## Materials and Methods  
### Robocasting  
#### 2.1.1 Design of CAD Models  
Three different types of scaffolds were designed for fabrication: lattice, dense and bi-layer FGM structures. The digital 3D models of the objects were obtained through CAD software (AutoCAD 2018, Autodesk, Inc., Mill Valley, CA, USA) as can be seen in Figure 1. The designs were then converted into stereolithography files (STL) which were subsequently sliced (Cura 3.6, Ultimaker, Geldermalsen, The Netherlands) into two-dimensional (2-D) layers in a virtual environment along with printing instructions in three different G-code files. The G-code files were translated by a 3D printing client software instructing the 3D printer to achieve computer-controlled layer-by-layer extrusion of the paste.  
Figure 1: Schematic illustration of the 3D CAD designs for robocasting; lattice (**A**), dense (**B**) and bi-layer (**C**) models.  
The lattice porous part was a network of struts comprising a tetragonal geometry with the dimensions of 9.6 \(\times\) 9.6 \(\times\) 7.3 mm\({}^{3}\). It was made of parallel linear rods aligned in the \(x\)- and \(y\)-axis in such a way that subsequent layers oriented at right angles. The part consisted of 24 layers and each layer contained 12 parallel struts with a diameter of 400 \(\upmu\)m. The parallel rods in each layer were separated equal to the struts sizes resulting in the center-to-center rod spacing of 800 \(\upmu\)m. To provide appropriate consolidation between parallel layers and maintain the integrity of the whole part, the spacing in the \(z\)-axis was defined as 300 \(\upmu\)m. As several parameters such as paste formulation, the geometry of the structure, slicing procedure and printing parameters affect the robocasting outcome, in order to focus on the paste formulation and minimize the effects of other parameters, the cubic geometry was selected for the lattice structure to have the simplest possible raster pattern consisting of parallel straight rods without any circular travels or sharp edges.  
The dense part was a monolithic cylinder with a diameter of 10 mm and a height of 15 mm. With the infill of 100%, the rod spacing between the filaments was defined as equal to d/1.2, where d is the diameter of the nozzle. The rod spacing was found sufficiently small to cause the filaments to completely squeeze together during printing, but not too small to cause overfill of the area being printed [27].  
The bi-layer structure was a combination of dense and lattice parts designed as an FGM structure mimicking the natural bone assembly. A radial gradient was achieved by designing the porous part around a dense cylindrical core. The porous part was composed of successive repetitions of two different layers: a layer of radial rods and another layer consisting of concentric rings with different diameters in such a way that the porosity increased from the center to the periphery. Each rod in the lattice part was composed of two adjacent 400 \(\upmu\)m filaments printed next to each other forming struts of 800 \(\upmu\)m thickness. The final design had a diameter of 20 mm and a height of 19.6 mm. The central dense core diameter was 8.8 mm.