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

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Progetto e realizzazione di inserti acetabolari di spessore sottile in UHMWPE reticolato additivato con vitamina E

Il basso tasso d'usura e l'alta resistenza meccanica dell'UHMWPE reticolato e additivato ha posto l'attenzione verso l'uso di teste femorali con diametri maggiori per diminuire il rischio d'impingement e dislocazioni. Questo richiede l'utilizzo d'inserti acetabolari più sottili di quelli attualmente in commercio. In quest'ottica è necessario porre particolare attenzione alla resistenza meccanica d'inserti più sottili, e all'efficacia della vitamina E nel combattere l'effetto dell'ossidazione che si manifesta in seguito al processo di reticolazione. Lo scopo del lavoro è quindi di studiare un inserto più sottile di quelli attualmente in commercio per verificarne le performance. Tale studio è svolto su una serie di taglie (compreso inserto prodotto ad-hoc con spessore di 3,6 mm) con spessore di 5,6 mm e di 3,6 mm dalle quali viene isolato il worst-case tramite analisi FEM. Con prove sperimentali è testata la resistenza meccanica del worst-case, e sono monitorate le deformazioni subite e l'ossidazione del campione. Dagli studi FEM è risultato che le tensioni sono mediamente le stesse in tutti i campioni, anche se si sono registrate tensioni leggermente superiori nella taglia intermedia. A differenza delle attese la taglia in cui si sono riscontrate le tensioni massime è la F (non è l'inserto che ha diametro inferiore). A seguito della messa a punto del modello FEM si è identificato un valore d'attrito inferiore a quello atteso. In letteratura i valori d'attrito coppa-inserto sono più grandi del valore che si è identificato tramite simulazioni FEM. Sulla base dei risultati FEM è isolato il worst-case che viene quindi sottoposto a un test dinamico con 6 milioni di cicli atto a valutarne le performance. Gli inserti di spessore ridotto non hanno riportato alcun danno visibile, e la loro integrità strutturale non è stata modificata. Le considerazioni preliminari sono confermate dalla verifica al tastatore meccanico e dall'analisi chimica, dalle quale non si sono evidenziate particolari problematiche. Infatti, da queste verifiche si rileva che l'effetto del creep nella prova accelerata è pressoché trascurabile, e non si riscontrano variazioni dimensionali rilevanti. Anche dall'analisi chimica dei campioni non si evidenzia ossidazione. I valori d'ossidazione dell'inserto testato sono analoghi a quelli del campione non testato, anche quando viene confrontato con l'inserto in UHMWPE vergine si evidenzia un'ossidazione di molto superiore. Questo prova che la vitamina E inibisce i radicali liberi che quindi non causano l'ossidazione con susseguente fallimento dell'inserto. Dai risultati si vede che i campioni non subiscono danni rilevanti, le deformazioni elastiche monitorate nel test dinamico sono pressoché nulle, come gli effetti del creep misurati analizzando i dati ottenuti al tastatore meccanico. Grazie alla presenza della vitamina E non si ha ossidazione, quella rilevata è vicina a zero ed è da imputare alla lavorazione meccanica. Secondo tali considerazioni è possibile affermare che la riduzione dello spessore degli inserti da 5,6 mm a 3,6 mm non ha conseguenze critiche sul loro comportamento, e non comporta un fallimento del dispositivo.

Vitamin E-stabilised UHMWPE for orthopaedic implants:quantitative determination of vitamin E and characterisation of its transformation products

The fate of vitamin E and the formation and identification of its transformation products were investigated at different stages of the manufacturing process of commercially produced cross-linked (by γ-irradiation) UHMWPE stabilised with vitamin E (vitamin E infused-post irradiation) used for tibia-components (as articulating surfaces) in total knee arthroplasty (total knee replacement). Vitamin E (α-tocopherol) and its transformation products were extracted from microtomed Tibia films and the different products were separated, isolated, purified using high performance liquid chromatography (HPLC), and characterised by spectroscopic methods and LC-MS. The amount of vitamin E and that of the products formed in the different Tibia samples and in their extracts were also quantified using FTIR and HPLC analysis and calibration curves. Thorough analysis of the Tibia extracts has shown that a number of vitamin E transformation products were formed at different concentrations at two selected stages of the implant manufacturing process that is before and after sterilisation by γ-irradiation. The identified products were found to correspond mainly to different stereoisomeric forms of a small number of vitamin E transformation products. Most of the observed products were of dimeric and trimeric nature with their identity confirmed through a detailed study of their spectral and chromatographic characteristics. It was found that the products of vitamin E, prior to the sterilisation step but after the crosslinking and doping of vitamin E, were mainly the dihydroxydimers and trimers (Tibia samples at this stage are referred to as “Tibia-VEPE”). After sterilisation and completion of the manufacturing process, additional dimers of vitamin E were also formed (Tibia samples at this stage are referred to as ‘Tibia-VEPE-Sterile’), Furthermore, two tocopherol-derived aldehydes (aldehyde 5-formyl-γ-tocopherol and aldehyde 7-formyl-γ-tocopherol) were also formed but at very low concentrations especially in the Tibia-VEPE-Sterile samples. The question of whether vitamin E becomes chemically reacted (grafted) onto the polymer matrix during the manufacturing process of the Tibia is also addressed.

UHMWPE/SBA-15 nanocomposites synthesized by in situ polymerization

Different nanocomposites have been attained by in situ polymerization based on ultra-high molecular weight polyethylene (UHMWPE) and mesoporous SBA-15, this silica being used for immobilization of the FI catalyst bis [N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato] titanium (IV) dichloride and as filler as well. Two distinct approaches have been selected for supporting the FI catalyst on the SBA-15 prior polymerization. A study on polymerization activity of this catalyst has been performed under homogenous conditions and upon heterogenization. A study of the effect of presence of mesoporous particles and of the immobilization method is also carried out. Moreover, the thermal characterization, phase transitions and mechanical response of some pristine UHMWPEs and UHMWPE/SBA-15 materials have been carried out. Relationships with variations on molar mass, impregnation method of catalyst and final SBA-15 content have been established.

A Study of the Tribological Behavior of Carbon-Nanotube-Reinforced Ultrahigh Molecular Weight Polyethylene Composites

The tribological properties of the high-strength and high-modulus ultrahigh molecular weight polyethylene (UHMWPE) film and the UHMWPE composites reinforced by multiwalled carbon nanotubes (MWCNT/UHMWPE) were investigated using a nanoindenter and atomic force microscope (AFM). The MWCNT/UHMWPE composites films exhibited not only high wear resistance but also a low friction coefficient compared to the pure UHMWPE films. We attribute the high wear resistance to the formation of the new microstructure in the composites due to the addition of MWCNTs.

The high strain rate response of Ultra High Molecular-weight Polyethylene: From fibre to laminate

The effect of strain rate upon the uniaxial response of Ultra High Molecular-weight Polyethylene (UHMWPE) fibres, yarns and laminates of lay-up [0/90]48 has been measured in both the 0/90 and ±45 configurations. The tensile strength of the matrix-dominated ±45 laminate is two orders of magnitude less than that of the fibre-dominated 0/90 laminate, and is more sensitive to strain rate. A piezoelectric force sensor device was developed to obtain the high strain rate data, and this achieved a rise time of less than 1 μs. It is found that the failure strength (and failure strain) of the yarn is almost insensitive to strain rate within the range (10 -1-103 s-1). At low strain rates (below 10 -1 s-1), creep of the yarn dominates and the failure strain increases with diminishing strain rate. The tensile strength of the dry yarn exceeds that of the laminate by about 20%. Tests on single fibres exceed the strength of the yarn by 20%. © 2013 Elsevier Ltd. All rights reserved.

The soft impact response of composite laminate beams

The quasi-static and dynamic responses of laminated beams of equal areal mass, made from monolithic CFRP and Ultra high molecular weight Polyethylene (UHMWPE), have been measured. The end-clamped beams were impacted at mid-span by metal foam projectiles to simulate localised blast loading. The effect of clamping geometry on the response was investigated by comparing the response of beams bolted into the supports with the response of beams whose ends were wrapped around the supports. The effect of laminate shear strength upon the static and dynamic responses was investigated by testing two grades of each of the CFRP and UHMWPE beams: (i) CFRP beams with a cured matrix and uncured matrix, and (ii) UHMWPE laminates with matrices of two different shear strengths. Quasi-static stretch-bend tests indicated that the load carrying capacity of the UHWMPE beams exceeds that of the CFRP beams, increases with diminishing shear strength of matrix, and increases when the ends are wrapped rather than through-bolted. The dynamic deformation mode of the beams is qualitatively different from that observed in the quasi-static stretch-bend tests. In the dynamic case, travelling hinges emanate from the impact location and propagate towards the supports; the beams finally fail by tensile fibre fracture at the supports. The UHMWPE beams outperform the CFRP beams in terms of a lower mid-span deflection for a given impulse, and a higher failure impulse. Also, the maximum attainable impulse increases with decreasing shear strength for both the UHMWPE and CFRP beams. The ranking of the beams for load carrying capacity in the quasi-static stretch-bend tests is identical to that for failure impulse in the impact tests. Thus, the static tests can be used to gauge the relative dynamic performances of the beams. © 2013 Elsevier Ltd. All rights reserved.

The effect of shear strength on the ballistic response of laminated composite plates

The ballistic performance of clamped circular carbon fibre reinforced polymer (CFRP) and Ultra High Molecular Weight Polyethylene (UHMWPE) fibre composite plates of equal areal mass and 0/90 lay-up were measured and compared with that of monolithic 304 stainless steel plates. The effect of matrix shear strength upon the dynamic response was explored by testing: (i) CFRP plates with both a cured and uncured matrix and (ii) UHMWPE laminates with identical fibres but with two matrices of different shear strength. The response of these plates when subjected to mid-span, normal impact by a steel ball was measured via a dynamic high speed shadow moiré technique. Travelling hinges emanate from the impact location and travel towards the supports. The anisotropic nature of the composite plate results in the hinges travelling fastest along the fibre directions and this results in square-shaped moiré fringes in the 0/90 plates. Projectile penetration of the UHMWPE and the uncured CFRP plates occurs in a progressive manner, such that the number of failed plies increases with increasing velocity. The cured CFRP plate, of high matrix shear strength, fails by cone-crack formation at low velocities, and at higher velocities by a combination of cone-crack formation and communition of plies beneath the projectile. On an equal areal mass basis, the low shear strength UHMWPE plate has the highest ballistic limit followed by the high matrix shear strength UHMWPE plate, the uncured CFRP, the steel plate and finally the cured CFRP plate. We demonstrate that the high shear strength UHMWPE plate exhibits Cunniff-type ballistic limit scaling. However, the observed Cunniff velocity is significantly lower than that estimated from the laminate properties. The data presented here reveals that the Cunniff velocity is limited in its ability to characterise the ballistic performance of fibre composite plates as this velocity is independent of the shear properties of the composites: the ballistic limit of fibre composite plates increases with decreasing matrix shear strength for both CFRP and UHMWPE plates. © 2013 Elsevier Masson SAS. All rights reserved.

Collapse of a composite beam made from ultra high molecular-weight polyethylene fibres

Hot-pressed laminates with a [0/90]48 lay-up, consisting of 83% by volume of ultra high molecular-weight polyethylene (UHMWPE) fibres, and 17% by volume of polyurethane (PU) matrix, were cut into cantilever beams and subjected to transverse end-loading. The collapse mechanisms were observed both visually and by X-ray scans. Short beams deform elastically and collapse plastically in longitudinal shear, with a shear strength comparable to that observed in double notch, interlaminar shear tests. In contrast, long cantilever beams deform in bending and collapse via a plastic hinge at the built-in end of the beam. The plastic hinge is formed by two wedge-shaped microbuckle zones that grow in size and in intensity with increasing hinge rotation. This new mode of microbuckling under macroscopic bending involves both elastic bending and shearing of the plies, and plastic shear of the interface between each ply. The double-wedge pattern contrasts with the more usual parallel-sided plastic microbuckle that occurs in uniaxial compression. Finite element simulations and analytical models give additional insight into the dominant material and geometric parameters that dictate the collapse response of the UHMWPE composite beam in bending. Detailed comparisons between the observed and predicted collapse responses are used in order to construct a constitutive model for laminated UHMWPE composites. © 2013 Elsevier Ltd.

耐热树脂的增强与增韧研究

耐热树脂或称特种工程塑料主要包括聚芳飒类、聚醚酮类、聚芳酷、液晶类、聚酞亚胺和聚苯硫醚等。长春应化所已获得有关聚芳醚酮（PEK-C），聚芳醚砜（PES-C）和聚酞亚胺（PEI）等特种工程塑料的专利12项。在特种工程塑料中，PEK-C，PES-C和PEI有着最高的机械强度（室温下的拉伸强度在100MPa以上）。PES-C和PEI的耐热等级最高（热变形温度分别为225℃和220-260℃）。PE工有极好的阻燃性（氧指数为47）和耐磨性。而PEK-C的加工性好、韧性高、耐磨损和抗电击穿等性质突出，其综合物性与英国ICI公司的聚醚醚酮（PEEK）相近，是良好的高性能复合材料基体树脂。具有优异综合物性的PEK-C、PES-C、PEI及其改性系列材料在机械、电子电气、军工、医疗及食品等许多领域有着广泛的应用前景。近年来，长春应化所在酞侧基聚芳醚酮和聚芳醚矾的结构一加工一物性关系及开发应用等方面作了大量的研究工作，主要涉及此两种聚合物的粘弹性、屈服行为、断裂行为、转变与松弛以及复合与共混等方面内容。这些研究工作表明，酞侧基聚芳醚酮和聚芳醚矾经过共混和复合改性能够具有更优异的使用性能，而且这些研究中的一些方法同样可以应用于其它耐热树脂的改性。高分子材料科学的发展趋势就是在更深层次上把握材料的结构特点及其与宏观物性间的相互关耽达到高分子分子设计和材料设计的目标，实现高性能化和高功能化使现有的高分子材料找到更广泛而合理的应用。工程塑料的高性能化是高分子材料科学近年来发展的一个主要方向。为满足航天航空、电子信息、汽车工业家用电器以及机械等多方面技术领域的需要，要求材料的机械性能、耐热性、耐腐蚀性和长期使用性等性能进一步提高。在现有工程塑料品种的基础上通过共混增韧、复合增强等改性方法使其成为高性能的结构材料，是高分子材料私｝学的前沿课题及重要任务。本文采用熔融加工的方法制备了PEK-C和PES-C耐热树脂的共混与复合材料，利用热分析、力学性能检测、微观形貌观察、加工性能检测等手段研究了共混物及复合材料的结构与性能。通过对PEK-C和PES-C的冲击断裂过程的研究，我们发现，两种材料在裂纹起始扩展时所能承受的最大应力值相同，但PES-C的裂纹引发（ti）和扩展（tp）所需时间仅是PEK-C的一半，此即PES-C的冲击强度（I）和断裂韧性（KIC）较低的原因。因此，如能延长裂纹引发和扩展的时间，也就是说如能扩大断裂过程区，抑制裂纹的早期形成就能达到增韧的目的。在PES-C的增韧研究方面，我们可以借鉴通用塑料的增韧方法，即在高聚物基体中，以适当的手段掺加第二相粒子，通过粒子的变形和引发基体在粒子周围产生剪切屈服或银纹化等作用机理，实现增韧目的。所不同的是，对PES-C类耐热树脂来说，实现增韧的同时应保持材料原有的高强度和高耐热性等优良险质。另一方面，由于耐热树脂的加工温度极高，适合于通用塑料的偶联剂等界面改性技术已不再适用于特种工程塑料。针对PES-C的增韧方法和机理的研究工作可归纳为以下三个方面：①刚性有机粒子（PPS，LCP等）增韧；②柔性有机粒子（UHMWPE）增韧；③刚性无机粒子（硅灰石）增韧。物理老化或结构松弛效应使得高聚物材料的结构和宏观物性随时间而发生变化。随时间的增长，PES-C和PES-C／PPS共混物的拉伸强度增加、冲击韧性减小，而且这种变化趋势表现出物理老化过程的自衰减特性。研究结果表明，PES-C／pps共棍物的结构松弛速率比PES-C慢。具有良好界面相互作用的PES一C／PPS共混物材料的强度和韧性始终高于PES-C纯组份聚合物。因此说，pES-C／PPS共混物不仅具有良好的短期性能，而且在高温下长期使用过程中，其力学性能将始终优于纯组份聚合物。我们研究了热固性聚酞亚胺预聚物（P01）增容聚芳醚酮／聚苯硫醚共混物的热学性能、力学性能、形态结构及加工性，对POI在聚芳醚酮／聚苯硫醚共混物中所起的增容作用机理进行了初步探讨。实验发现，PEK-C／PPS共混物在保持PEK一C原有的高强度和高模量的同时，加工流动性和韧性得到一定程度的改善。PEK-C／ppS／Pm三元共混物中，少量的POI能够控制PPS分散相的相区尺寸，防止分散相粒子的自凝聚，起到了增容剂的作用。热固性高聚物预聚体可用于增容热塑性高聚物共混体系，这种增容方法有其特殊性和新颖性，增容后的聚芳醚酮／聚苯硫醚共棍物的力学性能得以改善。利用纤维可以作为结晶性高聚物的异相成核剂的特性，将合适的结晶性高聚物与非晶高聚物共混，可以在在一定程度上改善非晶高聚物与纤维间的界面粘结，提高纤维增强效率。这种方法对PEK-C类耐热树脂尤为重要。通过与即S共混，玻纤增强PEK-C复合材料中纤维与基体间的界面粘结以及纤维的长径比明显增加，因而复合材料的强度和模量显著提高，而且加工流动性也得到一定程度的改善。从考虑综合物性的角度出发，利用结晶性高聚物改善纤维与非晶树脂基体间的界面粘结时，结晶性高聚物的用量存在一个最佳值。与PEEK／GF复合材料相比，PEK-C／PPS／GF复合材料在加工能耗、价格等方面存在很大优势，可以预期这一高性能复合材料可应用于制造高强度、高耐热、耐腐蚀、耐磨损、耐疲劳的往复运动部件、振动或转动等机械零部件。