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Due to their cost-effective fabrication, easy integration, and low frequency working range, triboelectric nanogenerators (TENGs) demonstrate tremendous potential in green energy harvesting to power smart devices and the internet of things (IoT). However, there is an urgent need to synergistically maximize their output and improve their durability to ensure a long-lasting high performance. This study aims at elucidating the performance of protective thin films deposited on the wear-prone PTFE surface of TENGs including doped and undoped, single- and multi-layer hydrogenated DLC films, MoS 2 coatings fabricated by physical vapor deposition and multi-layer Ti 3 C 2 T x (MXene) films. The deposited coatings are characterized by electron microscopy, and Raman spectroscopy. Their triboelectric performance is analyzed for TENGs operating in contact separation and freestanding sliding modes. We verified that MXenes outperformed the other films in contact separation mode due to the good electron gain ability of functional oxygen and fluorine groups. In sliding mode, the undoped a-C:H coating performed on a comparable level to the uncoated reference and superior to the tungsten-doped DLC and MoS 2 films. The film withstood long-term tests without notable signs of wear; merely the output slowly decreased with time due to graphitization and thus potential material transfer to the mating body. [Display omitted] • DLC, MoS 2 , and Ti 3 C 2 T x thin films were deposited on PTFE surface of TENGs. • Triboelectric performance was studied in contact separation and sliding mode. • Ti 3 C 2 T x thin films performed best in contact separation mode. • Undoped DLC showed promising results in long-term sliding tests. [ABSTRACT FROM AUTHOR]

Despite the excellent clinical performance of joint replacements, wear-induced aseptic loosening is a main cause of premature implant failure. Tribological testing is usually carried out using bovine serum as an artificial synovial fluid. In order to gain new insights into the suitability to simulate human synovial fluid and provide recommendations for the conditions of tribological testing, accurate rheological measurements on the influence of temperature, shear rate and pressure on density and viscosity were performed. Thus, a temperature dependence of density and viscosity could be verified, whereas both values decreased with higher temperatures. The temperature dependency of viscosity could be approximated by an Arrhenius model. Moreover, shear-thinning characteristics could be demonstrated and fitted to a Cross model, which agreed well with investigations on human synovial fluid reported in literature. Furthermore, an anomaly of pressure dependence of viscosity was found and correlated with the behavior of water as a main constituent. At room temperature, the viscosity initially decreased to a minimum and then increased again as a function of pressure. This was no longer distinct at human body temperatures. Consequently, the present study confirms the suitability of bovine serum as a substitute synovial fluid and emphasizes the importance of realistic testing conditions in order to ensure transferability and comparability. Image 1 • Rheological measurements on bovine serum as artificial synovial fluid. • A temperature dependence of density and viscosity could be verified. • Shear-thinning characteristics could be demonstrated. • An anomaly of pressure dependence of viscosity was found. • This study emphasized the importance of realistic testing conditions for transferability. [ABSTRACT FROM AUTHOR]

Total knee replacements are an effective surgical treatment to restore the function of the knee. For an adequate design, knowledge about stresses and lubrication conditions is vital. Part II proposes a fully-coupled transient 3D model for the soft elastohydrodynamically lubricated contact based upon the generalized Reynolds equation and Finite Element Method while Part I of this study focused on experimental observations. Within the scope of this contribution, a numerical model is presented and validated with experimental data. Good agreement between model predictions and experimental data was found. A strong influence of fluid and geometry assumptions and transient effects were found. Besides, it was demonstrated that the rheological synovial fluid properties have a decisive role in the tribological behavior. Image 1 • Combination of experimental investigations with FEM-based numerical modeling. • Good agreement when all relevant influencing factors are considered. • Load and geometry dominated the pressure and film formation in the stance phase. • The swing phase was largely influenced by the kinematics. • The fluid film formation was largely affected by reversal points of motion. [ABSTRACT FROM AUTHOR]

This contribution is aimed at the detailed understanding of lubrication mechanisms within total knee replacement. While Part I is focused on the experimental investigation, Part II deals with the development of a predictive numerical model. Here, a knee simulator was used for direct optical observation of the contacts between a metal femoral and a transparent polymer components. Transient dynamic conditions were applied. Mimicked synovial fluids with fluorescently labelled constituents were used as the test lubricants. The results showed that γ-globulin forms thin boundary lubricating film, being reinforced by the interaction of phospholipids and hyaluronic acid. Further development of lubricating film is attributed to albumin layering. Based on the results, a novel lubrication model of the knee implant is proposed. Image 1 • Employment of a knee simulator for exploration of lubrication mechanisms within TKA. • Comparison of experimental results with predictive numerical model. • Consideration of transient loading and kinematic conditions corresponding to ISO. • Fluorescent labeling enabling to determine the role of specific fluid constituents. • Novel lubrication model of TKA is proposed. [ABSTRACT FROM AUTHOR]