Design and Fabrication of 3D printed Scaffolds with a Mechanical Strength Comparable to Cortical Bone to Repair Large Bone Defects

Abstract

A challenge in regenerating large bone defects under load is to create scaffolds with large and interconnected pores while providing a compressive strength comparable to cortical bone (100-150 MPa). Here we design a novel hexagonal architecture for a glass-ceramic scaffold to fabricate an anisotropic, highly porous three dimensional scaffolds with a compressive strength of 110 MPa. Scaffolds with hexagonal design demonstrated a high fatigue resistance (1,000,000 cycles at 1-10 MPa compressive cyclic load), failure reliability and flexural strength (30 MPa) compared with those for conventional architecture. The obtained strength is 150 times greater than values reported for polymeric and composite scaffolds and 5 times greater than reported values for ceramic and glass scaffolds at similar porosity. These scaffolds open avenues for treatment of load bearing bone defects in orthopaedic, dental and maxillofacial applications.

Figure 1. Computer aided design models (left column) and SEM images of examined scaffolds (Scale bars: 500 μm unless stated otherwise).

(a) Hexagonal, (b) Curved, (c) Rectangular and (d)Zigzag design. (e) SEM images of fracture surface of a Sr-HT-Gahnite scaffold prepared by direct ink writing (perpendicular to the deposition plane, z direction), revealing the solid struts without any microporosity in the microstructure. ( f ) The microstructure of Sr-HT-Gahnite scaffolds consisting of three phases of (1) Sr-HT grains, (2) ZnAl2O4 crystals and (3) a glass phase between the grains. *Pore sizes calculated from SEM images at XY direction from the average of maximum and minimum distances between layers at intersections (Refer to supplementary table). +Porosity of scaffolds was calculated by Archimedes and micro–computed tomography (μ-CT) and average numbers were rounded to reported values (Refer to supplementary table). (g) Compressive strength of Sr-HT-Gahnite scaffolds with distinct pore geometries vs porosity. Comparison with compiled data from literature studies for polymer, composite, bioactive ceramic and glass scaffolds at porosities between 50 and 95%. (h) Flexural strength of Sr-HT-Gahnite scaffolds with hydroxyapatite and bioactive glass scaffolds. Each style of point corresponds to a different literature value. Standard deviations from average values are reported in Table 1 at the supplementary information.

Figure 2. Weibull plots of compressive strength, Weibull modulus (m) and Weibull scale parameter (σ_o) for Sr-HT-Gahnite scaffolds with (A) ~70% and (B) ~60% porosity.

Purple area indicates the Weibull modulus for porous hydroxyapatites and pink area is that for porous bioactive glasses.

Figure 3. Fatigue life (average number of cycles to failure) of Sr-HT-Gahnite scaffolds under cyclic compressive stress at 60 and 70% porosity.

(*Significant difference between groups, p < 0.05). The numbers on top of each bar indicates the number of scaffolds that survived 106 cycles after finishing the test.

Figure 4. Compressive strength–density Ashby chart showing the compressive strength of Sr-HT-Gahnite scaffolds fabricated by direct ink writing compared with other materials at various densities.