## 1 Introduction  
The thermoplastic polymer polyetheretherketone (PEEK) is a member of the polyaryletherketone (PAEK) polymer family, with remarkable mechanical and physicochemical properties similar to natural bone [1, 2, 3, 4, 5]. Three decades ago, since confirming the biocompatibility of PAEK polymers, this polymeric material has been increasingly used as a biomaterial for medical implants, especially in orthopedic and spinal applications [1, 2, 3, 4, 5]. PAEK was commercialized in the industry in the 1980s and was offered as a biomaterial for implants in 1998 [6, 7, 8]. This unique material was studied and investigated for years in different fields of medicine, and can be used as patient-specific implants [4]. Properties possessed by PEEK materials--such as excellent biocompatibility, radiolucency, durability, strength, light weight, mechanical properties resembling human bone, cortical bone-like young'smodulus, and chemical and thermal resistance--have shown the potential such material as an alternative to conventional materials, considering titanium as a gold standard [4; 9; 10; 11]. Additive manufacturing (AM), commonly known as three-dimensional (3D) printing technology has provided an excellent possibility creating complex structures in PEEK biomaterial [4; 11]. AM of polymeric material can be conducted via two main printing technologies, such as powder bed fusion and material-extrusion (MEX) [12; 13; 14; 15; 16; 17; 18; 19]. According to the study of [PERSON] et al. [20], the MEX technology allows the manufacturing of perfectly fitted customized patients-specific implants (PSIs). Moreover, the possibility of preoperative planning, shorter surgery time, and cost reduction are other benefits of integrating MEX technology at the point-of-care [11; 21; 22]. Commercially formulating PEEK into filaments for MEX printing is relatively new. High melting temperature (over 340 \({}^{\circ}\)C) and thermal gradients make the fabrication of high-performance PEEK parts challenging [10; 22]. The studies of [PERSON] et al. [4] and [PERSON] et al. [22] showed that high melting temperatures, fast crystallization kinetics, and significant dimensional shrinkage when cooled might generate cavities inside the printed parts, which degrade the mechanical properties, printing geometry, surface roughness, and reliability of PEEK 3D-printed parts. In the MEX method, it is essential to manage the temperature, as the strength of interlayer bonding, crystallinity of the polymer, and the deformation of printed parts are related to the heating and cooling rates [23; 24; 25; 26; 27; 28]. Another notable barrier of implementation of MEX printing is the anisotropic mechanical characteristics of the 3D-printed parts, i.e., the mechanical strength between layers (in either the build-direction or z-direction) is weaker than the mechanical strength in the x- and y-direction. This anisotropic behavior often leads to layer delamination, which usually leads to brittle fracture properties. Some studies showed that other printing parameters, such as speed and orientation of printing and the temperature of the nozzle, bed, chamber, and drying protocol of PEEK filaments can also influence the properties of printed PEEK parts [22; 24; 29; 30; 31; 32; 33]. [PERSON] et al. [34] looked into the relationship between different thermal conditions and crystallinity as well as mechanical properties. Their findings revealed that ambient and nozzle temperatures have a significant impact on the degree of crystallinity and tensile properties. [PERSON] et al. [35] performed tensile tests on horizontally printed PEEK dog bone samples and suggested that an optimal tensile strength can be achieved for annealed parts post-printing. However, we would like to state that the majority of these studies either tested the mechanical properties of horizontally printed specimens or compared the mechanical properties of horizontally and vertically printed specimens.