Design and characterization of thermally treated polyvinyl alcohol hydrogels for articular cartilage repair

Articular cartilage (AC) is a highly specialized connective tissue characterized by an avascular nature, low cell density and limited proliferative activity. Consequently, it has a restricted capacity for intrinsic healing and regeneration. One of the most common degenerative diseases that affect AC is Osteoarthritis (OA), which leads to progressive joint dysfunction and significant patient disability. Due to the unique and complex structure of this tissue, its treatment and repair present significant clinical challenges. Tissue engineering represents a new and integrated approach to cartilage repair and hydrogels have been identified as one of the most promising biomaterials for soft tissue engineering, as their three-dimensional hydrophilic network can mimic the high-water content and mechanical properties of the native extracellular matrix. In this context, polyvinyl alcohol (PVA) hydrogels stand out for their excellent biocompatibility, non-toxicity and tunable mechanical properties. However, standard PVA hydrogels often lack the mechanical strength required to match healthy AC. The objective of this thesis was to develop and optimize specific thermal treatments to enhance the mechanical performance of PVA-based hydrogels, making them suitable substitutes for damaged cartilage. In this work, PVA hydrogel samples were prepared using two distinct freeze-thawing (FT) protocols to induce physical crosslinking. To further enhance their properties, the scaffolds were subjected to thermal treatments: all samples were dried at 60°C, while a subset of samples was subsequently subjected to an annealing process at 120°C. The influence of these thermal treatments on the final properties of the hydrogels was evaluated through uniaxial tensile and compressive tests. A Scanning Electron Microscopy (SEM) and thermogravimetric analysis (TGA) were carried out to evaluate, respectively, the surface and cross-section morphology and the thermal behavior of PVA hydrogels. Additionally, weight and geometry changes were evaluated through the hydrogel fabrication and rehydration phases. The experimental results demonstrated that thermal treatments induce a structural transition and significantly enhance the performance of PVA hydrogels. SEM and TGA analysis revealed that annealed samples exhibit a substantial reduction in porosity and water content, along with superior thermal stability. Mechanical characterization showed a substantial increase in stiffness and strength, particularly after the annealing treatment, which provided the highest reinforcement in both tensile and compressive behavior. This improvement is crucial for sustaining the high mechanical loads typical of healthy AC, which the non-treated hydrogels could not withstand.