## 1 Introduction  
Tissue engineering is an alternative to bone grafting procedures, which aims to integrate engineering and life science criteria to develop reliable and physiologically acceptable solutions for bone regeneration [1]. From the tissue engineering point of view, an ideal bone graft must meet strict requirements regarding its biological, physical and mechanical characteristics. An ideal bone graft must be osteeconductive and osteoinductive and also gradually be degraded in the body fluid providing adequate space for the new bone to grow [2,3]. Such biological properties can be provided using calcium phosphate (CaP) bioceramics in the form of highly porous scaffolds with suitable architecture [4]. The scaffolds must fulfill special physical characteristics regarding porosity level, and pore and interpore channel sizes to supply proper cell attachment and vascularization [1,5, 6, 7, 8, 9]. Besides, surface topography, as well as surface charge and crystallinity, are also claimed to play key roles in guiding protein adsorption and consequently cell adhesion [10]. An ideal bone graft must also possess enough mechanical strength to structurally support the newly formed tissue. However, high mechanical properties are generally in contrast with the high biological performance of an implant [11]. In other words, improved biological properties of highly porous structures take place along with a decrease in their mechanical integrity [12,13]. One solution to accomplish such contradictory properties is to design the scaffold as a functionally graded structure (FGM) made from layers with different porosity levels. FGMs are defined as structures that show compositional or microstructural transition along with at least one direction designed to fulfill two or more functions simultaneously [14]. Natural bone is an example of a FGM structure in which the variation of the pore sizes and distributions across the transverse section provides a graded body. The fabrication of artificial bone grafts can be inspired from this morphological variation and be designed in multi-layer structures composed of different porous and dense layers, to fulfill both biological and mechanical requirements. Production of FGM structures from CaP bioceramics has been reported in several works where the FGM structures were produced in two [15, 16, 17, 18] or more layers [19], with improved mechanical and biological properties compared to single-layer materials. However, in all cases, the structures were produced using conventional ceramic processing methods such as freeze casting [15, 16, 18, 20], pressing [17] and gelcasting [21, 22] with limited control on the scaffold architecture [14]. An alternative to conventional ceramic processing methods is additive manufacturing (AM). AM is a novel manufacturing method that is inherently suitable for the production of complex parts like biomedical scaffolds. Contrary to conventional methods, AM makes it possible to precisely control the scaffold architecture, pore size and pore network [14]. Robocasting is an AM technique particularly suitable for the fabrication of ceramic parts in which the feed material is a highly concentrated ceramic paste. The paste is deposited continuously through a nozzle in the form of filaments and the whole part is fabricated from these filaments [23]. A robotic arm controls the displacement of the nozzle in accordance with a computer-aided design (CAD) model. Production of complex structures from different ceramics such as alumina [24, 25, 26], silicon carbide [27, 28], biofilases [29, 30, 31], and electroceramics [32] has been reported in the literature. Due to the high green density, high fired densities and therefore high mechanical properties can be achieved after sintering [33, 34]. Production of CaP structures by robocasting technique was also reported in several papers. These studies range from the fabrication of simple geometries to customized complex structures designed based on the individual patient's data [35, 36, 37, 38, 39, 40, 41].