Design and Synthesis of Bilayer Nanocomposite Hydrogels for Minimally Invasive Cartilage Replacement
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The complex structure of healthy articular cartilage facilitates the joint withstanding the imposed pressures and retaining interstitial fluid to lessen stresses on its soft tissue, while easing the locomotion and minimizing friction between cartilage mates. Avascular nature of this tissue results in unrecoverable damaged lesions and severe pain over time. Polymeric hydrogels are promising candidate materials for the replacement of the damaged cartilage. Recently, bilayer hydrogels have been developed with distinct techniques as artificial cartilage due to their resemblance to the native cartilage structure. Bilayer hydrogels contain bulk and lubricious layers that enhance water retention in their lubricious layer and improve tribological properties such as wear resistance and coefficient of friction (CoF). However, design and manufacturing bilayer hydrogels with desirable mechanical and tribological performance is challenging, because promoting mechanical properties results in mitigation of tribological properties and vice versa. Specifically, the lubricious layer of the bilayer hydrogels was found susceptible to wear under sliding motions. Therefore, the current study focused on design and manufacturing of bilayer hydrogels strengthened with nanoparticles to overcome this problem and achieve enhanced mechanical and tribological performances. In the first phase of this research, the formation of the lubricious and bulk layers was conducted with appropriate synthesis processes. The second phase was focused on optimizing the mechanical properties by variations in monomers and crosslinkers amounts. In the third phase, tribological properties were evaluated; and finally in the fourth phase, mathematical approaches were applied to determine mechanical surface energy, viscoelastic or poroelastic relaxation and define effective parameters in the design of artificial soft tissue with respect to both mechanical and tribological properties. In this study, two common types of nanoparticles in orthopaedic research, titania nanoparticles (TiO2 NPs) and Silica nanoparticles (SNPs), were utilized to strengthen both bulk and lubricious layers for the sake of mechanical and tribological improvements. Wide ranges of experiments on monomers and nanoparticles were conducted to assess mechanical properties such as elastic modulus, hardness, compressive strength, compressive moduli, tangent modulus, and relaxation parameters. Also, wide ranges of tribological tests were performed to evaluate CoF, wear-loss volume, surface topography, wear mechanisms, and lubrication regimes. All mechanical and tribological experiments were conducted according to the required tests for the native and artificial cartilage according to U.S. Food and Drug Administration (FDA) and the International Cartilage Repair Society (ICRS). Hydrogel formulation with 0.2wt% TiO2 NPs was found to have superior mechanical properties compared to other NPs-loaded samples and non-reinforced hydrogels (NRHs). Tribological test results showed a low mean coefficient of friction values of 0.007 and 0.014 for NRHs and nanocomposite hydrogels (NCHs), respectively, and wear resistance of NCHs improved significantly. SEM images showed that wear mechanisms are a combination of adhesive wear and fatigue wear. Silica NCHs, however, showed optimum results by topping 0.6wt% SNPs into the designed bilayer hydrogels. It is shown that poroelastic relaxation occurred before viscoelastic relaxation according to diffusion rate theory. Interfacial surface energy was also analyzed, and NCHs showed superior surface energy compared to NRHs. Lubrication regimes models were developed, and CoF results were addressed in the elastoviscous transition regime based on the Stribeck curve framework. SEM images showed a strengthened lubricious layer after sliding tests and an increased wear resistance compared to NRHs. The outcomes of the research presented in this thesis creates a framework for designing, deficiencies-free synthesizing, testing, and analyzing essential elements of bilayer hydrogels for cartilage replacement.