# Optimizing the Shape of the Spinning Electrode for Needleless Coaxial Electrospinning
## 1 Introduction  
Thanks to their properties, nanofibrous structures are among the most important materials of this millennium, and their application potential is constantly increasing. For this reason, basic and applied research is devoted to these materials. The areas in which nanofibers appear are diverse and constantly expanding. Currently, nanofibrous structures can be found in tissue engineering [1, 2], filtration [3, 4], chemical and medical [5] applications [6] and the electrical [7] and electronic industries [8]. Recently, very progressive applications have appeared that expand the fields of application of nanofibrous structures. Flexible sensors and wearable devices can be considered among the most important of these applications [9, 10, 11]. Nanofibrous structures can be incorporated with various additives for specific applications. Various types of additives can be used, such as activated carbon, drugs, or sorbents [12]. A variety of methods can create pure nanofibers. Among the most used are AC and DC electrospinningning [13], melt-blowing [14] and force-spinning [15, 16], which are used not only for producing nanofibers for laboratory and research purposes but also for products that can be produced on a mass scale. On the other hand, there are other methods whose products can produce nanofibers, but their technological level ranks them among methods used for laboratory purposes. This is due to their low productivity, and drowning is a typical example [17]. DC electrospinningning is currently the most commonly used method for producing nanofibers for research and commercial production, especially for flat nanofiber textiles in many fields. The essence of electrospinningning is generally the use of a high electric voltage between the spinning electrode and the collector [18]. Using an electric current, nanofibers can be produced via the DC electrospinningning (DC) and AC electrospinningning (AC) methods [19]. Although AC electrospinningning is a technology with several times higherproductivity [20], it has a significantly different fiber morphology than DC [21]. The most significant difference is in the diameters of the fibers produced. However, with DC electrospinning, fibers that are up to several hundred nanometers finer can be achieved than with AC. It is also worth noting that the DC method has a more homogeneous structure [22]. For parametric adjustments of the production technology from the point of view of electrical quantities, the values of the voltages on the positive and negative electrodes and the distance between the electrodes are changed. While AC electrospinning is more variable, it is possible to modify the shapes of the excitation signals, especially their amplitude and frequency. Based on the mentioned differences, the fiber structures obtained using different technologies suit different applications. DC-made structures are more suitable for medical applications or tissue engineering. On the other hand, structures produced using AC are mainly used, for example, in protective textiles, hygiene aids or filter applications. Needless electrospinning is spinning from the surface of the free layer of the polymer solution when the action of an electric field destabilizes this layer. This phenomenon is known as Tonks-Frenkel instability [23] This phenomenon uses the so-called self-organization of nozzles on the spinning solution's free surface using the fastest-growing wave mechanism. The electrospinning process occurs after reaching a specific electric field value (critical electric field value). The so-called Taylor cones are created on the surface of the polymer solution, from which nozzles and nanofibers are formed. The solution is drawn from the spinning electrode in nozzles, passes through the bending instability zone and is transformed into nanofibers [24].