Articular cartilage loss, due to diseases such as osteoarthritis or traumatic injury of the tissue, typically results in permanent damage because of cartilage tissue’s avascular and aneural nature. There are conventional treatment options, such as subchondral abrasion, microfracture or mosaicplasty, but these treatments have their limitations, ultimately resulting in a total joint replacement, even if the cartilage damage is limited to a small area within the whole joint. This is undesirable, especially in younger active patients. A synthetic material which mimics natural cartilage behavior would be advantageous to repair focal defects in cartilage less invasively than a joint replacement to prolong life of an injured joint. Hydrogels have been proposed as a promising option to replace articular cartilage because of their cartilage mimicking properties. Even though hydrogels show promise for mimicking mechanical properties, a crucial limitation of hydrogels is that their tribological properties do not compare with natural cartilage tissue.
To achieve improved friction properties, novel hydrogels were developed by functionalizing the biocompatible hydrogel polymer, polyvinyl alcohol, with an organic boundary lubricant carboxylic acid derivative. Different hydrogels were investigated by means of chemical, physical, mechanical and tribological experiments. Functionalization was verified by results of Fourier transform infrared spectroscopy and attenuated total reflectance spectroscopy which displayed formation of an ester peak, diminishment of the alcohol peak, amplification of the alkyl peaks, and elemental analysis that displayed an increase in carbon and hydrogen content, further confirming boundary lubricant attachment. Physical characterization by water content and contact angle measurements revealed that the physical properties varied depending on the processing method used to make the hydrogels. Mechanical characterization was performed using nanoindentation and the Oliver-Pharr technique to explore the change in hydrogel elastic modulus, and was compared to matching experiments performed on bovine cartilage samples. Results revealed that the processing method had an effect on the elastic modulus and that the values were within the lower range of cartilage. Tribological characterization was performed by sphere-on-flat friction tests at the macro-scale and compared to experiments on natural cartilage. It was demonstrated that a significant decrease in friction occurred for the boundary lubricant functionalized hydrogels when compared with neat polyvinyl alcohol hydrogels, and that friction coefficients were within the range of reported values for cartilage.
Hydrogels display viscoelastic time dependent behavior, and experimental analysis of stresses at the surface and within the gel is difficult to perform. To explore the stresses that develop at the surface of the hydrogel, a three dimensional model of a hydrogel was created in the commercial finite element software ABAQUSTM. The simulated hydrogel was created by implementing a poro-viscoelastic constitutive model with contact-dependent flow and friction conditions. The material property inputs for the hydrogel were found by nanoindentation, friction, and water content experiments which were performed on boundary lubricant functionalized and pure polyvinyl alcohol hydrogels. A comparison study performed with the model showed typical results that viscosity and modulus had the greatest influence on indentation depth while friction coefficient had the greatest influence on tangential force. Simulation results showed that the maximum compressive surface pressure in the hydrogel occurred at the leading edge of movement, and the maximum tensile stress occurred at the trailing edge of the slider. The functionalized hydrogels displayed a decrease in both tensile and compressive stress when compared to neat polyvinyl alcohol hydrogels. This decrease in tensile stress shown by the boundary lubricant functionalized hydrogels implies improved tribological performance with better wear potential than neat polyvinyl alcohol hydrogels.
The bulk tribological behavior of the boundary lubricant functionalized hydrogels was investigated by wear testing in a pin-on-flat reciprocating sliding apparatus. The friction and wear behavior of the functionalized hydrogels with increasing hydrocarbon boundary lubricant was compared to neat polyvinyl alcohol gels while sliding against a titanium pin in deionized water. Polymer weight loss results revealed that at high wear cycles the neat polyvinyl alcohol displayed the highest weight loss, and the boundary lubricant hydrogels exhibited greater wear resistance. Optical microscopy qualitatively showed that the polyvinyl alcohol hydrogels exhibit a predominant abrasive wear mechanism, while the boundary lubricant functionalized hydrogel exhibited reduced wear, with the hydrogel having the lowest hydrocarbon chain length boundary lubricant displaying the best wear performance with barely any noticeable wear. Wear testing was also performed with polyvinyl alcohol and boundary lubricant functionalized hydrogels articulating against bovine cartilage pins in phosphate buffer saline solution as a fluid. The results showed that there was no statistical difference between the weight loss of the neat polyvinyl alcohol hydrogels and the boundary lubricant functionalized hydrogels, and all hydrogels displayed low wear for all tests. Qualitative microscopy showed virtually no wear patch development on the boundary lubricant functionalized hydrogel surface at 2,000 cycles as well as minimal wear of the cartilage pins. This established that the boundary lubricant functionalized hydrogels exhibited improved wear performance as compared to the polyvinyl alcohol neat hydrogel.