Characterization of high-strength porous polyparaphenylene scaffolds for use as a potential orthopedic biomaterial

The purpose of this study is to serve as a preliminary investigation into the potential of polyparaphenylene (PPP) as an orthopedic biomaterial, and develop and mechanically characterize a porous PPP scaffold. PPPs are advertised as the world's strongest and stiffest thermoplastics exhibiting strength values 3 to 10 times higher and an elastic modulus nearly an order of magnitude higher than traditional polymers such as poly(methyl methacrylate) (PMMA), polycaprolactone (PCL), ultra-high molecular weight polyethylene (UHMWPE), and polyurethane (PU) and is significantly stronger and stiffer than polyetheretherketone (PEEK). Although PPP exhibits exceptional mechanical properties it is not currently used in any biomedical applications. By utilizing PPP we can overcome the mechanical limitations of traditional porous polymeric scaffolds since the outstanding stiffness of PPP allows for a highly porous structure appropriate for osteointegration that can match the stiffness of bone, while maintaining suitable mechanical properties for load-bearing orthopedic applications. Samples were manufactured by powder-press sintering; porous samples were developed by subsequent particle leaching. Nuclear Magnetic Resonance (NMR) was used to determine the polymer's chemical structure. Minimum Essential Medium (MEM) elution assay with fibroblasts confirmed that the PPP was non-toxic. Solid tensile PPP specimens along with samples from several other polymers often used for orthopedic applications were soaked in phosphate buffered saline for one hour, one day, one week, and one month, and then elongated to failure. Results showed that water absorption of the PPP plateaued after approximately one week at values of 0.66 wt% due in part to the hydrophobicity of the polymer. Water absorption did not affect elastic modulus (5.0 GPa), yield strength (141 MPa), fracture strength (120 MPa) or strain-to-failure (17%) more than one standard deviation. In porous samples the pore volume fraction was systematically varied from 50-90 vol% for small (150--250 microm) and large (420--500 microm) pore sizes, as indicated by previous studies for osteointegration. Both modulus and strength decreased with increasing porosity and matched well with theoretical predictions. Pore size did not have a clear influence the mechanical behavior of the scaffolds. Porous scaffolds showed a significant decrease in strain-to-failure (<4%) under tensile loading and experienced linear elasticity, plastic deformation, and densification under compressive loading. Relative to other polymers used for load-bearing biomedical applications, PPP displays a promising combination of biocompatibility, exceptional mechanical properties that are virtually unaffected by soaking, and low absorption. In addition, it is capable of being manufactured into a porous scaffold with pore sizes tailored for osteointegration and porosities matching the stiffness of trabecular bone.