible wearable sensors have attracted a lot of attention because of their great potential for application in wearable medical health detection, human activity detection, and artificial electronic skin. However, achieving both high sensitivity and a large measurement range on a single sensor remains a serious challenge. Here, we prepared piezoresistive sensors by dip-coating polypyrrole-coated halloysite nanotube (PPy/HNT)–silver nanoflower (AgNF) composite conductive materials on polyurethane sponges. The fabrication process is simple and reproducible on a large scale. The assembled sensor has a large sensing range (0–400 kPa), high sensitivity (3.97 kPa−1), ultra-fast response/recovery time (40 ms/50 ms), and reliable repeatability (12 000 s of cyclic testing). In addition, halloysite nanotubes greatly increase the measurement range of the piezoresistive sensor. The synergy of the crack sensing mechanism and the contact sensing mechanism allows the sensor to maintain high sensitivity over a large strain range while also detecting different levels of human motion. What's more, it has shown outstanding performance in acting as an artificial electronic skin to quantify the magnitude of external pressure distribution and in wireless motion detection in real time. It has potential application in the next generation of artificial electronic skin, medical health detection, and real-time detection of motion signals.

An in situ polymerized polypyrrole/halloysite nanotube–silver nanoflower based flexible wearable pressure sensor with a large measurement range and high sensitivity

Intrinsically conducting polymers in flexible and stretchable resistive strain sensors: a review

A smart and flexible textile strain sensor was developed based on a commercial core-spun yarn coated with carbon black (CB) nanoparticles and polydopamine (PDA). The morphology of the textile was studied using scanning electron microscopy (SEM), and the distribution of CB and self-polymerized PDA was observed. Thermogravimetric analysis (TGA) revealed the applicable temperature range of the strain sensor. The smart textile showed an outstanding strain sensor response range up to 450%, and its mechanical properties were only slightly reduced as compared to the original textile. The cyclic strain sensor behavior of the smart textile was investigated by stretching the textile 100% for 100 cycles, yielding a stable and durable output electrical signal. Application tests for various human motions were also performed. Finally, two mathematical equations based on tunneling theory were proposed to accurately predict the electrical signal during stretching and to reveal the fundamental properties of the textile. Our findings clearly expand the current knowledge of strain sensors. 

Carbon Black Nanoparticle/Polydopamine-Coated Core-Spun Yarns for Flexible Strain Sensors

Polypyrrole-coated copper nanowire-threaded silver nanoflowers for wearable strain sensors with high sensing performance

In this paper, amorphous alloy thin strips with Ca60-xMg20Zn20+x (x=2,4,6,8,10) composition ratio were prepared by single roll spin quenching and the effect of Zn on the microstructure and biological corrosion behavior of these alloys was investigated. It is found that Ca60-xMg20Zn20+x alloy strip is absolutely amorphous, and Ca56Mg20Zn24 alloy has the best glass forming ability. When Zn content is lower than 24 at.%, the glass-forming ability (GFA) and corrosion properties of Ca-Mg-Zn alloy are improved gradually with the increase of Zn content. However, when Zn content exceeds 24 at.%, the GFA and corrosion properties of Ca-Mg-Zn alloy decrease gradually with the increase of Zn content. After the crystallization treatment of Ca56Mg20Zn24 alloy strip, it is found that the greater the crystallization degree of Ca56Mg20Zn24 alloy, the weaker the corrosion resistance of the alloy, and the corrosion resistance of the alloy has a linear relationship with the crystallization degree. 

Effect of Zn on glass-forming capacity and corrosion resistance of Ca-based amorphous alloys

AZ91D magnesium alloy rolled under four rolling conditions, namely cold rolling, electric pulse cold rolling, hot rolling and electric pulse hot rolling, and the first principles calculation of Mg with or without external electric field carried out. The results show that: The application of pulse current in the rolling process of AZ91D magnesium alloy can effectively improve the edge crack of the sample, optimize the texture of AZ91D magnesium alloy and reduce its texture strength, promote the generation of tensile twins and the transition from small Angle grain boundaries to large Angle grain boundaries, and thus improve the plastic forming ability of AZ91D magnesium alloy. Make it more prone to plastic deformation. Compared with ordinary rolling, the microhardness of α-Mg matrix decreases by 15%. The tensile strength and elongation increased from 137MPa and 3.4% to 169MPa and 4.7%, respectively. The results show that the stiffness of Mg decreases and the Poisson's ratio of Mg increases when the electric field applies. When the B/G value is greater than 1.75, the plasticity of Mg is improved. The fault energy at the base surface of Mg does not change much, while the fault energy at the prismatic surface of Mg decreases obviously, showing the external electric field mainly affects the prismatic surface slip of Mg, which makes the prismatic surface slip easier to start, and thus improves the plastic forming ability of Mg.

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Wenjie Bo

Papers

Polypyrrole-coated copper nanowire-threaded silver nanoflowers for wearable strain sensors with high sensing performance

Effect of Zn on glass-forming capacity and corrosion resistance of Ca-based amorphous alloys

Effect of electric pulse rolling on plastic forming ability of AZ91D magnesium alloy