Toward Strong and Tough Glass and Ceramic Scaffolds for Bone Repair

Abstract

The need for implants to repair large bone defects is driving the development of porous synthetic scaffolds with the requisite mechanical strength and toughness in vivo. Recent developments in the use of design principles and novel fabrication technologies are paving the way to create synthetic scaffolds with promising potential for reconstituting bone in load-bearing sites. This article reviews the state of the art in the design and fabrication of bioactive glass and ceramic scaffolds that have improved mechanical properties for structural bone repair. Scaffolds with anisotropic and periodic structures can be prepared with compressive strengths comparable to human cortical bone (100-150 MPa), while scaffolds with an isotropic structure typically have strengths in the range of trabecular bone (2-12 MPa). However, the mechanical response of bioactive glass and ceramic scaffolds in multiple loading modes such as flexure and torsion - as well as their mechanical reliability, fracture toughness, and fatigue resistance - has received little attention. Inspired by the designs of natural materials such as cortical bone and nacre, glass-ceramic and inorganic/polymer composite scaffolds created with extrinsic toughening mechanisms are showing potential for both high strength and mechanical reliability. Future research should include improved designs that provide strong scaffolds with microstructures conducive to bone ingrowth, and evaluation of these scaffolds in large animal models for eventual translation into clinical applications.

Keywords: Bioactive glass and ceramics; bone tissue engineering; mechanical strength; porous scaffolds; reliability.

Figure 1

The prevailing toughening mechanisms in cortical bone.^[14]

Figure 2

Schematic representation of commonly applied methods for the fixation of segmental defects in large animal models. A) plate fixation, B) external fixator, and C) intramedullary nail.^[18]

Figure 3

Representative images of bioactive glass 13–93 scaffolds prepared by different techniques: (a) isotropic scaffold by a polymer foam replication technique; (b) anisotropic scaffold prepared by a freeze casting technique; (c) periodic scaffold by a direct-ink writing technique

Figure 4

(a) Compressive strength of bioactive glass scaffolds compiled from literature studies, and grouped based on their structures. Purple: isotropic scaffolds; green: anisotropic scaffolds; pink: peroidic scaffolds;^{[40, 41, 42, 44, 52, 62, 66, 70]} (b) Influence of porosity on the compressive strength of bioactive glass scaffolds. The black dotted line indicates a linear fitting by Rossi model,^[72] while red and blue dotted lines indicates the upper and lower theoretical strength predicted by Gibson and Ashby model.^[75]

Figure 5

(a) Compressive strength of bioactive ceramic scaffolds compiled from literature studies, and grouped based on their structures. Purple: isotropic scaffolds; green: anisotropic scaffolds; pink: peroidic scaffolds;^{[46, 47, 48, 49, 61, 63, 65, 78, 79, 80]} (b) Influence of porosity on the compressive strength of bioactive ceramic scaffolds The black dotted line indicates a linear fitting by Rossi model^[72], while red and blue dotted lines indicates the upper and lower theoretical strength predicted by Gibson and Ashby model.^[75]

Figure 6

Flexural strength of (a) bioactive glass, and (b) ceramic scaffolds compiled from literature studies, and grouped based on their structures.^{[40, 53, 79, 84, 85]} Purple: isotropic scaffolds; pink: peroidic scaffolds.

Figure 7

Tensile strength of isotropic bioactive ceramic scaffolds compiled from literature studies.^{[48, 87, 88]}

Figure 8

Ashby map of the damage tolerance (toughness versus strength) of materials. Ranges of fracture toughness versus yield strength are shown for oxide glasses, engineering ceramics, engineering polymers, engineering metals and metallic glasses. Yield-strength data shown for oxide glasses and ceramics represent ideal limits.^[99]

Figure 9

Toughening mechanisms observed in bioactive glass-ceramics: (a) SEM image of the Vickers indentation in a polished, unetched glass-ceramic; (b) crack bridging; (c) tortuous crack path; (d) crack deflection by the presence of crystalline phase.^[132]

Figure 10

Mechanical response and toughening mechanisms in the synthetic hybrid composites. (a) SEM image of the structure of nacre; (b) SEM image taken during an in situ R-curve measurement of a lamellar structure; (c) Bending stress-strain curves for the Al₂O₃/PMMA hybrid materials mimic those of nacre and show >1% inelastic deformation before failure; (d) exceptional toughness for crack growth, similar to that of natural composites, and display significant rising R-curve behavior.^[138]

Figure 11

On the preoperative radiograph a 4-cm-long gap of the proximal tibia is shown. At 2 months, bone callus formation around the implant was evident, but the radiolucent line of the bone-implant interface was still detectable in the lateral view. At 6 months, formation of extensive callus and peri-implant bone with a good integration between the implant and the tibia was evident. At 2.5 years follow-up, complete bone-implant integration with no evidence of implant fractures was detected. CT scan analysis at 7 years demonstrated complete healing of the gap, presence of a medullary channel within the implant, and persistence of new bone formation within the bioceramic scaffold pores. The HA ceramic was still present.^{[5, 7]}