Influence of Texture on Wear Behavior of 3D Printed HDPE/UHMWPE Bioimplant Material

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Movva, Sahitya
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Abstract
Polyethylene (PE) has been extensively used for implants as an acetabular cup in prosthetic hips and as a tibial plateau and patellar surface in prosthetic knees for total joint arthroplasty since the mid-twentieth century. Ultra-high molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) have been first adopted by Sir Charnley in the early 1960s as bearing surfaces against a metallic or ceramic part in the total joint replacement and have been used in clinical practice since then. However, long-term clinical use suggests that the friction between these parts leads to the failure of the PE component predominantly through surface wear resulting in the release of polymer debris that stimulates osteolysis causing loosening of the implant and eventually leading to its failure. These prosthetics require the PE component to have minimum wear in the directions of joint movement. Hence, tailoring the material response by altering the microstructure via texturing PE to have minimal wear in the essential directions will prove beneficial for improving the durability of the PE component and hence the longevity of the implant. In this study, the effect of texture and microstructure on wear behavior of two types of 3D printed HDPE/UHMWPE/HDPE_wax trimodal reactor blends, RB1 and RB2, along with 3D printed HDPE has been investigated and compared to that of injection molded samples. Both RB1 and RB2 contain 10% of UHMWPE by weight with RB2 having a higher overall weight averaged molecular weight (Mw) than RB1. Texture has been induced in the polymer samples using injection molding or fused deposition modeling where 3D printing speed and orientation have been varied to study their influence on texture and wear behavior. The texture components have been characterized through pole figures and orientation distribution functions using Wide Angle X-ray Diffraction. To determine the effect of texture on the wear behavior of the polymer samples, the wear properties at different loads, frequencies, fret orientations and number of fret cycles have been characterized using the fretting module of the NanoTest system. The fretting results provided the variation of wear amount and elastic recovery response with different fretting and processing parameters. The variation of wear with printing speed/orientation depended on the combination of fretting parameters used. Presence of high intensity <001> texture components in RB1 and RB2 increased their wear resistance compared to that of HDPE where there are no such components present. More specifically, combination of {110}<001>, {320}<001> and {310}<001> or the combination of {110}<001>, {320}<001> and {100}<001> with either {430}<001> or {120}<001> produced the least wear in general. The wear resistance increased by two to three times depending on the texture components present and fretting parameters used. The variations of hardness (H) and elastic modulus (E) with load have been obtained from NanoTest’s microindentation experiments. Presence of UHMWPE in RB1 increased both the hardness and elastic modulus, obtained from indentation, compared to those of HDPE and increase in Mw of RB1 resulting in RB2 further increased these properties. H/E ratio and (load* H/E2) ratio, where load is the applied normal load during microindentation, have been investigated to look at the correlation of these properties with wear. However, the correlation has been limited to some cases only but not all, possibly due to the direction of wear mode not coinciding with the direction in which these mechanical properties have been measured and also not taking the effect of all fretting parameters into account for mechanical properties’ measurements. The morphological features have been investigated using SEM and TEM. The SEM images showed the presence of shish-kebab structures in RB1 and RB2 formed by HDPE and HDPE_wax crystallizing onto extended-chain UHMWPE. From TEM images, it has been observed that in all the 3D printed samples, the fiber orientation is along the direction of 3D printing orientation and for injection molded ones, it is along the length of the sample. The physical property, bulk density, has been characterized using a density meter to look at its correlation with mechanical or wear properties but not much correlation has been found. The thermal and thermo-mechanical properties have been characterized using DSC and DMA respectively. On an average, crystallinity increased very slightly with the addition of UHMWPE in RB1 compared to HDPE and further increased slightly in RB2 with increase in Mw suggesting that the wear behavior of the samples is mostly affected by crystalline regions’ orientations i.e., texture. The thermo-mechanical behavior showed that there is a considerable decrease in storage modulus and increase in tan δ of all the samples with temperature. The storage moduli start decreasing at a temperature of about 40 oC, slightly higher than the body temperature of 37 oC, which might be a concern to the arthroplasty application unless the properties of the specimen in consideration are far higher than those required by the PE component making them comparable at in vivo conditions to those of the traditional PE component used in implants. The results of this work indicate that the evolution of texture through change in composition, processing and its parameters affects the wear behavior of HDPE, RB1 and RB2. By altering the microstructure, wear resistance is found to increase by two to three times depending on the fretting parameters used. Therefore, it is possible to tailor the material response to obtain minimal wear and thereby decrease surface wear of the PE component improving its durability. Further investigation using modeling will help correlate mechanical properties with wear behavior to obtain optimum performance of the PE component in implants. Overall, this work helps determine the global effect of texture on wear behavior and other properties of the 3D printed HDPE/UHMWPE/HDPE_wax trimodal reactor blends useful to improve the longevity of implants.
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2021-12-15
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