Experimetal investigation on mechanical and tribological behavior of multi-walled carbon nanotube reinforced acrylamide/acrylic acid hybrid hydrogels

ABSTRACT Hydrogels are of great interest in cartilage repair applications as their physical properties similar to the native extracellular matrix. However, the poor mechanical properties of hydrogels greatly limit their application in cartilage repair. In this thesis, nanocomposite hydrogel was synthesized by adding multi-walled carbon nanotube (CNT) consisting of multilayer graphene lamella at different ratios (0.5 wt% and 1%) to covalently bonded polyacrylamide/polyacrylic acid (PAAm/PAAc) hydrogel for load-bearing applications such as cartilage replacement. The structural, morphological, swelling and mechanical behaviors of the developed hydrogels were investigated. The tribological characterization of the hydrogels was evaluated under linear reciprocating motion, 5 mm stroke length, 10 mm/s sliding speed, 0.5-2 N applied load in three different condition (dry-sliding, phosphate buffered saline–PBS and bovine serum–BCS). Successful synthesis of PAAm/PAAc hydrogels and incorporation of CNT into the structure were confirmed by Fourier transform infrared (FTIR) analysis. By scanning electron microscopy (SEM) analysis, it was observed that the CNT was homogeneously distributed in the structure. It was determined that the amount of swelling decreased with the increase in the amount of hydrophobic CNT in the structure. Accordingly, an increase in the compressive modulus values was observed with the addition of CNT to the PAAm/PAAc hydrogel. The compression modulus of the synthesized hydrogels (0.094-0.299 MPa) was found to be comparable to that of natural articular cartilage. The average friction coefficient (0.15-0.20) obtained in the experiments performed in BCS was found to be lower than the experiments performed in PBS (0.22-0.26). This can be attributed to the more effective boundary lubrication on the contact surface with the proteins present in the BCS. In experiments performed in BCS, the friction coefficient of the PAAm/PAAc-1CNT hydrogel was reduced by 25% compared to the unreinforced PAAm/PAAc hydrogel. In dry-sliding condition, the average friction coefficient tramendously increased to 0.55-0.65. The wear volume was significantly reduced with the addition of CNT to the PAAm/PAAc hydrogel. This situation can be attributed to the high hardness and strength of the CNT nanomaterial. SEM analyzes show that adhesion and plastic deformation wear mechanisms are dominant on hydrogel surfaces.