Fatigue behaviour of titanium scaffolds with hierarchical porosity produced by material extrusion additive manufacturing

Metallic porous structures (scaffolds) produced by additive manufacturing represent an important class of personalised implants used in load-bearing orthopaedic applications. As such, their fatigue performance must be excellent to prevent the need for revision surgery. This paper provides insight into the high-cycle fatigue behaviour of novel titanium scaffolds with hierarchical porosity and properties comparable to those of human bone that were produced by direct ink writing (DIW) and tested under cyclic loading typical for bone implants. Opposite to traditional expectations in the field of metal fatigue, scaffolds produced with an open intrastrand pore network (14.3%) endured nearly an order of magnitude more cycles than those with relatively compact strands (5.9%) and their normalized fatigue strength (62% of their yield strength in one milion cycles) was competitive with many current titanium scaffolds produced by other additive manufacturing technologies. Improved fatigue performance was related to fatigue crack growth shielding effects that reduced the crack growth rate, prolonged the crack path, and increased energy absorption. Consequently, these novel hierarchically porous titanium structures prepared by DIW can serve safely under cyclic loading conditions, and at the same time can provide multiple open porosity-related functionalities in advanced biomedicine and other industrial sectors.