Development of polymer grafting technique onto the surface of additively manufactured Titanium substrate for improved hip arthroplasty performance
Creators: name: Ghosh, Subir
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400302
subject: Biomaterials

description: Total hip arthroplasty (THA) is one of the common surgical procedures, however, a significant number of the replacements fail permanently due to wear and lack of osseointegration of the implants. In spite of the success of medical procedure, there is not a single implant that can avoid aseptic loosening or prosthetic dislocation during its envisaged operating life. Regardless of tremendous acceptance of additively manufactured (AM) titanium alloys (Ti6Al4V) in the field of biomedical engineering, the high surface roughness due to partially-melted particles (fabricated in selective laser melting (SLM) process), limits their uses as hip implants. Therefore, this project aims to develop cartilage-mimicking poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC) grafting onto the surface of additively manufactured titanium implants, to improve surface properties, wear resistance and lubricating ability of the implant. <br> <br>Three different grafting techniques; ultraviolet (UV) irradiation and thermal heating both under normal atmosphere and UV irradiation under N2 gas atmosphere were applied. The energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) results evidenced the existence of continuous PMPC layer on the Ti6Al4V surface using the UV irradiation under a nitrogen atmosphere, due to the elimination of oxygen from the system. As indicated in the Fourier transform infrared spectroscopy (FTIR) results, one of the advantages of this technique is the presence of phosphorylcholine, mostly on the surface, which demonstrated that the polymer chains had been successfully anchored onto the surface of Ti6Al4V. Filmetric analysis identified the thickest layer of the grafted polymer for the surface grafted using UV irradiation under a nitrogen atmosphere. Therefore, this technique has been considered as optimal grafting technique considering surface morphology, polymer composition onto the top surfaces and film thickness of the grafted polymer. <br> <br>Three different monomer concentrations, 0.4 M, 0.6 M and 0.8 M were then examined to optimise the monomer concentration for grafting polymer onto the surface of AM Ti6Al4V. The nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC), results revealed that the monomer had been polymerised successfully. Samples grafted with 0.6 M monomer concentration showed more uniform surface and less surface roughness in comparison with other samples and untreated Ti6Al4V surfaces. 0.6 M monomer concentration was found to be the best option for grafting PMPC to the hip implant interfaces due to the improvement in surface morphology, roughness and polymerisation rate. Cell study was performed on the PMPC grafted surface to determine the biological response of PMPC layer. Scanning electron microscopy (SEM) and confocal imaging, results revealed that PMPC grafted surfaces prevent the implant interfaces from uncontrollable cell attachment which is of utmost importance in smoothing the motion of hip implant under cyclic loading.<br> <br>The thermogravimetric analyser (TGA) and differential scanning calorimetry (DSC), results confirmed the PMPC is thermally stable for implant applications regardless of the monomer concentration and can stand the thermal sterilisation process. Furthermore, the surface grafted with 0.6 M monomer concentration demonstrated improved wettability. <br><br>Nanoindentation studies were performed to evaluate the mechanical properties of the implant surface both before and after polymer grafting. Results showed 0.6 M monomer concentration was found to be the optimal concentration for grafting hip implant interfaces with MPC considering their surface properties, penetration depth and hardness results. A significant reduction in Young's modulus of PMPC grafted samples (33.2 - 42.9%), in comparison with untreated Ti6Al4V samples, indicated the capability of PMPC layers in avoiding stress shielding effect under cyclic loadings. The stress-strain curves revealed the improvement in toughness and elasticity behaviour of PMPC grafted surfaces in comparison with the control sample. The nano-scratch test revealed that the PMPC layer protects the underlying implant substrate from scratching even under high loads. Nano-tribological wear test was performed to investigate the effect of PMPC layer on wear resistance of the implant under loading conditions. The PMPC grafted surface exhibited a significant enhancement in wear resistance compared to the untreated surface even under application of high load. The concomitant improvement of wear resistance proved the potentiality of polymer films for implant applications. <br> <br>Finally, theoretical modelling was designed to predict the lubricating film formation under different physiological conditions of hip joints. The theoretical model revealed the boundary lubrication mechanism for hip joint conditions even for the polymer grafted surface. The polymer grafted implants showed improved film formation compared to untreated implants. Therefore, the surface grafting with cartilage-mimicking PMPC layer could play a significant role in boundary lubrication by protecting the underlying metallic implant and thus help to reduce surgical recurrence and to increase implant lifetime.