An Investigation of the Effects of Interfaces on the Fracture Resistance of 3D Printed Biopolymer Nanocomposites

Abstract

Biopolymer-based bone-inspired nanocomposites are potential alternatives to conventional allografts for reconstruction of segmental bone defects if engineered to be mechanically competent and osteoconductive. The high surfaces area to volume fraction of nanoparticles contributes to enhance the mechanical properties and cell-material interaction of bone-inspired nanocomposites. Nanocomposites prepared by dispersing appropriate volume fractions of nanohydroxyapatite (nHA) into a resorbable/degradable biopolymer resin matrix can mimic the inorganic and organic phases of bone composition, respectively. Such nanocomposites mixed with relevant photoinitiator can be used as 3D printing feedstock for fabricating patient specific synthetic grafts. Direct ink writing (DIW) is an effective material extrusion 3D printing method that offers flexibility to fabricate complex parts using diverse materials and programable deposition of the extruded feedstock (raster) allows the user to manipulate the mechanical properties of a printed part. Free radical polymerization of the nanocomposite matrix upon exposure to ultraviolet (UV) light of appropriate intensity cures the deposited raster during DIW printing and also bonds the newly deposited raster with a previously cured raster. Fracture resistance is an important mechanical attribute for bone substitutes in order to avoid catastrophic failure while enduring physiological loading after defect reconstruction. Natural bone has acquired remarkable fracture resistance and mechanical properties through its hierarchically organized microstructure. Mimicry of such microstructure is a novel approach in 3D printing to enhance the mechanical properties of the printed structures. This thesis reports upon an experimental investigation of photocurable bone-inspired nanocomposite biomaterials towards the goal of achieving robust DIW-printed structures. The aim was to enhance fracture resistance of the structures fabricated using these nanocomposites. The goal was achieved by proposing and testing approaches inspired by bone. Nanocomposite rasters were deposited and simultaneously UV cured in concentric layers on a rotating mandrel bed of a custom designed and built DIW printer. Multilayer nanocomposite microstructures were achieved partially mimicking the microstructure of lamellar bone. Free radical polymerization of the nanocomposite rasters resulted in detectable interfaces in the printed microstructures because of differences in crosslink density. Each printed microstructure revealed distinct morphology of interfaces. The contributions of these interfaces and the resulting microstructures on mechanical properties and fracture resistance of the nanocomposites were further evaluated with other printed microstructure configurations. The printed anisotropic nanocomposite microstructures showed higher fracture resistance than the isotropic cast control with marginal reduction in flexural strength and modulus. Fracture testing results indicated that weak interfaces in the printed microstructures dissipated a portion of mechanical energy and contributed towards enhancing the fracture resistance of nanocomposite, especially crack stability. Fracture resistance of these nanocomposites can be tuned by altering the morphology of the interfaces and therefore the microstructure using DIW printing. Crosslink density significantly contributes to mechanical properties of UV curable resins. In another approach, crosslink density of nanocomposite matrix compositions was altered by changing composition and additional functionalization. Biopolymer functionalization improved the crosslink density of the nanocomposites and exhibited flexural properties in the recommended flexural property range of bone cement according to ISO-5833 standard. Higher crosslink density of functionalized biopolymer improved resistance to crack growth initiation but induced brittle fracture behaviour. Contrarily, the addition of the functional oligomer (tri-glycerol diacrylate- TGDA) to nonfunctionalized biopolymer matrix functioned as a plasticizer in the crosslinked network of biopolymer and enhanced the crack growth resistance of 3D printed nanocomposite by three (3) folds. The added oligomer also contributed to enhance the shape holding and morphology of interfaces in both functionalized and nonfunctionalized biopolymer nanocomposites. Alteration to crosslink density of nanocomposite matrix significantly influenced the mechanical properties of the interfaces and fracture resistance of DIW printed structures. Finally, in an effort to further enhance the fracture resistance, microstructures were printed using coextrusion of functionalized and non-functionalized biopolymer nanocomposites, principally to organize discrete mechanical phases in the microstructures in addition to the preexisting interfaces. The combination of functionalized and non-functionalized biopolymer nanocomposites with novel coextrusion printing significantly improved the fracture resistance of brittle functionalized biopolymer nanocomposites from single point fracture toughness behaviour to rising resistance curve behaviour. High magnification images of the fractured surfaces indicated that the plastic deformation at the softer nanocomposite phases in the coextruded microstructures dissipated mechanical energy and enhanced the fracture resistance. However, interfaces were not detected at the intersection wall of core-shell in coextruded raster. The custom-built mandrel bed DIW printer (SkelePrint) along with its tailored modifications, demonstrates strong potential for fabricating complex bioinspired concentric-layer structures with functionally graded properties. The findings from this thesis provide key insights into the role of interfacial bonding in DIW-printed structures and its influence on mechanical performance of printed structure. These findings will foster a path for designing robust, bone-mimicking nanocomposite grafts with tunable mechanical properties, advancing their applicability in bone tissue engineering.