## 1 Introduction  
Additive manufacturing (AM), also referred to as 3D printing (3 DP), is a technology that is used to fabricate complex structures from 3D model data [1]. For this reason, 3D printing has been widely used in various fields, including machinery, architecture, automobiles, and electronics [1; 2; 3]. In recent years, the 3 DP of biomaterials or biocompatible materials has been actively investigated, including in biology and biomedical applications [4; 5; 6]. Accordingly, various biomaterials or biocompatible materials that are suitable for 3D printing have been developed [7; 8]. For printing these biocompatible materials or biomaterials, several types of 3D printer have been used, such as fused deposition modeling (FDM), stereolithography (SLA), and digital light processing (DLP). However, there is a limited choice of biocompatible materials available for 3 DP, and it is even more difficult to find suitable materials for FDM 3D printing, the most widely used 3D printing method. Thus, development of biocompatible 3 DP filament for FDM 3D printers can promote and expand the application of FDM 3 DP for bioengineering and biomedical research. Specifically, this method can be useful for the preparation of personalized bone scaffolds and artificial bone testbeds. Used in tissue engineering, a bone scaffold promotes bone cell migration, which is important for vascularization and bone in-growth, and also provides temporary mechanical support to fractured or injured sites [9; 10]. To be adoptable as a bone scaffold, however, the material must be self-degradable without producing toxic byproducts [9]. For bone scaffold fabrication, composites of hydroxyapatite (HA) and polycaprolactone (PCL) have been widely used. HA has a similar composition to native bone and has been used in various biomedical applications, for example, as a bone scaffold, as a filler, as an implant coating and as a drug delivery system, due to its osteoconductive and biocompatible properties [11; 12; 13]. HA is an essential material for bone scaffolds, as it shares 70% of the native bone's composition; however, its brittle nature limits its use as a scaffold [12; 13]. PCL is also a widely used biocompatible and biodegradable polymer [13], which makes it similarly suitable for various biomedical applications, such as bone scaffolds or long-term implantable drug delivery systems. [14]. Thus, PCL is generally used together with HA to provide the appropriate mechanical strength required for a scaffold [13; 15; 16]. For the fabrication of bone scaffolds using a PCL/HA composite, various fabrication methods, such as salt leaching [13], thermally induced phase-separation (TIPS) [15], and 3D printing [17; 18] have been employed. Briefly, salt leaching is a method of fabricating a porous structure by pouring composites mixed with NaCl particles into a mold to make a shape, then rinsing it in water to remove the NaCl. The TIPS process is a method of removing the solvent by freezing the composite solution into a gel and soaking it in excess cold water. Both methods are suitable for fabricating well-defined porous structures, but it is difficult to readily create customized structures. Among these fabrication methods, 3D printing is the best at facilitating the formation of complex structures, as well as customized scaffold preparation. Thus, several 3D-printed scaffolds have been reported over the years [11; 16; 19; 20; 21; 22; 23; 24; 25; 26; 27]. However, the aforementioned FDM (fused deposition modeling) 3 DP method is rarely used to fabricate scaffolds, even though it offers several advantages over other 3 DP methods, such as versatility, convenience, and accessibility [4; 5]. It is because there are no PCL/HA composite filaments available for FDM 3 DP, to the best of our knowledge. In this work, we report on the fabrication of PCL/HA composite filaments for FDM 3 DP and demonstrate the mechanical and electrical properties of the filaments and the printed specimens.