Ultra-high Molecular Weight Polyethylene Materials Research Articles

Initial response of human bone marrow–derived stem cells after contact with ultrahigh-molecular-weight polyethylene (UHMWPE) material: An in vitro study on cell viability and interleukin-6 expression

Introduction:Ultrahigh-molecular-weight polyethylene (UHMWPE) is a thermoplastic polymer useful in biomaterial applications, especially in orthopedic field. Yet, little is known concerning its initial effect on human bone marrow stem cells (hBMSCs) after implantation.Materials and Methods:A cytotoxicity analysis was performed with a 3-(4,5-dimethylthiazol 2-yl)-2,5-diphenyltetrazolium assay after 24, 48, and 72h of incubation of hBMSC culture. Expression of interleukin-6 (IL-6) was measured using enzyme-linked immunosorbent assay. Cell viability was measured with Inhibitory concentration 50% (IC50) formula.Results:All treatment groups showed a cell viability of >50% ranging from 78% to >100%. Lower expression of IL-6 of hBMSC compared to control group was found in 48h of incubation period.Conclusion:hBMSC showed high cell viability after initial contact with UHMWPE material. Modulation of IL-6 expression was present at the initial stage as a response to foreign material.

A Micro Constitutive Model for Ultra-High Molecular Weight Polyethylene Orthopaedic Implant Bearings

Ultra-high molecular weight polyethylene (UHMWPE) is a tough semi-crystalline polymer employed widely as a bearing material in total joint replacements. The micromechanical model has been presented that predicts stiffness of UHMWPE as an aggregate of crystalline inclusions (lamellae) embedded in a rubbery matrix of amorphous polymer chains. The differential scheme was chosen for its ability to represent the interaction between an inclusion and the matrix. Numerical simulations show that increasing lamellar thickness results in less stiffness, less shear stress imposed on the lamellae, indicates that thick lamellae are desirable for UHMWPE materials utilized in total joint replacement bearings.

Multi layered natural rubber coated woven P‐aramid and UHMWPE fabric composites for soft body armor application

In this work, the authors have investigated the impact performance of natural rubber (NR) coated high strength, high modulus fabrics. P‐aramid (Kevlar 129) and ultra high molecular weight polyethylene (UHMWPE) fabric were coated with different concentrations of NR solutions (20%, 30%). Samples were produced with different add on percentage. The NR solutions with 20% and 30% solid content provided 4 and 6% add‐on on p‐aramid fabric and 6 and 9% add‐on on UHMWPE fabric. Multilayered fabric structures coated with NR were also studied. Compression molding technique was used to prepare the layered fabric structures. The effect of coating with different add‐on percentages of NR on fabrics was studied. The pull out force increased by ∼480% and 360% on NR‐coated p‐aramid and UHMWPE fabrics, respectively. Although the impact energy absorption of single layer NR‐coated fabrics was lower than that of neat fabrics, there was substantial improvement in the impact energy absorption of the two layered p‐aramid and UHMWPE rubber coated fabrics as compared to untreated two layered ones. The increase was 44% and 81%, respectively, for Kevlar and UHMWPE fabrics. Further, the effect of NR latex treatment on tensile strength of fabrics has also been studied. POLYM. COMPOS., 39:3636–3644, 2018. © 2017 Society of Plastics Engineers

Effects of fabric construction and shear thickening fluid on yarn pull-out from high-performance fabrics

Yarn pull-out is one of the major modes of fabric failure during an event of impact. In this article, the effects of weave and fabric sett on yarn pull-out force have been investigated for untreated and shear thickening fluid (STF) treated p-aramid fabrics. Further, the effect of different fluid treatments on yarn pull-out behavior has been analyzed using p-aramid and ultra-high molecular weight polyethylene (UHMWPE) fabrics. For all levels of fabric sett, plain woven fabrics exhibited higher yarn pull-out force compared to twill, matt and satin weaves. STF impregnation increased the yarn pull-out force considerably for p-aramid and UHMWPE fabrics and this was found for all the weaves in the case of the former. However, increase in fabric sett beyond a threshold level caused yarn breakage before yarn pull-out in case of STF-treated plain woven p-aramid fabrics. Yarn pull-out force was found to have good association with the energy absorption by the high-performance fabrics during low-velocity impact (6 m s–1). The normalized yarn pull-out force was higher for two consecutive yarns than for two yarns with a single gap.

Dynamic stab resistance of ultra-high molecular weight polyethylene fabric impregnated with shear thickening fluid

The dynamic stab resistance properties of ultra-high molecular weight polyethylene (UHMWPE) fabrics impregnated with shear thickening fluids (STFs) were investigated. The chemical composition of dispersing medium in STF was varied. The resistance mechanism of woven fabrics against knife and spike threats was discussed and the role of STF in stab resistance improvement of UHMWPE fabrics was analyzed in detail. To further improve the stab resistance performance, the influence of polyethylene glycol (PEG) additives with different concentrations and molecular chain lengths on the stab resistance of STF/UHMWPE composites was investigated. The results demonstrate that the dynamic stab resistance of UHMWPE fabric is significantly enhanced due to the presence of STF. STF effectively decreases the yarn mobility and accelerates the transverse response of UHMWPE fabric. With the molecular weight of dispersing medium increasing, the energy dissipated by STF/UHMWPE fabrics decreases. Further, the stab resistance properties of STF/UHMWPE composites, especially the spike resistance, are improved with the increase of the concentration and the molecular chain length of additive.

A theoretical study on hybrid ultra-high molecular weight polyethylene for use in pin-shaped steel backing gear teeth of a cycloid drive

To improve performances of cycloid drive in a sluice hoisting equipment, pin-shaped steel backing gear teeth composite structure using ultra-high molecular weight polyethylene was designed. Based on the viscoelasticity of ultra-high molecular weight polyethylene material, geometric models and mechanical models of the hybrid polymer pin tooth were built up in this paper. Meanwhile, the model of traditional steel pin tooth was built up for comparison. Through the comparative analysis of the stress distribution and carrying capacity by using the simulation module of Inventor, it was proved that the composite structure polymer pin teeth can significantly reduce the contact stress and provide superior mechanical properties to the traditional cycloid drive, being an effective method to optimise the low-speed overloaded cycloid drive.

Preliminary Study on the Biocompatibility of Stainless Steel 316L and UHMWPE Material

Stainless steel AISI 316L (SS316L) and ultra-high molecular weight polyethylene (UHMWPE) are widely used materials for artificial hip joint components. The SS316L material is typically used for the acetabular cup, femoral head and the stem, while the UHMWPE material is used for the acetabular liner in an artificial hip joint. The aim of this work is to study the biocompatibility of SS316L and UHMWPE materials by implanting and installing these materials in the tissues of rabbits. The tissues around the implants were examined after eight weeks of the installment. Results showed that the reaction of the rabbit tissues around the implants was positive. It was concluded that the SS316L and the UHMWPE materials are biocompatible and the applications of these materials for implants seems conceivable.

Mechanical Properties and Three-Dimensional Topological Characterisation of Micron, Submicron and Nanoparticles from Artificial Joints

Study of the geometry, nanomechanical properties and three-dimensional topographic characteristics of ultra-high-molecular-weight polyethylene (UHMWPE) wear particles provides an insight into understanding the wear mechanisms in artificial joints, which have not been fully explored. In this study, UHMWPE particles were generated, collected and then separated into three groups based on their size: micrometre, submicrometre and nanometre. Particle size distribution and shape features were investigated using scanning electron microscopy. Atomic force microscopy was used to quantify wear particle topographic features and nanomechanical properties. From the wear particle geometry analysis, it was found that nano- and submicron-sized particles had a granular shape while micrometre-sized particles had a flake-like morphology. Moreover, the mechanical properties and topographic characterisation of the UHMWPE material indicated that the nano- and submicron-sized particles had a much greater modulus and smoother surface than that of the micron-sized particles and bulk polymer sample. These differences in the geometric, topographic and mechanical properties between the nanoparticles, submicron particles and the micron particles revealed that the micron particles were most likely to be produced under macroscopic polymer asperity wear, while the nano- and submicron-sized particles were generated under microscopic polymer asperity wear. These findings provide a deeper understanding of the wear mechanism and processes of wear particle generation. Furthermore, the information may be useful to develop strategies for controlling and minimising the production of wear particles with particular features.

Thermo-Mechanical Investigation of Ultra-High Molecular Weight Polyethylene for High Strain-Rate Applications

Advanced polymeric materials, such as ultra-high molecular weight polyethylene (UHMWPE) are used in lightweight body armour because of their combination of good impact resistance with light weight. However, a broader use of such materials is limited by the complexity and cost of the manufacturing processes and the lack of experimental data on their behaviour and failure evolution under high strain-rate loading conditions. The current study deals with an investigation of the internal heat generation during the tensile testing of UHWMPE and polymer nano-composite blends at various strain rates. A 3D finite element (FE) model of the tensile test is developed and validated with the experimental work. An elastic-plastic material model is used with adiabatic heat generation. The temperature and stresses obtained with the FE analysis are found to be in a good agreement with the experimental results for UHMWPE materials. The model can be used as a simple and cost effective tool to predict the thermo-mechanical behaviour of parts made from these materials under various loading conditions.

Technologies for Manufacturing Ultrahigh Molecular Weight Polyethylene-Based Porous Structures for Bone Implants

Efficacy of the use of bone implants and their reparative capacity are determined by the implant frame architecture. The implant should provide optimal ratio of porosity and pore size intended to maintain intensive pullulation of bone tissue. The goal of this work was to describe methods of formation of porous structure in ultrahigh molecular weight polyethylene (UHMWPE). The UHMWPE materials are intended for use in maxillofacial surgery. The following parameters of UHMWPE were estimated in this work: volume porosity, pore size, and mechanical characteristics of porous samples as functions of manufacturing procedures. Pore size was found to be determined by water-soluble filler, whereas mechanical characteristics depended on working pressure.

OS1009 打ち抜き試験による高度架橋超高分子量ポリエチレンの機械特性評価

Since small punch tests utilize small specimens that can be fabricated from retrieved components of artificial joints, it is useful method to evaluate in-vivo deterioration of mechanical properties of ultra-high molecular weight polyethylene (UHMWPE) used as bearing materials of artificial joints. However, standardized method, which requires strict dimensional accuracy of specimens, is not easy to perform. We have developed a modified small punch test, applicable to specimens with wider dimensional range, by the use of thickness gauges as spacers. In this study, tensile tests, compression tests, micro hardness tests and modified small punch tests were carried out, using three UHMWPE materials. Similar results were obtained from all tests, and usefulness of the modified small punch tests was confirmed.

In Vivo Oxidation in Remelted Highly Cross-Linked Retrievals

Elimination of free radicals to prevent oxidation has played a major role in the development and product differentiation of the latest generation of highly cross-linked ultra-high molecular weight polyethylene bearing materials. In the current study, we (1) examined oxidation in a series of retrieved remelted highly cross-linked ultra-high molecular weight polyethylene bearings from a number of device manufacturers and (2) compared the retrieval results with findings for shelf-stored control specimens. The hypothesis was that radiation-cross-linked remelted ultra-high molecular weight polyethylene would maintain oxidative stability in vivo comparable with the stability during shelf storage and in published laboratory aging tests. Fifty remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners and nineteen remelted highly cross-linked ultra-high molecular weight polyethylene tibial inserts were received after retrieval from twenty-one surgeons from across the U.S. Thirty-two of the retrievals had been in vivo for two years or more. Each was measured for oxidation with use of Fourier transform infrared spectroscopy. A control series of remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners from three manufacturers was analyzed with electron paramagnetic resonance spectroscopy to measure free radical content and with Fourier transform infrared spectroscopy to measure oxidation initially and after eight to nine years of shelf storage in air. The never-implanted, shelf-aged controls had no measurable free-radical content initially or after eight to nine years of shelf storage. The never-implanted controls showed no increase in oxidation during shelf storage. Oxidation measurements showed measurable oxidation in 22% of the retrieved remelted highly cross-linked liners and inserts after an average of two years in vivo. Because never-implanted remelted highly cross-linked ultra-high molecular weight polyethylene materials had no measurable free-radical concentration and no increase in oxidation during shelf storage, these materials were expected to be oxidation-resistant in vivo. However, some remelted highly cross-linked ultra-high molecular weight polyethylene retrievals showed measurable oxidation after an average of more than two years in vivo. This apparent departure from widely expected behavior requires continued study of the process of in vivo oxidation of ultra-high molecular weight polyethylene materials.

Microscale and nanoscale surface strain mapping of single asperity wear in ultra high molecular weight polyethylene: Effects of materials, load, and asperity geometry

Wear of ultra high molecular weight polyethylene (UHMWPE) is a limiting factor in longevity of joint replacements. Therefore, there is a desire to create new materials and enhance processing conditions associated with current materials to reduce wear. This requires understanding the effects of processing on performance of implants and of micron-scale wear mechanisms ongoing. Our goal is to generate detailed understanding of the micron-scale deformation-structure-properties relationships associated with UHMWPE subject to asperity wear processes and ultimately to be predictive of material success, in vivo. In this work, a surface strain analysis technique is developed and used to measure permanent strain from asperity deformations on the nano and micro scales. Deformation was applied to four material types (GUR 1050, GUR 1020, Hylamer, and Marathon) varying in molecular weight, crystallinity, and crosslinking. Surface strains were determined by mapping surface deformation fields and were compared across loading conditions and spatial scale, with variations in tip geometry and size, contact load, and material. Surface strains increased with asperity load for a fixed tip and were dependent on UHMWPE material, with a highly crystalline form exhibiting the most plastic strain and a crosslinked form exhibiting the least. Different asperity geometry [spherical microindenters with radius of 20 and 1500 mum, and a nanoindenter (Berkovich-type)] resulted in different surface strains (e.g., Berkovich vs. spherical were not similar) even when the nominal contact stress was similar. Finally, the extent of deformation during asperity wear correlates to the level of viscoelastic recovery of the materials observed after indentation testing.

Lubrication and Wear of Artificial Knee Joint Materials in a Rolling∕Sliding Tribotester

This paper describes the influence of lubrication on wear during testing of materials for artificial knee joints in a rolling∕sliding tribotester built to simulate contact conditions in a total knee replacement. The test configuration consists of parallel cylinders (pucks) of ultrahigh molecular weight polyethylene (UHMWPE) and polished cobalt-chrome alloy in oscillatory rolling∕sliding contact in a bath of dilute (25%) bovine serum. Wear tests of three different UHMWPE materials were run under constant load at 40% sliding for 1.5 million oscillation cycles at 1.5cycles∕s. Wear of the UHMWPE was determined by measuring the profile of the cylindrical contact surface of the puck before and after each test. Profile measurements were repeated after at least 53days to eliminate the contribution from creep. Differences between initial and final profiles were attributed to wear of the UHMWPE. It was found that the largest wear depth in the lubricated tests occurred near the ends of the oscillatory contact area, while dry (unlubricated) tests of the same materials showed a peak wear depth near the center of the contact area. In the lubricated tests, the worn depth was lowest for the most heavily irradiated material. Analysis of the elastohydrodynamic lubrication in the rolling∕sliding contact was carried out assuming a line-contact situation with smooth cylindrical surfaces. The time-dependent modified Reynolds equation and the elasticity equation with initial conditions were solved numerically using a multigrid technique with full approximation scheme, and using a Newton Raphson method to solve the highly nonlinear system of equations. The thickness of the lubricating film of bovine serum was determined for points along the length of the wear track. It was found that the smallest film thickness (hmin) occurs very close to the location in the oscillating contact where the greatest wear occurs, owing to the very low entraining velocity near the ends of the oscillation cycle. The coefficient (K) for wear of the UHMWPE was found to be relatively constant over the central section of the oscillatory motion, but increased to a higher value where hmin decreased to near zero. Thus, the important influence of lubrication on wear of artificial knee bearings was demonstrated.

Load Profile and Fluid Composition Influence the Soak Behavior of UHMWPE Implants

Abstract Wear of total joint replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in vivo. Load soak stations are used to correct for weight gain due to fluid absorption. Although the implementation of a soak station is considered only as an option for wear testing, this study indicated that fluid absorption depends on the lubricant composition as well as on the applied load profile. Two different fluid recipes were tested with three load scenarios each. Standard ultra-high molecular weight polyethylene (UHMWPE) pins acted against commercially available ceramic hip balls. Dynamic (0 to 1000 N), static (1000 N), and no load were applied for 23.1 days (or 2·106 cycles). Both lubricant recipes based on the same mix of deionized water and newborn calf serum (final protein content: 30 g/L), and differed in the chemicals admixed to the fluid to minimize bacterial growths and to prevent protein precipitation. After test completion, the group of samples tested in fluid B gained 2.4-, 3.6-, and 4.2-fold the weight of samples submersed in fluid A for the dynamically, statically, and nonloaded samples, respectively. The specimens showed a significantly higher weight gain under dynamic load than subjected to static or no load for both fluids. The results of this study suggest the implementation of a load-soak control station for every wear test when UHMWPE materials are examined.

Compliance calibration for fatigue crack propagation testing of ultra high molecular weight polyethylene

Ultra High Molecular Weight Polyethylene (UHMWPE) total joint replacement components under certain conditions are at risk of fatigue fracture. Thus, the fatigue crack inception/propagation resistance of UHMWPE is of interest. During fatigue crack propagation tests of UHMWPE, crack growth is often followed visually; however, this approach can be time consuming and requires that the specimen be accessible during testing. The objective of this study was to demonstrate the applicability of the compliance method for fatigue crack propagation tests of UHMWPE. We hypothesized that the standard calibration coefficients developed for metals may not be appropriate for UHMWPE and that different UHMWPE materials would require different compliance calibration coefficients. Three UHMWPE materials: sterilized (30 kGy); highly crosslinked and annealed (100 kGy, 130 degrees C); and highly crosslinked and remelted (100 kGy, 150 degrees C) were examined under ambient conditions. The results support the applicability of the compliance method for determination of crack length during fatigue crack propagation testing of UHMWPE. As hypothesized, the standard calibration coefficients were found to be inaccurate for UHMWPE. New UHMWPE-specific calibration coefficients were determined which predicted the crack growth behavior accurately. Also, as hypothesized, the compliance calibration coefficients for the three materials were significantly different. This is the first reported study to demonstrate the applicability of a compliance method to measure crack length in UHMWPE.

Numerical analysis of the effects of material parameters on the lubrication mechanism for knee prosthesis

The tibial component of current knee prostheses made of ultra high molecular weight polyethylene (UHMWPE) has a high degree of wear that causes knee inflammation, prosthesis loosening and subsequent replacement in not more than 15 years. In order to know which UHMWPE material properties have more influence on wear, a steady state lubrication model with non-Newtonian synovial fluid has been studied through numerical solution. The results show that UHMWPE has a very high elastic modulus that makes difficult a well lubricated artificial joint and induces the formation of very thin lubricating films between the moving surfaces with the same magnitude of roughness components. This study shows that the use of deformable porous materials in the tibial component could cause the lubricating film thickness to be higher than the average roughness and the pressure levels to be lower than the one predicted for UHMWPE. These two facts imply friction and wear reduction.

Anisotropy and oxidative resistance of highly crosslinked UHMWPE after deformation processing by solid-state ram extrusion

Solid-state deformation processing is a promising technique for modifying the physical and mechanical properties of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) beyond simple thermal treatment cycles that have been employed previously. This study evaluates anisotropy and oxidative resistance in a novel, radiation crosslinked (50 kGy) UHMWPE material (ArComXL: Biomet, Inc., Warsaw, IN), incorporating solid-state, deformation processing by extrusion below the melt transition for application in total hip arthroplasty. Tensile, compression, and small punch tests were conducted to evaluate the material properties in the three principal axes of the resulting material. Furthermore, short-term oxidative resistance was evaluated using Fourier transform infrared spectroscopy and the small punch test in conjunction with accelerated shelf aging protocols. The results of this testing indicate that the material is anisotropic, with significantly enhanced strength oriented along the long axis of the rod. For certain other properties, the magnitude of the anisotropy was relatively slight, especially in the elastic regime, in which only a 20% difference was noted between the long axis of the rod and the orthogonal, radial direction. The highly crosslinked material contains detectable free radicals, at a concentration that is 90% less than control, gamma inert sterilized UHMWPE. An unexpected finding of this study was evidence of oxidative stability of the deformation-processed material, even after 4 weeks of accelerated aging in a pressure vessel containing five atmospheres of oxygen (ASTM F2003), which resulted in macroscopic embrittlement of the control material. The oxidative stability observed in ArComXL suggests that the deformation-processed material may be suitable for air-permeable packaging and gas sterilization, which has thus far been reserved for remelted highly crosslinked UHMWPE.

The wear and fracture behaviour of ultra high molecular weight polyethylene subjected to ?-irradiation in an atmosphere of acetylene

The wear and mechanical properties of GUR 1020 (Perplas IMP 2000-2) Ultra High Molecular Weight Polyethylene (UHMWPE) subjected to gamma-irradiation in an atmosphere of acetylene, were evaluated for a range of processing conditions of irradiation, annealing and ageing. The results were compared with those obtained for the virgin UHMWPE material and material processed using conventional gamma-irradiation in nitrogen. Cross-linking produced by irradiation in acetylene, followed by subsequent annealing was found to be significantly more effective in improving the mechanical and wear properties of UHMWPE compared to when the material was irradiated in nitrogen. Gel fraction analysis on its own, while being able to detect the degree of cross-linking, was found to be insufficient in determining the effectiveness of the cross-links and the resulting mechanical properties of the UHMWPE material. The results suggest that gamma-irradiation in an atmosphere of acetylene may provide significant advantages over conventional UHMWPE processing and irradiation cross-linking techniques.

Rate effects on the microindentation-based mechanical properties of oxidized, crosslinked, and highly crystalline ultrahigh-molecular-weight polyethylene.

The surface micromechanical properties of ultrahigh-molecular-weight polyethylene (UHMWPE) are critical in determining the wear, deformation, and fracture in the surface region. These properties have not been accessible to simple mechanical testing on a spatial scale relevant to these mechanical processes until recently. The structural factors associated with surface mechanical properties (crosslinking, oxidation state, local orientation of polymer, crystallinity, etc.) can be highly variable and localized and may vary on micron spatial scales or smaller. Furthermore, time/frequency-dependent behavior of the surface may have an important role in the overall surface mechanical behavior. Recent work has shown the utility of depth sensing microindentation/nanoindentation testing to interrogate local surface mechanical properties. The goal of the present study was to measure the effect of loading rate on the depth-sensing microindentation testing of UHMWPE. Three different UHMWPE materials (Hylamer, a large crystal material; GUR 1020, a standard medical-grade material; and Marathon, a crosslinked material) were tested using a microindentation method at loading rates ranging from 0.01 to 1 Hz. Similarly, a gamma-irradiated in air and 15-year shelf-aged tibial component was tested through its cross-section to assess the variations in mechanical properties with location and to compare the micromechanical profile with the oxidation profile. It was found that rate of testing affected the microhardness of each material, however, only GUR 1020 and Hylamer showed rate-dependent behavior for modulus and energy dissipation factor. Micromechanical profiles through oxidized regions of the tibial component showed a high correlation with the oxidation profile. Increases in modulus, hardness, and energy dissipation factor were seen with increasing oxidation and each property was loading-rate dependent. These results show that depth-sensing microindentation/nanoindentation testing on the micron scale provides highly consistent and reproducible measurements of surface mechanical properties. This scale of testing minimizes the potential variations caused by local heterogeneity in crystallinity, surface orientation, and other submicron structural features.