Height-to-Diameter Ratio and Porosity Strongly Influence Bulk Compressive Mechanical Properties of 3D-Printed Polymer Scaffolds

Abstract

Although the architectural design parameters of 3D-printed polymer-based scaffolds-porosity, height-to-diameter (H/D) ratio and pore size-are significant determinants of their mechanical integrity, their impact has not been explicitly discussed when reporting bulk mechanical properties. Controlled architectures were designed by systematically varying porosity (30-75%, H/D ratio (0.5-2.0) and pore size (0.25-1.0 mm) and fabricated using fused filament fabrication technique. The influence of the three parameters on compressive mechanical properties-apparent elastic modulus E_app, bulk yield stress σ_y and yield strain ε_y-were investigated through a multiple linear regression analysis. H/D ratio and porosity exhibited strong influence on the mechanical behavior, resulting in variations in mean E_app of 60% and 95%, respectively. σ_y was comparatively less sensitive to H/D ratio over the range investigated in this study, with 15% variation in mean values. In contrast, porosity resulted in almost 100% variation in mean σ_y values. Pore size was not a significant factor for mechanical behavior, although it is a critical factor in the biological behavior of the scaffolds. Quantifying the influence of porosity, H/D ratio and pore size on bench-top tested bulk mechanical properties can help optimize the development of bone scaffolds from a biomechanical perspective.

Keywords: 3D printing; height:diameter ratio; mechanical properties; polymer scaffolds; pore size; porosity.

Figure 1

Scaffold test specimen geometrical design process to match theoretical values of porosity, heights and pore size: (A) the first layer was designed, the region of interest (ROI) of the struts that are created with a width (SW), length (SH) and a height (PH), as denoted by red lines. PS corresponds to the pore size and is equal to the pore height (PH). (B) A second layer is added by rotating the first one by 90° and placing it on top of it, a circumference with diameter (CD) is designed and everything outside it is removed producing (C). (D–F) The remained part is duplicated along the cylindrical principal axis (z-axis) as required for the specimen height. The final specimen geometry with length of 5, 10 or 20 mm was exported for 3D printing as a STL file.

Figure 2

Scaffold test specimen geometries: specimens with different inner architectures were created due to the combinations between pore size and porosity. (A) Representative specimen design of 10 mm height with a H/D ratio of 1.0, is shown for the different combinations of porosity and pore size. (B) Actual printed samples based on (A).

Figure 3

Qualitative evaluation of the accuracy of a printed specimen geometry versus theoretical CAD design: A comparison between the CAD model (grey) and the acquired volume with micro CT data (blue). (A) A cross-sectional plane at the midpoint along the horizontal plane showing the inner correlation; (B) along central vertical plane; (C) along the vertical plane with a small angle of rotation; and, (D) Isometric view of micro CT data (blue) and CAD model (grey) overlayed.

Figure 4

Mechanical properties of the scaffold test specimens: (A) Stress–strain curves showing the average and standard error for all the samples grouped by height. (B) Elastic modulus versus porosity for three specimen heights. (C) Yield strain versus porosity for three specimen heights. (D) Yield stress versus porosity for three specimen heights. Grey, shaded bands in (A) represent the standard error.

Figure 5

Standardized modulus (measured elastic Modulus E_p divided by the material modulus E₀) versus the porosity for three specimen heights. Data from the current study is overlayed with published curves [45,46,47,48,49,50,51,52].

Figure 6

Correlation between mechanical properties of the scaffold test specimens: (A) Yield Stress with respect to Apparent Elastic modulus and (B) Yield Strain with respect to Apparent Elastic modulus. Lines represent a linear fit with R² (%) being the coefficient of determination.

Figure 7

Mechanical analysis of the scaffold test specimens with respect to the H/D ratio: (A) Apparent Elastic modulus, (B) Yield Strain and (C) Yield Stress. Data are presented as notched box plots. Boxes represent the second and third quartile around the median, which is represented by the thick horizontal line within the block. Whiskers represent 100% of the data within each group, including outliers. Notches represent a 95% confidence interval (CI_notch) of the median and extend to [±1.58 × IQR/((n)^0.5)]. IQR = interquartile range between first to third quartile and “n” = number of non-missing observations within the group. Non-overlapping notches represent significant differences [60,61].