Development of Hdpe Composite for Biomedical Application Using 3d Printing

Abstract

Bioactive Glass (BAG) is a biomaterial well known to interact with physiological fluids to form strong bonds with hard (bone) and soft (muscle, tendons, ligaments, etc.) tissue. BAG tends to release ions, prompting amicable cellular response and formation of bone. The inherent brittleness of BAG inhibits its direct use in load-bearing orthopaedic applications. A viable alternative is to develop polymer-based BAG composites. The most affordable way to develop such intricate orthopaedic polymeric implants is by fused filament fabrication (FFF) based on material extrusion additive manufacturing (MEAM). FFF is a 3D printing (3DP) technique that deposits molten particulate reinforced thermoplastics through a heated liquefier and a nozzle. FFF process is one of the enablers of Industry 4.0 in orthopaedics because of its ability to produce customized implants in both lower time and cost. The initial part of the present work aims to develop high density polyethylene (HDPE) reinforced with BAG (H/BAG) composites as feedstock for the FFF process. HDPE is the bio-inert constituent responsible for structural integrity, and BAG is the bioactive constituent responsible for biological interactions. Additionally, neat HDPE (H) filament feedstock is developed to evaluate the baseline properties. The degradation behaviour of as received HDPE granules (HG) was evaluated to find the operable temperature ranges for composite development and filament extrusion. The composite blends are developed using the melt compounding technique. Micrography and elemental mapping of the composite blends reveal uniform distribution of BAG in HDPE matrix without any agglomeration. Neat H and composite H/BAG filament feedstock are extruded using a single screw extruder. Diameter, density, void content, thermal stability, thermal properties, and tensile properties of the developed filament feedstock are evaluated. The diameter of the feedstock is found to be within the permissible limit favourableable for 3DP. Density and void content increase with BAG addition of 5 (H5) , 10 (H10), and 20 (H20) wt.% respectively. Furthermore, the thermal stability of the filament feedstock is marginally improved when compared with HG. Crystallization temperature (Tc) and tensile modulus increase with BAG addition, although melting temperature (Tm) remains unaffected. Crystallinity and tensile strength decrease with BAG addition. The stiffer composite filament feedstock is effortlessly spooled and stored. Additionally, the filaments resist the forces exerted at the printer head and maintain the circular cross- section without any slippage during printing at elevated material flow rates. A deeper understanding of the process parameters influencing the FFF process is essential to print the newly developed filament feedstock. The second part of the present work aims to fabricate stable, defect-free printed parts by carefully identifying the influencing print parameters and their effect on print quality. The effect of BAG on the thermal and melt behaviour of developed semicrystalline feedstock is evaluated, and print parameters are manipulated effectively to achieve high quality prints. From both calorimetric and diffraction analysis, the crystallinity of the prints is found to be decreasing with BAG addition. The printed samples exhibit higher crystallinity than their respective filaments. The complex viscosity of the prints increases with BAG addition, which denotes the increased resistance to the material flow of the composites. The coefficient of thermal expansion (CTE) is inversely proportional to the prints' dimensional stability. As CTE decreases with BAG addition, the composite prints are observed to be dimensionally stable. Optical warpage and shrinkage analysis show the reduction in warpage with BAG addition. Furthermore, optical analysis confirms that the combined warpage and shrinkage of the prints lie in a narrow range. This indicates the effective selection of material flow rate despite having different melt viscosity. Storage and loss modulus of the composites increases with BAG addition. The samples are printed using styrene-ethylene-butylene-styrene (SEBS) adhesive, which exhibited adequate adhesion during printing and effortless detachment after printing. The influencing process parameters such as printing temperature (Tp), bed or substrate temperature (Tb), build envelope temperature (Te), printing speed, and material flow rate are found to be affected by feedstock properties such as crystallinity, melting temperature, crystallization temperature, and viscosity. The resulting print induced defects are observed to be warpage, shrinkage, inept diffusion, delamination, voids, nozzle clogging, underfill and overfill. Adhesion is improved by enhancing Tb (value closer to crystallization temperature), thereby controlling the warpage and peeling of the initial first layer. Uniform temperature distribution is achieved by increasing Te, thereby nullifying delamination of prints because of homogeneous thermal stress distribution. Print induced defects such as inept diffusion and voids are eliminated by increasing Tp and material flow rate. Furthermore, all the completed prints are left inside the print chamber and allowed to cool by natural convection, thereby reducing thermally induced stress. It is observed that the prints fabricated at elevated temperatures are dimensionally stable. Process parameters are optimized to obtain stable, defect-free, dense prints. The mechanical behaviour of the printed samples is evaluated as the final part of the present work. The effect of BAG addition on tensile, flexural, static, and quasi-static compression, impact, and DMA behaviour of the prints is investigated. Structure- property correlation of the properties is explained with extensive micrographic images to understand the deformation mechanism. Tensile modulus is observed to be increasing with BAG addition, and ultimate tensile strength is observed to be decreasing with BAG addition. The fracture strength of the samples increases with BAG addition. Printed samples exhibit higher tensile modulus and ultimate tensile strength than their respective filament. The printed neat H samples display a 26% higher modulus than injection moulded samples. The deformation of H/BAG composites under tensile load is observed in three stages. Micrographs show various features like voids, dimples, fibrils, and coalescence. The flexural modulus and strength of the composites are higher than the neat H samples. Stiffer BAG acts as stress concentrators, thereby increasing flexural strength. The printed neat H samples show 1.85 times higher flexural modulus when compared with injection moulded counterparts. All the compression samples exhibit the same levels of strain and similar deformation features irrespective of strain rate (static and quasi-static). Stress-strain curves are divided into three distinct regions to understand the deformation mechanism. The fractured surface shows brittle surface, wrinkles, fibrils (buckled, elongated, ruptured, peeled), voids, crazes, cracks, and ductile bands. Irrespective of strain rate (static and quasi-static), compression strength and modulus increase with BAG addition. Energy absorption of the developed composites improved with BAG addition. Yield strain is observed to be decreasing with BAG addition. The impact properties of the prints are studied. The impact strength of the samples decreases with BAG addition. Micrographs show that the insufficient adhesion between HDPE matrix and BAG results in swift propagation of cracks. Localized plastic deformation followed by void formation is also observed from the micrographs. Viscoelastic properties of the developed feedstock are evaluated by dynamic mechanical analysis. Storage and loss modulus is found to be increasing with BAG addition. Storage modulus is observed to be decreasing with sweep temperature. Loss modulus increases with temperature until  - relaxation of the HDPE matrix and then starts decreasing. The changes in storage modulus, loss modulus, and loss factor with increase in temperature below the melting point indicate the disappearance of crystal phases that restrict the mobility of the amorphous phase. This emphasizes the necessity of considering the temperature-dependent viscoelastic properties while designing the H/BAG implants. At the clinical temperature of 37 C, storage modulus, loss modulus, and loss factor are observed to increase with BAG addition highlighting the enhanced ability to dissipate energy. Selective mechanical properties of the developed feedstock are compared with other bioactive thermoplastic composites processed via the traditional manufacturing processes. Furthermore, the obtained results are compared with trabecular and cortical bone properties. The obtained results illustrate strong potential of FFF process to fabricate customized orthopaedic implants to mimic human bone.