Mechanics and tribology of a zwitterionic polymer blend: Impact of molecular weight.

Despite decades of biomimetic materials development, the tribological properties of articular cartilage remain unrivalled. This manuscript presents the design and material properties of a polymer blend composed of poly (vinyl alcohol) (PVA) and a zwitterionic polysulfobetaine (PMEDSAH) prepared into hydrogel form using a cyclic freeze-thaw method. The PVA hydrogel matrix provides mechanical strength while the zwitterionic polymer, PMEDSAH, is intended to act as a boundary lubricant. The formation of PVA-PMEDAH hydrogel blends was found to result in unique biomimetic system where the boundary lubricant elutes from the bulk material to the surface in response to applied pressure. This behavior is attributed to the high-water content of the PVA hydrogel matrix and the solubility of PMEDSAH in aqueous solution. In addition to characterizing the effects of boundary lubricant molecular weight on the diffusive properties of the hydrogel blend, we report the coefficient of friction, µ, versus sliding speed for the hydrogel/glass interface. Consistent with our prior findings, PMEDSAH was found to engender lubricious behavior and the dependence of µ on sliding velocity indicated a repulsive interaction with glass rather than an attractive one. This result agrees with the hydration lubrication hypothesis. Contact mechanics analyzed within the context of Hertzian biphasic theory were also investigated, revealing that the introduction of PMEDSAH enhances the hydrogel's ability to provide interstitial fluid load support.

Synthesis and Characterization of Zwitterionic Polymer Brush Functionalized Hydrogels with Ionic Responsive Coefficient of Friction.

Freeze-thaw poly(vinyl alcohol) hydrogels (PVA-H) offer great potential for several biomedical applications due to their biomimetic mechanical properties and biocompatibility. Despite these advantages, the use of PVA-H for load bearing applications has been limited due to poor performance in boundary lubrication compared to natural tissue such as articular cartilage. Recently, zwitterionic polymer brushes have been shown to act as effective boundary lubricants on rigid substrates; however, to the best of our knowledge, the synergistic effects of zwitterionic brushes coupled with the biomimetic fluid load support exhibited by hydrogels have not been reported. We report here on our investigation involving the synthesis and characterization of two unique types of polymer brush functionalized PVA hydrogels. The zwitterionic polymers that were compared contained either [2-(methacryloyloxy)ethyl]dimethyl-3-sulfopropylammonium hydroxide, PMEDSAH, or 2-methacryloyloxyethylphosphorylcholine, PMPC, repeating units. Both hydrogels coated with zwitterionic polymers were found to be cytocompatible. We report further on micrometer-scale surface properties via water contact angle goniometry, surface roughness measurements, and scanning electron microscopy. Finally, the impact of brush functionalization on the mechanics of the tribologically enhanced gels is reported with comparison to natural articular cartilage within the context of Hertzian contact theory.

The use of a hydrogel implant in the repair of osteochondral defects of the knee: A biomechanical evaluation of restoration of native contact pressures in cadaver knees.

BACKGROUND: Osteochondral injuries have been treated by a variety of methods, each having its own drawbacks. The purpose of this study was to determine the biomechanical feasibility of using a hydrogel implant replacement for an osteochondral core defect. The hypothesis of this study was that the contact pressure of the native knee can be recreated with the use of a hydrogel implant. METHODS: Six cadaver knees were tested in a knee simulator while contact pressures were measured on the tibial plateau. Pressure data was collected in the intact knee, after coring of the condyle and after insertion of a hydrogel implant. Following 1000 gait cycles of fatigue testing, each knee was taken through axial loading indentation testing where the stiffness of the in situ implant was compared to the contralateral condyle. FINDINGS: While coring significantly reduced the peak pressure at the coring site from 1.8 MPa in the intact knee to 0.0 MPa after coring, implant insertion significantly increased it to 1.2 MPa. There was no significant difference in the peak pressures or the average pressures at the hole location between the intact knee and following implant insertion. After fatigue testing, no macroscopic loosening or implant damage was observed. Based on indentation testing, the stiffness of the medial condyle, 157 N/mm, was significantly less than the lateral condyle, 696 N/mm. INTERPRETATION: The insertion of the hydrogel implant was able to achieve restoration of contact pressures in the knee supporting the viability of hydrogel implants in the treatment of osteochondral lesions of the knee.

Synthesis and characterization of a zwitterionic hydrogel blend with low coefficient of friction.

Hydrogels display a great deal of potential for a wide variety of biomedical applications. Often times the performance of these biomimetic materials is limited due to inferior friction and wear properties. This manuscript presents a method inspired by the tribological phenomena observed in nature for enhancing the lubricious properties of poly(vinyl alcohol) (PVA) hydrogels. This was achieved by blending PVA with various amounts of zwitterionic polymer, poly([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide) (pMEDSAH). Our results indicate that pMEDSAH acts as an effective boundary lubricant, allowing for reduction in coefficient of friction by more than 80%. This reduction in friction coefficient was achieved while maintaining comparable mechanical and physical properties to that of the neat material. Also, these zwitterionic blends were found to be cytocompatible. Analysis of the structure to property relationships within this system indicate that the zwitterionic polymer served as a boundary lubricant and promoted a reduction in friction through hydration lubrication. This novel approach provides a promising platform for further investigations enhancing the tribological properties of hydrogels for biomedical applications. STATEMENT OF SIGNIFICANCE: The novelty of this work stems from showing that zwitterionic polymers can be used as an extremely effective hydrogel boundary lubricant. This work will have significant scientific impact because to date, design of hydrogels has emphasized replication of mechanical properties, but in order for these types of materials to be fully utilized as biomaterials it is imperative that they possess improved tribological and lubrication properties, because ignoring the surface and boundary lubrication mechanism, make these potential load-bearing substitutes incompatible with other natural articulating surfaces, leading the constructs to wear, fail, and damage healthy tissue. Our work also provides unique insight to the structure-property-function relationships of these biomaterials which will be of great interest to the readership of the journal.

Low friction hydrogel for articular cartilage repair: evaluation of mechanical and tribological properties in comparison with natural cartilage tissue.

The mechanical and tribological properties of a novel biomaterial, a boundary lubricant functionalized hydrogel, were investigated and compared to natural cartilage tissue. This low friction hydrogel material was developed for use as a synthetic replacement for focal defects in articular cartilage. The hydrogel was made by functionalizing the biocompatible polymer polyvinyl alcohol with a carboxylic acid derivative boundary lubricant molecule. Two different gel processing techniques were used to create the hydrogels. The first method consisted of initially functionalizing the boundary lubricant to the polyvinyl alcohol and then creating hydrogels by physically crosslinking the reacted polymer. The second method consisted of creating non-functionalized polyvinyl alcohol hydrogels and then performing the functionalization reaction on the fully formed gel. Osteochondral bovine samples were collected and replicate experiments were conducted to compare the mechanical and tribological performance of the boundary lubricant functionalized hydrogels to non-functionalized hydrogels and native cartilage. Friction experiments displayed a maximum decrease in friction coefficient of 70% for the functionalized hydrogels compared to neat polyvinyl alcohol. Indentation investigated the elastic modulus of the hydrogels, demonstrating that stability of the hydrogel was affected by processing method. Hydrogel performance was within the lower ranges of natural cartilage tested under the exact same conditions, showing the potential of the boundary lubricant functionalized hydrogels to perform as a biomimetic synthetic articular cartilage replacement.

Experimental and numerical tribological studies of a boundary lubricant functionalized poro-viscoelastic PVA hydrogel in normal contact and sliding.

Hydrogels are a cross-linked network of polymers swollen with liquid and have the potential to be used as a synthetic replacement for local defects in load bearing tissues such as articular cartilage. Hydrogels display viscoelastic time dependent behavior, therefore experimental analysis of stresses at the surface and within the gel is difficult to perform. A three-dimensional model of a hydrogel was developed in the commercial finite element software ABAQUS™, implementing a poro-viscoelastic constitutive model along with a contact-dependent flow state and friction conditions. Water content measurements, sliding, and indentation experiments were performed on neat polyvinyl alcohol (PVA), and on low friction boundary lubricant functionalized (BLF-PVA) hydrogels, both manufactured by freeze-thaw processes. Modulus results from the indentation experiments and coefficient of friction values from the sliding experiments were used as material property inputs to the model, while water content was used to calculate initial flow conditions. Tangential force and normal displacement data from a three-dimensional simulation of sliding were compared with the experiments. The tangential force patterns indicated important similarities with the fabricated hydrogels that included an initially high force value due to time dependent deformation followed by a decrease in a stabile value. A similar trend was observed with the normal displacement. These comparisons rendered the model suitable as a representation and were used to analyze the development and propagation of stresses in the immediate surface region. The results showed that in a three-dimensional stress field during sliding, the maximum stress shifted to the surface and rotated closer to the leading edge of contact. This occurred because the stress field becomes dominated by an amplified compressive stress at the leading edge due to the biphasic viscoelastic response of the material during sliding. Also, the complex multi-axial contact stress field was reduced to focus predominately on stress in the contact surface region in the direction of sliding. The results showed that during biphasic viscoelastic frictional sliding, the maximum tensile stress develops at the trailing edge of contact and a compressive stress develops at the leading edge in the direction of motion. The BLF-PVA hydrogels displayed a decrease in this tensile and compressive stress as compared to the standard PVA. The diminishment of these stresses would be expected to give the BLF-PVA hydrogels lower material wear with greater life expectancy as a synthetic articular cartilage implant.

A novel polyvinyl alcohol hydrogel functionalized with organic boundary lubricant for use as low-friction cartilage substitute: synthesis, physical/chemical, mechanical, and friction characterization.

A novel material design was developed by functionalizing polyvinyl alcohol hydrogel with an organic low-friction boundary lubricant (molar ratios of 0.2, 0.5, and 1.0 moles of lauroyl chloride). The hydrogels were fabricated using two different techniques. First, the boundary lubricant was initially functionalized to the polymer, then the hydrogels were created by physically crosslinking the reacted polymer. Second, hydrogels were initially created by crosslinking pure polyvinyl alcohol, with the functionalization reaction performed on the fully formed gel. After the reaction, Fourier transform infrared spectroscopy and attenuated total reflectance spectra revealed a clear ester peak, the diminishment of the alcohol peak, and the amplification of the alkyl peaks, which confirmed attachment of the hydrocarbon chains to the polymer. Additional chemical characterization occurred through elemental analysis where an average increase of 22% carbon and 40% hydrogen provided further confirmation of attachment. Physical characterization of the boundary lubricant functionalized hydrogels was performed by water content and contact angle measurements. Water content dependency showed that method 1 had a direct relationship with boundary lubricant concentration, and method 2 displayed an inverse relationship. The contact angle increased as boundary lubricant concentration increased for the pure matrix material for both processing methods, suggesting that the hydrocarbons produced surface properties that mimic natural cartilage, and contact behavior of the biphasic system was dependent on processing method. Friction tests demonstrated a significant decrease in friction coefficient, with a maximum decrease of 70% and a minimum decrease of 24% for boundary lubricant functionalized hydrogels compared with nonfunctionalized polyvinyl alcohol hydrogels.