Morphology, Micromechanical, and Macromechanical Properties of Novel Waterborne Poly(urethane-urea)/Silica Nanocomposites

Morphology, macro-, and micromechanical properties of novel poly(urethane-urea)/silica nanocomposites were analyzed by electron microscopy, dynamic mechanical thermal analysis, and microindentation. The studied nanocomposites were based on a poly(urethane-urea) (PUU) matrix filled by nanosilica, and were prepared from waterborne dispersions of PUU (latex) and SiO₂. The loading of nano-SiO₂ was varied between 0 (neat matrix) and 40 wt% in the dry nanocomposite. The prepared materials were all formally in the rubbery state at room temperature, but they displayed complex elastoviscoplastic behavior, spanning from stiffer elastomeric type to semi-glassy. Because of the employed rigid and highly uniform spherical nanofiller, the materials are of great interest for model microindentation studies. Additionally, because of the polycarbonate-type elastic chains of the PUU matrix, hydrogen bonding in the studied nanocomposites was expected to be rich and diverse, ranging from very strong to weak. In micro- and macromechanical tests, all the elasticity-related properties correlated very strongly. The relations among the properties that related to energy dissipation were complex, and were highly affected by the existence of hydrogen bonding of broadly varied strength, by the distribution patterns of the fine nanofiller, as well as by the eventual locally endured larger deformations during the tests, and the tendency of the materials to cold flow.

Effect of Polydopamine Coating of Cellulose Nanocrystals on Performance of PCL/PLA Bio-Nanocomposites

In bio-nanocomposites with a poly(lactic acid) (PLA)/poly(ε-caprolactone) (PCL) matrix with neat and polydopamine (PDA)-coated cellulose nanocrystals (CNCd), the use of different mixing protocols with masterbatches prepared by solution casting led to marked variation of localization, as well as reinforcing and structure-directing effects, of cellulose nanocrystals (CNC). The most balanced mechanical properties were found with an 80/20 PLA/PCL ratio, and complex PCL/CNC structures were formed. In the nanocomposites with a bicontinuous structure (60/40 and 40/60 PLA/PCL ratios), pre-blending the CNC and CNCd/PLA caused a marked increase in the continuity of mechanically stronger PLA and an improvement in related parameters of the system. On the other hand, improved continuity of the PCL phase when using a PCL masterbatch may lead to the reduction in or elimination of reinforcing effects. The PDA coating of CNC significantly changed its behavior. In particular, a higher affinity to PCL and ordering of PLA led to dissimilar structures and interface transformations, while also having antagonistic effects on mechanical properties. The negligible differences in bulk crystallinity indicate that alteration of mechanical properties may have originated from differences in crystallinity at the interface, also influenced by presence of CNC in this area. The complex effect of CNC on bio-nanocomposites, including the potential of PDA coating to increase thermal stability, is worthy of further study.

[Repeated Sterilization of Articular Components of Joint Replacements with Ethylene Oxide Does Not Affect the Structure and Properties of UHMWPE]

PURPOSE OF THE STUDY In clinical practice UHMWPE is the most commonly used material for manufacturing articular components of joint replacements. The purpose of this study is to find out whether repeated ethylene oxide sterilization results in oxidative degradation of UHMWPE or not and also whether the oxidative degradation of various types of ethylene oxide-sterilized UHMWPE depends on storage time or not. MATERIAL AND METHODS The set included 12 samples of UHMWPE (three samples with different modifications (virgin PE, with E vitamin and cross-linked with thermal treatment) and different number of sterilizations (0×-3×)). The set also included 8 samples of commercial components of hip or knee replacements sterilized with ethylene oxide and stored for different storage periods. The oxidative degradation was assessed by infrared microspectroscopy, based on which the oxidation index (OI), transvinylene index (VI), crystallinity index (CI) and E vitamin index (EI) were calculated. Mechanical properties of UHMWPE were obtained through microhardness measurements. Statistical processing of the results was performed. RESULTS In all the samples, very low oxidative degradation values were reported (most OI values < 0.1). All radiation crosslinked UHMWPE samples showed an increased VI index and a slightly lower crystallinity index. All unmodified samples (irrespective of whether or not and how many times or how long ago the samples were sterilized with EtO) had almost zero value of VI. Changes in crystallinity were negligible (in the rage of 0.56-0.63), which required very accurate measurements of micromechanical properties. Yet, linear correlation was established between microhardness and crystallinity. DISCUSSION All the mentioned indices changed as anticipated: OIs were very low and slightly increased with time of storage, VIs of radiation crosslinked samples grew in proportion to the total gama radiation dose, CIs decreased in samples thermally treated by remelting, and EIs were very low due to negligible concentration of stabiliser (0.1%) in the samples of medical grade UHMWPE. CONCLUSIONS All samples showed zero or minimum oxidative degradation. This confirmed that neither ethylene oxide sterilization, nor multiple EtO sterilization or longer storage of polymer after ethylene oxide sterilization result in major oxidative degradation. Key words: UHMWPE, ethylene oxide, sterilization, oxidation, infrared spectroscopy, microhardness.

Chiral Templating of Polycarbonate Membranes by Pinene Using the Modified Atomic Layer Deposition Approach

In this article, chiral templating of a polycarbonate (PC) membrane by (-)-α-pinene using the atomic layer deposition (ALD) approach is investigated. The templating with the enantiomer of (-)-α-pinene, used as a case compound, was performed either on the original commercial PC membrane or on the PC membrane with a beforehand deposited Al₂O₃ layer. The efficiency of the templating was assessed by a difference in the membrane ability to adsorb/absorb (-)-α-pinene, (+)-α-pinene, and their racemic mixture, using a very sensitive gas sorption analyzer. The results clearly show that the solution-diffusion mechanism rather than the sieving mechanism applied for adsorption/absorption of (-/+)-α-pinene enantiomers, which have the same size of the molecule. The PC membrane with the predeposited Al₂O₃ before the (-)-α-pinene templating shows significantly higher sorption of (-)-α-pinene compared to (+)-α-pinene and racemate, which clearly demonstrates the presence of a chiral recognition effect.

Synergistic effects in Methylcellulose/Hydroxyethylcellulose blend: Influence of components ratio and graphene oxide

A specific feature of water-soluble polysaccharides is formation of organized structures in solutions. This study deals with an unexpected effect of 2-hydroxyethylcellulose (HEC) on structure and mechanical performance of methylcellulose (MC) films. The values of modulus with 5 and 10 % HEC content exceed those of the linear model, which indicates synergistic effect consisting in formation of ordered structures. However, higher content of HEC leads to worse properties corresponding to contribution of its lower parameters. The structural transformations are confirmed by XRD and polarized-light microscopy. Ability of HEC to support formation of ordered structures in MC solutions is indicated by rheology. Important fact is that low graphene oxide (GO) content has a high reinforcing effect on neat MC or HEC, but its presence in blends is accompanied by elimination of HEC-induced structural transformations. The results confirm complex effect of blending and GO on structure and properties of the MC/HEC system.

Nano-modified epoxy: the effect of GO-based complex structures on mechanical performance

The application of nanofillers (NFs) in multicomponent polymer systems is accompanied by important structure-directing effects that are more marked in partially miscible systems, such as polymer-modified epoxy. This study deals with rubber-modified epoxy using different combinations of GO and amine-terminated butadiene-acrylonitrile copolymer (ATBN), including in situ and pre-made grafting. Moreover, GO grafted via planar epoxy groups or solely edge-localized carboxyls was used. It is shown that the grafted ATBN chains promote the assembly of GO-g-ATBN into nacre-mimicking lamellar structures instead of usual exfoliation in thermoplastics. This complex structure of elastically embedded GO leads to the best mechanical performance. It is obvious that a small concentration of the grafted polymer exceeds the contribution of a higher concentration of separately added ATBN. The results highlight the important effect of the degree of grafted chains and geometry of the internal structure of the self-assembled arrays and their effect on the mechanical performance.

ATBN-grafted GO forms nacre-mimicking lamellar structures in epoxy; the effect of grafting geometry on the structure/property relationship is highlighted.

[Comparison of the Quality of the Most Frequently Used New UHMWPE Articulation Inserts of the Total Hip Replacement]

PURPOSE OF THE STUDY For an orthopedic surgeon it is difficult or even impossible to estimate the real quality of ultrahigh molecular weight polyethylene (UHMWPE) liners that are used in the total joint replacements (TJR) just on the basis of information given by the manufacturers. At the same time, the quality of the UHMWPE liner can impact strongly on the total lifespan of the implanted TJR. This work aims at independent, objective comparison of properties of the UHMWPE liners for total hip replacements (THR), which are most frequently used in the Czech Republic. MATERIAL AND METHODS We analyzed seventeen most frequently implanted UHMWPE cups of different manufacturers implanted in the Czech Republic between 2014 and 2015 and four control samples prepared by standard industrial-scale procedures according to our instructions, whose modification (crosslinking, thermal treatment, stabilization and sterilization) was known in detail. The UHMWPE polymer was characterized by four independent microscale methods, suitable for relatively small and irregular specimens such as THR cups: infrared microspectroscopy (IR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). RESULTS The employed methods yielded quite complete information about the investigated UHMWPE materials. IR spectra enabled us to calculate oxidation indexes (OI, measure of oxidative damage), trans-vinylene indexes (VI, measure of absorbed radiation dose during crosslinking and/or sterilization) and crystallinity indexes (CI, amount of crystalline phase that strongly influences mechanical performance). DSC curves were employed in calculation of crystallinities (wc, proportional to CI) and melting points (Tm, proportional to the average thickness of crystalline lamellae). MH measurements confirmed that the observed structure changes showed a real impact on mechanical properties. TGA experiments gave rough estimate of stabilization and, consequently, possible long-term oxidation resistance. Significant correlations among oxidative damage (OI), crystallinity (CI, wc) and microhardness (Hv) were statistically proven. The highest oxidative degradation was usually observed in samples thermally treated by annealing and/or sterilized by gamma irradiation. DISCUSSION The results confirmed our expectations that the UHMWPE liners from various manufacturers can be significantly different as far as their molecular structure, supermolecular structure, and mechanical properties are concerned. The differences among the various UHMWPE can be expected to increase after the implantation during in vivo. CONCLUSIONS From the clinical practice point of view, the results showed the following facts: (i) In the field of THR, all manufactures prefer crosslinked types of UHMWPE due to their increased wear resistance; non-crosslinked UHMWPEs are regarded as obsolete. (ii) Most of the manufacturers prefer ethylene oxide or gas plasma sterilization to gamma sterilization because the gammasterilized UHMWPEs exhibit lower long-term oxidation resistance. (iii) Modern trend is the stabilization of UHMWPEs with vitamin E. Key words: UHMWPE, hip replacements, oxidative degradation, infrared spectroscopy, microhardness.

[The Risk of Total Joint Replacement Infectious Complications in Relation to the UHMWPE Particle Adhesion Area]

PURPOSE OF THE STUDY Periprosthetic joint infection is a major complication which in most of the cases requires a long-term administration of antibiotics and often necessitates undergoing multiple challenging surgeries. Bacterial adhesion to foreign material is one of the key risk factors associated with periprosthetic joint infection. The foreign material with large adhesion area might be also the UHMWPE (Ultrahigh molecular weight polyethylene) particles released during the wear process from the surface of articulating components. The purpose of this study is to evaluate potential adhesion areas of wear particles in relation to diverse distribution of the size and shapes of wear particles in periprosthetic tissue and to assess an increase in the risk of infectious complications associated with an increase in the adhesion area of wear particles. MATERIAL AND METHODS The size and morphology of model and real UHMWPE particles were determined with the use of light microscopy and scanning electron microscopy. By determining the morphological descriptors, the surfaces of individual particles for different distributions of polyethylene particles were calculated. When measuring the model wear particles, 6 model situations were simulated, in which comparisons with the control measurement by the BET (Brunauer-Emmet-Teller) method were made. RESULTS The variability of individual morphological descriptors demonstrates the effect on the total surface of particles. The calculated coefficient defines how many times the particle surface increases when corrected to the given descriptor (elongation, flattening, roughness, porosity). The total area of real wear particles at 1 year is 4,622 cm2, at 20 years it is 92,440 cm2. Based on our calculations, the area of particles where a biofilm is actually formed (approximately 50 bacteria may adhere to a particle of 3µm in diameter) is 809.5 cm2 at 1 year and 16,190 cm2 at 20 years. DISCUSSION According to the measurements, the size of the potential adhesion area of metal parts and polyethylene particles becomes equal already after several weeks of endoprosthesis usage and after a few years it is many times larger. The question is whether the risk of bacterial adhesion, i.e. also the risk of infectious complications of TEP actually increases. The clinical practice suggests that the number of infections e.g. 10, 15 or 20 years after the primary implantation is not statistically higher, despite the confirmed growth of potential adhesion area in the form of UHMWPE particles. This fact could be explained by a partially equal regulatory pathway of infection and polyethylene disease. The immune system stimulated by wear particles might better resist the hematogenic infection. CONCLUSIONS The study outcomes clearly indicate that the area of polyethylene wear particles considerably increases over time. In spite of the fact that only approximately 10% of wear particles show parameters (also with respect to the size of particles and bacteria) for potential bacterial adhesion, this area is many times larger than the area of metal parts of the endoprosthesis. Key words: UHMWPE particle, adhesion, biofilm, wear, TJR infection.

[Comparison of Explanted UHMWPE Hip Replacement Components of Various Manufacturers after 10 Years in vivo]

PURPOSE OF THE STUDY Considered to be one of the most common causes of aseptic loosening of endoprostheses is the THA failure due to the wear of articulating components, UHMWPE in particular. The purpose of this study is to verify, in terms of oxidative damage and other parameters, the differences between the UHMWPE implants made by various manufacturers explanted for aseptic loosening with the same life span in vivo. MATERIAL AND METHODS In the period 2010-2015, a total of 21 THA articulating components (cups) made of Ultrahigh molecular weight polyethylene by seven different manufacturers were explanted. For each manufacturer, three UHMWPE cups with the same life span (10-12 years after the primary implantation) were evaluated. The damage to the examined joint replacements was described in complexity using three different criteria, namely independently by three evaluators - experienced orthopaedic surgeons. The evaluated criteria were the following: degree of osteolysis determined based on the preoperative radiographs, wear rate of the explanted UHMWPE component, and extent of perioperatively detected granuloma. Oxidative damage and other structural characteristics of explanted cups were studied by means of infrared spectroscopy and microhardness testing. The correlation between the clinical orthopaedic assessment and oxidative damage were statistically processed. RESULTS Strong correlations between the oxidative damage and crystallinity, strong correlations between all types of orthopaedic assessments, negligible correlations between trans-vinylene index and all the other quantities, and moderate correlations between the oxidative damage and clinical evaluation were identified. It was confirmed by experimental measurement that the observed high oxidative damage, resulting in increased crystallinity, manifested itself also in micromechanical properties of the material at the respective site of the THA articulating component. DISCUSSION The discussion includes the comparison of correlations of individual quantities as well as potential effects on the differences in values of components made by individual manufacturers. The values are related to the data in literature and generally accepted claims. CONCLUSIONS At the time of failure almost all the components showed severe or even critical oxidative damage that strongly correlated with the overall clinical evaluation of the damage to the implant. This confirmed that the oxidative degradation is one of the main causes of THA failure. Key words: UHMWPE, oxidation index, crystallinity, THA failure, wear.