Ultra-high Molecular Weight Polyethylene Substrate Research Articles

One-Pot Construction of Articular Cartilage-Like Hydrogel Coating for Durable Aqueous Lubrication.

Articular cartilage has an appropriate multilayer structure and superior tribological properties and provides a structural paradigm for design of lubricating materials. However, mimicking articular cartilage traits on prosthetic materials with durable lubrication remains a huge challenge. Herein, an ingenious three-in-one strategy is developed for constructing an articular cartilage-like bilayerhydrogel coating on the surface of ultra-high molecular weight polyethylene (BH-UPE), which makes full use of conceptions of interfacial interlinking, high-entanglement crosslinking, and interface-modulated polymerization. The hydrogel coating is tightly interlinked with UPE substrate through hydrogel-UPE interchain entanglement and bonding. The hydrogel chains are highly entangled with each other to form a dense tough layer with negligible hysteresis for load-bearing by reducing the amounts of crosslinker and hydrophilic initiator to p.p.m. levels.Meanwhile, the polymerization of monomers in the top surface region is suppressed via interface-modulated polymerization, thus introducing a poroussurface for effective aqueous lubrication. As a result, BH-UPE exhibits an ultralow friction coefficient of 0.0048 during 10000 cycles under a load of 0.9MPa, demonstrating great potential as an advanced bearing material for disc prosthesis. This work may provide a new way to build stable bilayer coatings and have important implications for development of biological lubricating materials.

Evolution of the microstructure of amorphous polyethylene under friction-induced plastic flows: A reactive molecular investigation.

The plastic flow of ultra-high molecular weight polyethylene (UHMWPE) at a frictional interface, which is critical to the wear behavior, was investigated by reactive molecular dynamics simulations. The UHMWPE substrate was found to experience various deformations during the friction process. First, some polyethylene (PE) chains could detach from the substrate because of their rapid movement. Second, the frequent motion of PE chains also resulted in the intermittent formation and breaking of cavities between intermolecular PE chains. These deformations were more obvious on a surface with a convex protrusion, where the plowing effect exacerbated the cavitation and elastic deformation of PE chains. Correspondingly, the plastic flow in turn reconstructed the convex protrusion by displacing the surface atoms on the Fe slab. The plastic flow of PE chains broke the C-C bonds, and the carbon moieties were then chemically bonded onto the metal surface. A rapid change of atomic charge, hence, happened when the bonds broke. Meanwhile, PE chains release short alkyl radicals gradually after bond breakage, indicating gradual wear of the substrate during friction. This work provides molecular insight into the evolution of interfacial microstructure under plastic flow on a UHMWPE substrate.

Influence of processing conditions on microstructural, mechanical and tribological properties of graphene nanoplatelet reinforced UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) is a relevant thermoplastic in industry and a well-proven standard biomaterial in joint replacements. To enhance its tribological properties while preserving its bulk ones, composite coatings on a UHMWPE substrate were prepared using non-functionalised graphene nanoplatelet (GNP) at reinforcement concentration of 0.1–5 wt% and two mechanical mixing techniques (ball mill or blade mixer) with different consolidation temperatures of 175–240 °C. Changes in morphology and size of the UHMWPE particles before hot-pressing were observed in function of the mechanical mixing techniques applied. Wear rate was affected by graphene content, reaching a minimum at 0.5 wt% GNP, with a reduction of 20 and 15%, for ball milling and blade mixer, respectively. However, blade mixer increased the wear rate by around twice respect the ball milling results, for all the studied materials. The coefficient of friction decreased notably, by ~25%, below 3 wt% GNP content, and hardness increased by 24%, regardless of the mechanical mixing process used. Finally, consolidation temperature had a positive influence on wear rate at temperatures of around 195 °C, which could be related to the free radical scavenger effect of the GNP.

Optimizing thickness of ceramic coatings on plastic components for orthopedic applications: A finite element analysis.

Realizing hard ceramic coatings on the plastic component of a joint prosthesis can be strategic for the mechanical preservation of the whole implant and to extend its lifetime. Recently, thanks to the Plasma Pulsed Deposition (PPD) method, zirconia coatings on ultra-high molecular weight polyethylene (UHMWPE) substrates resulted in a feasible outcome. Focusing on both the highly specific requirements defined by the biomedical application and the effective possibilities given by the deposition method in the perspectives of technological transfer, it is mandatory to optimize the coating in terms of load bearing capacity. The main goal of this study was to identify through Finite Element Analysis (FEA) the optimal coating thickness that would be able to minimize UHMWPE strain, possible insurgence of cracks within the coating and stresses at coating-substrate interface. Simulations of nanoindentation and microindentation tests were specifically carried out. FEA findings demonstrated that, in general, thickening the zirconia coating strongly reduced the strains in the UHMWPE substrate, although the 1 μm thickness value was identified as critical for the presence of high stresses within the coating and at the interface with the substrate. Therefore, the optimal thickness resulted to be highly dependent on the specific loading condition and final applications.

Improvement effects of CaO nanoparticles on tribological and microhardness properties of PMMA coating

Poly(methyl methacrylate)/calcium oxide (PMMA/CaO) nanocomposite in the form of solution has been successfully prepared by mixing CaO nanoparticles and PMMA obtained by a free radical polymerization. For comparative studies, pure PMMA coating was also prepared to determine the influence of CaO nanoparticles incorporation into PMMA matrix on the properties behavior. Fourier transform infrared technique was used to study the synthesis of PMMA and PMMA/CaO nanocomposite. The coating thickness was determined by profilometry; it was increased from 97 to 107 µm, when CaO nanoparticles were added. The surface morphology of the coatings was characterized by scanning electron microscopy. The wear tests for PMMA/CaO and PMMA coatings on ultra-high molecular weight polyethylene substrates were carried out on a pin-on-disk tribometer in dry conditions with normal loads ranging from 2 to 10 N. The friction coefficient behavior of PMMA coating was improved by the addition of CaO nanoparticles; however, volume loss and wear rate values were slightly higher for PMMA/CaO coatings than those corresponding to PMMA coatings. Microhardness of PMMA and PMMA/CaO coatings was also evaluated by Vickers measurements, revealing an increase of one order of magnitude for PMMA/CaO coating.

Performance evaluation of chitosan/hydroxyapatite composite coating on ultrahigh molecular weight polyethylene

The aim of the present study is to evaluate the performance of chitosan/hydroxyapatite (CTS/HA) coatings on ultrahigh molecular weight polyethylene (UHMWPE). We focused on adhesion measurements, micro-hardness indentation, friction and wear tests. Using commercial compounds of medical grade, CTS/HA composite was prepared by a simple mixture using acetic acid as solvent. The CTS/HA solution obtained was used to prepare coatings on UHMWPE substrates by the immersion method. Scanning electronic microscopy (SEM) results showed that the composite coating had flake-like morphology characteristic of chitosan and calcium phosphate powders with a porous structure. The adhesion strength between CTS-HA coating and UHMWPE was approximately 0.15 MPa, according the ASTM D5179 method. The micro-hardness of CTS/HA coating on UHMWPE was evaluated by using a Vickers hardness tester, while friction and wear were studied by a pin-on-disk method in dry conditions.

Effects of screen-grid bias voltage on the microstructure and properties of the ultrahigh molecular weight polyethylene (UHMWPE) modified by oxygen plasma

The specimens of ultrahigh molecular weight polyethylene (UHMWPE) were treated by oxygen plasma using a pulse-biased screen-grid technique under different negative bias conditions. The screen-grid was used to provide an electric field to accelerate the oxygen ions towards the UHMWPE substrate during the plasma treatment process. The effects of the screen-grid voltage on the surface microstructure, wettability, mechanical properties and wear resistance of UHMWPE were investigated. It was found that the degree of crosslinking, oxidation, wettability and surface roughness of UHMWPE can be increased with the increasing of the screen-grid voltage. Owing to the increase of the degree of crosslinking, the hardened layer formed on the surface of the UHMWPE samples was also strengthened greatly with the increase of the grid voltage. However, the wear results indicated that the UHMWPE sample modified at higher bias voltage exhibits poor wear performance, which could be mainly related to the embrittlement resulted from the aggravation of oxidation.

Friction and wear properties of poly(methyl methacrylate)–hydroxyapatite hybrid coating on UHMWPE substrates

Poly(methyl methacrylate)–hydroxyapatite (PMMA/HA) composite has been successfully prepared for its application as coating on ultra high molecular weight polyethylene (UHMWPE). The hybrid coating was prepared by mixing PMMA obtained from methyl methacrylate (MMA) polymerization and medical grade hydroxyapatite (HA). FTIR technique was used to study the PMMA synthesis and hybrid composite coating. For comparative studies, pure PMMA coatings were also prepared in order to determine the influence of HA incorporation into PMMA matrix. The coating thickness was determined using image analysis software and increased from 32.61μm to 34.01μm when HA was added. The surface morphology of the coatings was characterized by Scanning Electron Microscopy (SEM). The wear tests for PMMA/HA and PMMA coatings on UHMWPE were carried out on a pin-on-disk tribometer in dry conditions with normal loads ranging from 2 to 10N. The wear performance of PMMA coatings was improved by HA additions, obtaining lower friction coefficients and volume loss for PMMA/HA coatings than the corresponding values for PMMA coating under 10N load.

Deposition of a-C:H films on UHMWPE substrate and its wear-resistance

In prosthetic hip replacements, ultrahigh molecular weight polyethylene (UHMWPE) wear debris is identified as the main factor limiting the lifetime of the artificial joints. Especially UHMWPE debris from the joint can induce tissue reactions and bone resorption that may lead to the joint loosening. The diamond like carbon (DLC) film has attracted a great deal of interest in recent years mainly because of its excellent tribological property, biocompatibility and chemically inert property. In order to improve the wear-resistance of UHMWPE, a-C:H films were deposited on UHMWPE substrate by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-PECVD) technology. During deposition, the working gases were argon and acetylene, the microwave power was set to 800 W, the biased pulsed voltage was set to −200 V (frequency 15 kHz, duty ratio 20%), the pressure in vacuum chamber was set to 0.5 Pa, and the process time was 60 min. The films were analysed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, nano-indentation, anti-scratch and wear test. The results showed that a typical amorphous hydrogenated carbon (a-C:H) film was successfully deposited on UHMWPE with thickness up to 2 μm. The nano-hardness of the UHMWPE coated with a-C:H films, measured at an applied load of 200 μN, was increased from 10 MPa (untreated UHMWPE) to 139 MPa. The wear test was carried out using a ball (Ø 6 mm, SiC) on disk tribometer with an applied load of 1 N for 10000 cycles, and the results showed a reduction of worn cross-sectional area from 193 μm 2 of untreated UHMWPE to 26 μm 2 of DLC coated sample. In addition the influence of argon/acetylene gas flow ratio on the growth of a-C:H films was studied.

ATR-FTIR as a thickness measurement technique for hydrated polymer-on-polymer coatings.

Hydrated polymer coatings on polymer substrates are common for many biomedical applications, such as tissue engineering constructs, contact lenses, and catheters. The thickness of the coatings can affect the mechanical behavior of the systems and the cellular response, but measuring the coating thickness can be quite challenging using conventional methods. We propose a new method, that is, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to determine the relative thickness, combined with atomic force microscopy to calibrate the ATR-FTIR measurements. This technique was successfully employed to determine the hydrated thickness of a series of crosslinked tetraglyme coatings on ultrahigh molecular weight polyethylene substrates intended to reduce wear of acetabular cups in total hip replacements. The hydrated coatings ranged from 30 to 200 nm thick and were accurately measured despite the relatively high root-mean-square (RMS) roughness of the substrates, 20-35 nm (peak-to-peak roughness 55-100 nm). The calibrated ATR-FTIR technique is a promising new method for measuring the thickness of many other polymer-on-polymer and hydrated coatings.

Biomimetic apatite formation on Ultra-High Molecular Weight Polyethylene (UHMWPE) using modified biomimetic solution

Modifications were performed on a biomimetic solution (SBF), according to previous knowledge on the behavior of ions present in its composition, in order to obtain apatite coatings onto Ultra-High Molecular Weight Polyethylene (UHMWPE) without having to use polymer pre-treatments that could compromise its properties. UHMWPE substrates were immersed into a 30% H(2)O(2) solution for a 24-h period and then submitted to a biomimetic coating method using standard SBF and two other modified SBF solutions. Apatite coatings were only obtained onto UHMWPE when the modified SBF solutions were used. Based on these results, apatite coatings of biological importance (calcium-deficient hydroxyapatite-CDHA, amorphous calcium phosphate-ACP, octacalcium phosphate-OCP, and carbonated HA) can be obtained onto UHMWPE substrates, allowing an adequate conciliation between bonelike mechanical properties and bioactivity.

Diamond-like carbon films for polyethylene femoral parts: Raman and FT-IR spectroscopy before and after incubation in simulated body liquid

In artificial prosthetics for knee, hip, finger or shoulder joints, ultrahigh molecular weight polyethylene (UHMW-PE) is a significant material. Several attempts to reduce the wear rate of UHMW-PE, i.e. the application of suitable coatings, are in progress. A surface modification of polyethylene with wear-resistant hydrogenated diamond-like carbon is favourable, owing to the chemical similarity of polyethylene (-C-H(2)-)(n) and C:H or amorphous C:H (a-C:H) coatings with diamond-like properties. In the present study, the microstructure of a-C:H coatings on UHMW-PE substrates was investigated by Raman and Fourier transform infrared (FT-IR) spectroscopy. FT-IR spectroscopy shows very broad absorption lines, which point to the disorder and diversity of different symmetric, asymmetric aromatic, olefin sp(2)-hybridized or sp(3)-hybridized C-H groups in the amorphous diamond-like carbon coating. Following a long incubation of 12 months in a simulated body liquid, the structural investigations were repeated. Furthermore, fractured cross-sections and the wetting behaviour with polar liquids were examined. After incubation in simulated body liquid, Raman spectroscopy pointed to a reduction of the C-H bonds in the diamond-like carbon coatings. On the basis of these findings, one can conclude that hydrogenated diamond-like carbon is able to interact with salt solutions by substituting the hydrogen with appropriate ions.