The interaction between two surfaces leads to the generation of friction and wear of material. Friction and wear are some of the major challenges that may readily be overcome by the third part of tribology called lubrication. Utilizing solid lubricants including polymers, carbon-based materials, soft metals, transition metal dichalcogenides, along with their potential benefits and drawbacks in dry environments can reduce friction. Recently, an emerging class of two-dimensional (2D) transition metal nitrides, carbides or carbonitrides commonly known as MXenes have emerged as an attractive alternative for solid lubrication because of their ability to establish wear-resistant tribo layers and well as low friction and shear strength. Furthermore, the inherent hydrophilic nature of these substances has led to limited dispersion stability and phase compatibility when combined with pure base oils. As a result, their potential use as solid lubricants and lubricant additives has been impeded. To address this issue and enhance the applicability of MXenes as solid lubricants, their surface modification can be an attractive tool. Therefore, this review provides a succinct summary of the current state-of-the-art in surface functionalization of MXenes, a subject that has not yet been thoroughly addressed. Further, the mechanical behavior of MXenes and composites has been discussed, followed by the potential of MXenes as a solid lubricant at micro- and macro-scale. Finally, the existing opportunities and challenges of the research area have been discussed with possible future research directions. We believe, this article will be a valuable resource for MXenes and opens the door to improve the chemical, physical and mechanical properties of MXenes in various applications, such as solid lubrication.

The durability of photografted zwitterionic hydrogel coatings on cochlear implant biomaterials was examined to determine the viability of these antifouling surfaces during insertion and long-term implant usage. Tribometry was used to determine the effect of zwitterionic coatings on the lubricity of surfaces with varying hydration levels, applied normal force, and time frame. Additionally, flexural resistance was investigated using mandrel bending. Ex vivo durability was assessed by determining the coefficient of friction between tissues and treated surfaces. Furthermore, cochlear implantation force was measured using cadaveric human cochleae. Hydrated zwitterionic hydrogel coatings reduced frictional resistance approximately 20-fold compared to uncoated PDMS, which led to significantly lower mean force experienced by coated cochlear implants during insertion compared to uncoated systems. Under flexural force, zwitterionic films resisted failure for up to 60 min of desiccation. The large increase in lubricity was maintained for 20 h under continual force while hydrated. For loosely cross-linked systems, films remained stable and lubricious even after rehydration following complete drying. All coatings remained hydrated and functional under frictional force for at least 30 min in ambient conditions allowing drying, with lower cross-link densities showing the greatest longevity. Moreover, photografted zwitterionic hydrogel samples showed no evidence of degradation and nearly identical lubricity before and after implantation. This work demonstrates that photografted zwitterionic hydrogel coatings are sufficiently durable to maintain viability before, during, and after implantation. Mechanical properties, including greatly increased lubricity, are preserved after complete drying and rehydration for various applied forces. Additionally, this significantly enhanced lubricity translates to significantly decreased force during insertion of implants which should result in less trauma and scarring.

Contact lenses are one of the most successful applications of biomaterials. The chemical structure of the polymers used in contact lenses plays an important role in determining the function of contact lenses. Different types of contact lenses have been developed based on the chemical structure of polymers. When designing contact lenses, materials scientists consider factors such as mechanical properties, processing properties, optical properties, histocompatibility, and antifouling properties, to ensure long-term wear with minimal discomfort. Advances in contact lens materials have addressed traditional issues such as oxygen permeability and biocompatibility, improving overall comfort, and duration of use. For example, silicone hydrogel contact lenses with high oxygen permeability were developed to extend the duration of use. In addition, controlling the surface properties of contact lenses in direct contact with the cornea tissue through surface polymer modification mimics the surface morphology of corneal tissue while maintaining the essential properties of the contact lens, a significant improvement for long-term use and reuse of contact lenses. This review presents the material science elements required for advanced contact lenses of the future and summarizes the chemical methods for achieving these goals.

Despite the extensive use of intermittent catheters (ICs) in healthcare, various issues persist for long-term IC users, such as pain, discomfort, infection, and tissue damage, including strictures, scarring and micro-abrasions. A lubricous IC surface is considered necessary to reduce patient pain and trauma, and therefore is a primary focus of IC development to improve patient comfort. While an important consideration, other factors should be routinely investigated to inform future IC development. An array of in vitro tests should be employed to assess IC\'s lubricity, biocompatibility and the risk of urinary tract infection development associated with their use. Herein, we highlight the importance of current in vitro characterisation techniques, the demand for optimisation and an unmet need to develop a universal \'toolkit\' to assess IC properties.

Low temperature and high humidity conditions significantly degrade the performance of solid-state lubricants consisting of van der Waals (vdW) atomic layers, owing to the liquid water layer attached/intercalated to the vdW layers, which greatly enhances the interlayer friction. However, using low temperature in situ atomic force microscopy (AFM) and friction force microscopy (FFM), we unveil the unexpected ultralow friction between two-dimensional (2D) ice, a solid phase of water confined to the 2D space, and the 2D molybdenum disulfides (MoS2). The friction of MoS2 and 2D ice is reduced by more than 30% as compared to bare MoS2 and the rigid surface. The phase transition of liquid water into 2D ice under mechanical compression has also been observed. These new findings can be applied as novel frictionless water/ice transport technology in nanofluidic systems and promising high performance lubricants for operating in low temperature and high humidity environments.

Poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm-co-AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0-12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength (I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient (µ), which decreased with increasing AA concentration. P(AAm-co-AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity (µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) (µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm-co-AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface.

Our experiments aimed to study the influence of layered materials with nanometric-scale particles, which are part of lubricant oils, on their tribological properties. The object of this study was a lubricant oil made using base oil PAO4, which contained nanoparticle hexagonal boron nitride (nano h-BN) and a dispersant based on succinic acid imide. Comparative tests for engine oil (CB30) were also performed. The paper presents the method of preparing the test material and the tribological test results, including wear spot diameter (wear mark), limit wear load, and seizure load. The test results obtained demonstrate that nano-hexagonal boron nitride improves the tribological properties of lubricant oils. However, oil preparation and the quantitative selection of components markedly influence the results.

Reduced-fat food products can help manage diet-related health issues, but consumers often link them with poor sensory qualities. Thus, high-quality fat replacers are necessary to develop appealing reduced-fat products. A full-fat model emulsion was reduced in fat by replacing fat with either water, lactose, corn dextrin (CD), inulin, polydextrose, or microparticulated whey protein (MWP) as fat replacers. The effect of fat reduction and replacement, as well as the suitability of different types of fat replacers, were determined by analyzing fat droplet size distribution, composition, rheological and tribological properties, and the dynamic aroma release of six aroma compounds prevalent in cheese and other dairy products. None of the formulations revealed a considerable effect on droplet size distribution. MWP strongly increased the Kokini oral shear stress and viscosity, while CD exhibited similar values to the full-fat emulsion. All four fat replacers improved the lubricity of the reduced-fat samples. Butane-2,3-dione and 3-methylbutanoic acid were less affected by the changes in the formulation than butanoic acid, heptan-2-one, ethyl butanoate, and nonan-2-one. The aroma releases of the emulsions comprising MWP and CD were most similar to that of the full-fat emulsion. Therefore, CD was identified as a promising fat replacer for reduced-fat emulsions.

Reduced-fat products can help to fight obesity and its associated health risks. To develop appealing products, both product-specific fat replacers and suitable analytical methods for the characterization of fat-associated properties are important. The rheology, tribology, texture, and spreadability of a reduced-fat mayonnaise with different concentrations of corn dextrin were analyzed to determine properties such as flow behavior, viscosity, lubricity, firmness, and stickiness. Additionally, a sensory panel analyzed the samples for their mouthfeel (creaminess, firmness, and stickiness). Correlations between the results of the instrumental methods suggested that the analytical effort for the future development of appealing reduced-fat food products can be reduced. In addition, several correlations were identified between the instrumental and the sensory data. Results from tribological measurements correlated with the sensory attribute of stickiness, suggesting that tribometry can complement or constitute an alternative to complex and expensive human sensory tests. Additionally, the use of Stevens\' power law showed a high correlation between the Kokini oral shear stress and the sensory attribute of creaminess. The instrumental texture properties (firmness, stickiness) also correlated with the sensory sensation. The identified correlations obtained from comparing different methods may help to estimate the possible applications of new fat replacers and facilitate innovative product development.

UNASSIGNED: The American Society for Testing and Materials (ASTM), considered the gold standard worldwide, requires only testing in physiological saline solution to simulate in vivo conditions in standard testing of spinal implants.
UNASSIGNED: We conducted an in vitro study to identify an industrial lubricant with characteristics that are most similar to those of biologically lubricating fat, blood, and tissue fluids. The use of such a material could standardize the results of in vitro mechanical tests for better clinical applications.
UNASSIGNED: Our study has shown that the lubricity of physiological saline was well below that of human soft tissues and tissue fluids, and among the motor oils, Castrol GTX3 provided a testing environment similar to that of a living organism.
UNASSIGNED: With the intention of standardizing and preventing a biological hazard, we have developed a reproducible mechanical testing proposal based on our experiments, which, in addition, would allow us to avoid many misunderstandings and contingencies.