Monotonic and cyclic loading behavior of porous scaffolds made from poly(para-phenylene) for orthopedic applications

Porous poly(para-phenylene) (PPP) scaffolds have tremendous potential as an orthopedic biomaterial; however, the underlying mechanisms controlling the monotonic and cyclic behavior are poorly understood. The purpose of this study was to develop a method to integrate micro-computed tomography (μCT), finite-element analysis (FEA), and experimental results to uncover the relationships between the porous structure and mechanical behavior. The μCT images were taken from porous PPP scaffolds with a porosity of 75 vol% and pore size distribution between 420 and 500 µm. Representative sections of the image were segmented and converted into finite-element meshes. It was shown through FEA that localized stresses within the microstructure were approximately 100 times higher than the applied global stress during the linear loading regime. Experimental analysis revealed the S-N fatigue curves for fully dense and porous PPP samples were parallel on log-log plots, with the endurance limit for porous samples in tension being approximately 100 times lower than their solid PPP counterparts (0.3-35 MPa) due to the extreme stress concentrations caused by the porous microarchitecture. The endurance limit for porous samples in compression was much higher than in tension (1.60 MPa). Through optical, laser-scanning, and scanning-electron microscopy it was found that porous tensile samples failed under Mode I fracture in both monotonic and cyclic loading. By comparison, porous compressive samples failed via strut buckling/pore collapse monotonically and by shearing fracture during cyclic loading. Monotonic loading showed that deformation behavior was strongly correlated with pore volume fraction, matching foam theory well; however, fatigue behavior was much more sensitive to local stresses believed to cause crack nucleation.