With the increasing interest in research on the development of next-generation technologies such as flexible smartphones, displays, and wearable devices, interest in the development of materials and processes for transparent electrodes constituting them is also increasing. The most widely used material for manufacturing transparent devices is indium tin oxide (ITO). However, ITO is scarce, expensive, and brittle, making it is essential to replace it with new materials. In this study, we successfully fabricated a transparent electrode by electrospinning polyvinylpyrrolidone (PVP) and copper electroless deposition on the polyimide film. Especially, this study suggests a new combined heat treatment that uses both the hot plate and the convection oven. Through the combined heat treatment, the junctions between the nanofibers overlapped removed consequently reducing contact resistance. The mechanical stability of the fabricated electrode was evaluated by using a highly repeated bending test. Also, through the tape-peeling test, we confirmed that the adhesive strength of the electrode was high. This method can be applied to various polymer-based, substrate which are vulnerable to annealing process.

Recently, applying nanoscale functional materials, there have been great advances in the flexible sensor system, which provides a large number of applications for soft electronics, such as skin-attachable sensors, artificial electronic skins, and soft robotic systems. Here, we developed a highly sensitive and flexible device on the basis of polymeric piezoelectric nanofibers and elastomeric packing structures. To produce the nanofibers, we applied the electrospinning process with a representative piezoelectric co-polymer, poly (vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE). Unlike the conventional electrospinning, we applied an anisotropic fiber collection system, which could obtain uniaxially aligned nanofiber array. The aligned nanofibers were sandwich-packed with bridge-shaped PDMS substrates, thereby integrating the flexible piezoelectric sensor. As an external force made a deflection of the bridge in the sensor, the embedded nanofibers generated piezoelectricity in a longitudinal direction of the fibers. The piezoelectric sensor generated good discernable outputs versus the varied mechanical input deflection from tens of micrometers to the sub-micrometer. With this great sensing ability, we could monitor heart pulse signals on the wrist skin by measuring tiny deflections generated from the expansion of the radial artery underneath the skin. Our study suggests a potential application of flexible sensor in the field of wearable health-monitoring medical systems.