Electrospun polyvinylidene fluoride (PVDF) nanofiber has shown its advantages of producing piezoelectric devices. Although the effect of electrospinning parameters on the formation of β crystalline phase has been investigated, their mixed effect has not been fully understood. In this study, two dependent design of experiments were utilized to systematically control the transformation of crystalline phases by three typical electrospinning parameters, i.e. applied voltage, needle tip diameter and feeding flow rate, and the mixed interaction of these parameters in improving the fraction of β crystalline phase was investigated. The structures and the contents of crystalline phases within PVDF nanofibers were analyzed by Fourier transform infrared spectra and x-ray diffraction patterns. The results of single factorial experiments showed that all of three electrospinning parameters have a significant effect on the formation and fraction of β -phase crystallinity of electrospun PVDF nanofibers. Furthermore, both the fraction and the crystallinity of β phase initially increase and then decrease with an increase of applied voltage, and consistently decrease with the needle-tip diameter. However, the β -phase fraction parabolically increases with the increasing feeding flow rates, whereas its crystallinity linearly deceases. The orthogonal experimental results demonstrated that the mixed effect of three electrospinning parameters is different from that of single parameter on the fraction of β -phase crystalline, and that the feeding flow rates among three parameters have the greatest effect. Taken together, these results suggested a new strategy to improve the β -phase fraction of electrospun PVDF fibers via sequence of priority of electrospinning parameters.

Flexible textile strain sensors that can be directly integrated into clothing have attracted much attention due to their great potential in wearable human health monitoring systems and human–computer interactions. Fiber- or yarn-based strain sensors are promising candidate materials for flexible and wearable electronics due to their light weights, good stretchability, high intrinsic and structural flexibility, and flexible integrability. This article investigates representative conductive materials, traditional and novel preparation methods and the structural design of fiber- or yarn-based resistive strain sensors as well as the interconnection and encapsulation of sensing fibers or yarns. In addition, this review summarizes the effects of the conductive materials, preparation strategy and structures on the crucial sensing performance. Discussions will be presented regarding the applications of fiber- or yarn-based resistive strain sensors. Finally, this article summarizes the bottleneck of current fiber- or yarn-based resistive strain sensors in terms of conductive materials, fabrication techniques, integration and performance, as well as scientific understanding, and proposes future research directions.

The high piezoelectric response capacity of electrospun PVDF nanofibers is currently a challenging area of research, and this study attempts to optimize the piezoelectric properties of electrospun PVDF nanofibrous mat under controlled electrospinning process. Some experiments were designed to study the relationship between electrospinning processing parameters and the piezoelectric properties of PVDF mats under cyclic compression, and this relationship was compared with the change of crystalline structure features with the electrospinning parameters. The excitation method is based on low velocity compression test by cyclically pressing flat contactor on samples. The impact of each of processing parameters has been investigated. It was found that three electrospinning parameters including: applied voltage, feeding volume flow rate and syringe needle tip diameter, are significant influential factors to the piezoelectric properties of the electrospun PVDF nanofibrous mats. However, the feeding volume flow rate has a greater effect on the piezoelectric responses to cyclic compression than both applied voltage and volume flow rate does, and its effect on the piezoelectric responses is different from that of β-phase fraction of crystalline structure within PVDF nanofiber. It was also observed that the relationship between the piezoelectric peak intensity and the applied voltage as well as the feeding volume flow rate is nonlinear.

Carbon nanotubes have been widely used to formulate printed conductive ink in recent years due to their excellent conductivity, chemical stability and mechanical properties. However, the common problems of this ink, such as poor adhesion and low resistance stability under large deformation, hinder its application in the fabric electrodes (FEs) of wearable and stretchable e-textiles. Herein, conductive inks with a simple preparation process, high adhesion and conductive stability were formulated by mixing the conductive filler, multi-walled carbon nanotubes (MWCNTs), into a thermoplastic polyurethane matrix with a reversible cross-linked structure. Then it is evaluated whether the MWCNT-based conductive ink is suitable for the screen-printing fabrication of highly durable and washable FEs. The experimental results showed that the screen-printed fabric electrode exhibited remarkable resistance stability under bending and folding deformation. In particular, after 1000 bending cycles and 100 folding–unfolding cycles under additional pressure, the sheet resistance of FEs only increased by 0.8% and 5.0%, respectively. Moreover, the screen-printed conductive pattern has strong adhesion to polyamide fabric substrate by the Scotch tape and washing tests, and there are no significant changes in resistance and surface morphology. High-performance conductive inks with facile and large-scale production potential are developed, and show great prospect in the development of wearable printing e-textiles.