3D Printable Ethylene- <i>co</i> -vinyl Acetate-Hydroxyapatite Composites for Bone Substitute Applications

Over the years, 3D printing has become a multidisciplinary research hotspot and state-of-the-art technology for developing bioinspired structures with intricate geometry, mechanical robustness and verified designs. However, the extrusion complexity of thermoplastic elastomeric filaments makes it challenging to design complex shapes in filament-based extrusion 3D printing. Herein, the paucity of low-modulus ethylene-<i>co</i>-vinyl acetate (EVA) polymer for the fabrication of bone tissue mimetic scaffolds was addressed by compounding with hydroxyapatite (HAP) and the effects of HAP incorporation on extrudability, printability, mechanical properties and osteoblast-material interactions were studied. The systematic optimization of printability and printing parameters enabled successful 3D printing of composite scaffolds with controlled deposition, pore geometry and architecture using a pellet-extrusion 3D printer. The die swell, unstable extrudate deposition and warpage of the EVA polymer melt subsided upon HAP addition. Confocal Raman microscopy and scanning electron microscopy (SEM) confirmed the uniform dispersion of HAP in EVA matrix, necessary to yield stable extrusion of the polymer melt. Dynamic mechanical analysis (DMA) revealed a 5-fold increase in storage modulus as well as a shift in Tg of the composites from -13°C to -9.8°C for 40 vol % HAP, confirming the possible polymer-HAP interactions. Biocompatibility studies demonstrated robust viability, proliferation and cellular integrity, especially in scaffolds with 40 vol % HAP. Moreover, F-actin staining of MG-63 cells revealed expanded cell pseudopods distributed evenly across the scaffold surface with a polygonal spreading pattern, confirming the cell adhesion and proliferation conducive for osteogenesis on the composite scaffolds. Osteogenic differentiation, as evidenced by ALP activity and Alizarin red S staining, indicated statistically higher levels of osteogenic-related factors and mineralization in composite scaffolds relative to neat EVA. These primary findings collectively support that the EVA-HAP composite, especially with 40 vol % HAP loading, provides a suitable microenvironment for osteoblast activities and is expected to promote bone tissue formation.