## 1 Introduction  
Polycaprolactone (PCL) is a high-molecular-weight aliphatic polyester that is biodegradable and non-toxic, making it an eco-friendly polymer with various medical applications [1; 2; 3; 4; 5]. PCL has a low melting point (ca. 60 \({}^{\circ}\)C) but is stable and easy to process, even at temperatures of 200 \({}^{\circ}\)C or higher. [6]. This is an important characteristic that makes PCL useful in various medical products, such as sutures, dental implants, and wound-covering tapes [7; 8; 9]. The global market size of PCL was 5530 million in 2018 and is expected to grow to $1230 million by 2026 [10]. Among the methods of fabricating fiber products using PCL, electrospinning is the most commonly used. The diameter of a fiber manufactured during electrospinning is determined by various spinning conditions, among which the electrical conductivity and viscosity of the spinning solution are the most decisive variables [11]. If the electrical conductivity of the spinning solution is low, a low surface charge density is formed on the jet during electrospinning, causing a decrease in the mobility of the ions and electrostatic repulsive forces [12]. This phenomenon also reduces the elongation of the solution, resulting in thicker fibers [13]. In addition, because the diameter of the spun fiber is proportional to the square of the viscosity of the polymer solution, thick fibers are generally obtained when the viscosity of the spun solution is high [14; 15]. PCL has very high viscosity and low conductivity, making electrospinning difficult without the use of an appropriate solvent [16]. Because PCL is hydrophobic, it does not dissolve well in non-toxic or almost non-toxic solvents, such as water or dimethyl sulfoxide. Currently, the solvents used for electrospinningn PLC include tetrahydrofuran [17], \(N\),\(N\)-dimethylformamide [18], chloroform/methanol [19], 1,1,1,3,3-hexafluoro-2-propanol [20], dichloromethane [21], 1-methyl-2-pyrrolidone [22], and 1,4-dioxane [23]. Most solvents used for the electrospinningn of PCL are highly toxic and adversely affect the environment. During the electrospinning process, most of the solvent is removed by evaporation, but a low concentration of toxic solvent remains in the PCL fibers. Although the concentration of toxic solvents remaining in PCL fibers is low, the residual toxic solvents impart a very negative effect on medical products, which is the most important application field of PCL [24]. Despite heat treatment under a vacuum to remove toxic solvents remaining in PCL, studies have reported that toxic solvents are still not completely removed [24; 25]. To solve the problem of PCL electrospinningning toxic solvents, a solvent-free melt electrospinningning technique has been actively developed [26; 27]. During melt electrospinningning, because the spinning solution has low conductivity and high viscosity, the diameter of the fiber is larger than that of fibers manufactured by solution electrospinningning [28; 29; 30]. According to recently published papers on electrospinningning, despite attempting to reduce the diameter of the fiber as much as possible in the case of melt electrospinningning, the realistic limit is from two to several tens of micrometers [26]. However, in the case of solution electrospinningning, it is possible to reduce the fiber diameter to several tens of nanometers [31; 32], where the diameter of the fibers produced by the two methods differs significantly from approximately 50 to 1000 times. Even in the case of PCL, owing to its hydrophobicity, high viscosity, and low conductivity, the desired fiber diameter (e.g., tens of nanometers) cannot be achieved by melt electrospinningning [33]. To solve the problem of fiber diameter limitation in melt electrospinningning, a method of increasing the electrical conductivity and lowering the viscosity by mixing a low-molecular-weight polar additive with a spinning polymer solution was proposed [34; 35; 36].