Biotribological properties of UHMWPE grafted with AA under lubrication as artificial joint

Osteolysis caused by wear particles from polyethylene in the artificial hip joints is a serious issue. In order to endow the low friction and wear of the bearing surface of ultra-high molecular weight polyethylene (UHMWPE) artificial joint for a longer term, hydrophilic acrylic acid (AA) was grafted on UHMWPE powders with the method of ultraviolet irradiation and then the modified powders were hot pressed. The tribological properties of modified UHMWPE sliding against CoCrMo metallic plate on reciprocating tribometer under calf serum, saline and distilled water lubrication during a long-term friction were investigated. The measurement of Fourier-transform infrared spectroscopy indicates that AA is successfully grafted on the surface of UHMWPE powders by photo-induced graft polymerization. Contact angles of UHMWPE are decreased from 83° to 35° by grafting and the surface wettability is effectively improved. The tensile strength of modified sample decreases. The friction coefficient and wear rate of UHMWPE-g-PAA under calf serum, saline and distilled water lubrication are lower than that of untreated UHMWPE. With the increase of grafting ratio, the wear rate of UHMWPE-g-PAA decreases firstly and then increases. The modified UHMWPE with grafting ratio of 3.5 % has the lowest wear rate, which is just quarter of the untreated UHMWPE. The hydrated PAA polymer brushes enclosed in the UHMWPE bulk material provide continuous lubrication during long term sliding.     

Surface grafting of artificial joints with a biocompatible polymer for preventing periprosthetic osteolysis

Periprosthetic osteolysis-bone loss in the vicinity of a prosthesis-is the most serious problem limiting the longevity of artificial joints. It is caused by bone-resorptive responses to wear particles originating from the articulating surface. This study investigated the effects of graft polymerization of our original biocompatible phospholipid polymer 2-methacryloyloxyethyl phosphorylcholine (MPC) onto the polyethylene surface. Mechanical studies using a hip-joint simulator revealed that the MPC grafting markedly decreased the friction and the amount of wear. Osteoclastic bone resorption induced by subperiosteal injection of particles onto mouse calvariae was abolished by the MPC grafting on particles. MPC-grafted particles were shown to be biologically inert by culture systems with respect to phagocytosis and resorptive cytokine secretion by macrophages, subsequent expression of receptor activator of NF-kappaB ligand in osteoblasts, and osteoclastogenesis from bone marrow cells. From the mechanical and biological advantages, we believe that our approach will make a major improvement in artificial joints by preventing periprosthetic osteolysis.     

Influence of a foreign body on the wear of metallic femoral heads and polyethylene acetabular cups of total hip prostheses

Excessive wear of polyethylene in total replacement hip prostheses elicits deleterious biologic reactions and may be thus a limiting factor that compromises the long-term performance of these devices. This study is based on the report of two clinical failures of total hip prostheses with metallic femoral heads and polyethylene acetabular cups. The investigations reveal that foreign bodies (titanium fibermesh pieces) can migrate into the joint space of total hip prostheses and participate in abrasive third-body wear of the polyethylene cups. This excessive wear of polyethylene enhanced by the modification of the metallic counterface roughness is likely to induce the early loosening of the devices.     

Observations of residual sub-surface shear strain in the ultrahigh molecular weight polyethylene acetabular cups of hip prostheses

Analysis of bearing surfaces of explanted cups can help to determine the wear mechanisms that are responsible for generation of wear debris. In this study a microscope polariscope was used to detect residual subsurface shear strains, deformation and subsurface cracks in explanted Charnley acetabular cups. The wear surfaces were compared to an acetabular cup from a hip joint simulator test. The six explanted cups that were studied had all failed after long periods of implantation, with penetrations ranging from 2.1 to 3.8 mm. The explanted and simulator cups both had a smooth, high-wear region. High residual subsurface shear strains were found in the high-wear region of most cups, with certain cups possessing subsurface cracks running parallel with the surface 5–10 m deep, close to the areas of high residual subsurface shear strain. This was caused by plastic deformation and subsurface fatigue of the polymer surface.     

PIRAC Ti nitride coated Ti–6Al–4V head against UHMWPE acetabular cup–hip wear simulator study

Wear behaviour of TiN(titanium nitride)-coated Ti and Ti-6AI-4V alloy against UHMW polyethylene was studied in hip simulation test. Ti alloys possess an excellent combination of mechanical properties and biocompatibility, however, they suffer from inadequate wear resistance. Thus, their use as articulating components of total joint replacements requires surface hardening, e.g. by TiN. Thirty-two millimetre diameter cp-Ti and Ti-6AI-4V femoral heads were coated with several micrometre thick TiN layers employing an original PIRAC nitriding method based on interaction of Ti alloy substrate with highly reactive monatomic nitrogen. The heads were tested against UHMWPE cups at 37 degrees C in Ringer's solution or in distilled water. Simulator tests were performed at peak pressures of 1.5 and 2.0 MPa in a constant rotation mode at the frequency of 1.5 Hz. The wear of UHMWPE was estimated by weight loss, and the worn metallic and polyethylene surfaces were examined in SEM. The wear rate of UHMWPE cups articulating against PIRAC coated Ti and Ti-6AI-4V after up to 4 x 10(6) cycles was significantly lower than that of UHMWPE articulating against 316L stainless steel. No delamination of TiN coatings was observed after 4 x 10(6) cycles. These results suggest that TiN PIRAC coating on Ti-6AI-4V heads could minimise the wear of total hip replacements without compromising the mechanical properties of the femoral component.     

Status of surface modification techniques for artificial hip implants

Surface modification techniques have been developed significantly in the last couple of decades for enhanced tribological performance of artificial hip implants. Surface modification techniques improve biological, chemical and mechanical properties of implant surfaces. Some of the most effective techniques, namely surface texturing, surface coating, and surface grafting, are applied to reduce the friction and wear of artificial implants. This article reviews the status of the developments of surface modification techniques and their effects on commonly used artificial joint implants. This study focused only on artificial hip joint prostheses research of the last 10 years. A total of 27 articles were critically reviewed and categorized according to surface modification technique. The literature reveals that modified surfaces exhibit reduced friction and enhanced wear resistance of the contact surfaces. However, the wear rates are still noticeable in case of surface texturing and surface coating. The associated vortex flow aids to release entrapped wear debris and thus increase the wear particles generation in case of textured surfaces. The earlier delamination of coating materials due to poor adhesion and graphitization transformation has limited the use of coating techniques. Moreover, the produced wear debris has adverse effects on biological fluid. Conversely, the surface grafting technique provides phospholipid like layer that exhibited lower friction and almost zero wear rates even after a longer period of friction and wear test. The findings suggest that further investigations are required to identify the role of surface grafting on film formation and heat resistance ability under physiological hip joint conditions for improved performance and longevity of hip implants.     

Effect of the design parameters on the in vitro wear performance of total shoulder arthroplasties

The loosening of the glenoid component is the main reason for the failure of a total shoulder arthoplasty. It may be caused either by high tensile stresses or by osteolysis of the surrounding bone in response to the presence of particle debris. This failure might be associated with the wear of the implant as occurs with replacement hip and knee joints.The paper reports the findings of a study of the in vitro performance of the currently used total shoulder prostheses to determine the effects of implant geometry on the wear of the polyethylene components and the friction conditions operating within the shoulder prosthesis.The wear performance of the implants was evaluated using a self-developed tribotester, simulating the physiological conditions of a shoulder joint. This study revealed that significantly different wear occurred with conforming and non-conforming articulation and revealed the influence of the thickness of the polyethylene glenoid on the wear and friction occurring in the joint. In this preliminary study significant wear of the polyethylene glenoid component occurred, estimated to be up to 19 mm³/year, which is similar to that found in retrieved implants. The conforming implants demonstrated significantly greater wear than the non-conforming implants (p < 0.05). A significantly lower friction factor, about 0.05 ± 0.01 (p < 0.05), was obtained for the less conforming implants.     

Corrosion behaviour and mechanical properties of functionally gradient materials developed for possible hard-tissue applications

Artificial hip joints have an average lifetime of 10 years due to aseptic loosening of the femoral stem attributed to polymeric wear debris; however, there is a steadily increasing demand from younger osteoarthritis patients aged between 15 and 40 year for a longer lasting joint of 25 years or more. Compliant layers incorporated into the acetabular cup generate elastohydrodynamic lubrication conditions between the bearing surfaces, reduce joint friction coefficients and wear debris production and could increase the average life of total hip replacements, and other human load-bearing joint replacements, i.e. total knee replacements. Poor adhesion between a fully dense substrate and the compliant layer has so far prevented any further exploitation. This work investigated the possibility of producing porous metallic, functionally gradient type acetabular cups using powder metallurgy techniques – where a porous surface was supported by a denser core – into which the compliant layers could be incorporated. The corrosion behaviour and mechanical properties of three biomedically approved alloys containing two levels of total porosity (>30% and <10%) were established, resulting in Ti–6Al–4V being identified as the most promising biocompatible functionally graded material, not only for this application but for other hard-tissue implants.     

高性能陶瓷人工髋关节材料摩擦磨损研究发展

陶瓷已经在人工髋关节假体制作中获得一定的应用,并且有更大的潜力以待开发.对于陶瓷人工髋关节假体,其摩擦磨损性能是决定其寿命的最重要的性能之一,而体外的摩擦磨损测试对于确定其摩擦磨损性能有着重要意义.本文综述了国内外对于氧化铝、氧化锆、氮化硅和碳化硅四种陶瓷的摩擦磨损性能的研究结果,并且结合作者的工作对其进行评论.     

Surface functionalization of biomaterials by radical polymerization

One effective strategy in the field of biomaterials is to develop biomimetic interfaces to modulate the cell behavior and promote tissue regeneration and surface modification is the best way to obtain biomaterial surfaces with the desired biological functions and properties. Surface radical polymerization offers many advantages compared to other methods, for instance, low cost and simplicity, ability to control the surface chemistry without changing the properties of the bulk materials by introducing high-density graft chains and precisely controlling the location of the chains grafted to the surface, as well as long-term chemical stability of the chains introduced by this method due to the covalent bonding. Because of the precise control of the macromolecules and easy preparation, controlled/living radical polymerization has been widely used to modify biomaterials. There are three main techniques: atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible radical addition-fragmentation chain transfer (RAFT) polymerization. Some other grafting methods such as plasma-induced polymerization, irradiation-induced polymerization, and photo-induced polymerization also have great potential pertaining to functionalization of biomaterials and tailoring of surface chemistry. This paper summarizes recent advances in the various grafting polymerization methods to enhance the surface properties and biological functions of biomaterials.     

Preparation and characterization of poly(N-isopropylacrylamide) films on a modified glass surface via surface initiated redox polymerization

A functionalization with 3-aminopropyltriethoxysilane (APTES) monolayer of a hydroxylated glass surface, followed by the surface initiated graft radical polymerization of N-isopropylacrylamide (NIPAm) using amino groups of APTES monolayer chemical bonded with glass surface and Ce⁴⁺ as a redox initiating system. The microstructure of poly(N-isopropylacrylamide) (PNIPAm) film obtained from the redox graft polymerization on the modified glass surfaces was examined by water contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), and the results showed that about 60 nm thickness of thermosensitive polymer (PNIPAm) film successfully formed.     

Gleitpaarungen Keramik/Polyethylen und Keramik/Keramik: Vergleich von in vitro und in vivo Abrieb

Wear Couple Ceramic‐on‐polyethylene and Ceramic‐on‐ceramic: In‐vitro Wear Versus In‐vivo Wear For more than 30 years ceramics are used for heads and acetabular cups of hip joint arthroplasty. The results of wear rates of metal‐on‐polyethylene, ceramic‐on‐polyethylene and ceramic‐on‐ceramic determined from a hip simulator testing were not confirmed when comparing to results of clinical follow ups using either X‐rays methods or investigating retrieved implants. There is consensus that the wear couple of metal‐on‐polyethylene is twice or three times larger than the one for ceramic‐on‐poly‐ethylene. Ceramic‐on‐ceramic offers the option of 5μm/year. When evaluation novel concepts the best is to investigate retrievals because the hip simulator does not mimic all in‐vivo effects. Hip simulator testing is not “worse case testing” as it should be, hip simulator testing is “best case testing”. The results may be used for screening purposes.     

不锈钢表面粗糙度对超高分子量聚乙烯摩擦磨损性能的影响

以超高分子量聚乙烯软骨材料为销样,316不锈钢硬骨材料为盘样,在自制的销－盘式磨损试验机上考察了不锈钢盘样表面粗糙度对超高分子量聚乙烯摩擦磨损性能的影响,并利用光学显微镜观察了摩擦副表面的形貌,结果表明,在干摩擦条件下,表面粗糙度对超高分子量聚乙烯的摩擦磨损有较大影响,存在着适合的表面粗糙度范围,使超高分子量聚乙烯摩擦系数,磨损率最小。     

Application of a stereometric structural analysis for an evaluation of surface and for a forecast of the wear of endoprothesis acetabular cups

To improve the resistance to wear and permanent deformation of polyethylene operating in a polymer/metal friction couple, initial plastic deformation of the polyethylene and its electron irradiation was applied. This contributed to a change of the polymer structure, visible already while machining when the sample surfaces were being prepared for a tribological test. The study shows that the interactions that shape the structure of polyethylene, at the same time cause adequate changes to the stereometric structure of its surface. The parameters of surface microgeometry characterize the future tribological behavior of polyethylene during its operation in a friction couple. It has been shown that an analysis of stereometric parameters may constitute the first projection of polymer wear resistance.     

Examination of 2-methacryloyloxyethyl phosphorylcholine polymer coated acrylic resin denture base material: surface characteristics and <Emphasis Type="BoldItalic">Candida albicans</Emphasis> adhesion

The aim of this study is to evaluate the effects of 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer coating with various concentrations onto acrylic resin denture base material on surface characteristics such as contact angle and surface roughness and on Candida albicans adhesion which is the major factor of denture stomatitis. Specimens, prepared from heat-polymerized acrylic denture base material, were divided into control and three test groups, randomly. Surfaces of the specimens in test groups were coated with poly(MPC) (PMPC) by graft polymerization of MPC in different concentrations (0.25?mol/L; 0.50?mol/L and 0.75?mol/L), while no surface treatment was applied to the control group. Contact angles and surface roughness were examined, and chemical composition of the surfaces was analyzed by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (FTIR) to verify the presence of PMPC coatings. Then, specimens were incubated with C. albicans for 18?h and the number of adhered cells was determined. Upon PMPC coating, the contact angle values statistically decreased, but no difference was found in surface roughness values. A statistically significant decrease was observed in C. albicans adhesion in parallel with the increase in the MPC polymer concentration. There was no significant difference between 0.50?mol/L and 0.75?mol/L groups in terms of adhesion. These findings indicated that graft polymerization of MPC on acrylic denture base material reduces the adhesion of C. albicans, and may be evaluated as a coating for prevention of denture stomatitis.     

Structural modifications induced by ion implantation in metals and polymers used for orthopaedic prostheses

Abstract

Ti–6Al–4V titanium alloy, AISI 316L stainless steel, and UHMW (ultrahigh molecular weight) polyethylene are commonly used as friction materials in orthopaedic joint prostheses. The most interesting property of the titanium alloy is its high corrosion fatigue resistance. However, its friction and wear behaviour is unsatisfactory, even when rubbing against a soft material such as polyethylene; it cannot be used without a surface treatment. The 316L stainless steel has superior friction and wear characteristics, but its mechanical properties are inferior and crevice corrosion is often associated with wear. The UHMW polyethylene wears and flows. Wear is related to the surface properties, whereas creep is linked to the bulk mechanical properties. Ion implantation is a surface treatment mainly used in industry for increasing the wear resistance of metals, such as in cutting tools, and for changing the electrical or optical properties of ceramics and polymers. In the present work, the technique is applied to orthopaedic materials, to improve their surface properties and to increase the lifetime of the prostheses. The treatment does not deteriorate the bulk characteristics of the materials. As far as metals are concerned, structural modifications are produced in a thin layer of <1 μm thickness. They depend on the implantation parameters, namely, nature of the ions, energy, dose, and temperature, and are analysed using glow discharge spectrometry and grazing incidence X-ray diffraction. For polymers, ion implantation leads to two different effects: a modification of the first atomic layers, which induces an increase in wettability, and a variation of the structure at greater depths, which influences the mechanical properties. These effects are analysed using specific methods such as electron spectroscopy for chemical analysis, electron spin resonance, and attenuated total reflectance infrared spectroscopy.

MST/1689     

Tribological performance of the biological components of synovial fluid in artificial joint implants

Abstract

The concentration of biological components of synovial fluid (such as albumin, globulin, hyaluronic acid, and lubricin) varies between healthy persons and osteoarthritis (OA) patients. The aim of the present study is to compare the effects of such variation on tribological performance in a simulated hip joint model. The study was carried out experimentally by utilizing a pin-on-disk simulator on ceramic-on-ceramic (CoC) and ceramic-on-polyethylene (CoP) hip joint implants. The experimental results show that both friction and wear of artificial joints fluctuate with the concentration level of biological components. Moreover, the performance also varies between material combinations. Wear debris sizes and shapes produced by ceramic and polyethylene were diverse. We conclude that the biological components of synovial fluid and their concentrations should be considered in order to select an artificial hip joint to best suit that patient.     

Amphiphilic Graft Copolymers as a Versatile Binder for Various Electrodes of High‐Performance Lithium‐Ion Batteries

It is known that grafting one polymer onto another polymer backbone is a powerful strategy capable of combining dual benefits from each parent polymer. Thus amphiphilic graft copolymer precursors (poly(vinylidene difluoride)‐graft‐poly(tert‐butylacrylate) (PVDF‐g‐PtBA)) have been developed via atom transfer radical polymerization, and demonstrated its outstanding properties as a promising binder for high‐performance lithium‐ion battery (LIB) by using in situ pyrolytic transformation of PtBA to poly(acrylic acid) segments. In addition to its superior mechanical properties and accommodation capability of volume expansion, the Si anode with PVDF‐g‐PtBA exhibits the excellent charge and discharge capacities of 2672 and 2958 mAh g^?1 with the capacity retention of 84% after 50 cycles. More meaningfully, the graft copolymer binder shows good operating characteristics in both LiN_0.5M_1.5O₄ cathode and neural graphite anode, respectively. By containing such diverse features, a graft copolymer‐loaded LiN_0.5M_1.5O₄/Si‐NG full cell has been successfully achieved, which delivers energy density as high as 546 Wh kg^?1 with cycle retention of ≈70% after 50 cycles (1 C). For the first time, this work sheds new light on the unique nature of the graft copolymer binders in LIB application, which will provide a practical solution for volume expansion and low efficiency problems, leading to a high‐energy‐density lithium‐ion chemistry.     

Adhesion of PE modified by capacitively coupled radio frequency plasma-induced graft polymerization of glycidyl methacrylate

A graft polymerization of glycidyl methacrylate (GMA) on the pretreated polyethylene (PE) sheet samples by oxygen capacitively coupled radio frequency (RF) plasma was carried out to improve the adhesive properties of PE. The PE samples were treated with a RF power of 200 W for a treatment time of 40 s and then exposed to an oxygen atmosphere for a saturation time of 10 min. The grafting of the plasma pretreated PE performed in an aqueous GMA solution with the monomer concentration from 20 vol.% to 100 vol.% at a temperature from 20°C to 90°C for a reaction period up to 50 h. The optimum wettability of the graft polymerized PE surface with the concentration of 40 vol.% at the temperature of 70°C and for the time of 24 h was obtained as the static contact angle decreased from 104.2° for the original PE to 67.6° for the graft polymerized. After the graft polymerization, a strong absorption peak of C-O bonding was shown at 1050 cm⁻¹ in Fourier transform infrared spectrum, indicating an introduction of epoxy groups on the graft polymerized surface. Correspondingly, the surface roughness (Ra) increased from 0.137 μm for the original PE to 1.660 μm for the graft polymerized. The maximal lap adhesive strength of the graft polymerized PE samples lapped using a mixture of epoxy resin and curing agent was achieved to about 160 N·cm⁻². The fractured surfaces by tearing of the PE sheet matrix were observed on the tensioned PE samples due to the higher adhesive strength than that of the PE matrix.     

Immobilization of chitosan gel with cross-linking reagent on PNIPAAm gel/PP nonwoven composites surface

This study was to immobilize chitosan (CS), which is a biodegradable and antibacterial polymer, on poly(N-isopropylacrylamide) (PNIPAAm) gel/polypropylene (PP) nonwoven composites surface for wound dressing applications. PP nonwoven has been extensively used due to its porosity, allowing ventilation, high surface area and excellent mechanical properties. However, the hydrophobic surface of PP nonwoven limits its applications; in this study, we used the plasma-activation treatment and subsequently UV-light graft polymerization of NIPAAm gel to improve its hydrophilicity. Chitosan was immobilized onto PNIPAAm gel/PP nonwoven composites surface using the cross-linking agent, glutaraldehyde (GA). This complex was characterized by scanning electron microscopy (SEM). The results indicated that the wettability of the composite was improved after plasma treatment and photo-induced graft polymerization and chitosan was successfully immobilized onto the surface of PNIPAAm gel/PP nonwoven composites through cross-linking process. Finally, the preliminary result shows that chitosan hydrogels displayed antibacterial ability to Escherichia coli and Staphylococcus aureus. The (3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide) (MTT) method indicated that the porous chitosan sponge exhibited good biocompatibility to fibroblast cells.