# Computed Tomography-Driven Design and Fused Filament Fabrication of Graded Density Bone Scaffolds
## 3 Test Case and Discussion
### Preliminary Process Calibration
Potential approaches include using filtering functions to smooth the data or applying functions to enforce minimum and maximum limits. These adjustments can help fine-tune the scaffold's graded design, ensuring the desired properties and characteristics are achieved. Furthermore, while it is feasible to obtain a low-density structure by controlling the extrusion width and cell size, and optionally the layer height, 100% dense structures are not always attainable. As highlighted in the literature [56; 57], there is a delay in deposition; the actual change in the extruded material is offset relative to the corresponding G-code command. This delay arises from complex interactions involving the material properties, material flow, heat transfer, and elasticity, which lead to discrepancies between the intended design and the printed part. Using a direct-drive extruder, as we did, decreases the presence of delay, since the distance between the motor and the heating system is reduced. Figure 12 illustrates this phenomenon in the case study. Figure 11: (**a**) An image of the 3D-printed femur segment. (**b**) A top-down view, with magnified insets showing (**c**) the central region and (**d**) an area near the segment’s outer boundary (higher relative density). Figure 12: Magnification of femur sample with the first 10 layers (top view). Furthermore, the effect becomes more evident depending on the order of the commands. Assuming a constant motion speed, transitioning from high to low extrusion rates may be more detrimental than the reverse. This is due to the higher backpressure that builds up in the nozzle during high extrusion rates. When the extrusion rate suddenly decreases, this residual backpressure causes excess material to be extruded, as the system seeks to relieve the pressure. As a result, this behavior limits the ability to create sharp transitions during extrusion. Rapid changes in the extrusion rate inherently introduce gradients, making it difficult to achieve discrete transitions without implementing additional strategies (such as retraction commands) that account for the material's behavior within the nozzle. Nevertheless, even when a slight shift in deposition occurs, the overall relative density of the printed part remains unchanged [42]. The design of lattice structures is constrained by the infill patterns available in each slicer software. These patterns are software-dependent, meaning that the options and implementation may vary across different platforms--even though many slicers (such as Slic3r, PrusaSlicer, Cura, and IceSL) offer similar standard infill types [58; 59]. When customizing infill patterns, some slicers such as Cura allow the customization of 2D patterns. While others, such as IceSL, offer 3D pattern capabilities [60], these are not always straightforward or may require workarounds [61]. Plugins and tools within CAD software also exist to generate G-code. One example is the Rhinoceros/Grasshopper add-on Silkworm, among many others, which utilizes curves and polylines as inputs. Alternatively, it should be possible to develop a custom slicer. The absence of a CAD model for the lattice structure limits its applicability in computer-aided engineering (CAE) analyses. This characteristic identifies the proposed method as CAM-oriented, as the actual design is embedded within the G-code and defined through the control of process parameters rather than explicit geometric modeling. Nevertheless, the volumetric model and homogenization approach can estimate the mechanical behavior of the graded lattice, by treating the bone as a continuum where the mechanical properties depend on the porosity, avoiding the complexities of modeling intricate lattice structures [62; 63; 64].